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Sunday, January 11, 2015

Stop worrying, the bees are doing just fine, thank you.

Bee experts shred ‘Harvard’ neonics-Colony Collapse Disorder study, upbraid journalists for ‘activist science’

| December 19, 2014 |
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2014-12-14-deadbee-thumb

Chensheng Lu was in his element last month, delivering an impassioned speech before a green group at Harvard Law School. The School of Public Health professor was lecturing on his favorite topic–his only subject these days, as it has become his obsession: why he believes bees around the world are in crisis.

Lu is convinced, unequivocally, that a popular pesticide hailed by many scientists as a less toxic replacement for farm chemicals proven to be far more dangerous to humans and the environment is actually a killer in its own right.
“We demonstrated that neonicotinoids are highly likely to be responsible for triggering Colony Collapse Disorder in bee hives,” claimed Lu. The future of our food system and public health, he said, hangs in the balance.

Lu is the Dr. Doom of bees. According to the nutritionist — but not clear to most other experts in the field — colony collapse disorder (CCD), which first emerged in 2006, can be directly linked to “neonics,” as the now controversial class of pesticides is often called, and also to genetically modified crops. Phased in during the 1990s, neonics are most often used by farmers to control unwanted crop pests. They are coated on seeds, which then produce plants that systemically fight pests.

To many environmental activists, the pesticide does more harm than good, and they’ve found their champion in Chensheng Lu. It’s been a busy fall for the professor, jetting back and forth between Boston and Washington, with forays around the United States to talk to adoring audiences. He presents himself as the defender of bees, and this fiery message has transformed a once obscure academic into a global “green” rock star, feted at events like last month’s lunch talk at Harvard.

The sudden abandoning of hives by honey bees known as Colony Collapse Disorder has emerged as one of the hottest science mysteries in recent years. Lu has authored two extremely controversial papers on CCD: one in 2012 and a second published this past spring. He and his two beekeeper colleagues – there were no entomologists on his tiny research team – contend that neonicotinoids present a mortal threat to bees. Not only that, Lu claims, neonics endanger humans as well, accelerating Parkinson’s Disease.

Lu reached folk hero status among environmentalists last May when the Harvard School of Public Health launched a promotional campaign touting his latest, controversial research: “Study strengthens link between neonicotinoids and collapse of honey bee colonies,” the press release claimed. Before the study was even circulated, stories in some mainstream publications including Forbes ran the release with only a pretense of a rewrite.

The story exploded on the Internet. Many environmental and tabloid journalists painted an alarmist picture based on Lu’s research: “New Harvard Study Proves Why The Bees Are All Disappearing,” “Harvard University scientists have proved that two widely used neonicotinoids harm honeybee colonies,” and “Neonicotinoid Insecticide Impairs Winterization Leading to Bee Colony Collapse: Harvard Study” are three of hundreds of blog posts and articles.

Behind the headlines

Although public opinion has coalesced around the belief that the bee death mystery is settled, the vast majority of scientists who study bees for a living disagree–vehemently.

How could a “Harvard study” and a sizable slice of the nation’s press get this story so wrong?.

The buzz that followed the publication of Lu’s latest study is a classic example of how dicey science can combine with sloppy reporting to create a ‘false narrative’–a storyline with a strong bias that is compelling, but wrong. It’s how simplistic ideas get rooted in the public consciousness. And it’s how ideology-driven science threatens to wreak public policy havoc.

Bees are important to our food supply. They help pollinate roughly one-third of crop species in the US, including many fruits, vegetables, nuts and livestock feed such as alfalfa and clover. That’s why the mystery of CCD is so troubling.

One of the central problems with Lu’s central conclusion–and much of the reporting–is that despite the colony problems that erupted in 2006, the global bee population has remained remarkably stable since the widespread adoption of neonics in the late 1990s. The United Nations reports that the number of hives has actually risen over the past 15 years, to more than 80 million colonies, a record, as neonics usage has soared.

Country by country statistics are even more revealing. Beehives are up over the past two decades in Europe, where advocacy campaigns against neonics prompted the EU to impose a two-year moratorium beginning this year on the use of three neonics. 2014-12-14-european_union_beehive_totals.jpg
Last February, the government of Australia, where neonics are used extensively, reaffirmed that “honeybee populations are not in decline despite the increased use of [neonicotinoids] in agriculture and horticulture since the mid-1990s.” Its central finding was just the opposite of what many in the media have reported: The APVMA (Australian equivalent of the EPA) concluded, “[T]he introduction of the neonicotinoids has led to an overall reduction in the risks to the agricultural environment from the application of insecticides.”
According to statistics Canada honey bee colonies have increased from 521,000 in 1995 to 672,000 in 2013, a record. North American managed beehive numbers have held stable over the last two decades. 2014-12-14-NA.pngSources: USDA and Statistics Canada

So how did the narrative that the world faces a beepocalypse become settled wisdom? The media have widely conflated two parallel but different phenomena: Bee deaths related to CCD and bees dying from other causes.

Bee health took a sharp hit in the 1980s and has been struggling during the winter months for decades coinciding with the global spread of the parasitical Varroa destructor mite and the sub-lethal effects of miticides used to control the parasite. But these overwinter losses, while troubling, haven’t translated into declines in the overall bee population because bees reproduce rapidly in warmer months.

The bee health issue erupted into the public consciousness in 2006, when bee die-offs mysteriously spiked–in California to as high as 80%.

GMOs and cell Phones did it?

The event was dubbed CCD by a team of entomologists because of the unique characteristics of the deaths: the unusual abandonment of hives by the oldest bees leaving behind larvae, the queen and food stores.

Advocacy groups originally pointed to cell phones and genetically modified crops as the likely culprits, and some fringe organizations, like the fringe activist group the Organic Consumers Association, still do. But CCD gradually subsided.

Dennis van Engelsdorp, a University of Maryland entomologist who was part of the research team that named CCD, has written to me that there has not been a single field CCD incident in the last three years, except cases linked to the Nosema fungus. Confusing the picture, overwinter bee deaths also increased in the years after the CCD scare, reaching 30% or more in the US and in some European countries. Confounding doomsayers, losses plummeted to 21.9% over the winter of 2011-2012, jumped again during the following year’s frigid weather, then settled back into the low 20s.

In some states, like North Dakota, which is the largest honey producer in the US, the number of bee colonies has hit an all-time high.

The recent trend in Europe is also encouraging. In April, the EU released a report called Epilobee that surveyed bee health in 2012-2013. Seventy-five percent of bees suffered overwinter losses of 15% or less, a level considered well within the acceptable range in the US. Only countries in Europe’s far north, home to 5% of the bee population, and which suffered through a bitter winter, experienced losses of more than 20%.

In short, most entomologists scoff at media references to a beemageddon.

But that’s exactly what Lu claims.

Hyping the “Harvard” studies

Mother Jones, in its coverage led by food reporter Tom Philpott, has been particularly myopic in its promotion of Lu’s controversial views and the scientifically dubious claim that neonics is the prime driver of bee deaths. It’s run more than a dozen articles about the alleged mortal threat posed by neonics. Upon the release of Lu’s most recent study, Philpott titled his article, “Did Scientists Just Solve the Bee Collapse Mystery?”

There were no “scientists” behind the Lu study, of course–only Lu himself. But rather than seeking out views of established experts in the field, he had Lu and only Lu answer the question he posed.

“[C]oming on the heels of a similar [study] he published in 2012, the CCD mystery has been solved,” he wrote. Philpott now unqualifiedly, and incorrectly say mainstream entomologists, refers to neonics as “bee killer chemicals.”

Who is Chensheng (Alex) Lu, the Dr. Doom of honey bees? He is an environmental researcher with the Harvard School of Public Health with no formal training in entomology. His two bee papers are “Harvard studies” only in the sense that the only scientist who conducted the studies has a Harvard faculty appointment; his co-authors are local beekeepers. Both studies appeared in one of the most obscure science journals in the world, a marginal Italian journal.

Lu emerged out of academic obscurity two years ago with the publication of his first study on bee deaths. He promoted a simple explanation, the kind that energizes activists: A new class of pesticides, promoted by large chemical companies as a safer alternative to older chemicals, was a hidden killer.

“I kind of ask myself,” Lu told Wired in 2012. “Is this the repeat of Silent Spring? What else do we need to prove that it’s the pesticides causing Colony Collapse Disorder?”

The second coming of Silent Spring? Almost from the day his first study was published, Lu was making grandiose claims. By his own admission, he is the definition of an activist scientist. He is on the board of The Organic Center, an arm of the multi-million dollar Organic Trade Association, a lobby group with strong financial interest in disparaging conventional agriculture, synthetic pesticides and neonics in particular–a conflict of interest that Lu never acknowledges and to my knowledge no other journalist has reported.

Earlier this month, OTA announced it is partnering with Lu to tout the benefits of organics, including promoting the dangers of neonics.

Many of the world’s top scientists have challenged his research. Dennis vanEngelsdorp called Lu’s first study “an embarrassment” while Scott Black, executive director of the bee-hugging Xerces Society for
Invertebrate Conservation, characterized it as fatally flawed, both in its design and conclusions.
University of Illinois entomologist May Berenbaum, who chaired the National Academy of Sciences 2007 National Research council study on the Status of Pollinators in North America called it “effectively worthless” to serious researchers. “The experimental design and statistical analysis are just not reliable,” she said.

Beekeepers have been skeptical as well. Lu’s findings contradicted what they witnessed in the fields. If neonics were a mystery killer, then not using them should translate into healthier bee stocks; but that’s not what has happened.

“In places where neonicotinoid pesticides have been banned, such as France and Italy, there’s no evidence that honeybee populations have rebounded,” noted Hannah Nordhaus, beekeeper and author of the bestseller The Beekeepers’ Lament.

Lu has been defiant since the stinging expert rejection of his first paper. He sees the fingerprints of a Big Ag conspiracy of chemical companies, USDA and entomologists who he believes are ignoring the dangers to bees. Those are damning charges if true, but Lu had yet to present any evidence to back them up–until the publication of his newest paper last May.

Lu monitored 18 hives, a small number for such a complex study, comparing two different pesticides in different locations. He fed bees high fructose corn syrup laced with two neonics, imidacloprid and clothianidin, for 13 weeks. It was an odd choice because bees in fields usually only feed for as few as two weeks. Six of the 12 colonies fed neonics eventually ended up showing substantial deaths over the winter, as did one of the six control colonies.

According to Lu and his beekeeper co-authors, this proved that neonics cause CCD.

To seasoned observers of the bee controversy, the “new” study looked like more of the same. “Lu’s sample sizes are astonishingly small,” May Berenbaum told me, ticking off a litany of problems. “He never tested for the presence of pathogens, so his conclusions dismissing other likely causes don’t follow from his data. The whole study just doesn’t hold together. And I’m not being a fusspot here. It’s unfortunate this was presented as a Harvard paper because it gives this credibility that it doesn’t deserve.”

Twitter lit up with critical comments, starting with Nordhaus: 2014-12-14-Nordhaus.png
Many other critical posts followed, including by Brian Ames, a prominent apple grower, artisanal honeymaker and beekeeper: 2014-12-14-Ames.png

Even rudimentary digging by reporters would have turned up the revealing fact, unreported by the adulatory environmental press, that first study was rejected by Nature, as Lu himself has acknowledged, before ending up in the Bulletin of Insectology, a marginal “pay for play” publication that is known to publish research often rejected by mainstream peer-reviewed journals.

(The Bulletin of Insectology has an “impact factor” (IF) of 0.375, which means that the average paper from that journal is cited by another journal approximately once every three years; in contrast, Nature, which rejected Lu’s first paper, has an IF of 51).

The second study faced the same fate. Unable to get his work published by credible journals, Lu returned to the same publication that put out his first piece–perhaps the only journal in the world that would publish it.

“Anyone at this point in time who wishes to make a contribution to the study of potential effects of neonicotinoids on honey bees–or any other aspect of honey bee health–and publishes this data in the extremely obscure journal Bulletin of Insectology is very hard to take seriously,” Colorado State University entomologist Whitney Cranshaw emailed me.

A week does not go by without one advocacy group or government official or activist scientist making sensational claims about the supposed catastrophic dangers that neonics supposedly present.

In November, for example, advertisements began appearing across Ontario in Canada warning, “neonic pesticides hurt our bees and us,” one of them accompanied by a young boy gazing sadly at a dead bee. 2014-12-14-ontario_2.jpg

They were placed by a fringe advocacy group, the Canadian Association of Physicians for the Environment; its primary funder is David Suzuki, a once prominent but now long retired geneticist who more recently has become known for rants against GMO foods.

That kind of hyperbole, scientists say, obscures the complex story of what’s really happening to bees and why–and the risks of advocacy groups and activist journalists driving science and agricultural regulations into a policy ditch.

Which brings us back to the curious case of Alex Lu.

Although Lu’s most recent paper, published last spring, was not clear on this point, the nutritionist has publicly maintained that neonic seed treatments are the driving cause of CCD. Let’s be clear. Neonics are an appropriate subject for serious research. They are neurotoxic pesticides. Because they rely on a complex set of behaviors, bees exposed to high volumes could conceivably become drunk and ill. Scientists are and should continue to examine this chemical and all agricultural chemicals.

But the emphasis of many popular articles, and Lu’s study, is way out of whack with the potential dangers that scientists believe are presented by neonics. The pesticide is applied to seeds sparingly — only about 1-3 ppb is commonly found in pollen or nectar after application, levels way below safety concerns. Plants grown from a treated seed often need no further insecticidal treatment, unlike many competing chemicals. And in contrast to earlier generation insecticides that required multiple applications, when infestations are severe a single additional spraying generally suffices.

Lu steadfastly claims that bees that died in his studies were fed field realistic levels doses–statements echoed uncritically by reporters without, it turns out, cross checking with beekeepers or entomologists. “Chensheng Lu and his team treated 12 colonies with tiny levels of neonics,” Mother Jones maintained.

Tiny?

As Randy Oliver, a well known beekeeper, wrote on his Scientific Beekeeping blog, Lu fed his test colonies a pesticide brew of about 135 parts per billion (ppb). That’s 100 times higher then the 1-3 ppb commonly found in pollen or nectar, a level far below safety concerns. Rather than citing the chemicals’ ppb, some reporters touted the physical size of the dose, a worthless measurement. Lu also fed bees every week for 13 straight weeks when the real world application is just a few weeks at most.

“It’s hard to imagine anyone even reviewed this paper,” Oliver concluded.

What’s remarkable, numerous scientists and beekeepers told me, is that Lu’s bees didn’t just keel over in the first few weeks after sucking down what amounted to a lethal cocktail every day.

“It’s surprising those colonies lasted so long given the stratospheric quantities of insecticide [Lu] pumped into them for 13 weeks,” wrote Jonathan Getty on Bee-L Chat, a discussion forum for bee experts. “Lu has convincingly demonstrated, again, as in his previous study … that a high dose of an insecticide will kill an insect. Has anyone learned anything from all this? Looks like junk science at its worst.”

There was also scant evidence to back up Lu’s central claim that he had solved the mystery of CCD. “His description of the hives just didn’t show that,” University of Maryland entomologist Dennis vanEngelsdorp told me. Bee die offs, he said, have occurred mysteriously and periodically since at least the mid-19th century but became the focus of widespread public concern only in 2006. It’s clear that what Lu observed–bee deaths–“was not CCD. Looks like a typical bee colony death over the winter–which often includes bees abandoning the hive–but it’s a slow dwindle not a sudden collapse.”

Joe Ballenger, an entomologist writing for the independent sustainability site Biology Fortified, outlined how little Lu appears to know about CCD. “There are very important differences between the colonies Lu poisoned with insecticide and those which have been affected by CCD,” Ballenger wrote. “Despite these differences, Lu claims he has replicated CCD. However, his data demonstrates that he did not replicate CCD.”

Ballenger drew up a chart of Lu’s mistakes: 2014-12-14-neonics_donts_cause_ccd.jpg

Are there any prominent entomologists who endorse Lu’s findings? I couldn’t find any. Mother Jones quoted Jeffrey Pettis, an entomologist and research leader at USDA’s Beltsville’s Bee Laboratory, as appearing to be supportive. “Pettis told me that he thought Lu’s study ‘adds to the list’ of studies showing that pesticides pose a significant threat to honeybees,” Tom Philpott wrote.
I emailed Pettis about that quote:
I was trying to be diplomatic when I talked to Philpott but the Lu study should not have been published. It is not good science. I was trying to say that it adds to the list that pesticides and bees don’t mix but it is not a paper that shows that neonics cause problems simply because it was poorly replicated with high dosages used.
So what was going on in the hives that Lu monitored? The bee deaths that Lu found suggest a quite different cause, said vanEngelsdorp; the bees appear to have been killed by Lu himself–entirely expected if hives are overdosed during a frigid winter.

Are there potential advantages to using neonics to control pest infestations?

A telling fact emerges when you view the landscape of studies on neonics: on the whole, those done in a laboratory or that use unrealistic high doses (e.g. Lu’s studies) raise precautionary concerns. In contrast, field observations show few if any serious problems.

The latest example? Four Canadian scientists led by Cynthia D. Scott Dupree, an environmental biologist at the University of Guelph, undertook a large-scale study of honey bee exposure to one neonic, clothianidin, which is applied as a seed treatment. The study was centered in southern Ontario, which advocacy groups have contended has been particularly hard hit by neonic-related bee deaths.

Designed in cooperation with the U.S. Environmental Protection Agency and Health Canada, it was industry funded, but executed under Good Laboratory Practice Standards.

The scientists observed bees foraging heavily on the canola. As numerous other studies have suggested, they found, “Although various laboratory studies have reported sublethal effects in individual honey bees exposed to low doses of neonicotinoid insecticides, the results of the present study suggest that foraging on clothianidin seed-treated crops, under realistic conditions, poses low risk to honey bee colonies.”

Assertions by entomologists that neonics play a limited role in bee health infuriates some environmentalists convinced this mystery is solved: Let’s just ban neonics, they say, and move on.

“For its part, the pesticide industry is doing its best to shroud the phenomenon in uncertainty,” Mother Jones wrote in its article hyping the Lu study, “promoting a ‘multifactorial’ explanation that points the finger at mites, viruses, and ‘many other factors, but not…the use of insecticides,’ as neonic producer Bayer puts it in its ‘Honey Bee Health’ pamphlet.”

But it’s not Bayer making those claims, as Philpott seemed to suggest; it’s independent and government scientists. Noting the complexity of the phenomenon, the US Department of Agriculture and the Environmental Protection Agency took a cautious, science-based approach to the emerging controversy three years ago, commissioning a broad-based assessment of the evidence. This panel, reflecting views by most beekeepers and academic expertsconcluded that neonics were unlikely to be the major driver of bee deaths.

Rather, the experts identified a complex set of causes likely linked to a surge in pathogens, such as Varroa mites that feed on the bodily fluids of bees and which first surfaced in the U.S. in the 1980s and began infesting beehives in California in 1993; and Nosema, a common parasite that invades their intestinal tracts; and the use and perhaps misuse of miticides to control them. Other issues include the stress put on bees by large commercial beekeepers, particularly to service the agri-business demand for bees needed for the California almond crop in late winter before bees normally repopulate, as well as climate change and breeding issues.

Few experts or practitioners believe banning neonics or GMOs would improve bee health and could in fact result in farmers going back to spraying insecticides known to harm pollinators and humans.

“If we took pesticides out of the equation tomorrow, I think there’s no doubt we would have reduced colony losses,” vanEngelsdorp told me. “But even without pesticides, we’d still be seeing significant losses–losses that are unsustainable.”

Neonics present in corn dust at planting have been shown definitively to contribute to bee mortality, but that’s a result of faulty formulation, scientists have concluded. When used properly, there is intriguing evidence that neonics may actually improved bee health in some circumstances. Hints can be found, ironically, in Alex Lu’s own data, of all places.

Lu’s 2012 paper raises red flags because he used two separate dosing regimens as the experiment progressed, noted Richard Cowles, a prominent entomologist with the state of Connecticut, in an email to me. During the first four weeks of his study, the bees were fed concentrations of imidacloprid that, as it turns out, were in fact field realistic. At three weeks into testing using these concentrations, the health of the bee colonies was positively correlated with exposure to imidacloprid, as measured by the number of capped brood cells. In other words, the bees appeared healthier.

“Rather than continue the experiment with these concentrations, Dr. Lu inexplicably increased the dosages for the last nine weeks of feeding-by 40 times,” Cowles told me.

Why?

Cowles couldn’t get an answer from Lu and neither could I. This is one of the many questions that I had hoped to put to Lu in an interview. He at first agreed by email but then stopped communicating. I contacted him again and also reached out to the Harvard School of Public Health, but got no reply. Entomologists have volunteered as to what they thought might have been going on when Lu changed feeding tactics.

“Dr. Lu probably was trying to hide the fact that he observed an unexpected result contrary to his expectations, which led to him increase the dosages to poison the bees,” Cowles, emailed me. “Whether this sub-lethal effect is actually therapeutic to honey bees is a very interesting question, and one that I’d like to investigate.”

In other words, Lu’s data suggests the opposite of his stated conclusion–bees appear to do fine when exposed to field realistic doses and even increasingly higher amounts of neonics, but ultimately succumb to astronomical levels.

This is not the first time a neonic study has shown that bee health might improve when crops are treated with new generation insecticides. In a 2013 PLOS ONE study, a team led by vanEngelsdorp and Jeffrey Pettis studied the real world impact of 35 pesticides including three neonics–acetamiprid, imidacloprid and thiacloprid–by examining hives from seven major crops. Intriguingly, bee health improved although the results would need to be confirmed with follow up research. This study remains the only lab research to date that has evaluated how real world pollen-pesticide blends affect honey bee health.

The researchers found a striking reduction in the risk from Nosema infection when neonics were used, bee health improved. Why would that be? It seems neonics may suppress the parasite associated with the disease. vanEngelsdorp and Pettis are not yet sure this is a real effect; good science requires that results be confirmed in multiple studies. That said, the intriguing but startling finding directly challenges the belief that neonics pose an unusually unique danger to bees.

What is the future for bees, neonics and agriculture?

Are there replacement insecticides if neonics should be banned? Sure. Those based on pyrethroids and organophosphates some of which are more toxic to bees and humans, are not as effective as neonics for many uses–and are not in the political crosshairs.

That’s not slowed demands for an immediate ban. Advocacy groups recently widened the scope of their concerns, claiming neonics could have an unknown environmental impact, and waterways are being polluted. But evidence for that is scant. A US Geological Society study published in July found the highest levels detected were at least 40 times lower than benchmarks established by EPA to be protective of aquatic life, and most were up to 1,000 times below that level.

What would happen if U.S. officials do institute sharp restrictions, as the White House may be contemplating?

Neonics are not only important to major row crops such as corn, soy and canola, they also remain the most effective weapon against Asian psyllid, an insect that spreads the deadly virus that threatens America’s citrus crop. They are the key pesticide keeping in check whitefly infestations, which could otherwise devastate winter vegetables. They are the primary insecticide used to counter leafhoppers in the grape-growing Northwest as well as thrips in cotton and water weevil in rice. They’ve been hugely successful in combating aphids and beetles in potatoes.

I found scant support among entomologists for the two-year precautionary moratorium adopted by European politicians in the wake of near hysterical media reports in 2012 and 2013, many generated by coverage of Lu’s research. That ban looks like a textbook case of “shooting before you aim,” resulting in unintended but predictable consequences. As Matt Ridley reported in November in The Times of London:
All across southeast Britain this autumn, crops of oilseed rape are dying because of infestation by flea beetles. The direct cause of the problem is the two-year ban on pesticides called neonicotinoids brought in by the EU over British objections at the tail end of last year. … Farmers in Germany, the EU’s largest producer of rape, are also reporting widespread damage. Since rape is one of the main flower crops, providing huge amounts of pollen and nectar for bees, this will hurt wild bee numbers as well as farmers’ livelihoods.
There are now growing concerns that Lu’s studies will carry weight with politicians facing pressure to “do something”. That’s what happened in late November in Ontario, where the government has proposed to restrict the sale of corn and soybean seeds treated with neonics to farmers by 80 percent over the next two years.

The very same week, Health Canada issued a report after a long investigation that found bee mortality, which was not an issue until 2012, dropped 70 percent over last winter.

Activists are trying to jack up political pressure in the United States, perhaps concerned as signs that a temporary global surge in bee deaths appears over, undercutting their campaign. In September, a coalition of environmental groups co-wrote a letter signed by 60 Congressional Democrats urging the EPA to restrict neonicotinoid use citing Lu’s work in arguing that “native pollinators” have “suffered alarming declines.”

Those calls send chills down the back of entomologists concerned that Lu’s claims that he has solved the mystery of the beemageddon that doesn’t actually exist will have a bullying impact on public policy.

“Lu’s work is clearly biased, sensational,” said Richard Cowles. “It is horrendously incompetent. This is just hogwash. We will all pay a price for bad research.”

May Berenbaum was appointed this past summer to chair a National Academy of Sciences study on the health of pollinators ordered by the White House. I asked her if there is anything of value in Lu’s study to guide scientists and regulators? Do neonicotinoids threaten the health of this beleaguered arthropod?

Berenbaum paused. A dedicated environmentalist, she is known for her understated fairness.

“I’m no fan of pesticides and they are overused in agriculture, but you won’t find any confirmation of that in this study.”

Science is not a set of results; it is a method. If the method is wrong, the results are useless. The uncomfortably high number of bee deaths eludes the kind of definitive but potentially reckless conclusion that could result in precipitous regulations.

“This is a really complex issue with no quick and easy solutions,” Berenbaum said. “I can’t imagine a situation in which I would cite the findings of this paper as rigorous and reliable. This is just not good science.”

Jon Entine, executive director of the Genetic Literacy Project, is a Senior Fellow at the World Food Center Institute for Food and Agricultural Literacy, University of California-Davis and at the Center for Health and Risk Communication, George Mason University. Follow @JonEntine on Twitter.

Asteroid

From Wikipedia, the free encyclopedia
 
253 Mathilde, a C-type asteroid measuring about 50 kilometres (30 mi) across, covered in craters half that size. Photograph taken in 1997 by the NEAR Shoemaker probe.

Asteroids are minor planets, especially those of the inner Solar System. The larger ones have also been called planetoids. These terms have historically been applied to any astronomical object orbiting the Sun that did not show the disc of a planet and was not observed to have the characteristics of an active comet, but as minor planets in the outer Solar System were discovered, their volatile-based surfaces were found to resemble comets more closely and so were often distinguished from traditional asteroids.[1][not in citation given] Thus the term asteroid has come increasingly to refer specifically to the small bodies of the inner Solar System out to the orbit of Jupiter.[citation needed] They are grouped with the outer bodies—centaurs, Neptune trojans, and trans-Neptunian objects—as minor planets, which is the term preferred in astronomical circles.[2] In this article the term "asteroid" refers to the minor planets of the inner Solar System.

There are millions of asteroids, many thought to be the shattered remnants of planetesimals, bodies within the young Sun's solar nebula that never grew large enough to become planets.[3] The large majority of known asteroids orbit in the asteroid belt between the orbits of Mars and Jupiter, or are co-orbital with Jupiter (the Jupiter Trojans). However, other orbital families exist with significant populations, including the near-Earth asteroids. Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, S-type, and M-type. These were named after and are generally identified with carbon-rich, stony, and metallic compositions, respectively.

Only one asteroid, 4 Vesta, which has a relatively reflective surface, is normally visible to the naked eye, and this only in very dark skies when it is favorably positioned. Rarely, small asteroids passing close to Earth may be visible to the naked eye for a short time.[4] As of September 2013, the Minor Planet Center had data on more than one million objects in the inner and outer Solar System, of which 625,000 had enough information to be given numbered designations.[5]

On 22 January 2014, ESA scientists reported the detection, for the first definitive time, of water vapor on Ceres, the largest object in the asteroid belt.[6] The detection was made by using the far-infrared abilities of the Herschel Space Observatory.[7] The finding is unexpected because comets, not asteroids, are typically considered to "sprout jets and plumes". According to one of the scientists, "The lines are becoming more and more blurred between comets and asteroids."[7]

Naming

2013 EC, shown here in radar images, has a provisional designation

A newly discovered asteroid is given a provisional designation (such as 2002 AT4) consisting of the year of discovery and an alphanumeric code indicating the half-month of discovery and the sequence within that half-month. Once an asteroid's orbit has been confirmed, it is given a number, and later may also be given a name (e.g. 433 Eros). The formal naming convention uses parentheses around the number (e.g. (433) Eros), but dropping the parentheses is quite common. Informally, it is common to drop the number altogether, or to drop it after the first mention when a name is repeated in running text.

Symbols

The first asteroids to be discovered were assigned iconic symbols like the ones traditionally used to designate the planets. By 1855 there were two dozen asteroid symbols, which often occurred in multiple variants.[8]
 
Asteroid Symbol Year
1 Ceres Old planetary symbol of Ceres Variant symbol of Ceres Other sickle variant symbol of Ceres Ceres' scythe, reversed to double as the letter C 1801
2 Pallas Old symbol of Pallas Variant symbol of Pallas Athena's (Pallas') spear 1801
3 Juno Old symbol of Juno Other symbol of Juno Symbol 3.jpg A star mounted on a scepter,
for Juno, the Queen of Heaven
1804
4 Vesta Modern astrological symbol of Vesta Old symbol of Vesta Old planetary symbol of Vesta 4 Vesta Unsimplified Symbol.svg The altar and sacred fire of Vesta 1807
5 Astraea 5 Astraea symbol alternate.svg 5 Astraea Symbol.svg A scale, or an inverted anchor, symbols of justice 1845
6 Hebe 6 Hebe Astronomical Symbol.svg Hebe's cup 1847
7 Iris 7 Iris Astronomical Symbol.svg A rainbow (iris) and a star 1847
8 Flora 8 Flora Astronomical Symbol.svg A flower (flora) (specifically the Rose of England) 1847
9 Metis 9 Metis symbol.svg The eye of wisdom and a star 1848
10 Hygiea 10 Hygeia symbol alternate.svg 10 Hygiea Astronomical Symbol.svg Hygiea's serpent and a star, or the Rod of Asclepius 1849
11 Parthenope 11 Parthenope symbol alternate.svg 11 Parthenope symbol.svg A harp, or a fish and a star; symbols of the sirens 1850
12 Victoria 12 Victoria symbol.svg The laurels of victory and a star 1850
13 Egeria Astronomical symbol of 13 Egeria A shield, symbol of Egeria's protection, and a star 1850
14 Irene Symbol 14 Irene.png A dove carrying an olive branch (symbol of
irene 'peace') with a star on its head,[9] or
an olive branch, a flag of truce, and a star
1851
15 Eunomia 15 Eunomia symbol.svg A heart, symbol of good order
(eunomia), and a star
1851
16 Psyche 16 Psyche symbol.svg A butterfly's wing, symbol of
the soul (psyche), and a star
1852
17 Thetis 17 Thetis symbol.png A dolphin, symbol of Thetis, and a star 1852
18 Melpomene 18 Melpomene symbol.svg The dagger of Melpomene, and a star 1852
19 Fortuna 19 Fortuna symbol.svg The wheel of fortune and a star 1852
26 Proserpina 26 Proserpina symbol.svg Proserpina's pomegranate 1853
28 Bellona 28 Bellona symbol.svg Bellona's whip and lance[10] 1854
29 Amphitrite 29 Amphitrite symbol.svg The shell of Amphitrite and a star 1854
35 Leukothea 35 Leukothea symbol.png A lighthouse beacon, symbol of Leucothea[11] 1855
37 Fides 37 Fides symbol.svg The cross of faith (fides)[12] 1855

In 1851,[13] after the fifteenth asteroid (Eunomia) had been discovered, Johann Franz Encke made a major change in the upcoming 1854 edition of the Berliner Astronomisches Jahrbuch (BAJ, Berlin Astronomical Yearbook). He introduced a disk (circle), a traditional symbol for a star, as the generic symbol for an asteroid. The circle was then numbered in order of discovery to indicate a specific asteroid (although he assigned ① to the fifth, Astraea, while continuing to designate the first four only with their existing iconic symbols). The numbered-circle convention was quickly adopted by astronomers, and the next asteroid to be discovered (16 Psyche, in 1852) was the first to be designated in that way at the time of its discovery. However, Psyche was given an iconic symbol as well, as were a few other asteroids discovered over the next few years. (See chart above.) 20 Massalia was the first asteroid that was not assigned an iconic symbol, and no iconic symbols were created after the 1855 discovery of 37 Fides.[14] That year Astraea's number was increased to ⑤, but the first four asteroids, Ceres to Vesta, were not listed by their numbers until the 1867 edition. The circle was soon abbreviated to a pair of parentheses, which were easier to typeset and sometimes omitted altogether over the next few decades, leading to the modern convention.[9]

Discovery

243 Ida and its moon Dactyl. Dactyl is the first satellite of an asteroid to be discovered.

The first asteroid to be discovered, Ceres, was found in 1801 by Giuseppe Piazzi, and was originally considered to be a new planet.[note 1] This was followed by the discovery of other similar bodies, which, with the equipment of the time, appeared to be points of light, like stars, showing little or no planetary disc, though readily distinguishable from stars due to their apparent motions. This prompted the astronomer Sir William Herschel to propose the term "asteroid",[15] coined in Greek as ἀστεροειδής asteroeidēs 'star-like, star-shaped', from Ancient Greek ἀστήρ astēr 'star, planet'. In the early second half of the nineteenth century, the terms "asteroid" and "planet" (not always qualified as "minor") were still used interchangeably; for example, the Annual of Scientific Discovery for 1871, page 316, reads "Professor J. Watson has been awarded by the Paris Academy of Sciences, the astronomical prize, Lalande foundation, for the discovery of eight new asteroids in one year. The planet Lydia (No. 110), discovered by M. Borelly at the Marseilles Observatory [...] M. Borelly had previously discovered two planets bearing the numbers 91 and 99 in the system of asteroids revolving between Mars and Jupiter".

Historical methods

Asteroid discovery methods have dramatically improved over the past two centuries.

In the last years of the 18th century, Baron Franz Xaver von Zach organized a group of 24 astronomers to search the sky for the missing planet predicted at about 2.8 AU from the Sun by the Titius-Bode law, partly because of the discovery, by Sir William Herschel in 1781, of the planet Uranus at the distance predicted by the law. This task required that hand-drawn sky charts be prepared for all stars in the zodiacal band down to an agreed-upon limit of faintness. On subsequent nights, the sky would be charted again and any moving object would, hopefully, be spotted. The expected motion of the missing planet was about 30 seconds of arc per hour, readily discernible by observers.
First asteroid image (Ceres and Vesta) from Mars - viewed by Curiosity (20 April 2014).

The first object, Ceres, was not discovered by a member of the group, but rather by accident in 1801 by Giuseppe Piazzi, director of the observatory of Palermo in Sicily. He discovered a new star-like object in Taurus and followed the displacement of this object during several nights. His colleague, Carl Friedrich Gauss, used these observations to find the exact distance from this unknown object to Earth. Gauss's calculations placed the object between the planets Mars and Jupiter. Piazzi named it after Ceres, the Roman goddess of agriculture.

Three other asteroids (2 Pallas, 3 Juno, and 4 Vesta) were discovered over the next few years, with Vesta found in 1807. After eight more years of fruitless searches, most astronomers assumed that there were no more and abandoned any further searches.

However, Karl Ludwig Hencke persisted, and began searching for more asteroids in 1830. Fifteen years later, he found 5 Astraea, the first new asteroid in 38 years. He also found 6 Hebe less than two years later. After this, other astronomers joined in the search and at least one new asteroid was discovered every year after that (except the wartime year 1945). Notable asteroid hunters of this early era were J. R. Hind, Annibale de Gasparis, Robert Luther, H. M. S. Goldschmidt, Jean Chacornac, James Ferguson, Norman Robert Pogson, E. W. Tempel, J. C. Watson, C. H. F. Peters, A. Borrelly, J. Palisa, the Henry brothers and Auguste Charlois.

In 1891, Max Wolf pioneered the use of astrophotography to detect asteroids, which appeared as short streaks on long-exposure photographic plates. This dramatically increased the rate of detection compared with earlier visual methods: Wolf alone discovered 248 asteroids, beginning with 323 Brucia, whereas only slightly more than 300 had been discovered up to that point. It was known that there were many more, but most astronomers did not bother with them[citation needed], calling them "vermin of the skies", a phrase variously attributed to Eduard Suess[16] and Edmund Weiss.[17] Even a century later, only a few thousand asteroids were identified, numbered and named.

Manual methods of the 1900s and modern reporting

Until 1998, asteroids were discovered by a four-step process. First, a region of the sky was photographed by a wide-field telescope, or astrograph. Pairs of photographs were taken, typically one hour apart. Multiple pairs could be taken over a series of days. Second, the two films or plates of the same region were viewed under a stereoscope. Any body in orbit around the Sun would move slightly between the pair of films. Under the stereoscope, the image of the body would seem to float slightly above the background of stars. Third, once a moving body was identified, its location would be measured precisely using a digitizing microscope. The location would be measured relative to known star locations.[18]

These first three steps do not constitute asteroid discovery: the observer has only found an apparition, which gets a provisional designation, made up of the year of discovery, a letter representing the half-month of discovery, and finally a letter and a number indicating the discovery's sequential number (example: 1998 FJ74).

The last step of discovery is to send the locations and time of observations to the Minor Planet Center, where computer programs determine whether an apparition ties together earlier apparitions into a single orbit. If so, the object receives a catalogue number and the observer of the first apparition with a calculated orbit is declared the discoverer, and granted the honor of naming the object subject to the approval of the International Astronomical Union.

Computerized methods

2004 FH is the center dot being followed by the sequence; the object that flashes by during the clip is an artificial satellite.

There is increasing interest in identifying asteroids whose orbits cross Earth's, and that could, given enough time, collide with Earth (see Earth-crosser asteroids). The three most important groups of near-Earth asteroids are the Apollos, Amors, and Atens. Various asteroid deflection strategies have been proposed, as early as the 1960s.

The near-Earth asteroid 433 Eros had been discovered as long ago as 1898, and the 1930s brought a flurry of similar objects. In order of discovery, these were: 1221 Amor, 1862 Apollo, 2101 Adonis, and finally 69230 Hermes, which approached within 0.005 AU of the Earth in 1937. Astronomers began to realize the possibilities of Earth impact.

Two events in later decades increased the alarm: the increasing acceptance of Walter Alvarez' hypothesis that an impact event resulted in the Cretaceous–Paleogene extinction, and the 1994 observation of Comet Shoemaker-Levy 9 crashing into Jupiter. The U.S. military also declassified the information that its military satellites, built to detect nuclear explosions, had detected hundreds of upper-atmosphere impacts by objects ranging from one to 10 metres across.

All these considerations helped spur the launch of highly efficient surveys that consist of Charge-Coupled Device (CCD) cameras and computers directly connected to telescopes. As of spring 2011, it was estimated that 89% to 96% of near-Earth asteroids one kilometer or larger in diameter had been discovered.[19] A list of teams using such systems includes:[20]
The LINEAR system alone has discovered 138,393 asteroids, as of 20 September 2013.[21] Among all the surveys, 4711 near-Earth asteroids have been discovered[22] including over 600 more than 1 km (0.6 mi) in diameter.

Terminology

A composite image, to scale, of the asteroids which have been imaged at high resolution. As of 2011 they are, from largest to smallest: 4 Vesta, 21 Lutetia, 253 Mathilde, 243 Ida and its moon Dactyl, 433 Eros, 951 Gaspra, 2867 Šteins, 25143 Itokawa.
The largest asteroid in the previous image, Vesta (left), with Ceres (center) and the Moon (right) shown to scale.

Traditionally, small bodies orbiting the Sun were classified as asteroids, comets or meteoroids, with anything smaller than ten metres across being called a meteoroid.[23][24] The term "asteroid" is ill-defined. It never had a formal definition, with the broader term minor planet being preferred by the International Astronomical Union. In 2006, the term "small Solar System body" was introduced to cover both most minor planets and comets.[25] Other languages prefer "planetoid" (Greek for "planet-like"), and this term is occasionally used in English for larger minor planets such as the dwarf planets. The word "planetesimal" has a similar meaning, but refers specifically to the small building blocks of the planets that existed when the Solar System was forming. The term "planetule" was coined by the geologist William Daniel Conybeare to describe minor planets,[26] but is not in common use. The three largest objects in the asteroid belt, Ceres, 2 Pallas, and 4 Vesta, grew to the stage of protoplanets. Ceres is a dwarf planet, the only one in the inner Solar System.

When found, asteroids were seen as a class of objects distinct from comets, and there was no unified term for the two until "small Solar System body" was coined in 2006. The main difference between an asteroid and a comet is that a comet shows a coma due to sublimation of near surface ices by solar radiation. A few objects have ended up being dual-listed because they were first classified as minor planets but later showed evidence of cometary activity. Conversely, some (perhaps all) comets are eventually depleted of their surface volatile ices and become asteroids. A further distinction is that comets typically have more eccentric orbits than most asteroids; most "asteroids" with notably eccentric orbits are probably dormant or extinct comets.[27]

For almost two centuries, from the discovery of Ceres in 1801 until the discovery of the first centaur, 2060 Chiron, in 1977, all known asteroids spent most of their time at or within the orbit of Jupiter, though a few such as 944 Hidalgo ventured far beyond Jupiter for part of their orbit. When astronomers started finding more small bodies that permanently resided further out than Jupiter, now called centaurs, they numbered them among the traditional asteroids, though there was debate over whether they should be considered as asteroids or as a new type of object. Then, when the first trans-Neptunian object (other than Pluto), 1992 QB1, was discovered in 1992, and especially when large numbers of similar objects started turning up, new terms were invented to sidestep the issue: Kuiper-belt object, trans-Neptunian object, scattered-disc object, and so on. These inhabit the cold outer reaches of the Solar System where ices remain solid and comet-like bodies are not expected to exhibit much cometary activity; if centaurs or trans-Neptunian objects were to venture close to the Sun, their volatile ices would sublimate, and traditional approaches would classify them as comets and not asteroids.

The innermost of these are the Kuiper-belt objects, called "objects" partly to avoid the need to classify them as asteroids or comets.[28] They are believed to be predominantly comet-like in composition, though some may be more akin to asteroids.[29] Furthermore, most do not have the highly eccentric orbits associated with comets, and the ones so far discovered are larger than traditional comet nuclei. (The much more distant Oort cloud is hypothesized to be the main reservoir of dormant comets.) Other recent observations, such as the analysis of the cometary dust collected by the Stardust probe, are increasingly blurring the distinction between comets and asteroids,[30] suggesting "a continuum between asteroids and comets" rather than a sharp dividing line.[31]

The minor planets beyond Jupiter's orbit are sometimes also called "asteroids", especially in popular presentations.[32] However, it is becoming increasingly common for the term "asteroid" to be restricted to minor planets of the inner Solar System.[28] Therefore, this article will restrict itself for the most part to the classical asteroids: objects of the asteroid belt, Jupiter trojans, and near-Earth objects.

When the IAU introduced the class small Solar System bodies in 2006 to include most objects previously classified as minor planets and comets, they created the class of dwarf planets for the largest minor planets—those that have enough mass to have become ellipsoidal under their own gravity. According to the IAU, "the term 'minor planet' may still be used, but generally the term 'Small Solar System Body' will be preferred."[33] Currently only the largest object in the asteroid belt, Ceres, at about 950 km (590 mi) across, has been placed in the dwarf planet category, although there are several large asteroids (Vesta, Pallas, and Hygiea) that may be classified as dwarf planets when their shapes are better known.[34]

Formation

It is believed that planetesimals in the asteroid belt evolved much like the rest of the solar nebula until Jupiter neared its current mass, at which point excitation from orbital resonances with Jupiter ejected over 99% of planetesimals in the belt. Simulations and a discontinuity in spin rate and spectral properties suggest that asteroids larger than approximately 120 km (75 mi) in diameter accreted during that early era, whereas smaller bodies are fragments from collisions between asteroids during or after the Jovian disruption.[35] Ceres and Vesta grew large enough to melt and differentiate, with heavy metallic elements sinking to the core, leaving rocky minerals in the crust.[36]

In the Nice model, many Kuiper-belt objects are captured in the outer asteroid belt, at distances greater than 2.6 AU. Most were later ejected by Jupiter, but those that remained may be the D-type asteroids, and possibly include Ceres.[37]

Distribution within the Solar System

The asteroid belt (white) and the Trojan asteroids (green)

Various dynamical groups of asteroids have been discovered orbiting in the inner Solar System. Their orbits are perturbed by the gravity of other bodies in the Solar System and by the Yarkovsky effect. Significant populations include:

Asteroid belt

The majority of known asteroids orbit within the asteroid belt between the orbits of Mars and Jupiter, generally in relatively low-eccentricity (i.e. not very elongated) orbits. This belt is now estimated to contain between 1.1 and 1.9 million asteroids larger than 1 km (0.6 mi) in diameter,[38] and millions of smaller ones. These asteroids may be remnants of the protoplanetary disk, and in this region the accretion of planetesimals into planets during the formative period of the Solar System was prevented by large gravitational perturbations by Jupiter.

Trojans

Trojans are populations that share an orbit with a larger planet or moon, but do not collide with it because they orbit in one of the two Lagrangian points of stability, L4 and L5, which lie 60° ahead of and behind the larger body.
The most significant population of trojans are the Jupiter trojans. Although fewer Jupiter trojans have been discovered as of 2010, it is thought that they are as numerous as the asteroids in the asteroid belt.
A couple of trojans have also been found orbiting with Mars.[note 2]

Near-Earth asteroids

Near-Earth asteroids, or NEAs, are asteroids that have orbits that pass close to that of Earth. Asteroids that actually cross Earth's orbital path are known as Earth-crossers. As of November 2014, 11,600 near-Earth asteroids are known[19] and the number over one kilometre in diameter is estimated to be 900–1,000.
Frequency of small asteroids roughly 1 to 20 meters in diameter impacting Earth's atmosphere.

Characteristics

Size distribution

Sizes of the first ten asteroids to be discovered, compared to the Moon
HST image of the dwarf planet Ceres

Asteroids vary greatly in size, from almost 1,000 km for the largest down to rocks just tens of metres across.[note 3] The three largest are very much like miniature planets: they are roughly spherical, have at least partly differentiated interiors,[39] and are thought to be surviving protoplanets. The vast majority, however, are much smaller and are irregularly shaped; they are thought to be either surviving planetesimals or fragments of larger bodies.

The dwarf planet Ceres is by far the largest asteroid, with a diameter of 975 km (610 mi). The next largest are 2 Pallas and 4 Vesta, both with diameters of just over 500 km (300 mi). Vesta is the only main-belt asteroid that can, on occasion, be visible to the naked eye. On some rare occasions, a near-Earth asteroid may briefly become visible without technical aid; see 99942 Apophis.

The mass of all the objects of the asteroid belt, lying between the orbits of Mars and Jupiter, is estimated to be about 2.8–3.2×1021 kg, or about 4% of the mass of the Moon. Of this, Ceres comprises 0.95×1021 kg, a third of the total.[40] Adding in the next three most massive objects, Vesta (9%), Pallas (7%), and Hygiea (3%), brings this figure up to 51%; whereas the three after that, 511 Davida (1.2%), 704 Interamnia (1.0%), and 52 Europa (0.9%), only add another 3% to the total mass. The number of asteroids then increases rapidly as their individual masses decrease.

The number of asteroids decreases markedly with size. Although this generally follows a power law, there are 'bumps' at 5 km and 100 km, where more asteroids than expected from a logarithmic distribution are found.[41]
The asteroids of the Solar System, categorized by size and number
Approximate number of asteroids (N) larger than a certain diameter (D)
D 100 m 300 m 500 m 1 km 3 km 5 km 10 km 30 km 50 km 100 km 200 km 300 km 500 km 900 km
N ~25,000,000 4,000,000 2,000,000 750,000 200,000 90,000 10,000 1,100 600 200 30 5 3 1


Largest asteroids

The relative masses of the twelve largest asteroids known,[42] compared to the remaining mass of the asteroid belt.[43]

Although their location in the asteroid belt excludes them from planet status, the four largest objects, Ceres, Vesta, Pallas, and Hygiea, are remnant protoplanets that share many characteristics common to planets, and are atypical compared to the majority of "potato"-shaped asteroids.

Ceres is the only asteroid with a fully ellipsoidal shape and hence dwarf planet.[44] Vesta has—aside from the large crater at its southern pole, Rheasilvia—an ellipsoidal shape. Ceres has a much higher absolute magnitude than the other asteroids, of around 3.32,[45] and may possess a surface layer of ice.[46] Like the planets, Ceres is differentiated: it has a crust, a mantle and a core.[46] Vesta, too, has a differentiated interior, though it formed inside the Solar System's frost line, and so is devoid of water;[47] its composition is mainly of basaltic rock such as olivine.[48] Pallas is unusual in that, like Uranus, it rotates on its side, with its axis of rotation tilted at high angles to its orbital plane.[49] Its composition is similar to that of Ceres: high in carbon and silicon, and perhaps partially differentiated.[50] Hygiea is a carbonaceous asteroid and, unlike the other largest asteroids, lies relatively close to the plane of the ecliptic.[51]
Attributes of protoplanetary asteroids
Name Orbital
radius (AU)
Orbital period
(years)
Inclination
to ecliptic
Orbital
eccentricity
Diameter
(km)
Diameter
(% of Moon)
Mass
(×1018 kg)
Mass
(% of Ceres)
Density[52]
g/cm3
Rotation
period
(hr)
Axial tilt Surface
temperature
Vesta 2.36 3.63 7.1° 0.089 573×557×446
(mean 525)
15% 260 28% 3.44 ± 0.12 5.34 29° 85–270 K
Ceres 2.77 4.60 10.6° 0.079 975×975×909
(mean 952)
28% 940 100% 2.12 ± 0.04 9.07 ≈ 3° 167 K
Pallas 2.77 4.62 34.8° 0.231 580×555×500
(mean 545)
16% 210 22% 2.71 ± 0.11 7.81 ≈ 80° 164 K
Hygiea 3.14 5.56 3.8° 0.117 530×407×370
(mean 430)
12% 87 9% 2.76 ± 1.2 27.6 ≈ 60° 164 K

Rotation

Measurements of the rotation rates of large asteroids in the asteroid belt show that there is an upper limit.
No asteroid with a diameter larger than 100 meters has a rotation period smaller than 2.2 hours. For asteroids rotating faster than approximately this rate, the inertia at the surface is greater than the gravitational force, so any loose surface material would be flung out. However, a solid object should be able to rotate much more rapidly. This suggests that most asteroids with a diameter over 100 meters are rubble piles formed through accumulation of debris after collisions between asteroids.[53]

Composition

Cratered terrain on 4 Vesta

The physical composition of asteroids is varied and in most cases poorly understood. Ceres appears to be composed of a rocky core covered by an icy mantle, where Vesta is thought to have a nickel-iron core, olivine mantle, and basaltic crust.[54] 10 Hygiea, however, which appears to have a uniformly primitive composition of carbonaceous chondrite, is thought to be the largest undifferentiated asteroid. Most of the smaller asteroids are thought to be piles of rubble held together loosely by gravity, though the largest are probably solid. Some asteroids have moons or are co-orbiting binaries: Rubble piles, moons, binaries, and scattered asteroid families are believed to be the results of collisions that disrupted a parent asteroid.
Artist's view of a watery asteroid in the white-dwarf system GD 61.[55]

Asteroids contain traces of amino acids and other organic compounds, and some speculate that asteroid impacts may have seeded the early Earth with the chemicals necessary to initiate life, or may have even brought life itself to Earth. (See also panspermia.)[56] In August 2011, a report, based on NASA studies with meteorites found on Earth, was published suggesting DNA and RNA components (adenine, guanine and related organic molecules) may have been formed on asteroids and comets in outer space.[57][58][59]
Asteroid collision - building planets (artist concept).

Composition is calculated from three primary sources: albedo, surface spectrum, and density. The last can only be determined accurately by observing the orbits of moons the asteroid might have. So far, every asteroid with moons has turned out to be a rubble pile, a loose conglomeration of rock and metal that may be half empty space by volume. The investigated asteroids are as large as 280 km in diameter, and include 121 Hermione (268×186×183 km), and 87 Sylvia (384×262×232 km). Only half a dozen asteroids are larger than 87 Sylvia, though none of them have moons; however, some smaller asteroids are thought to be more massive, suggesting they may not have been disrupted, and indeed 511 Davida, the same size as Sylvia to within measurement error, is estimated to be two and a half times as massive, though this is highly uncertain. The fact that such large asteroids as Sylvia can be rubble piles, presumably due to disruptive impacts, has important consequences for the formation of the Solar system: Computer simulations of collisions involving solid bodies show them destroying each other as often as merging, but colliding rubble piles are more likely to merge. This means that the cores of the planets could have formed relatively quickly.[60]

On 7 October 2009, the presence of water ice was confirmed on the surface of 24 Themis using NASA’s Infrared Telescope Facility. The surface of the asteroid appears completely covered in ice. As this ice layer is sublimated, it may be getting replenished by a reservoir of ice under the surface. Organic compounds were also detected on the surface.[61][62][63][64] Scientists hypothesize that some of the first water brought to Earth was delivered by asteroid impacts after the collision that produced the Moon. The presence of ice on 24 Themis supports this theory.[63]

In October 2013, water was detected on an extrasolar body for the first time, on an asteroid orbiting the white dwarf star GD 61.[65]

Surface features

Most asteroids outside the big four (Ceres, Pallas, Vesta, and Hygiea) are likely to be broadly similar in appearance, if irregular in shape. 50-km (31-mi) 253 Mathilde is a rubble pile saturated with craters with diameters the size of the asteroid's radius, and Earth-based observations of 300-km (186-mi) 511 Davida, one of the largest asteroids after the big four, reveal a similarly angular profile, suggesting it is also saturated with radius-size craters.[66] Medium-sized asteroids such as Mathilde and 243 Ida that have been observed up close also reveal a deep regolith covering the surface. Of the big four, Pallas and Hygiea are practically unknown. Vesta has compression fractures encircling a radius-size crater at its south pole but is otherwise a spheroid. Ceres seems quite different in the glimpses Hubble has provided, with surface features that are unlikely to be due to simple craters and impact basins, but details will not be known until Dawn arrives in 2015.

Color

Asteroids become darker and redder with age due to space weathering.[67] However evidence suggests most of the color change occurs rapidly, in the first hundred thousands years, limiting the usefulness of spectral measurement for determining the age of asteroids.[68]

Classification

Asteroids are commonly classified according to two criteria: the characteristics of their orbits, and features of their reflectance spectrum.

Orbital classification

Many asteroids have been placed in groups and families based on their orbital characteristics. Apart from the broadest divisions, it is customary to name a group of asteroids after the first member of that group to be discovered. Groups are relatively loose dynamical associations, whereas families are tighter and result from the catastrophic break-up of a large parent asteroid sometime in the past.[69] Families have only been recognized within the asteroid belt. They were first recognized by Kiyotsugu Hirayama in 1918 and are often called Hirayama families in his honor.
About 30–35% of the bodies in the asteroid belt belong to dynamical families each thought to have a
common origin in a past collision between asteroids. A family has also been associated with the plutoid dwarf planet Haumea.

Quasi-satellites and horseshoe objects

Some asteroids have unusual horseshoe orbits that are co-orbital with the Earth or some other planet. Examples are 3753 Cruithne and 2002 AA29. The first instance of this type of orbital arrangement was discovered between Saturn's moons Epimetheus and Janus.

Sometimes these horseshoe objects temporarily become quasi-satellites for a few decades or a few hundred years, before returning to their earlier status. Both Earth and Venus are known to have quasi-satellites.

Such objects, if associated with Earth or Venus or even hypothetically Mercury, are a special class of Aten asteroids. However, such objects could be associated with outer planets as well.

Spectral classification

This picture of 433 Eros shows the view looking from one end of the asteroid across the gouge on its underside and toward the opposite end. Features as small as 35 m (115 ft) across can be seen.

In 1975, an asteroid taxonomic system based on color, albedo, and spectral shape was developed by Clark R. Chapman, David Morrison, and Ben Zellner.[70] These properties are thought to correspond to the composition of the asteroid's surface material. The original classification system had three categories: C-types for dark carbonaceous objects (75% of known asteroids), S-types for stony (silicaceous) objects (17% of known asteroids) and U for those that did not fit into either C or S. This classification has since been expanded to include many other asteroid types. The number of types continues to grow as more asteroids are studied.

The two most widely used taxonomies now used are the Tholen classification and SMASS classification. The former was proposed in 1984 by David J. Tholen, and was based on data collected from an eight-color asteroid survey performed in the 1980s. This resulted in 14 asteroid categories.[71] In 2002, the Small Main-Belt Asteroid Spectroscopic Survey resulted in a modified version of the Tholen taxonomy with 24 different types. Both systems have three broad categories of C, S, and X asteroids, where X consists of mostly metallic asteroids, such as the M-type. There are also several smaller classes.[72]

The proportion of known asteroids falling into the various spectral types does not necessarily reflect the proportion of all asteroids that are of that type; some types are easier to detect than others, biasing the totals.

Problems

Originally, spectral designations were based on inferences of an asteroid's composition.[73] However, the correspondence between spectral class and composition is not always very good, and a variety of classifications are in use. This has led to significant confusion. Although asteroids of different spectral classifications are likely to be composed of different materials, there are no assurances that asteroids within the same taxonomic class are composed of similar materials.

Exploration

951 Gaspra is the first asteroid to be imaged in close-up (enhanced color).
Vesta, imaged by the Dawn spacecraft
Several views of 433 Eros in natural colour

Until the age of space travel, objects in the asteroid belt were merely pinpricks of light in even the largest telescopes and their shapes and terrain remained a mystery. The best modern ground-based telescopes and the Earth-orbiting Hubble Space Telescope can resolve a small amount of detail on the surfaces of the largest asteroids, but even these mostly remain little more than fuzzy blobs. Limited information about the shapes and compositions of asteroids can be inferred from their light curves (their variation in brightness as they rotate) and their spectral properties, and asteroid sizes can be estimated by timing the lengths of star occulations (when an asteroid passes directly in front of a star). Radar imaging can yield good information about asteroid shapes and orbital and rotational parameters, especially for near-Earth asteroids. In terms of delta-v and propellant requirements, NEOs are more easily accessible than the Moon.[74]

The first close-up photographs of asteroid-like objects were taken in 1971 when the Mariner 9 probe imaged Phobos and Deimos, the two small moons of Mars, which are probably captured asteroids. These images revealed the irregular, potato-like shapes of most asteroids, as did later images from the Voyager probes of the small moons of the gas giants.

The first true asteroid to be photographed in close-up was 951 Gaspra in 1991, followed in 1993 by 243 Ida and its moon Dactyl, all of which were imaged by the Galileo probe en route to Jupiter.

The first dedicated asteroid probe was NEAR Shoemaker, which photographed 253 Mathilde in 1997, before entering into orbit around 433 Eros, finally landing on its surface in 2001.

Other asteroids briefly visited by spacecraft en route to other destinations include 9969 Braille (by Deep Space 1 in 1999), and 5535 Annefrank (by Stardust in 2002).

In September 2005, the Japanese Hayabusa probe started studying 25143 Itokawa in detail and was plagued with difficulties, but returned samples of its surface to earth on 13 June 2010.

The European Rosetta probe (launched in 2004) flew by 2867 Šteins in 2008 and 21 Lutetia, the second-largest asteroid visited to date, in 2010.

In September 2007, NASA launched the Dawn Mission, which orbited the protoplanet 4 Vesta from July 2011 to September 2012, and is planned to orbit 1 Ceres in 2015. 4 Vesta is the largest asteroid visited to date.

On 13 December 2012, China's lunar orbiter Chang'e 2 flew within 2 miles (3.2 km) of the asteroid 4179 Toutatis on an extended mission.

Planned and future missions

The Japan Aerospace Exploration Agency (JAXA) plans to launch around 2015 the improved Hayabusa 2 space probe and to return asteroid samples by 2020. Current target for the mission is the C-type asteroid (162173) 1999 JU3.

In May 2011, NASA announced the OSIRIS-REx sample return mission to asteroid 1999 RQ36, and is expected to launch in 2016.

On 15 February 2013, an asteroid measuring approximately 18 metres (59 feet) with a mass of about 9,100 tonnes (10,000 short tons) exploded over Chelyabinsk, Russia causing 1,500 injuries and damaging 7,000 buildings. Small samples of the rocky Chelyabinsk meteorite were quickly recovered and analyzed with a larger fragment found several months later.

In early 2013, NASA announced the planning stages of a mission to capture a near-Earth asteroid and move it into lunar orbit where it could possibly be visited by astronauts and later impacted into the Moon.[75] On 19 June 2014, NASA reported that asteroid 2011 MD was a prime candidate for capture by a robotic mission, perhaps in the early 2020s.[76]

It has been suggested that asteroids might be used as a source of materials that may be rare or exhausted on earth (asteroid mining), or materials for constructing space habitats (see Colonization of the asteroids). Materials that are heavy and expensive to launch from earth may someday be mined from asteroids and used for space manufacturing and construction.

Fiction

Asteroids and the asteroid belt are a staple of science fiction stories. Asteroids play several potential roles in science fiction: as places human beings might colonize, resources for extracting minerals, hazards encountered by spaceships traveling between two other points, and as a threat to life on Earth by potential impact.

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