Introduction by David Morrison, 9 November 2001
One of the persistent topics in NEO studies is the frequency or rate of impacts of differing sizes. This impact rate depends on the population of near-earth asteroids (NEAs) and occasional comets, and the dynamics of their obits that ultimately may bring them into collision course with our planet. The current press is reporting on a recent paper from the Sloan Digital Sky Survey team at Princeton, which reports on the implications that the Sloan Survey data may have for NEA impact rates.
Before discussing these results, it may be worth noting that the exact statistical frequency, while of interest to asteroid scientists, is not directly relevant to the present impact hazard. Impacts by NEAs of any size are exceedingly rare, from the 5-megaton limit of atmospheric shielding up to the hundreds of millions of megatons associated with mass extinctions. Statistically, no impact is to be expected within a human lifetime. The real issue, therefore, is whether against the odds there is an NEA on an impact trajectory. If there is such a threat, we want to identify it and deal with it. That is the purpose of the Spaceguard Survey. The important issue is not the average frequency of impacts but whether we can expect an impact within our lifetime, or the lifetime of our grandchildren.
Still, the total population of NEAs and their average impact frequency are often discussed. Sometimes these issue are used as a "hook" to attract attention to work that might otherwise be overlooked by the media and the general public. This week there has been considerable press interest in the conclusion that the new Sloan Survey results have reduced the estimated frequency of impacts and therefore that we were safer than had been previously thought. It is true that the estimated frequency for impacts by 1 km and larger NEAs has dropped in recent years, but not by more than the probable uncertainties, which have been given by most scientists studying the problem as about a factor of 2 or 3. Since the mid-1990s, the most common estimate has been that the Earth is hit by a "civilization threatening" impact (by a 1.5-km-diameter asteroid) about twice per million years, which is equivalent to a 1-in-5000 chance per century. But it is hard to tie down such estimates, in part because there is also a range of uncertainty as to what constitutes a civilization-threatening impact, spreading over at least a factor of two in asteroid size (from 1 km to 2 km diameter).
Alan Harris of JPL has provided several estimates of the impact frequency for NEAs with absolute magnitude H less than 18 (approximately corresponding to a 1-km asteroid with average reflectivity or albedo). For these estimates he used various recent values of both the population of NEAs and their dynamical lifetime (how long they orbit the Sun, on average, before they hit the Earth). These estimates for the chances of an impact in the next century are: 1 in 8600, 1 in 7100, 1 in 4800, and 1 in 4000. Expressed in terms of NEAs 1.5 km in diameter, the estimated impact frequencies are 1 in 12000, 1 in 14000, 1 in 10500, and 1 in 8800. The range in these values, which us about a factor 3, is probably representative of our uncertainty in the impact frequency.
It should be noted that all of Harris's numbers are based on the directly observed population of NEAs, of which nearly 1500 are now known. In contrast, the Sloan Survey measured main belt asteroids and assumed that the main belt population had the same size-dependence as the NEAs. It is by no means clear that this assumption is correct, or that main belt asteroids are a satisfactory surrogate for NEAs.
I would also like to comment on the statement in the Sloan news release that "the new impact risk estimate, like most previous ones, relies on assumptions about a single event 65 million years ago when a 10-kilometer asteroid collided with earth and killed the dinosaurs. The researchers assumed that such impacts occur on roughly 100 million-year intervals and used that statistic to calculate the impact odds for the more common asteroids of smaller sizes." As one of the first to publish NEA impact frequencies (with Clark Chapman, see our Nature paper in 1994), I note that we made no such assumption. Nor, to my knowledge, have any previous estimates involved any assumption about the frequency of KT-size impacts. Indeed, we have no way from a single example 65 million years ago of estimating the average frequency of such impacts. To my knowledge, this Sloan impact frequency estimate is the first to depend on such an assumption.
The Sloan Survey is an exciting astrophysics project with great promise in a number of fields of astronomy. It has already contributed to our knowledge of the main belt asteroids. But it has not measured NEAs, and it has not made an independent estimate of the impact frequency. It is therefore probably premature to conclude from these observations of the main belt asteroids that we should revise our estimates of the impact frequency of NEAs or of the magnitude of the impact hazard.
The relevant parts of the press release from the Sloan Survey follow, along with a press report from Rob Britt of Space.com. Concluding this news item is an anonymous parody of the Sloan Press Release.
Public release date: 7-Nov-2001
Contact: Steven Schultz, Princeton University
Princeton, N.J. -- The odds of earth suffering a catastrophic collision with an asteroid over the next century are about one in 5,000, which is less likely than previously believed, according to research published this month.
Astronomers using data from the Sloan Digital Sky Survey found that the solar system contains about 700,000 asteroids big enough to destroy civilization. That figure is about one-third the size of earlier estimates, which had put the number at around two million and the odds of collision at roughly one in 1,500 over a one hundred-year period.
"Our estimate for the chance of a big impact contains some of the same uncertainties as previous estimates, but it is clear that we should feel somewhat safer than we did before we had the Sloan survey data," said lead researcher Zeljko Ivezic of Princeton University.
The results were published in the November issue of the Astronomical Journal.
The new estimate draws on observations of many more asteroids, particularly small faint ones, than were available in previous impact risk estimates, said Ivezic. The ability to detect faint objects in large numbers is a hallmark of the Sloan survey, a multi-institutional collaboration that is mapping one-quarter of the sky. While its main purpose is to look at objects outside our galaxy, the survey also records images of closer objects that cross the view of its telescope, which is located at the Apache Point Observatory in New Mexico.
The survey data also allowed the astronomers to gauge the size of asteroids with improved accuracy, which required categorizing the objects by their composition. Asteroids with a surface of carbon -- looking like giant lumps of coal -- are darker than those made of rock. A small rocky asteroid therefore looks just as bright as a much larger one made of carbon.
"You don't know precisely the size of an object you are looking at unless you know what type it is," Ivezic said, noting that the Sloan survey provides information about the color of objects, which allows astronomers to distinguish between carbon and rock.
Based on observations of 10,000 asteroids, the researchers estimated that the asteroid belt contains about 700,000 that are bigger than one kilometer (six-tenths of a mile) in diameter, which is the minimum size thought to pose a catastrophic risk to humans and other species. The asteroid belt is the source for a smaller group of asteroids called "near- earth objects," which have broken from the belt and have the potential to collide with earth. Although they did not specifically observe near earth objects, the researchers believe that their census of main belt asteroids reveals the likelihood of collisions with similarly sized near-earth asteroids.
Ivezic noted that the new impact risk estimate, like most previous ones, relies on assumptions about a single event 65 million years ago when a 10-kilometer asteroid collided with earth and killed the dinosaurs. The researchers assumed that such impacts occur on roughly 100 million-year intervals and used that statistic to calculate the impact odds for the more common asteroids of smaller sizes. This calculation required knowing how much more common one-kilometer asteroids are than 10-kilometer ones, which was hard to measure before the Sloan data was available.
"There is a lot of uncertainty when you have a sample of only one event," Ivezic said, referring to the dinosaur-killing impact. "But this is the best information we have."
Previous studies could detect only asteroids five kilometers or larger, so astronomers had to extrapolate to estimate the number of smaller ones, said Ivezic. The Sloan researchers found that this approach produced high estimates. When they could actually observe them, the small asteroids were not as plentiful as had been expected from observations of large ones.
By Robert Roy Britt
Space.com, 8 November 2001
There may be fewer large asteroids in the solar system than previously believed according to a new survey released Wednesday. Astronomers using data from the Sloan Digital Sky Survey say that the main asteroid belt, between Mars and Jupiter, contains about 700,000 space rocks larger than 1 kilometer (0.62 miles). Previous estimates had put the count at around two million.
The survey involved several institutions mapping a quarter of the sky, so the new estimate represents an extrapolation to the entire sky. The survey data also allowed the astronomers to better gauge the size of asteroids by studying their composition. An asteroid's size is estimated in part by the amount of light it reflects, which is tied to the surface composition.
"The Sloan study is a major advance in our understanding of the gross asteroid belt structure," said Robert Jedicke, an asteroid expert at the University of Arizona. Millions and millions of smaller rocks are thought to inhabit the asteroid belt.
The researchers involved in the study, led by Zeljko Ivezic of Princeton University, said their work reduced the odds that an asteroid larger than 1 kilometer would hit Earth sometime in the next 100 years. They say the odds had been reduced from 1-in-1500 over the next century to 1-in-5000.
But Donald Yeomans, an asteroid expert at NASA's Jet Propulsion Laboratory, said the 1-in-5000 figure has been accepted for years and is based on a population of Near Earth Asteroids (NEAs). Asteroids that are thought to be near enough to our planet that gravity could lure them in sometime soon.
Scientists at NASA and elsewhere work together to find and track NEAs, especially those 1 kilometer and larger. Experts say only rocks that large are capable of causing global catastrophe and possibly destroying civilizations. Between 700 and 1200 are thought to exist. Roughly 500 have been found. All are on courses that pose no threat to Earth.
Asteroids larger than 1 kilometer are suspected of hitting Earth every 100,000 to 300,000 years, according to widely accepted estimates based partly on a handful of terrestrial craters. But Earth tends to bury or erode the evidence. So the estimate is based also on craters on the Moon, which do not erode quickly but which provide a glimpse into what likely happens on Earth.
Smaller asteroids hit Earth more frequently and can wipe out a city. Objects the size of a bus or smaller tend to burn up as they zoom through Earth's atmosphere, and therefore they pose little or no threat.
At any rate, the Sloan survey did not study Near Earth Asteroids, but instead looked at objects primarily in the main asteroid belt. Asteroids that far away represent no threat to Earth over the next century and well beyond, Yeomans said. They are orbiting the Sun in relatively comfortable fashion.
The Sloan results were published in the November issue of the Astronomical Journal.
Copyright 2001, Space.com
From an anonymous British correspondent...
Press Release from the Slone Digital Survey, Princesstown University
The likelihood of the average American being attacked by a wild animal has been shown to be much less than previously estimated, according to new results from the Slone Digital Survey, operated by Princesstown University astronomers.
Records of attack rates compiled by mammal experts had indicated that a large fraction of deaths caused by wild animals in Africa are due to hippopotamus strikes. New observations by the Princesstown researchers, however, show that there are fewer hippopotamuses than earlier counts indicated.
Using their new data they have extrapolated the figures to obtain a kill rate for inhabitants of North America.
"Although we cannot be sure that our detections were all hippopotamuses - for example some may have been elephants or wildebeest - we are convinced that our figures represent a much-improved evaluation of the numbers of large gray-brown mammals in Africa" said a spokesman for the team, Dr Neophobius Snobbs. He continued: "While some detractors have criticized us for not including smaller beasts like lions and hyenas in our analysis, so far as we are aware these do not pose a danger to humans."
The team have also dismissed the arguments of public safety experts who point to grizzly bears as being a significant risk. "I've never seen a grizzly bear in Princesstown," countered another member of the team, "whereas I remember seeing a hippopotamus in Central Park Zoo when I was a kid. And that's only a handful of miles from here."
The purpose of the present project was to capitalize on an earlier press release, when much publicity was gained by the team's claim that their detection of a smaller number of main-belt asteroids in the 1-km size range had important implications for the probability of such a projectile hitting the Earth soon. "We were amazed. We never expected the media and the public to be so gullible. Admittedly this is something we know nothing about but, hey, look at all that coverage. So we figured we could pull the same stunt with hippopotamuses".
When asked about polar bears, the Dr Snobbs said that, according to their working definition, white animals cannot be dangerous. "To imagine otherwise would be as silly as thinking that a comet could strike the Earth," he said. "Clearly this makes no sense. We deal with white animals all the time ? cats and mice, mostly - and although a cat may scratch you occasionally, they are not life-threatening." When pressed further, he commented that in any case polar bears live only in parts of Canada, not the United States, and Canadians don't count.
Still other critics have argued that, even if there are a few hippopotamuses in zoos, the hazard they represent is tiny. "This shows a profound ignorance of recent scientific advances" said Snobbs. "Global warming is an established fact. This means that North America will get hotter, and soon will be like the African savannah lands, and so hippopotamuses will flourish in the Midwest." When asked how they would get there, Dr Snobbs said that the time scale involved was some millions of years, over which hippopotamuses would evolve so as to develop wings, just like pigs have now.