ASTEROID 1950 DA Science magazine for April 5 published a paper by Jon Giorgini and others on the orbit of near Earth asteroid (NEA) 1950 DA. This asteroid has the highest calculated probability of impact with the Earth for any NEA yet: about one part in 300. That is the bad news. The good news is that the impact, if it happens, will not take place for nearly a millennium. Following are the abstract of the published paper and several discussions, including an article from the Washington Post.
Asteroid 1950 DA's Encounter with Earth in 2880: Physical Limits of Collision Probability Prediction J. D. Giorgini, S. J. Ostro, L. A. M. Benner,
P. W. Chodas, S. R. Chesley, R. S. Hudson, M. C. Nolan,
A. R. Klemola, E. M. Standish, R. F. Jurgens,1 R. Rose,
A. B. Chamberlin, D. K. Yeomans, J.-L. Margot Abstract: Integration of the orbit of asteroid (29075) 1950 DA, which is based on radar and optical measurements spanning 51 years, reveals a 20-minute interval in March 2880 when there could be a nonnegligible probability of the 1-kilometer object colliding with Earth. Trajectory knowledge remains accurate until then because of extensive astrometric data, an inclined orbit geometry that reduces in-plane perturbations, and an orbit uncertainty space modulated by gravitational resonance. The approach distance uncertainty in 2880 is determined primarily by uncertainty in the accelerations arising from thermal re-radiation of solar energy absorbed by the asteroid. Those accelerations depend on the spin axis, composition, and surface properties of the asteroid, so that refining the collision probability may require direct inspection by a spacecraft.
This paper provides one of the most complete orbital analyses of any asteroid done to date. In the text of the paper, the authors note that the probability of a collision in the year 2880 is approximately equal to the probability of an impact by an unknown asteroid of comparable size sometime between now and 2880. As noted by Al Harris of JPL, there is zero chance of an impact from 1950 DA between now and 2880, and only a small probability then. For all unknown asteroids of that size collectively, there is about one chance in a million of an impact in any given year, and it could be any year, including tomorrow. In terms of impact hazard over the next century or two (which is all we usually consider), 1950 DA joins the ranks of "certified safe" objects. The interesting possibility of an impact in 2880 arises only because of the exceptionally good orbit determinations (primarily from radar data) that allow us to calculate the position of the asteroid for the next millennium.
Harris comments that if we wish to find out more about this asteroid, the next close approach, in 2032, will undoubtedly provide plenty of opportunity to refine the orbit and most likely eliminate any possibility of an impact. If not, there is a chance in 2074, still a generous 800 years before any active intervention would be needed in the very unlikely event that it really is on a collision course. Harris has also been asked if he thinks we should take any action now toward deflecting the asteroid or otherwise considering the mitigation of the hazard. His answer is "absolutely not", for the following reasons: (1) With the only possible impact 878 years in the future and complete certainty that an impact can be positively ruled out (or in) by further radar observations within the next century (leaving a generous 800 more years to deal with it if it is on a collision course), it would be a waste of resources to do anything until those definitive observations are taken. (2) Until we know the trajectory within an Earth diameter in 2880, a gentle shove could as well put the asteroid on a collision course as push it off one. Until you know exactly where it is heading, you don't know which way to push to make it miss. (3) The technology of our descendants 800 years from now is likely to be so superior to our own that it is entirely appropriate to let them worry about it. Our responsibility to that generation is no more than to take and preserve the observations from which more definitive orbits can be calculated with the addition of future observations. Would you expect William the Conqueror to anticipate and solve any of the major problems of our modern society? Or for Christopher Columbus to design a vehicle to transport several hundred people across the Atlantic in 6 hours?
The radar observations coupled with the long arc of 50 years since its discovery have yielded an orbit for this asteroid of such precision that we can actually follow it a millennium into the future. Only 1950 DA and a few other radar-observed asteroids have orbits good enough to say anything more than probabilistic generalizations that far in the future. So, what 19950 DA does, and the fact that a possible collision that far out depends on non-gravitational forces, is not unique to 1950 DA. What is unique about 1950 DA is that we are in a position to start monitoring these subtle effects. That's scientifically interesting, even exciting.
Steve Ostro, one of the authors of the paper, notes that most of the uncertainty in the impact probability comes from uncertainty in the asteroid's physical properties. Right now the collision probability is 1/300 or less. The geometry of the intersection of the asteroid's orbit and the Earth in 2880 are very well known, and there is a 20-min interval 2880 when there could be a collision. Virtually all the uncertainty is in the along-track component and arises from Yarkovsky accelerations, which depend on the object's physical properties, including not just the gross ones like mass, shape, and pole direction, but the detailed global distribution of optical and thermal properties. This paper establishes that trajectory prediction and physical characterization are closely connected, because long-term orbit calculations require knowledge of physical properties.
David Morrison
ASTEROID'S FAR-OFF DANGER DETAILED From The Washington Post, 5 April 2002
By Guy Gugliotta
Washington Post Staff Writer An asteroid nearly a mile wide could be headed for an apocalyptic collision with Earth. That's the bad news. The good news is that it won't arrive for 878 years, and it might be pretty easy for our descendants to move it out of the way. A team of researchers determined that an asteroid known as 1950 DA, a gigantic, near-spherical boulder hurtling through space on an elliptical orbit around the sun, has a one- in-300 chance of smacking into the Earth on March 16, 2880. One-in-300 is as close as the odds have ever been for an asteroid collision. "The orbits will meet up," said senior engineer Jon D. Giorgini of NASA's Jet Propulsion Laboratory in Pasadena, Calif. He led the team that reported the calculation in today's issue of the journal Science. "The question is, 'Will the Earth and the asteroid be there at the same time?' " But even if further study determines the asteroid and Earth are on a collision course, another researcher argues that asteroids such as 1950 DA could be thrown off line relatively easily by manipulating their ability to absorb sunlight and translate it into thermal energy. Once absorbed, solar energy radiates from the asteroid's surface like a tiny thruster engine. That's meaningless in the short-term when compared with the gravitational forces that give orbits most of their size and shape. But it's potentially decisive in moving a small celestial body a few degrees off-course over a period of centuries. "You just want to change something about the surface" of the asteroid to alter the way it processes sunlight, said University of Arizona planetary scientist Joseph N. Spitale, who outlined the approach in an accompanying article in Science. And it doesn't take nuclear warheads, as were used in the 1998 movies "Armageddon" and "Deep Impact." "There are a lot of ways," Spitale said, such as roughening the asteroid's surface with conventional explosives or covering it with dirt. Giorgini suggested coating it with charcoal or chalk, or "shrink-wrapping" a large piece of it with Mylar. "You could attach a rocket engine to it, like an outboard," added Clark R. Chapman, a senior space scientist at the Boulder, Colo., office of the Southwest Research Institute. "When you have a lead time of centuries, almost anything will do." There is, however, "the problem that you can make it worse," said Brian G. Marsden, director of the International Astronomical Union's Minor Planet Center. Altering the asteroid's orbit indiscriminately to avoid a 2880 catastrophe could set up an earlier or later collision. "A kilometer is where you start thinking about global catastrophe," Spitale said. A kilometer is six-tenths of a mile; the asteroid 1950 DA is seven-tenths of a mile wide. Something smaller than a kilometer can wipe out a metropolitan area or devastate a coastline with tidal waves. Something bigger throws up a huge cloud of dust that can dim the sun for years, causing a massive die-off of species. Astronomers say there are between 1,000 and 1,200 objects six-tenths of a mile in diameter or larger whose orbits intersect that of Earth; about half of these objects have been detected. Among those identified, only 1950 DA is a potential threat. Giorgini is looking for more information on 1950 DA, and fortunately there is plenty of time to get it. He and his colleagues have already gathered enough data about 1950 DA to create what Chapman called "the most definitive study of an asteroid orbit that's ever been done." This is fortunate, for premature warnings based on sketchy information about "Earth-crossing" asteroids and comets have a remarkable ability to alarm the public. In 1998, Marsden predicted that asteroid 1997 XF11 was "virtually certain" to pass relatively close to Earth in 2028 and had a slight chance of hitting the planet. He based his warning on three months of observation, but after further research, it became apparent that the asteroid had "zero chance" of collision. By that time, however, the first prediction had created an enormous explosion of publicity. "If you say something, you're accused of crying wolf," said Marsden with a wry chuckle. "If we don't say anything, we get accused of coverups. You can't win in this business." Astronomers first sighted 1950 DA in February 1950, but it was lost to observers until 2000, when it was reidentified. Giorgini said scientists were able to observe it for four days with radar and with optical telescopes until August 2001. What they found was a rounded body with craters "but no large dents," traveling on a 2.2 year-long elliptical solar orbit, Giorgini said. The asteroid comes to within 77 million miles of the sun -- inside Earth's orbit -- and reaches its farthest point 238 million miles away in the asteroid belt between Mars and Jupiter. The team could examine the range of effects of many secondary forces on the orbital shape. But an assessment of the thermal energy thrusters, known as the Yarkovsky Effect, for the Russian engineer who discovered it a century ago, was difficult. The asteroid rotated once every 2.1 hours, but the Giorgini team did not know where the poles were, crucial in assessing the direction of the thruster push. "We've got two possibilities," Giorgini said. "In one position, the probability of collision is nearly .33 percent. In the other position, it is zero." But either way, he added, "there's nothing to worry about . . . in a few hundred years, there will be ways of dealing with this we can't even imagine." © 2002 The Washington Post Company
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FOR IMMEDIATE RELEASE April 4, 2002 RADAR PUSHES LIMITS OF ASTEROID IMPACT PREDICTION Applying unprecedented refinements to the analysis of celestial hazards, NASA astronomers have identified a potential close encounter with Earth more than eight centuries in the future by an asteroid two-thirds of a mile (one kilometer) wide. What will most likely be a miss, even without preventive measures, will come on March 16, 2880, said Jon Giorgini, a senior engineer at NASA's Jet Propulsion Laboratory, Pasadena, Calif. Odds for a collision are at most one in 300, and probably even more remote, based on what is known about the asteroid so far. Still, that makes this space rock, named 1950 DA, a greater hazard than any other known asteroid. "This is not something to worry about," said Giorgini, leader of a team reporting about the asteroid in the April 5 edition of the journal Science. "We're showing that searches with optical telescopes and follow-up observations with radar telescopes can provide us centuries of advance notice about potential close encounters of asteroids with Earth. That's plenty of time to consider the options -- 35 generations, in fact." "This report is a success story for our efforts to identify potential troublemakers," said JPL's Dr. Don Yeomans, manager of the NASA Near Earth Object Program. "Radar observations are helping us push predictions 5 to 10 times further into the future." This report differs from previous ones about other asteroids' Earth-impact potential. Estimates of impact risks in earlier cases came from a few nights' optical observations of newly found asteroids. Astronomers soon ruled out the possible impacts after a few more observations narrowed uncertainties about the asteroids' orbits. The current orbit of 1950 DA has been mapped with great accuracy using precise radar data and a 51-year span of optical data. Uncertainty about how close it will come to Earth in 2880 stems from gaps in knowing physical details of the asteroid that could subtly alter its course over the centuries. "How close 1950 DA will approach Earth turns out to depend on the asteroid's physical attributes -- it's size, shape and mass, and how it spins, reflects light and radiates heat into space," Giorgini said. These things are unlikely to be known any time soon. The way the asteroid radiates energy absorbed from the Sun back into space has the biggest potential effect, he said. Releasing heat in one direction nudges the asteroid in the opposite direction. The resulting acceleration is tiny, but over the centuries acts like a weak rocket and could make the difference between a hit and a miss. Asteroid 1950 DA was discovered from Lick Observatory, Mount Hamilton, Calif., in 1950. It faded from view for five decades then was found from Lowell Observatory in Arizona in 2000. Astronomers used large dish antennas of NASA's Deep Space Network site at Goldstone, Calif., and the Arecibo Observatory in Puerto Rico to examine the asteroid with radar when it passed at a distance 21 times farther away than the Moon in March 2001. "Once an asteroid is discovered, radar is the most powerful way to find its exact orbit and, apart from sending a spacecraft, the only way to see what it looks like," said JPL's Dr. Steve Ostro, who led the radar observations of 1950 DA. Giorgini refined calculations of future orbits by including factors such as the push from sunshine and the potential gravitational tug from 7,000 other asteroids and nearby stars. Effects of each small influence on the asteroid's movement could be amplified by 15 gravitational tugs during close approaches to Earth and Mars -- none of which have any chance of an impact -- prior to 2880. "It's like predicting a 15-bank shot in a pool game," Giorgini said. "We know the cue stroke extremely well because it is right now and we can measure it. But at each future bank, small variations accumulate and change the next bounce, which changes the following one and so on. What we've done is find the range of changes possible due to tilt, imperfections and fuzz on the table, the bounce of the cushions, and wind blowing across the room. We need to know more about the 'cue ball' to really be sure of how the last three banks in 2809, 2840 and 2860 will line things up for 2880." If future generations' studies of 1950 DA indicate it ought to be diverted to prevent a collision, the subtle influences that its physical properties have on its motion might be manipulated to advantage. For example, Giorgini suggested, its surface could be dusted with chalk or charcoal to alter the way it reflects light, or a spacecraft propelled with a solar sail could collapse its reflective sail around the asteroid. In any event, determining asteroids' physical properties will be important for long-term calculations of impact hazards. In addition to Giorgini, Ostro and Yeomans, authors of the report include Dr. Lance Benner, Dr. Paul Chodas, Dr. Steven Chesley, Dr. Myles Standish, Dr. Ray Jurgens, Randy Rose and Dr. Alan Chamberlin, all of JPL; Dr. Scott Hudson, Washington State University, Pullman; Dr. Michael Nolan, Arecibo Observatory; Dr. Arnold Klemola, Lick Observatory; and Dr. Jean-Luc Margot, California Institute of Technology, Pasadena. Images and additional information are available at the JPL NEO website: http://neo.jpl.nasa.gov/1950da/ This section includes images of the asteroid, a radar movie, simulations of the asteroid's orbit, and a video segment narrated by Jon Giorgini. Arecibo Observatory is operated by the National Astronomy and Ionosphere Center at Cornell University, Ithaca, N.Y., under an agreement with the National Science Foundation. NASA's Office of Space Science, Washington, D.C supported the radar observations. JPL is managed for NASA by the California Institute of Technology.
JON GIORGINI <jdg@tycho.jpl.nasa.gov> ON 1950 DA This case of 1950 DA is rich in the sense of there being many different aspects to it. For example, in addition to the impact potential in 2880, 1950 DA's orbital uncertainty region is modulated by resonance. That is, the extent of the uncertainty region doesn't exceed 20000 km (~2 Earth diameters) for the next 600+ years. Resonance instead causes it to oscillate over the centuries. This resonance effect can be seen in animation #5 at the above web site. There is the idea of a probability range (instead of a single value); unknown physical parameters (spin, surface thermal conductibity, etc.) potentially bias the whole uncertainty region. There is the influence of factors not considered in previous cases (because the quality of the data was so poor it wasn't worthwhile): solar pressure, galactic tides, perturbations due to thousands of other asteroids, heat emission, solar oblateness, uncertainties in the masses of the planets, and the role imprecision in computer hardware doing the calculations might play. There is the non-linear gravitational "amplification" of these small factors, which turns such minor issues into things that make the difference between predicting a hit or a miss. The case especially illustrates the value of planetary radar; 1950 DA's impact potential was detected when the asteroid's position and velocity were measured by Goldstone at the 10's of meters and millimeter per second level. Such measurements routinely open up prediction windows 5-10 times greater in extent than with optical data. There is the issue of when (or even if) a conclusive yes/noo impact prediction for any object can be made without radar data and the length of buffer that it allows. This may seem obvious, but with the future of planetary radar opaque at the moment, it needs to be said. (Even after the 1950 DA detection, Arecibo and Goldstone were ordered to end their asteroid activities (Arecibo with 3 weeks notice over the holidays) -- only to have the directives later rescinded. The post-October future is currently unknown. The case of 1950 DA differs from previous hazard predictions. For past cases (and now likely to be the case for all future situations), a risk was detected based on a few days or weeks of data for a newly discovered object. The uncertainty region that surrounds an object then is large, sometimes spanning a big chunk of the inner solar system. Additional measurements made a few days or weeks later shrink the region such that the Earth falls out of it and the risk drops to near zero. This is normal and expected. Although other (currently unknown) asteroids may hit before 2880, the situation with 1950 DA is likely to be unique for future detections; observations spanning 51 years coupled with high-precision radar data. Future ground-based observations over years and decades are unlikely to change the prediction of 1950 DA unless adequate physical knowledge of it is obtained. While 878 years is a long time in the future, hundreds of years of warning is exactly what we should want, since a year or two (or 10 seconds, like the dinosaurs), leaves few if any options. With hundreds of years of warning there are many options. Some might argue mitigation can be left to the future. This assumes a uniform progression of technology. Others might note that although Apollo landed on the Moon several times 30 years ago, there is no longer the infrastructure and will to do something similar in the present. Things are not always "onward and upward". Thus it's interesting to consider how altering physical properties -- the way an object reflects and absorbs light -- using present methods can be used to mitigate hazards. Andrea Milani first proposed powdered sugar while seated around a table last June in Palermo (I recall a table of pastries in sight during a break), when the case of 1950 DA was introduced at that time. I like the idea of collapsing a solar sail mission spacecraft around; encasing it in a reflective substance. While the future may have better solutions, that's not a certainty. |
