Recent discussions, including those at the Torino meeting in June 1999, have noted the discrete nature of the hazard even for NEOs with orbits that bring them very close to the Earth. Most such flybys of the Earth pose no danger whatever. However, on a close flyby there are often a few very specific, discrete points in phase space that will bring the NEO back into a subsequent collision path. These points have recently been called keyholes. Only in the unlikely case that a NEO passes directly through one of the keyholes during a future flyby of Earth will it pose a subsequent hazard. In such a circumstance, all that is required to put the NEO in the (not hazardous) group is to verify that it will not (or did not) pass through a keyhole. A negative observation is thus sometimes enough to ensure that there is no future near-term risk. Even if the NEO is lost, in the sense that a highly accurate future orbit is not known, it may still be possible to eliminate it as a near-term hazard.
Following (for those interested in the details) are excerpts from recent correspondence dealing with such issues, in the cases of NEOs 1999 AN10 and 1998 OX4, both of which have been suggested as potential threats. A newly-discovered observation of AN10 now shows that it cannot impact the Earth within approximately the next century (that is, it will not pass through one of the small keyholes and cannot approach the Earth closely, at least until after 2076, when the orbit becomes less well determined).
From SpaceViews, 9 June 1999
For the third time in a little over a year, astronomers have found an asteroid that has a very small, but non-zero, probability of impacting the Earth next century.
Astronomers believe such impact probabilities as the one found for 1998 OX4 will become more commonplace in the future, though, as stepped-up search efforts turn up more asteroids whose orbits bring them close to Earth.
Italian astronomer Andrea Milani and colleagues reported the impact probability at the end of the IMPACT conference in Torino, Italy, earlier this month. They found that 1998 OX4, discovered last year at the Spacewatch telescope in Arizona, has a 1-in-10 million chance of hitting the Earth in January 2046.
This probability of impact is considerably less than the probability of an impact in any given year by an undiscovered asteroid 1 km or greater in diameter, so the discovery is of little more than academic curiosity. Moreover, Milani and colleagues note that this probability has yet to be confirmed by other researchers.
The discovery makes 1998 OX4 the third asteroid since last March which has been found to have a small impact probability at some point in the future. In April asteroid 1999 AN10 was found to have a 1-in-1 billion chance of hitting the Earth in 2039. Later analysis changed that probability to 1-in-10 million while uncovering another possible impact with significantly greater odds -- 1-in-500,000 -- in 2044.
In March 1998 asteroid 1997 XF11 was briefly thought to have a small possibility of impacting the Earth in 2029 [2028, BJP]. However, within a day of the public announcement new data eliminated the possibility of any impact in that year.
The London Times, June 14, 1999 By Nigel Hawkes, Science Editor
The good news is that astronomers have identified an asteroid that could be on a collision course with Earth. The bad news is that they have lost it. The object, called 1998 OX4, was found last year by a team at the University of Arizona, who tracked it for two weeks.
The information the scientists gathered gave an approximate orbit for the object, which is believed to be several hundred yards in diameter and capable of continent-wide destruction if it were to collide with Earth.
From Robert McMillan June 14 1999
NEO 1998 OX4 was not lost through lack of trying. The object was discovered on 1998 July 26 (UT) by Jim Scotti with the 0.9-m Spacewatch telescope on Kitt Peak. Those who are familiar with the climate of southern Arizona know that it can be difficult to observe in the summer owing to frequent (although localized) thunderstorms.
Jim reobserved the object on two epochs the following night, but was clouded out on the 28th. By the 31st the Spacewatch observer was now Dr. Jeff Larsen, who was ready to open and be on the object within 5 minutes, but was still clouded out on Kitt Peak. However, thanks to the localized weather pattern, it was clear at nearby Mt. Hopkins where Carl Hergenrother recovered the object again with the 1.2-m telescope there.
Kitt Peak remained completely clouded out through the rest of the dark run in early August, except for Jeff's attempt at recovery on Aug. 3 UT which was aborted by clouds after he opened. Dave Balam recovered the object on 1998 Aug 3 and 4 UT with the 1.8-m telescope of the Dominion Astrophysical Observatory in Victoria, BC, Canada.
Is it worthwhile to discover objects during a season of persistently bad weather? I think so, because even if the objects cannot be recovered, the discoveries contribute to the accumulation of statistics on the distributions of asteroids with absolute magnitude and orbital parameters, and to an extrapolation to their total number. Jedicke and Metcalfe (1998 Icarus 131, 245-260) showed how much can be done with very short arcs on a number of main belt asteroids. Spacewatch will be doing similar analyses on its discoveries of Earth-approachers.
From Scott Manley
OK... so we've lost it... but it's not all doom and gloom.
Hypothetically were the object to really be a threat to the Earth on its 2046 close approach then there must be a range of orbital solutions for which it will hit. In which case during the next opportunities it might be wise to search for the object on orbits matching these killer solutions. While this still represents an area of the sky the area is far smaller than the entire error based on the observations. (The searchable area will surely tend to grow linearly with time, rather than with the square of the time).
If we don't find it then we can be safe in the knowledge that we're likely to be safe from this object for at least the forseeable future.
Date: Tue, 15 Jun 1999 From: Alan W. Harris: email@example.com
Scott Manley's point is magnificent, and should be kept firmly in mind in any discussion of follow-up requirements. [A search limited to killer solutions] solves the problem of future impacts from "lost" objects. Scientifically, it's unsatisfying to simply solve the impact hazard issue while throwing away scientifically valuable results (actual orbits on "lost" objects), but you have to stick to doing what you're being paid to do.
From: Andrea Milani: firstname.lastname@example.org
[Harris is] perfectly right in stating that it is not necessary to recover a lost potential impactor to be able to exclude the possibility of impact. Indeed the impacting orbit is much better known that the lost asteroid orbit, because the impact can be considered as an observation. Thus the 'virtual impactor' (we are anticipating to you the title of our next paper) is essentially a two opposition orbit, and its ephemerides can be predicted with reasonable accuracy.
However, we need to remind you of the responsibility of the scientist in discussing these matters. We cannot just say that negative observations could be used to exclude an impact: we need to state with extreme accuracy and reliability where and when these observations should be done, how big the telescope needs to be, and the uncertainty of the 'virtual impactor' ephemerides, that is, how large is the region of the sky to be looked at.
We have been working full speed on the problem since (and during) the travel back from Torino, and we expect the complete calculation to be done within a few days.
E. L. G. Bowell wrote:
The discussion reminds me of work that Karri Muinonen and I did a few years ago on the lost asteroid Albert. Given a suite of possible orbits, straddling a +/- 3-sigma range, we and collaborators searched uncertainty loci on archive plates. When we failed to find Albert, which was consistently the case or you would have heard otherwise, we blacked out a portion of sigma space where we knew Albert was not lurking. Unfortunately, the number of plates to be looked at was large enough (several thousand, if I remember correctly) that the project was eventually abandoned.
Steve Chesley replied:
I find more important this report from Ted about Albert. This would, I think, be a clear precedent for using negative observations to rule out particular solutions. We have already talked about using this idea to validate shaky identifications and coordinate the recovery lost objects, but ruling out close approaches turns out to be a much more challenging problem.
Date: Fri, 18 Jun 1999 From: Clark Chapman: email@example.com
Steve Chesley, et al. -- Whatever threads of reasoning led to this idea (and I think it would be interesting to know what they were), it strikes me as a really important development...one of the most significant to arise from the XF11/AN10/OX4 events. I hope to be kept informed about the resolution of the question of how easy or difficult it will prove to be, in practice,to implement this elegant idea.
JPL Near-Earth Object Program Office July 13, 1999
As announced in MPEC 1999-N21, a trail of asteroid 1999 AN10 was discovered on plates taken in 1955 from the Palomar Sky Survey. The nominal and minimum-possible close-approach distance for the 2027 Earth encounter are now 0.00260 AU and 0.00258 AU respectively (about 389,000 km). Preliminary analyses indicate that the 2044 and 2046 impacting key-holes are well outside the current 2027 impact-plane error ellipse. This implies that those previously highly unlikely impacts are now virtually impossible. The next Earth close-approach is in 2076 and will likely be between 0.046 and 0.009 AU (6,900,000 and 1,300,000 km).
From Benny J Peiser: firstname.lastname@example.org July 13 1999
Now, two amateur and two professional astronomers have dispelled the concerns about potentially hazardous asteroid 1999 AN10 colliding with planet Earth in 40 years time. The discovery of 44-year-old photographic images of the asteroid by Arno Gnadig and Andreas Doppler, two German amateur astronomers, and orbital computations by Brian Marsden and Gareth Williams of the Minor Planet Center in Cambridge (Mass.), show that there is no longer any risk of asteroid 1999 AN10 colliding with Earth at least for most of the twenty-first century.
From Brian G. Marsden and Gareth V. Williams July 13 1999
So wrote Malcolm Browne in The New York Times on March 14 last year with reference to some eight-year old photographs of 1997 XF11. It was of course the identification and measurement by Eleanor Helin and Ken Lawrence of images of 1997 XF11 on films obtained with the 18-inch Schmidt telescope at Palomar on two consecutive nights in March 1990, and the resulting orbital computations by us and others, that removed any possibility that the infamous object could impact the earth during the foreseeable future. Now it seems that the recognition of 44-year-old Palomar images and the resulting orbital computations show that the earth is also quite safe from impact by the almost equally infamous 1999 AN10, at least through most of the twenty-first century.
The 130-fold increase in the arclength permits a very dramatic improvement in the orbit determination of 1999 AN10. The miss distance on 2027 Aug. 7 will be 0.0026 AU, just the distance of the moon, and therefore now the closest confirmed approach of a sizeable minor planet. For a few hours it should become as bright as eighth magnitude. Its subsequent revolution period of slightly more than 1.74 years completely eliminates the possibility of close approaches in 2044, 2046--and even 2034, when the minimum distance will be comparable to that at the time of the 1955 observations. At the time of the purported 2044 encounter the object will be more than 200 million miles away, on the far side of the sun! The next significant approach after 2027 will not be until Feb. 2076, when the distance is likely to become less than 0.05 AU, although a miss distance as small as 0.008 AU would not be possible. Computations into the past show miss distances of 0.0062 AU on 1946 Aug. 7 and 0.025 AU on 1990 Aug. 8.
13 July 1999
The discovery of a 44-year-old photo of a near-Earth asteroid has all but eliminated any possibility that the object could hit the earth next century, astronomers reported July 12.
Two German amateur astronomers, Arno Gnadig and Andreas Doppler, located a pre-discovery image of asteroid 1999 AN10 that dates back to 1955. The image, taken as part of the first Palomar Sky Survey, dates back to when the asteroid was making a close approach to the Earth and visible high in northern skies.
Asteroid 1999 AN10 attracted attention earlier this year shortly after its discovery, when astronomers computed its orbit and discovered a billion-to-one chance that it could collide with Earth in 2039. Later analysis discovered another possible impact with 500,000-to-1 in 2044.
Those predictions, though, were based on only few months' worth of observations and thus had large uncertainties. The discovery of the 1955 image allows astronomers to tie down the orbit with much greater accuracy.
The refined orbit essentially eliminated any possibility of an impact in 2039 and 2044. In fact, Brian Marsden and Gareth Williams of the Minor Planet Center note that in 2044, 1999 AN10 will be on the opposite side of the Sun, more than 320 million kilometers (200 million miles) from the Earth at the time of the previously-predicted impact.
The improved orbit also adjusted a close approach the asteroid will make to the Earth in 2027. Instead of passing as close as 32,600 km, the asteroid will pass at around 390,000 km, or about the Moon's distance from the Earth. The asteroid will not pass close to the Earth until 2076, when it will come no closer than 1.2 million kilometers (745,000 mi.) to the Earth.
The revised orbit underscores the need to not only ramp up current searches for near-Earth objects, but to dig into archives to look for images that include the object prior to its discovery.