On the night of January 13/14, 2004, the international asteroid science community faced an unprecedented situation, with an initial prediction of an impact of a small asteroid on the Earth within a few days. The asteroid in question, bearing the temporary designation AL00667, had been discovered the previous night by the LINEAR telescopes of the Spaceguard Survey. Routine and largely automatic processing of these observations led to the posting (by the IAU-supported Minor Planet Center in Cambridge MA) of an orbit that intersected the Earth within the next 36 hours. Although the observations were limited and the orbit solution was highly non-unique, there initially appeared to be a distinct possibility, verified by orbital calculations carried out at the NASA NEO Program Office at JPL, of an impact in the northern hemisphere.
The estimated size of asteroid AL00667, diameter 30 m, was small enough that no serious consequences were expected, but large enough that significant ground damage or possible injury could not be ruled out. The very limited data (from one night's observations) and the preliminary nature of the orbit at no time allowed an estimate of a possible impact site. The situation remained uncertain for several hours, until amateur astronomer Brian Warner, with a 20-inch aperture telescope in Colorado, searched the area where JPL calculations showed that the asteroid would have to be if it were on an actual collision course. Warner's observations were negative, showing that the true orbit for AL00667 did not intersect the Earth. Thus, as Clark Chapman of Southwest Research Institute writes in a paper presented at the American Institute of Aeronautics and Astronautics workshop on protection of the Earth (February 23), "instead of waking up to headlines and TV news specials of an asteroid strike about to happen in the next day or two, life went on as usual around the world." Only a handful of astronomers (professional and amateur) were involved, and there were no scare stories in the press.
Several members of the IAU Working Group on Near Earth Objects were involved in the activity surrounding the possible impact orbit of AL00667, and the Working Group on NEOs seeks to draw some lessons from these events.
First, we stress that there were no significant errors in either data or interpretation made that night. The observations were real, the object did exist (although it turned out to be much farther away than originally thought), and there were orbital solutions that predicted an impact with an apparent probability of at least 10%. Additional observations were required in order to verify that the object was not on a collision course. These observations were of the sort used previously to falsify a dangerous orbit by looking at the place where the asteroid would be if it were on a collision course (sometimes called a process of eliminating a virtual impactor). The IAU WGNEO congratulates those who worked to eliminate this particular threat in a highly professional way, and did so within a few hours, and without negative publicity.
Second, we note that the Spaceguard Survey was not designed to detect small asteroids a few days before collision. It does not have either the equipment or the resources to function as a warning system for imminent impacts. Its purpose is to carry out a long-term survey, with emphasis on discovering and calculating accurate orbits for asteroids larger than 1 km -- those with a possibility of triggering a global catastrophe if they collided with Earth. The Spaceguard goal is to discover 90% of the near-Earth asteroids larger than 1 km by the end of 2008. This approach should provide decades of warning for any impact by these larger asteroids.
Third, the probability that a small asteroid will be discovered within a few days of hitting the Earth is extremely small. While many asteroids might masquerade as impactors based on discovery observations spanning a single night, the great majority of these will be "false alarms."
Finally, however, we note that it is possible for the survey telescopes to pick up a small asteroid on its final approach, and the interested community should decide how cases like AL00667 should be handled in the future. The IAU WGNEO suggests to its members that we consider what actions should be taken to facilitate the determination of a more accurate orbit, and the ways this information should be made available to the public and to interested official groups. The current mode of verification for larger asteroids, including the provision for an optional IAU peer review of orbital calculations, does not apply when there is a possibility of an impact within a few days of discovery. The WGNEO therefore suggests that we should discuss these issues among ourselves and with other interested parties to determine protocols for dealing with such cases in the future. There should be planning at an international level of what action to take with respect to notifying governments and the public of a possible near-term impact. This planning must recognize that it is likely that in all such cases new observations will quickly eliminate any possibility of impact rather than confirm it.
In the meantime, the WGNEO suggests the following two interim guidelines:
(1) If any orbital solution for a newly discovered asteroid includes a possible near-term impact, every effort must be made to pursue that possibility (even if it is not the most likely of several possible orbits). The raw data should be made available immediately to the community so that experts on asteroid orbits all over the world can be involved in the interpretation and can freely check each other's calculations.
(2) Observatories, amateur and professional, should be informed of the high priority to be given to checking the possibility of an impact by searching for the virtual impactor and (hopefully) showing that it is not on a collision course. Searching positions corresponding to a virtual impact orbit should be given priority, as it is more important to eliminate the virtual impactor than to recover the asteroid and compute its correct orbit - the latter will happen routinely within a few days.
Chair, IAU WGNEO