Near Earth Asteroids

1998 OX4: Third Asteroid with a Non-Zero Impact Probability Found
from Benny J Peiser <b.j.peiser@livjm.ac.uk>

As a result of recent discussions at the IMPACT workshop in Turin, is has become clear that the rapid increase in NEO discoveries, which is expected in the next decade, will inevitably lead to significant numbers of PHAs with non-zero impact probabilities such as asteroids 1997XF11 and 1999 AN10. In contrast to XF11 and AN10, however, the vast majority of these PHAs will no longer be newsworthy due to their minuscule chances of actual impact (in the next century or so). The calculations of their orbital dynamics will be made routinely, and particularly interesting objects will be continuously monitored. Consequently, public interest will only arise in exceptional cases which prove to have *significant* impact risks (< 1:1000?, or in the event of the detection of a Tunguska-sized object). That this coolness is almost the norm already was evident when, at the last day of the IMPACT workshop, Andrea Milani presented new results he and his team (Steven Chesley and Giovanni Valsecchi) obtained from calculating the impact probabilities for asteroid 1998 OX4.

Asteroid 1998 OX4 was discovered by the SPACEWATCH search programme operated by the University of Arizona on Kitt Peak on July 26 1998. Since this PHA (size c. 300-600m) approaches the Earth fairly close several times during the next 100 years or so, the Minor Planet Centre has placed the NEO on its list of Potentially Hazardous Asteroids. Andrea Milani and his colleagues have now analysed all dangerous encounters asteroid 1998 OX4 will make with the Earth in the next 50 years and found that there is a tiny chance of an impact in January 2046. Yet, the probability of such an encounter is, once again, extremely small (1:100.000.000) and thus of actual interest only to the NEO search community which will have to monitor the object in the future. Unfortunately, this PHA has been lost (which raises the important questions of how to avoid such losses in the future!) and might only be recovered sometime in the year 2000 or 2004.

Andrea Milani tells me that the details of their calculations will be posted on the NEODys home page in the next few days.

Benny J Peiser