Science presentations announced for the Comm 20 sessions at GA24:



ANSERINE PURSUITS

Jeremy B. Tatum

Spaceguard Canada

I describe the observational programme carried out in Victoria, Canada, by David Balam, using the 72-inch Plaskett telescope of the Herzberg Institute of Astrophysics, National Research Council of Canada. This is a follow-up programme rather than a discovery programme, and our fortunate access to a large telescope enables us to follow up fainter objects and over a larger arc than is possible for smaller telescopes. I discuss some of the problems involved and their solutions, in particular the problem of "wild goose chases" or "anserine pursuits", which is the problem of the inefficient waste of large telescope time while attempting to follow up non-existent objects that arise from unconfirmed single-night observations.



NEO FOLLOW-UP ASTROMETRIC PROGRAMME AT KLET

Jana Ticha

Klet' Observatory

- confirmatory observations of new NEO candidates
- early and later NEO follow-up
- NEO recoveries
- our experience with using various WWW services for observers for planning such NEO observations



INITIAL ORBITAL RANGING AND COLLISION ASSESSMENT FOR ASTEROID 1998 OX4

Karri Muinonen1, Jenni Virtanen1, and Edward Bowell2

1) Observatory, University of Helsinki,
Kopernikuksentie 1 (P.O. Box 14), FIN-00014 U. Helsinki, Finland.
E-mail: Karri.Muinonen@Helsinki.Fi

2) Lowell Observatory, 1400 West Mars Hill Road,
Flagstaff, Arizona 86001, U.S.A.
E-mail: ebowell@lowell.edu

With the help of our novel initial-orbital-ranging technique, we compute the probability density of the orbital elements for the Earth-crossing asteroid 1998 OX4, and confirm non-vanishing Earth-collision probabilities in 2014, 2038, 2044, and 2046 and, in addition, have discovered several other orbits allowing Earth collision in different years (e.g., in 2012 and 2015). Furthermore, the novel technique can allow the reduction of errors from the observations: ranging a large number of sample orbits reveals systematic offsets of the observations and the discrete sets of theoretical positions. We study the effects of the offsets on the orbit determination, ephemeris prediction, and collision probability computation of asteroid 1998 OX4.



THE POPULATION OF NEAR-EARTH ASTEROIDS

A. W. Harris (JPL), A. W. Harris (DLR) and S. Werner (DLR)

For purposes of defining a population vs. size, we define a Near-Earth asteroid as one with a perihelion less than 1.3 AU, and we measure size in terms of absolute magnitude H, rather than actual size. Earlier estimates of the population with H < 18.0 (generally considered to correspond to diameter > 1 km) range from as many as 2000 to as few as 700. We have estimated the population in two ways: (1) by normalizing a model of lunar crater population vs. impactor size to agree with the numbers of NEAs observed in the range where surveys are complete already (H < 14.5), and exprapolating to smaller sizes, and (2) by dividing the numbers of already discovered asteroids by the ratio of the number of new discoveries to total detections (new plus re-detections) in the last year. Both methods yield strikingly similar estimates of the number of NEAs with H < 18.0, of about 850-1,000.

This research was supported at JPL under contract from NASA.



UPDATE ON SMALL SOLAR ELONGATION NEO SEARCH

D.J. Tholen, R.J. Whiteley

Institute for Astronomy, University of Hawaii

We have continued to develop and refine our techniques for finding NEOs at solar elongations of less than 90 deg, which is the only place in the sky where asteroids with aphelion distances of less than 1 AU can be found. To date we have found eight NEOs, in addition to our share of Mars crossers, and Phocaeas, while the hundreds of main belt objects go unmeasured due to lack of resources. Among our discoveries are two PHAs, the largest Apollo found during 1999 (4.6 km estimated diameter), the faintest NEO ever discovered (V= 23), and the smallest solar elongation of any NEO discovery (51 deg). Among these eight is one candidate for having an aphelion distance of less than 1 AU (1998 DK36). Unfortunately, this is the only one of our NEOs that become lost, due to instrumentation failure. Some good came out of it, however, as it served as motivation to develop orbit solution tools that work on short arcs and a small number of observations. The use of this software on 1998 DK36 indicates a high probability that the object's aphelion does indeed lie interior to the Earth's orbit. Practical application of these tools helped with the recovery of a Mars crosser 30 days after its discovery arc of only six minutes!