The strategy used to identify possible detections of known asteroids and comets by 2MASS is to consider the actual area covered during each survey scan, and the time each point on the sky within the scan was observed. The asteroid and comet ephemerides are then searched to determine which objects may have been within the scan boundaries at the specified epoch. Ephemerides are computed using orbital elements published by the Minor Planet Center for all numbered asteroids, and all multiple-opposition unnumbered asteroids, as well as all periodic comets, and recent nonperiodic comets. The orbits of the planets are included for completeness and are taken from the JPL DE403. The heliocentric position of the Earth is derived from DE403, and topocentric corrections to the two observing sites are included. Although the ephemeris computations are two-body in nature, the database of orbital elements is updated every 100 days to incorporate newly numbered asteroids and improved orbits, and the opportunity is taken to numerically integrate all the orbits to a current epoch of osculation. The ephemeris accuracy is typically 1". In addition to the predicted position of the object, the apparent magnitude is computed, which can be used to validate proper identification, though a large acceptance window is needed because of unknown lightcurve and color effects, and the line of variation is used to represent the major axis of the error ellipsoid. Distances and phase angle are also computed for purposes of reducing apparent magnitudes to absolute magnitudes.
If an asteroid or comet is predicted to have a position within the observed boundaries of a scan during the time of its observation, a search is made of the extracted 2MASS point source lists for objects that positionally correlate with the predicted position. Candidate 2MASS asteroid detections are first screened by searching for infrared sources within a coarse window of 30" in RA and DEC around the predicted position. For each 2MASS point source within that window, a two dimensional chi-square position parameter is computer using the separation between the 2MASS and predicted positions and the combined position error covariance matrix. If the value of the chi-square is less than 16.0, the association is acceptable (a threshold of 16.0 corresponds to a completeness of 0.000335; in other words. one correct match out of every 3000 will be missed in the attempt to avoid false matches). For example, for a predicted asteroid position uncertainty major axis of 3.0" and a minor axis of 1.0", this threshold just allows a match with a position discrepancy of 8.5" along the major axis and 2.8" along the minor axis.
Because the astrometric precision of 2MASS point source positions is typically <0.2"-0.3" with respect to the ACT (Hipparcos/Tycho) reference system, the dominant uncertainty in matching 2MASS candidate sources to asteroids and comets is the uncertainty in orbital predictions. Typical uncertainties are in the range 1"-5", and as expected, As expected, the major axis of the asteroid position uncertainty ellipse is generally parallel to the orbital plane. The astrometric precision of 2MASS also means that every siting of an asteroid or comet can be used to update orbital data for that object.
This table on the 2MASS Sampler CD-ROM is a compilation of all 2MASS Sampler Point Source Catalog entries that are probable detections of known asteroids. The positions of 75 asteroids and one comet (6P/d'Arrest) were scanned on the night of 971116 UT by the northern 2MASS facility, and there were 51 nominal detections (6P/d'Arrest was not detected). Note that several asteroids, including 4650 Mori, 3300 McGlasson, 2983 Poltava, 1993 XO, 1995 GJ7, 1997 YR3 and 1987 YB, were detected twice because they fall within the overlap region between adjacent scans.
There is currently no attempt made during 2MASS data processing to identify previously unknown minor planets or comets. Such a search might be possible using the repeated observations of the small areas in the overlapping regions in adjacent tiles.