The 2MASS Spring 1999 Point Source Catalog contains 1186 associations with known asteroids and 2 comets. Identification of the objects takes place as part of pipeline processing.
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 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 computed 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 error 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.
2MASS point sources that have a match chi-square value below the threshold of 16.0 will be flagged as being possible asteroid or comet detections in the PSC by having an "mp_flg" value set to 1. It is otherwise 0. Objects so noted are not positive asteroid or comet detections, but rather only positional associations. Some percentage of chance associations between predicted asteroid positions and background sources are to be expected, particularly in the Galactic Plane. For example, the two putative comet associations in the Spring 1999 PSC are clearly detections of relatively bright background stars. Users should examine in detail sources putatively associated with asteroids and comets to determine if the associations are in fact identifications. Information which is useful in determining this includes the 2MASS source color and whether the 2MASS object has a positional association with one of the optical reference catalogs.
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, 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 sighting of an asteroid or comet can be used to update orbital data for that object.
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.
[Last Update: 5 May 1999, by R. Cutri and M. Sykes]