You may use either the asteroid number, the Horizons designation (for comets) or the common name. If you use a common name the Horizons system will try to resolve this into an unambiguous identification.
If no name is given, then the specified orbital elements will be used. The first seven orbital elements must be given but the H magnitude is optional. The epoch of the elements and the time of perihelion passage may be expressed in Julian (or Modified Julian) Days or using an ISO date string (yyyy-mm-dd hh:mm:ss). All angles should be specified in decimal degrees. The perihelion distance is given in astronomical units.
This page uses an ephemeris generated dynamically using NASAs Horizons facility. This is compared with the databases of NEAT, DSS, DSS2 and USNO to find images in which the object may be found. The HST archive catalog may also be searched for observations within 1 arcminute of the specified position. Note that the position of the NEAT observatory is used for all ephemerides so there may be a non-trivial parallax correction for objects within ~20 million kilometers. The USNO search includes the AAON, AAOR, AAOREJ, and P2REJ catalogs at the USNO. Other catalogs available there are included in the DSS and DSS2 datasets.
Searching all sources can be quite slow. Rows may be returned where the object is actually slightly off the edges of the plate and for the DSS2, some plates may not be publically available. DSS2 images are retrieved from the Space Telescope Science Institute. For ST observations the instrument type is not considered when searching for matches and the nominal pointing position as given in the HSTAEC catalog is used. The version available in the HEASARC Browse service is searched.