Topocentric Parallax for Near-Earth Object Initial Orbit Determination

Near-Earth Object (NEO) initial orbit determination is crucial for establishing the identity of the NEO as a discovery. In general, astrometric observations over a sufficiently long arc of the orbit can yield a well-determined orbit. However, because small NEOs can only be conveniently observed when they are very close to the Earth, the observation time window tends to be short. When the arc is short, the topocentric parallax, from the diversity of positions of the observatories relative to the center of the Earth, plays a crucial role in NEO orbit determination by providing the NEO ranging information. Topocentric parallax can be measured by a single observatory observing a NEO with different hour angles or multiple observatories at different latitudes. We present our study on when the topocentric parallax becomes significant for NEO orbit determination. We find that if the product of arc length and distance of approach is less than 1 day AU, the topocentric parallax dominates the NEO orbit determination; increasing amplitudes of parallax would improve the accuracy of the orbit. We discuss scheduling observations for increasing the topocentric parallax amplitudes and recommend using synthetic tracking to enhance accuracy for observing fast-moving NEOs.