Unbiased orbit determination for the next generation asteroid/comet surveys

Abstract

In the next generation surveys, the discovery of moving objects can be successful only if an observation strategy and the identification/orbit determination procedure are appropriate for the diverse apparent motions of the target sub-populations. The observations must accurately measure the displacement over a short interval of time; observations believed to belong to the same object have to be connected into tracklets. Information contained in tracklets is in most cases not sufficient to compute an orbit: two or more of them must be identified to provide an orbit. We have developed a method for recursive identification of tracklets allowing an unbiased orbit determination for all sub-populations and efficient enough to cope with the data flow expected from the next generation surveys. The success of the new algorithms can be easily measured only in a simulation, by consulting a posteriori some "ground truth".We present here the results of a simulation of the orbit determination for one month of operations of the future Pan-STARRS survey, based upon a Solar System Model with a downsized population of Main Belt asteroids and a full size populations of Trojans, NEO, Centaurs, Comets and TNO. The results indicate that the method already developed and tested to find identifications of NEO and Main Belt asteroids are directly applicable to Trojans. The more distant objects often require modified algorithms, fitting orbits with only 4 parameters in a coordinate system specially adapted to handle very short arcs of observations. These orbits are mostly used as intermediate results, allowing to find full solutions as more tracklets are identified.When the number density of detections is as large as expected from the next generation surveys, both joining observations into tracklets and identifying tracklets can produce some false results. The only reliable way to remove them is a procedure of tracklet/identification management. It compares the tracklets and the identifications with a complex logic, allowing to discard almost all the false tracklets and all the false identifications. However, the distant objects still present a challenge for orbit determination: they require three tracklets in separate nights. If this requirement is met we have found no problem in achieving an unbiased orbit determination for all populations. Further work will lead to more advanced simulations, in particular by introducing a realistic model for astrometric and photometric errors.

Topocentric orbit determination: Algorithms for the next generation surveys

“…A number of these methods exist and most of them have one feature in common; the number of orbital parameters determined is less than 6. There are 4-fit methods in which 2 variables are kept fixed (e.g., [Milani et al 2006]) and 5-fit or constrained least squares solutions in which one parameter is fixed (the fixed parameter need not be one of the orbital elements but may be defined in some intrinsic way [Milani et al 2005a]). …”

“…It has already been tested in the context of a simulation of a next generation survey in [Milani et al 2006]. …”

“…We have performed a series of tests of our algorithms using a realistic Solar System Model (a catalog of orbits for synthetic objects [Milani et al 2006]) …”

“…Since the orbits of MBAs and Jupiter Trojans are easier to compute than those of NEOs and more distant objects [Milani et al 2006] we have prepared four targeted simulations: two containing only observations of NEOs and two with TNOs only. In both cases, one of the simulations covers the area near opposition and the other covers the so called sweet spots at solar elongations…”

“…Table 5 gives the size of the realized synthetic dataset after the simulation. While the focus of this paper is those objects that have tracklets available on three nights, those that are observed for less nights are part of the problem because their tracklets can be incorrectly linked to other objects [Milani et al 2006, figure 3]. …”