**DPS Pasadena Meeting 2000, 23-27 October 2000**

*Session 14. Asteroids II Posters - Discovery and Dynamics*

Displayed, 1:00pm, Monday - 1:00pm, Friday, Highlighted Tuesday and Thursday, 3:30-6:30pm, C101-C105, C211
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## [14.05] Short Arc Orbit Solutions and Ephemeris Computations: Application to NEOs

*D. J. Tholen, R. J. Whiteley (Inst. for Astronomy, Univ. of Hawaii)*

We had previously described a technique for computing orbits
and ephemerides using just two observations. The technique
involves assuming values for the topocentric distance and
projection angle of the topocentric velocity vector onto the
plane of the sky, along with using measured values for the
right ascension, declination, and their time derivatives at
the mid-observation time, to solve for a suite of orbits
that could be used to compute future ephemeris positions. We
have since generalized this technique to allow more than two
observations. The topocentric distance and projection angle
are still assumed, but we now use a least-squares approach
to find the best-fit values of the initial right ascension,
declination, and their time derivatives. With just two
observations, it is always possible to find a perfect fit,
so the resulting orbits are usually constrained to be
elliptical and prograde (though these restrictions can be
relaxed for special cases). With more than two observations,
the resulting orbits are constrained by their RMS residuals.
We will present several test cases involving NEOs that
demonstrate the success of this method in predicting the
positions of these typically fast-moving objects. One
object, 1999 XA152, was recovered an amazing thirty days
after its initial six-minute observational arc (bad weather
had prevented more immediate recovery). Even when the
individual orbital elements appear to be unconstrained, they
collectively can still constrain the ephemeris prediction
rather well. In particular, we will show that all orbit
solutions for 1998 DK36 with RMS residuals of less than 1
arcsec have aphelion distances between 0.97 and 0.99 AU,
despite having perihelion distances ranging from 0.14 to
0.76 AU and semimajor axes ranging from 0.56 to 0.87 AU,
which reinforces the case for this object being the first
asteroid known to have an orbit entirely interior to the
Earth's orbit. We acknowledge NASA Grant No. NAG5-4524 for
portions of this work.

The author(s) of this abstract have provided an email address
for comments about the abstract:
tholen@ifa.hawaii.edu

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