Peter G. Antreasian

INVESTIGATIONS INTO THE UNEXPECTED DELTA-V INCREASES DURING THE EARTH GRAVITY ASSISTS OF GALILEO AND NEAR

Unexpected energy increases during Earth flybys of both the Galileo and Near Earth Asteroid Rendezvous (NEAR) spacecraft have drawn evidence of spacecraft trajectory modeling errors, an unknown perturbing force or failure of Newtonian gravity. This paper will investigate the gravity field of Earth as a possible source of these anomalous AVs. Other possible sources of errors have been considered including: the mathematical models representing the perturbing forces acting on the spacecrafts while in the sphere of influence of Earth such as relativistic effects, tidal effects, Earth radiation pressure and atmospheric drag. However, most of these perturbations such as atmospheric drag can be ruled out because the imparted acceleration upon the spacecraft is several orders of magnitude less than observed. Since the oblateness effect is several orders of magnitude greater than the non-gravitational perturbations, errors in the spherical harmonic representation of Earths gravity field will be examined. Other sources that have already been examined and tentatively dismissed include numerical round-off, integration errors, spacecraft antenna phase center offset and spacecraft antenna switching during encounter.

Mars Exploration Rovers orbit determination filter strategy.

§†† ‡‡ §§ §§ , The successful delivery of the Mars Exploration Rover (MER) landers to well within the boundaries of their surface target areas in January of 2004 was the culmination of years of orbit determination analysis. The process began with a careful consideration of the filter parameters used for pre-launch covariance studies, and continued with the refinement of the filter after launch based on operational experience. At the same time, tools were developed to run a plethora of variations around the nominal filter and analyze the results in ways that had never been previously attempted for an interplanetary mission. In addition to achieving sub-kilometer Mars-relative orbit determination knowledge, the filter strategy and process detected unexpected error sources, while at the same time proving robust by indicating the correct solution. Consequently, MER orbit determination set a new standard for interplanetary navigation. Nomenclature

Recovery of near's mission to eros

Abstract On December 20, 1998, the main bipropellant thruster on the Near Earth Asteroid Rendezvous (NEAR) spacecraft was commanded to start a 15-minute burn. It was the first and largest of four planned maneuvers that would cancel NEARs velocity relative to (433) Eros to allow capture into orbit about the asteroid in January 1999. But the burn aborted and the spacecraft tumbled, causing loss of communication. The spacecraft corrected the problem, but used 29 kg of fuel in the process. A day later, NEAR was reacquired with just enough time to upload a sequence of commands to image Eros extensively as the spacecraft hurtled past the asteroid on December 23. The NEAR team designed a large bipropellant maneuver that successfully cancelled most of NEARs velocity relative to Eros on January 3, 1999. But then NEAR was almost 1 million kilometers from Eros, so a long “U-turn” journey was begun to return to the asteroid on February 14, 2000. The spacecraft remains healthy and enough fuel remains to enter orbit about Eros to complete all of the mission goals. This will be the first time that an interplanetary spacecraft has failed an orbital capture burn and returned to accomplish its objectives. This is possible because NEAR had a generous fuel supply and a robust contingency plan.