Periodic anomalies in Earth-Mars range
Abstract
The distance between Earth and Mars can be inferred, with an accuracy of 2-3 m, by measuring the round trip flight time of X-band radio signals between DSN tracking stations on Earth and the MGS spacecraft in orbit around Mars. Such measurements have been made frequently since January 1999, and mostly agree quite well with model values from the JPL export ephemeris DE 403. However, there are periodic variations in the residual range (observed minus computed) with half-amplitude ~30 m and dominant period of ~250 days. The cause of the difference is still unresolved. The most likely source of small sporadic anomalies in Earth-Mars range would appear to be close encounters between Mars and any of the numerous small asteroids. However, the periodic nature of the observed anomalies seems to make that alternative unlikely. Among other possible sources that we have considered and rejected are: (1) errors in the orbit of MGS about Mars, (2) oblateness of the Sun's mass distribution, (3) improper modeling of general relativistic effects. Among the possible sources we have considered, and not yet rejected, are: (1) free wobble of the spin pole of Mars, (2) Previously undetected satellites of Earth or Mars. To produce the observed period of ~250 days, when projected onto the rotating Earth-Mars line, a perturbation at either end of the line must have an inertial period of ~190 days.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.P21A0362B
- Keywords:
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- 5450 Orbital and rotational dynamics