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Phobos Ephemeris Improvement from Recent Spacecraft ObservationsDespite 127 years of observations, there is still room for improvement in the ephemeris of the Martian satellite Phobos. Early in this history, Earth-based astrometric observations of Phobos and Deimos were used to estimate the mass and oblateness of Mars. As more data accumulated, it became clear that a secular acceleration in the longitude of Phobos was occurring, and this was attributed to tidal dissipation within Mars, yielding rough estimates of the tidal quality factor, or Q. At the epoch of the earliest spacecraft observations of Phobos, from Mariner 9 and the Viking Orbiters and Landers, the gravitational field of Mars, and resulting forces on Phobos, were still not particularly well known. Thus observations of natural and artificial satellite motions continued to contribute, each in their own way, to knowledge of the mass distribution within Mars. Improvements in tracking system accuracy, and the placement of satellites, like the 1996 Mars Global Surveyor and 2001 Mars Odyssey, in circular polar orbits has lead to dramatic improvements to knowledge of the Mars gravity field. The direct gravitational influence on Phobos is no longer expected to be a limiting factor in predicting its orbital motion. Despite that progress, a variety of observations of Phobos from recent orbiters and landers suggest that the best satellite ephemeris still has along-track orbit errors which are accumulating at a rate of 1.75 kilometers per year, with Phobos gaining on the predicted positions. These recent observations alone do not span sufficient time to separately resolve the positional error into changes in mean motion and changes in secular acceleration. However, combining them with earlier observations will allow improvements in both the mean motion and its first derivative. This latter parameter is particularly interesting, as it relates to tidal dissipation, and thus uniquely constrains the internal structure of Mars. The current best estimate of the secular acceleration rate of Mars implies a surprisingly low value for the tidal dissipation factor Q = (94 plus or minus 1). For the Earth, most tidal dissipation occurs within the oceans. The terrestrial mantle tidal Q is 280. An issue of interest is then: why, where, and how does Mars dissipate tidal energy so effectively.
Document ID
20040171856
Acquisition Source
Goddard Space Flight Center
Document Type
Conference Paper
Authors
Bills, B. G.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Neumann, G. A.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Smith, D. E.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Zuber, M. T.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Date Acquired
August 22, 2013
Publication Date
January 1, 2004
Subject Category
Astronomy
Meeting Information
Meeting: IAG International Symposium on Gravity, Geoid and Space Missions
Location: Porto
Country: Portugal
Start Date: August 1, 2004
Sponsors: International Association of Geodesy
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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