Anticipated Improvements In Lunar Gravity Field Modeling With the Upcoming Lunar Missions

Maria T. Zuber1, David E. Smith2

1Dept of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 USA

2Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771 USA Phone: 617-253-0149

Email:

ABSTRACT

In the next two years there will be four orbital missions at the moon, all of which will contribute to our knowledge of the lunar gravity field. With SELENE, Chandrayaan, Chang’E, and LRO there will be an increase in new tracking data to be added to that from previous lunar missions, particularly Lunar Prospector that is presently the basis for the most accurate lunar gravity models [Lemoine et al; Konopliv et al]. All these missions will be tracked from Earth, some more than others, but the general data quality and coverage will be the same with the same coverage limitations as the existing data. Only SELENE has an operational configuration involving multiple spacecraft that will enable tracking over the lunar farside. All other missions are tracked when visible from Earth and limited to the lunar nearside. With the exception of LRO all the primary spacecraft are in 100 km orbits, and all are polar. LRO is in a nominal 50 km circular orbit although its altitude will vary between 30 km and 70 km during the course of each month. For the most part the tracking is microwave S-band Doppler, but LRO also has a laser ranging system operating at 532 nm capable of providing 10 cm precision ranges to LRO from Earth at 28 Hz over the lunar nearside. Each of these missions will carry laser a altimeter and have the goal to map the surface of the moon with radial accuracies of a few meters or better, with the limitation being the knowledge of the spacecraft position, which depends on the gravity field. LRO plans to use the altimeter as a tracking device [Smith et al] and employ the orbital cross-over technique to improve the along-track and radial position of LRO in its orbit and solve for a low resolution gravity field on the lunar far side. This approach was successfully employed in the development of a gravity field for Mars using the laser altimeter on Mars Global Surveyor in combination with Doppler tracking. The orbital cross-over method for improving the orbit is expected to be effective for LRO because of LRO’s greater accuracy, higher pulse rate of 28Hz, and multi-beam design. The ability to estimate the lunar gravity field is primarily dependent on the positional accuracy obtained for the spacecraft and the sensitivity of the orbit to gravity perturbations. At 50 km average altitude LRO will be more sensitive to the gravity field than the other missions with present LRO positional uncertainties due to gravity after 48 hours of 200 to 300 meters along track. LRO hopes to be able to reduce this to 50 meters with the laser ranging and altimeter cross-over data.

REFERENCES

Lemoine F. G., Smith D. E., Zuber M. T., Neumann G. A. and Rowlands D. D. (1997), A 70th degree lunar gravity model (GLGM-2) from Clementine and other tracking data, Journ. of Geophys. Res., 102(E7), 16339-16361, doi:10.1029/2008GL033494.
Konopliv A. S., Binder A. B., Hood L. L., Kucinskas A. B., Sjogren W. L. and Williams J. G. (1998), Improved Gravity Field of the Moon from Lunar Prospector, Science, 281, 1998, 1476-1480.

Smith D. E., Zuber M. T., Neumann G. A., Lemoine F. G., Robinson M. S., Aharonson O., Head J. W., Sun X., Cavanaugh J. F. and Jackson G. B. (2006), The Lunar Orbiter Laser Altimeter (LOLA) on the Lunar Reconnaissance Orbiter, AGU Fall Meeting

Abstracts, C826.