|
The gravity field of the Moon strongly influences the altitude
of a spacecraft in low circular orbit. The most dramatic example
is the Apollo 16 subsatellite. After being deployed in a near
circular orbit from the Command and Service Module, the eccentricity
increased quickly and the spacecraft impacted the lunar surface
35 days after the release strictly due to the force of the gravity
field. Understanding the precise nature of a planet's gravity
field is vital to all exploration and experimentation.
As presented at the March 5 science return press conference,
Lunar Prospector's Doppler Gravity Experiment (DGE) has provided
the first polar low altitude measurement of the lunar gravity
field. This provided the spacecraft with the first truly operational
gravity map of the Moon and immediately improved orbit and fuel
efficiency. Improved gravity information will not only help scientists
build better models of the role of impact processes on the history
and evolution of the Moon, but will also help in estimating the
lunar core size and metallic iron content. A more practical benefit
of the new lunar gravity data provided by Prospector's DGE experiment
is that a more precise gravity map of the Moon will inevitably
aid future mission planners in planning fuel-efficient journeys
to the Moon, and may even help identify potential resources.
Lunar Prospector's Doppler
Gravity Experiment
The Moon has a large asymmetry due to the fact that the lunar
crust is thicker on the far side than on the near side and a much
"bumpier" gravitational field than the Earth, with small
anomalies due to mass concentrations on the surface. The Apollo
missions helped demonstrate such sizable positive gravity anomalies.
Interestingly, they exist within the topographically low, large
circular mare basins. This discovery was unexpected and opposite
of any physical model at that time and started the development
of new models of the Moon's interior. The features were called
mascons (short for "mass concentrations"). These bumps
cause an orbiting spacecraft to speed up or slow down. The DGE
is, in effect, drawing a map of the bumps.
To hear more information about mascons, please select one of
the following:
The DGE, unlike the other experiments aboard Lunar Prospector,
requires no extra instrumentation. All of the data is collected
simply by communicating with the spacecraft. As the spacecraft
orbits the Moon, its speed can always be determined by the Doppler
effect, the same effect that causes a police siren to sound higher
when the police car is moving toward you and lower when it is
moving away from you. The "siren," in this case, is
the spacecraft's radio signal, whose frequency shifts slightly
as it moves toward Earth or away from it. Relative to the near
side, lunar farside gravity is poorly determined because the spacecraft
is not in view from the Earth when over the lunar far side. However,
some information is obtained by observing changes in the LP orbit
due to the accumulated acceleration of the farside gravity as
the spacecraft comes out of occultation (back into view).
By tracking the velocity of the spacecraft, mission scientists
can infer the forces acting upon it. For over 99 percent of the
duration of the mission (excepting only periods when the engines
are being fired) the only force on Lunar Prospector is gravity.
Thus, by simply circling the Moon and sending signals back to
Earth, Lunar Prospector has mapped the Moon's global gravitational
field. Lunar Prospector completed this gravitational map in the
first two months of the mission. However, the results of the DGE
will be greatly improved with data from the extended, low-altitude
phase of the mission. At this low altitude of 6 miles (10 km),
the precision of the gravity data will be improved by a factor
of over 100.
Lunar Prospector's DGE data
Þ Recent measurements have revealed three new mascons ("mass
concentrations") on the near side of the Moon coincident with
the large impact basins Mare Humboltianum, Mendel-Ryber, and Schiller-Zucchius.
Furthermore, although there is no direct measurement of the lunar
farside gravity, LP data indicate four additional new mascons
in the large farside basins of Hertzsprung, Coulomb-Sarton, Freundlich-Sharonov,
and Mare Moscoviense, and clearly show a central area of increased
gravity in these basin centers.
The newest gravity map based on Lunar Prospector data
This image shows the gravity field at the Mare Serenitatis (Sea
of Tranquility). The upper segment represents the topography-
a fairly flat low region-and the lower segment shows the corresponding
strong gravity field. This is a mascon.
This diagram shows the three new mascons identified on the near
side of the moon. The top segment for each location shows topography.
The segment beneath shows the gravity field which, surprisingly,
is particularly strong in the center of each area where the topography
is low. The Mare Humboldtianum location includes a third segment
below the gravity field as identified by Lunar Prospector. This
third segment shows the gravity field as measured before Prospector.
Lunar Prospector's high quality gravity data, improved by roughly
a factor of five over previous estimates, indicate the existence
of a lunar core, probably iron, with a radius of more than 300
km.
Such improvements to the lunar gravity field also offer the practical
benefits of modeling long-term spacecraft orbits about the Moon,
which allows more accurate planning of future mission fuel needs
and enables the development of fuel efficient orbital maintenance
strategies. LP engineers are currently relying on the improved
lunar gravity model in devising strategies for maintaining LP's
extremely low orbit during the extended mission phase.
To hear more about the Doppler Gravity Experiment, please select
one of the following:
Back to science
results main page
|
