An artist’s rendering of the twin GRAIL spacecraft orbiting the moon. The project uses two spacecraft flying in formation with known altitude amd separation distance to investigate the moon’s gravity field in unprecedented detail. Image: NASA/JPL |
Today’s
(September 9, 2011) expected launch of NASA’s Gravity Recovery And Interior
Laboratory (GRAIL) twin spacecraft, a carefully choreographed mission to
precisely map the moon’s gravitational field, could help scientists understand
fundamental questions about the moon’s composition, internal structure, and
evolution.
GRAIL’s
principal investigator—Maria Zuber, the Earle A. Griswold Professor of
Geophysics and Planetary Science at the Massachusetts Institute of Technology (MIT)—becomes
not only the first MIT professor to lead a NASA mission, but also the first woman
to lead a planetary spacecraft mission.
Following
the launch of the twin GRAIL spacecraft and a four-month journey, GRAIL-A is
scheduled to enter orbit around the moon Dec. 31; 2011, GRAIL-B will enter
lunar orbit the following day. GRAIL’s 90-day scientific mapping mission will
begin in March 2012.
“Technologically,
it is blazing the trail for other planetary missions,” says Zuber, the head of
MIT’s Department of Earth, Atmospheric and Planetary Sciences. “Using dual
spacecraft in tight-formation flight, doing precise timing and gravity
measurement, you could envision future planetary missions to Europa [one of
Jupiter’s moons] to map currents in its subsurface ocean; to Venus to map the
circulation of its atmosphere; and to Mars to observe the seasonal carbon
dioxide cycle between its atmosphere and its surface.”
Analysis
of data from the paired spacecraft will allow GRAIL to map the lunar
gravitational field with unprecedented accuracy and resolution. The precise
distance between the two spacecraft can be accurately measured using radio
interferometry and tracking beacons with ultra-stable oscillators.
By
precisely measuring changes in the distance between the twin orbiting
spacecraft as they traverse the moon’s gravitational field, scientists expect
to create a lunar gravity model up to 1,000 times more accurate than previous
models. “The ability to calibrate observations in space and time brings a lot
of value to scientific investigation,” Zuber says.
Dating
back to the Apollo lunar program in the 1960s, the trajectories of unmanned
orbiters circling the moon have been perturbed by its uneven gravitational
field. These gravitational anomalies are believed to be caused primarily by
subsurface geological features that could date back to the moon’s origin.
The
detailed lunar-gravity model obtained from GRAIL will be compared to
topographical models developed using the high-resolution laser altimeter on the
Lunar Reconnaissance Orbiter (LRO) mission launched in 2009—a project for which
Zuber serves as the deputy principal investigator. “We have over four billion
measurements of elevation on the moon from the LRO altimeter,” Zuber says, “so
we will be correlating these two data sets. What we stand to learn in terms of
crustal structure of the moon is really going to be unprecedented in planetary
science.”
GRAIL’s
close orbit—only 50 km above the lunar surface—should permit better spatial
resolution of the moon’s surface and subsurface geological features. “One of
the science objectives is to understand the structure of impact basins and the
role that impact basins played in the evolution of the moon,” Zuber says.
GRAIL
is also the first mission to fly an imaging system that does not have a
scientific objective: MoonKAM will be devoted entirely to education and
outreach. This imaging system includes four cameras on each spacecraft that
will be pointed at features on the lunar surface. Middle school students from
around the world can request MoonKAM pictures of the lunar surface.
Former
NASA astronaut Sally Ride, America’s
first female astronaut, is leading GRAIL’s public outreach effort, which
includes the development of an iPhone application that will allow students to
follow the trajectory of the GRAIL spacecraft over the lunar surface. “Students
are very interested in exploration,” Zuber says. “We hope they will share the
thrill of discovery along with us.”
The
GRAIL mission was selected as a NASA Discovery mission in 2007. Its scientific
team includes researchers from NASA’s Goddard
Space Flight
Center, NASA’s Jet Propulsion
Laboratory, the University of Arizona, the Southwest Research Institute, and the University of Paris. MIT research scientist David
Smith is GRAIL’s deputy principal investigator and Michael Watkins from the Jet
Propulsion Laboratory is GRAIL’s project scientist.
