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May
22, 2008: Meet MIT professor of physics Maria Zuber.
She's dynamic, intelligent, intense, and she's on a quest
for the Grail.
No,
not that Grail.
Zuber
is the principal investigator of the Gravity Recovery and
Interior Laboratory — "GRAIL" for short. It's a
new NASA mission slated for launch in 2011 that will probe
the moon's quirky gravity field. Data from GRAIL will help
scientists understand forces at play beneath the lunar surface
and learn how the moon, Earth and other terrestrial planets
evolved.
"We're
going to study the moon's interior from crust to core,"
says Zuber. "It's very exciting."
Above: An artist's concept of GRAIL in action.
Here's
how it works: GRAIL will fly twin spacecraft, one behind the
other, around the moon for several months. All the while,
a microwave ranging system will precisely measure the distance
between the two satellites. By watching that distance expand
and contract as the two satellites fly over the lunar surface,
researchers can map the moon's underlying gravity field1.
Scientists
have long known that the moon's gravity field is strangely
uneven and tugs on satellites in complex ways. Without course
corrections, orbiters end their missions nose down in the
moondust! In fact, all five of NASA's Lunar Orbiters (1966-1972),
four Soviet Luna probes (1959-1965), two Apollo sub-satellites
(1970-1971) and Japan's Hiten spacecraft (1993) suffered this
fate.
The
source of the gravitational quirkiness is a number of huge mascons
(short for "mass concentrations") buried under the
surfaces of lunar maria or "seas." Formed by colossal
asteroid impacts billions of years ago, mascons make the moon
the most gravitationally lumpy major body in the solar system.
The anomaly is so great—half a percent—that it actually would
be measurable to astronauts on the lunar surface. A plumb bob
held at the edge of a mascon would hang about a third of a degree
off vertical, pointing toward the central mass. Moreover, an
astronaut in full spacesuit and life-support gear whose lunar
weight was exactly 50 pounds at the edge of the mascon would
weigh 50 pounds and 4 ounces when standing in the mascon's center.
To
minimize the effects of mascons, satellite orbits have to
be carefully chosen. GRAIL's gravity maps will help mission
planners make those critical decisions. Moreover, the maps
GRAIL scientists will construct are essential to NASA's intended
human landing on the moon in the next decade. The gravity
of the moon's far side and polar regions, where future landings
are targeted, is least understood.
The
GRAIL team aims to map the moon's gravity field so completely
that "after GRAIL, we'll be able to navigate anything
you want anywhere on the moon you want," says Zuber.
"This mission will give us the most accurate global gravity
field to date for any planet, including Earth."
Above:
A gravity map of the moon made
by the Lunar Prospector spacecraft in 1998-99. Mascons are
shown in orange-red. The five largest all correspond to the
largest lava-filled craters or lunar "seas" visible
in binoculars on the near side of the Moon: Mare Imbrium,
Mare Serenitatus, Mare Crisium, Mare Humorum and Mare Nectaris.
Image reference: Alex S. Konopliv et al, Icarus 150,
1–18 (2001). [more]
GRAIL
will also help students learn about gravity, the moon, and
space. Each satellite will carry up to five cameras dedicated
to public outreach and education. Undergraduate students supervised
by trained adults will remotely operate the cameras from a
facility at the University of California, San Diego, that
currently operates similar cameras on the International Space
Station.
Middle
school students from all over the country will also get to
join in the excitement of lunar exploration. "We'll have
an interactive website where the middle school students can
make recommendations for targets to photograph and then view
the pictures of their suggested targets," she says. "This
just has incredible potential to engage students."
Clearly,
this is no ordinary Grail quest. Stay tuned to Science@NASA
for updates as the adventure unfolds.
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Author: Dauna Coulter
| Editor:
Dr. Tony Phillips | Credit: Science@NASA
| more
information |
| 1How
does the distance between satellites reveal the underlying
gravitational field of the moon? Zuber explains:
"The
change of distance with time is velocity, the change
of velocity with time is acceleration, and the unit
of gravity is acceleration," explains Zuber. "If
you very very precisely measure the tiny gravitational
perturbations of the two satellites at various locations,
and then put all those measurements together for the
whole moon, you get a gravity map."
It's
not as simple as it sounds. Like any good quest, this
one has its challenges. For starters, in making all
their calculations, Zuber and company will have to correct
for pesky factors such as atmospheric drag, gravitational
pull from other planets and general relativity, just
to name a few.
But
according to Zuber, the biggest challenge to GRAIL is
operating two spacecraft at the same time. "They
are launched together, but they have separate trajectories.
At some point they have to hook up and range to each
other. This requires great precision. We'll do a lot
of testing and simulations to make sure everything is
ready."
Zuber
proposed the concept for the GRAIL mission to meet an
objective NASA identified in a 2006 Announcement of
Opportunity for NASA's Discovery Program. Her proposal
was selected from among 24 submissions. She hand picked
a science team to carry out the investigation, and she
chose the Jet Propulsion Laboratory to support the mission
and JPL's Dave Lehman to serve as program manager. NASA's
Marshall Space Flight Center will oversee the mission,
with Bryan Barley as mission manager there.
EXTRA!
TEACHER WORKSHOP: Teachers in the Southeast
U.S. are invited to apply to attend a workshop entitled
"Paving the Way to the Moon and Beyond," held
June 12 – 14, 2008 in Huntsville, Alabama. The two day
workshop, sponsored by the Lunar Precursor Robotic Program
(LPRP), will focus on content that will explain the
who, the what, and the why of lunar exploration. Beginning
with the Lunar Reconnaissance Orbiter (LRO) and the
Lunar Crater Observation and Sensing Satellite (LCROSS),
teachers will research mission design and scientific
goals and experiments through hands on activities. Included
in the workshop will be a tour of NASA science labs
and an opportunity to talk with scientists involved
in exploration related activities. All materials will
be appropriate for elementary and middle school preservice
and inservice teachers and will be aligned with national
standards. Example activities: Earth-Moon comparisons
and motions, craters and lunar soils, solar influences
on the Moon. Most activities will take place at the
Educator Resource Center, located at the Space and Rocket
Center. Housing costs for three nights will be provided
(at the University of Alabama in Huntsville). Stipends
also will be provided. To attend, contact Mitzi Adams
256 961 7626 or mitzi dot adams @ nasa dot gov.
NASA's
Future: US
Space Exploration Policy |
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