MESSENGER readies for Mercury orbit insertion. Credit: Johns Hopkins Univ. |
When
a NASA spacecraft goes into orbit around Mercury Thursday evening (March 17,
2011), a team of Lawrence Livermore National Laboratory researchers will be
paying close attention.
During
2002 and 2003, the LLNL scientists developed a germanium-based gamma ray
spectrometer that has been winging its way aboard the Mercury MESSENGER (short
for MErcury Surface, Space Environment, GEochemistry and Ranging) for the past
six-and-half years.
If
everything goes as planned, MESSENGER will start a highly elliptical orbit of
Mercury at 6 p.m. (Pacific Daylight Time) Thursday, coming as close as 200 km
(120 m) to the planet and as far as 15,000 km (9,000 m) away. It will be the
first spacecraft to ever orbit Mercury, circling the planet every 12 hours for
one year.
The
MESSENGER mission is part of NASA’s Discovery Program and is led by the Johns
Hopkins Univ. Applied Physics Laboratory.
Livermore’s gamma ray spectrometer
will help determine the elemental and mineral composition of Mercury’s surface.
“Mercury’s
surface is highly radioactive, so it emits a large amount of gamma rays,”
said LLNL physicist Morgan Burks. “By measuring the energy of the gamma
rays with high resolution, it is possible to determine the composition of the
planet’s surface. Each element gives off a unique gamma ray signature.”
It
is anticipated that once the MESSENGER spacecraft starts its orbit of Mercury
that it will be one to two weeks before the gamma ray spectrometer starts
sending data back to Earth.
The
fundamental challenge faced by LLNL researchers was to build a gamma ray
spectrometer that could withstand the extreme heat radiating from Mercury’s
surface, as well as the heat from the sun itself, which is 11 times brighter there
than at Earth.
Mercury
can reach as high as 400 degrees Celsius (752 degrees Fahrenheit). In order for
the germanium-based spectrometer to operate properly, the crystal has to be
cooled to -200 degrees Celsius (-330 degrees Fahrenheit).
Before
this mission, it wasn’t clear whether it was possible to operate a
cryogenically cooled instrument near Mercury. But the team came up with a
thermal and mechanical cooling design that allows the germanium crystal to live
at -200 degrees Celsius while rejecting 98% of the infrared heat and energy from
the broiling surroundings.
Before
MESSENGER, only NASA’s Mariner 10 spacecraft had traveled to Mercury. The
spaceship performed three flybys in 1974-75 and returned the first images of
Mercury to Earth. However, only 45 percent of the surface was photographed and
the spacecraft contained no X-ray or gamma ray spectrometers.
Launched
in August 2004, MESSENGER will completely orbit Mercury with a suite of seven
instruments, including X-ray, gamma ray, neutron, and charged particle
spectrometers, a laser altimeter and a magnetometer.
To
orbit Mercury, MESSENGER has had to follow a path through the inner solar
system, including one flyby of Earth, two flybys of Venus and three flybys of
Mercury.
Marianne Ammendolia and Morgan Burks examine the next-generation radiation detector, GeMini. Credit: Lawrence Livermore National Laboratory |
Traveling
at 84,500 miles per hour and logging more than 4.88 billion miles so far,
MESSENGER performed its flybys of Mercury on Jan. 14, 2008; Oct. 6, 2008 and Sept.
29, 2009.
The
LLNL gamma ray spectrometer took data and has performed well multiple times,
including during one of the flybys and during two deep space tests in 2004 and
2010, according to Burks.
“Despite
the fact that the gamma ray spectrometer has passed every test performed on it
in deep space, I think we’ll all rest much easier once we see it operating well
in the harsh thermal environment and orbit of Mercury,” Burks said.
As
the MESSENGER has journeyed toward its orbit of Mercury, an on-board heater has
been used to repair radiation damage from the flight to the LLNL gamma ray
spectrometer. To date, the crystal has been heated to 85 degrees Celsius (185
degrees Fahrenheit) on five occasions, annealing the germanium crystal and
restoring it to an almost perfect crystalline structure.
There
has long been a synergy for some scientific instruments, such as gamma ray
spectrometers, for applications in outer space exploration and back on Earth
for homeland security applications, such as to detect nuclear materials.
“The
technologies that we’re developing have a wide range of applications on Earth
and in space,” Burks said. “What we develop for basic research also
allows us to do important homeland security applications. In turn, the homeland
security advances we achieve help us further our basic research.
“We’ve
taken the MESSENGER technology and used it to create the next generation
hand-held gamma ray detector,” he said.
The
MESSENGER detector is a spinoff of the Lab’s Cryo-3, a mobile hand-held
mechanically cooled germanium radiation detector that can detect gamma rays
from radioactive material, and was developed by Norm Madden and John Becker.
Several
new LLNL radiation detectors, the GeMini and the GN-5, were designed to be
about the size of a lunch box, lighter, smaller and more energy efficient than
the Cryo3. Using lithium polymer batteries, the new detectors can run for 10
hours at a time, while the Cryo3 could only operate for a two-to-three hour
stretch.
The
GeMini and GN-5 germanium-based gamma ray spectrometers have been licensed to
companies, with the GeMini is expected to go into commercial production next
year.
Burks
said the GeMini detector could be used at border crossings, ports, airports, or
anywhere else that inspectors need to check for nuclear materials, particularly
plutonium or uranium. The GN-5 has been developed as a search tool, particularly
for use by the Coast Guard.
High-resolution
germanium-based detectors not only detect the presence of radiation, but can
discriminate between benign radioactive substances, such as medical isotopes or
bananas, and nuclear materials that could be used in a weapon.
“Unlike
a Geiger counter, which only tells you that radioactive material is present, a
germanium spectrometer will identify it,” Burks said. “The readings
are like a fingerprint for the material. Germanium detectors have been around
for decades for use in the lab. But this instrument is so small you can carry
it around in your hand.”
Some
of the initial engineering work for the MESSENGER detector was performed at
Lawrence Berkeley National Laboratory by members of the instrument team, who
later joined LLNL. Also participating in the collaboration was the UC Berkeley
Space Sciences Laboratory.
