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Posted by baalke on February 27, 2007, 11:16 am
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MESSENGER Mission News
February 20, 2007
Priming Instruments to Map Mercury's Crust
Understanding if ice exists on the surface of Mercury, and if so what
types, will mark an important component of the investigations by the
MESSENGER spacecraft about the origin and evolution of the solar
system's inner planets. This month, instrument engineers at the Johns
Hopkins University Applied Physics Laboratory in Laurel, Md., turned
on
the Neutron Spectrometer (NS)-one of several sensors aboard MESSENGER
that will be key to sorting out the mystery of Mercury's surface.
The NS will collect data about the composition of the uppermost tens
of
centimeters of Mercury's crust by measuring the numbers and energies
of
neutrons that reach the MESSENGER probe as it passes near the planet.
The NS together with a gamma ray sensor make up the Gamma-Ray and
Neutron Spectrometer (GRNS) instrument.
The NS will map variations in the fast, thermal, and epithermal
neutrons
Mercury's surface emits when struck by cosmic rays. "Fast" neutrons
shoot directly into space; others collide with neighboring atoms in
the
crust before escaping. If a neutron collides with a small atom (like
hydrogen), it will lose energy and be detected as a slow (or thermal)
neutron. Scientists can look at the ratio of thermal to epithermal
(slightly faster) neutrons across Mercury's surface to estimate the
amount of hydrogen-possibly locked up in water molecules-and other
elements.
Subtracting the Background Noise
APL's Edgar Rhodes, an instrument scientist on the MESSENGER mission,
says the current NS calibration will "tweak the electronic thresholds
and voltages for the detectors to assure that the most interesting
parts
of the neutron spectra are within the energy ranges of the instrument
before the upcoming second Venus flyby on June 5."
In space, the spacecraft is bombarded continuously from all directions
by galactic cosmic rays, high-energy particles (mostly protons)
thought
to originate from remnants of supernovae in our galaxy, explains APL's
John Goldsten, the GRNS instrument engineer.
"On Earth, we are protected from these penetrating rays by our
atmosphere; but in the vacuum of space they collide directly with
spacecraft materials, smashing into atomic nuclei, and sending off
energetic neutrons," he says. "These energetic neutrons, in turn,
collide with other atoms and produce a host of lower-energy neutrons
and
gamma rays. These signals pose a serious background in the instrument
that needs to be 'subtracted out' from the total signal measured near
a
planet. After all, it's the composition of the planet and not the
spacecraft we are trying to measure."
To make matters more difficult, Goldsten adds, some components of this
induced spacecraft background build up over time, "and so it is
important to make periodic measurements to characterize this
background
to perform a proper analysis and interpretation of the science data."
Searching for Solar Neutrons
The NS will remain on during most of MESSENGER's cruise phase and
return
data from Venus flyby 2 in June, three Mercury flybys in 2008 and
2009,
and one Earth year in Mercury orbit starting in 2011.=20
The NS is a low-power instrument that can safely be powered on
indefinitely, Goldsten says. "We plan to take special advantage of the
unique measurements the NS can produce as it journeys through the
inner
solar system; a region of space never before studied with this type of
instrument."
"Looking for the presence of energetic neutrons streaming away from
the
Sun during solar flares is of particular scientific interest to
physicists trying to model and understand the underlying mechanisms of
solar activity," he continues. "Solar neutrons are very difficult to
observe from Earth because these sub-atomic particles by themselves-
not
bound inside an atomic nucleus-are not stable and decay away in about
ten minutes, so only the most energetic (fastest moving) neutrons
reach
Earth before disintegrating, and these are very few. But as the
MESSENGER spacecraft journeys closer to the Sun and gets inside the
orbit of Venus, the likelihood of observing solar neutrons increases
dramatically because we get a chance to capture them before they can
decay away."
Another reason to operate the NS during the long cruise to Mercury is
to
help the Interplanetary Network of satellites detect and locate Gamma
Ray Bursts-the most energetic events known in the Universe-which
produce
monstrous flashes of gamma rays that appear in the sky at random times
and from random locations.
"Detecting these gamma ray bursts simultaneously at distant spacecraft
helps to triangulate their direction so that observatories can quickly
aim powerful telescopes to study the optical counterpart or
'afterglow'
of these colossal events, hypothesized to mark the collapse of stars
into black holes or the collision of super dense neutron stars,"
Goldsten says. "While the NS is optimized to detect neutrons, gamma
rays
appear as a steady background signal, so any sudden changes in this
background signal can be used as a gamma-ray burst monitor. Gamma-ray
bursts are typically detected about once a day, and no two are exactly
alike; some last milliseconds, while others may continue for minutes."
>From an engineering point of view, the GRNS is a flexible instrument
with many controls and settings that can be adjusted remotely via
commands to the spacecraft. "This flexibility is important as we
cannot
easily simulate the galactic cosmic ray environment on the ground,"
Goldsten says. "Optimizing the in-flight settings is usually an
iterative process where we make small changes and then analyze the
results. We then collect long-term data with the final settings to
establish the instrument baseline prior to an encounter such as the
upcoming Venus flyby."
More information about the GRNS is available online at
http://btc.montana.edu/messenger/instruments/grns.php .
Stat Corner: Now 931 days after launch, MESSENGER is about 71.9
million
miles (115.7 million kilometers) from the Sun and 162.3 million miles
(261.2 kilometers) from Earth. At that distance, a signal from Earth
reaches the spacecraft in 14 minutes and 31 seconds. The spacecraft is
moving around the Sun at 73,111 miles (117,661 kilometers) per hour.
MESSENGER's onboard computers have executed 225,580 commands from
mission operators since launch on August 3, 2004.
Preview of Upcoming Venus Flyby
with this featured animation
<http://messenger.jhuapl.edu/the_mission/movies/encounters/Venus
%202%20s
c%20od076%20small.mov> of the event. The mission gallery, available
online at http://messenger.jhuapl.edu/the_mission/gallery.html ,
contains a
variety of images, photos, animation, and movies about all aspects of
the mission-from pre-launch preparations, to images returned from the
planetary flybys, to artists' renditions of the MESSENGER spacecraft
on
its voyage to the inner solar system. Check back often; new materials
are added as the mission progresses.
MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and
Ranging) is a NASA-sponsored scientific investigation of the planet
Mercury and the first space mission designed to orbit the planet
closest
to the Sun. The MESSENGER spacecraft launched on August 3, 2004, and
after flybys of Earth, Venus, and Mercury will start a yearlong study
of
its target planet in March 2011. Dr. Sean C. Solomon, of the Carnegie
Institution of Washington, leads the
mission as principal investigator. The Johns Hopkins University
Applied
Physics Laboratory built and operates the
MESSENGER spacecraft and manages this Discovery-class mission for NASA.
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