NASA's MErcury Surface, Space ENvironment,
GEochemistry and Ranging spacecraft
swung by Venus for the second time on Tuesday evening for a
gravity assist that shrank the
radius of its orbit around the sun, pulling it closer to
Mercury. At nearly 15,000 miles per
hour, this change in MESSENGER's velocity is the largest of
the mission.
Mission operators at
Johns Hopkins' Applied Physics Laboratory said
MESSENGER's
systems performed flawlessly as the spacecraft sped over
the cloud tops of Venus at a
relative velocity of more than 30,000 miles per hour,
passing within 210 miles of the surface
of the planet at 7:08 p.m.
For 20 minutes during this closest approach, MESSENGER
was within the shadow of
Venus, and in the absence of solar power the probe relied
solely on its internal battery. By
9:32 p.m. the battery had fully recharged, and the
spacecraft was operating as planned. "The
biggest milestone for mission operations was first
acquisition of telemetry following closest
approach, and confirmation that the battery was fully
recharged following the 20-minute solar
eclipse," said APL's Andy Calloway, MESSENGER's mission
operations manager. "Next stop,
Mercury on Jan. 14, 2008!"
This second Venus flyby was a critical mission
milestone in the probe's circuitous journey
toward Mercury orbit insertion, said principal investigator
Sean Solomon, from the Carnegie
Institution of Washington. "Not only did the maneuver
sharpen the spacecraft's aim toward
the first encounter with Mercury in more than three
decades, it presented a special
opportunity to calibrate several of our science instruments
and learn something new about
Earth's nearest neighbor."
According to APL's Eric Finnegan, MESSENGER systems
engineer, the spacecraft's
approach geometry is similar to that for the first Mercury
flyby, allowing — for the first time
in flight — the craft's seven instruments to be
turned on and operated collectively in science-
observing mode, just as they will be for Mercury.
"Gathering approximately six gigabits of
data, the spacecraft will take more than 630 images, as
well as make other scientific
observations over the next few days," Finnegan said after
the flyby.
The team plans to image the upper cloud layers at
visible and near-infrared wavelengths
for comparison with earlier spacecraft observations.
Magnetic field and charged particle
observations will be made to characterize solar wind
interaction and search for solar wind
pickup ions. Ultraviolet-visible and X-ray spectrometry
will permit detailed observations of the
composition of the upper atmosphere, and MESSENGER will
search for lightning on the Venus
night side.
"We are very excited with this next step in reaching
our ultimate destination, Mercury,"
said APL's Ralph McNutt, MESSENGER project scientist.
During this Venus encounter, MESSENGER joined forces
with the European Space
Agency's Venus Express spacecraft, currently orbiting
Venus, to make joint observations of
the Venus environment. "Although Venus' atmospheric
interaction with the solar wind was
studied extensively by Pioneer Venus Orbiter in the 1980s,
there has never before been an
opportunity to measure simultaneously both interplanetary
conditions and the particle-and-
field characteristics at Venus," McNutt said. "The combined
MESSENGER and Venus Express
observations will be the first opportunity to conduct such
two-spacecraft measurements and
should enable advances in our overall understanding of this
interaction."
Next up for MESSENGER is a trio of swings past Mercury
in January and October 2008
and September 2009. During these flybys, MESSENGER will map
most of the planet and
determine surface and atmospheric composition; these data
will be used to help plan priorities
for the yearlong orbital mission, which begins in March
2011.
"The spacecraft and its operations team are poised to
embark on the most intensive
period of trajectory activities of the mission," Finnegan
said. "Over the next 18 months, the
spacecraft will travel on a veritable inner-planetary
roller coaster. Three passive gravity
assists will be conducted, one by Venus and two by Mercury.
Three Deep Space maneuvers will
also be executed, using the large main engine of the
spacecraft. In addition, 13 maneuvers
utilizing smaller thrusters are possible in the event that
periodic corrections to the
trajectory are necessary along the way. All told, this adds
up to a very high tempo of
operations."
APL's Dave Grant, MESSENGER project manager, said the
work of the team of
engineers and scientists in completing this second Venus
flyby has been outstanding. "Our
adherence to the designed mission trajectory and
demonstrated performance of the science
payload lends confidence to the ultimate success of the
MESSENGER mission, and critical
experience has been gained for managing future flybys and
eventual orbit insertion at
Mercury," he said.
APL built and operates the MESSENGER spacecraft and
manages the mission for
NASA's Science Mission Directorate, Washington.
For the latest news and images about the MESSENGER
mission, go to
messenger.jhuapl.edu.
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