Six Months Of Adjustments Will Shrink Orbit Around Red
Planet
Last Thursday, NASA's Mars Reconnaissance Orbiter began its
crucial six-month campaign to gradually shrink its orbit into the
best geometry for the mission's science work.
The spacecraft is now in the "aerobraking" phase of its entry
into Martian orbit. The process uses friction with the upper
atmosphere of Mars to transform a very elongated 35-hour orbit
around the planet, to a much tighter, nearly circular two-hour
orbit needed for the mission's science observations.
On March 30, Mars Reconnaissance Orbiter fired its intermediate
thrusters for 58 seconds at the far point of the orbit. That
maneuver lowered its altitude to 207 miles when the spacecraft next
passed the near point of its orbit, at 9:46 am EST Friday.
Since its successful March 10 arrival at
Mars, the MRO has been flying about 265 miles above
Mars' surface at the nearest point of each loop, before swinging
more than 27,000 miles away before heading in again.
While preparing for aerobraking, the flight team tested several
instruments, obtaining the orbiter's first Mars pictures and
demonstrating the ability of its Mars Climate Sounder instrument to
track the atmosphere's dust, water vapor and temperatures. The
image below shows heat emission measurements conducted by the MCS
across different wavelengths.
"We're not low enough to touch Mars' atmosphere yet, but we'll
get to that point next week," said Dr. Daniel Kubitschek of NASA's
Jet Propulsion Laboratory, Pasadena, CA deputy leader for the
aerobraking phase of the mission.
Aerobraking includes about 550 dips into the atmosphere, each
carefully planned for the desired amount of braking. At first, the
dips will be more than 30 hours apart; by August, there will be
four per day.
"We have to be sure we
don't dive too deep, because that could overheat parts of the
orbiter," Kubitschek said. "The biggest challenge is the
variability of the atmosphere."
Using aerobraking to get the spacecraft's orbit to the desired
shape, instead of doing the whole job with thruster firings,
reduces how much fuel a spacecraft needs to carry when launched
from Earth.
"It allows you to fly more science payload to Mars instead of
more fuel," Kubitschek said.
Readings from accelerometers during the passes through the
atmosphere are one way the spacecraft can provide information about
upward swelling of the atmosphere due to heating.
The Mars Climate Sounder instrument also has the capability to
monitor changes in temperature that would affect the atmosphere's
thickness. "We demonstrated that we're ready to support
aerobraking, should we be needed," JPL's Dr. Daniel McCleese,
principal investigator for the Mars Climate Sounder, said of new
test observations.
Infrared-sensing instruments and cameras on two other Mars
orbiters are expected to be the main sources of information to the
advisory team of atmospheric scientists providing day-to-day
assistance to the aerobraking navigators and engineers. "There is
risk every time we enter the atmosphere, and we are fortunate to
have Mars Global Surveyor and Mars Odyssey with their daily global
coverage helping us watch for changes that could increase the
risk," said JPL's Jim Graf, project manager for the Mars
Reconnaissance Orbiter.
Once in its science orbit, NASA and JPL expect the Mars
Reconnaissance Orbiter to return more data about the planet than
all previous Mars missions combined. The data will help researchers
decipher the processes of change on the planet. It will also aid
future missions to the surface of Mars by examining potential
landing sites and providing a high-data-rate communications
relay.