2018-03-07 16:14:48 UTC
By Leonard David, Space.com's Space Insider Columnist
| May 30, 2006 06:11am ET
The Tricky Task of Aerobraking at Mars
Putting on the brakes! Mars Reconnaissance Orbiter is now dipping into
the martian atmosphere to adjust its orbit. The controlled use of
atmospheric friction is a process called “aerobraking”, a technique that
changes the initial, very elongated orbit of the orbiter into a rounder
shape optimal for science operations at Mars.
Image Credit: JPL/Corby Waste
DENVER, Colorado--Round and round it goes, and where it ends up is a
matter of aerobraking for the Mars Reconnaissance Orbiter, NASA's newest
spacecraft to explore the red planet.
Aerobraking is "science friction"--that is, using the friction of the
thin martian atmosphere to slow the orbiter. Mission officials keep an
eye on things like glide slope, drag passes and pop-ups while also
watching the weather and looking out for other spacecraft.
Mars Reconnaissance Orbiter (MRO) is making controlled skims through
that planet's tenuous upper atmosphere to convert the spacecraft's
initial, very elongated 35-hour orbit to a nearly circular two-hour
orbit needed for the mission's science observations.
A combined team both at the Jet Propulsion Laboratory (JPL) in Pasadena,
California and here at a Lockheed Martin Space Systems control center
plot out and perform MRO's dainty, ease-on-down, maneuvers.
Fall into the planet
"We're on schedule ... definitely right on the plan. It's going really
well," said Wayne Sidney, MRO Flight Engineering Team Lead for Lockheed
Martin Space Systems, the firm that designed and built the spacecraft.
MRO arrived at Mars on March 10 and inserted itself into orbit. Thanks
to aerobraking, the spacecraft's 35-hour orbit period has been shaved
down to a 20-hour orbit, but hundreds of drag passes are ahead. "We've
still got a long ways to go," Sidney told SPACE.com.
The process uses short blasts from MRO thrusters while the spacecraft is
at the far end of its orbit, called the "apoasis." The burns are used
only to adjust how deeply the spacecraft dips into the atmosphere at the
near end or "periapsis" end of its orbital ellipse--where the orbit
comes closest to the planet.
Sidney said the first steps to enter the atmosphere of Mars-- a
"walk-in" or "toe dipping" phase--have been completed. Now underway is
the main phase of aerobraking that lasts several months, concluding with
a "walk-out" of the atmosphere.
If all goes as planned, MRO aerobraking is scheduled to end around early
to mid-September, Sidney added. "The more you slow down ... the more you
kind of fall into the planet."
Compared to relying only on rocket engines to shape orbits, aerobraking
reduces the amount of fuel hauled on a spacecraft by nearly one-half.
This is the reason for aerobraking, a procedure NASA has used with
success for the Mars Global Surveyor and Mars Odyssey orbiters now
circling the red planet.
"MRO is a culmination of lessons learned from those two spacecraft,"
Sidney said. "For those two early pioneers it was kind of virgin
territory where we were going."
Clear and quiet
MRO was fabricated, optimized, and beefed up to take the buffeting and
heat from aerobraking. For example, the craft's large set of solar
arrays are designed to go up to 175 degrees centigrade (347 degrees
Fahrenheit). "The highest we've seen to date is minus-three. So we
haven't even cracked a zero at this point in time," Sidney explained.
Mars itself has been good to MRO ... so far. No dust storms appear to be
brewing. The atmosphere of the planet is "clear and quiet," Sidney
pointed out. "I'm not going to say it has been a walk in the park ...
but it has been a picnic in comparison" to the last two NASA orbiters
and their respective aerobraking activities, he said.
Generally, the density of a planetary atmosphere decreases with
increasing altitude. However, Mars' upper atmosphere is quite changeable
in how dense it is at any given altitude. Up to now, the amount of time
MRO is in the atmosphere at periapsis has been on the order of five
minutes duration. Later on, towards the end of aerobraking, MRO will
experience some 20 minutes of atmospheric drag.
If spacecraft operators run into a problem during aerobraking, say too
much stress and strain is being felt by MRO, a pop-up maneuver can be
done to prevent damaging the spacecraft.
Onboard MRO is a Periapsis Timing Estimator (PTE). Using very precise
accelerometers and a computer algorithm, the spacecraft can predict
orbit period changes from one orbit to the next. Trial-running this
think-for-itself, auto-navigation approach may lead to wider adoption of
the PTE in the future.
PTE number-crunching is being gauged against the output from the
traditional, navigation prediction team work here on Earth, with very
favorable results, Sidney said. "We kind of have our own little
navigator onboard MRO ... the two methods line up very, very well."
The whereabouts of MRO relative to other Mars orbiters now busy at
work--Mars Global Surveyor, Mars Odyssey, and Europe's Mars Express--is
a daily output from collision avoidance analysis, Sidney said. And there
has been an "interesting little dance" with Mars Odyssey, in particular,
While having a fleet of orbiters circuiting Mars has relayed a bonanza
of data, precise knowledge of MRO's orbit, given its aerobraking
aerobics, is top priority.
But there's more of a sweat factor when you mix in the orbits of the
Mars moons--Phobos and Deimos--and the unknown locations of two Viking
orbiters from the 1970s that are thought to be still zipping around Mars.
All orbiters are supposed to be in Quarantine Safe Orbits, which
requires a minimum lifetime of 50 years circling Mars. So it's believed
that both Viking Orbiters are swinging around Mars too--silent sentinels
that ran out of attitude control gas early on in their missions.
With each loop around the red planet, the $750 million MRO mission draws
closer to the day when its science gathering tasks can begin. Following
months of aerobraking--512 dips through the atmosphere--MRO will be
locked into the desired time-of-day pattern for the mission's science phase.
"It all has to come together," Sidney observed.
The orbit will remain at a fixed angle to the Sun; every time the
spacecraft passes northbound over the equator, it will see mid-afternoon
lighting on the ground. A complete orbit will then take about one hour
and 52 minutes. By the next time MRO loops around and crosses the
equator again, Mars will have rotated just enough that the spacecraft
sees the same time-of-day on the ground beneath it.
A key objective of MRO is to search for evidence that water persisted on
the surface of Mars for a long period of time. While other Mars missions
have shown that water flowed across the surface in the planet's past, it
remains a mystery whether water was ever around long enough to provide a
habitat for life.
"It's going extremely well ... it really is," Sidney concluded. "We're
on the glide slope ... right on track to keep moving forward."
VIDEO: MRO's Path to Mars
GALLERY: Visualizations of Mars
GALLERY: Mars Express Images