Mars orbiter relies on ‘science friction’

By Leonard David

Senior space writer

updated 1:57 p.m. ET May 30, 2006

DENVER - 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, or MRO, is making controlled skims through that planet’s tenuous upper atmosphere to convert the spacecraft’s initial, very elongated 35-hour orbit to the nearly circular two-hour orbit needed for the mission’s science observations.

A combined team both at the Jet Propulsion Laboratory in Pasadena, Calif., and here at a Lockheed Martin Space Systems control center plot out and perform MRO’s dainty, ease-on-down maneuvers.

Falling 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 under way is the main phase of aerobraking, which lasts several months, concluding with a "walk-out" of the atmosphere.

If all goes as planned, MRO aerobraking is scheduled to end in early to mid-September, Sidney added. "The more you slow down ... the more you kind of fall into the planet."

Compared with relying only on rocket engines to shape orbits, aerobraking reduces the amount of fuel hauled on a spacecraft by nearly a 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 Celsius (347 degrees Fahrenheit). "The highest we’ve seen to date is -3 (degrees C). 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, toward the end of aerobraking, MRO will experience 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.

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