Squeezing every penny of fuel efficiency out of your gas tanks is important when you're sending two spacecraft to Mars for less than $300 million. NASA's Jet Propulsion Laboratory is doing just that with the Mars Surveyor '98 mission, a venture that includes the Mars Climate Orbiter, and the Mars Polar Lander.
To cut the amount of fuel the Mars Climate Orbiter needed to carry in order to brake and enter orbit, controllers are sending the craft on a series of dives into the planet's atmosphere and using friction, not just fuel, to slow down the craft and circularize its orbit.
For the first two months after its arrival, the Mars Climate Orbiter will be using every lap around the planet to decelerate.
Once each orbit, at its closest approach to Mars, the craft will streak through Mars' carbon dioxide atmosphere, slowing a little each time from atmospheric drag. First tested in 1993 with NASA's Magellan spacecraft at Venus, the procedure drastically reduces the propellant the spacecraft needs to brake and enter Mars orbit.
The orbiter's solar panels will help create atmospheric drag. Its edges are specially outfitted with extensions called drag flaps, which are thermally protected and designed to increase the spacecraft's cross-section during each swoop.
After the initial thrust maneuver, controllers will fire the spacecraft's thrusters each time the machine reaches the point in its orbit most distant from Mars. This will bring the point of closest approach steadily down into Mars' upper atmosphere, until it passes within about 60 miles (100 kilometers) of the surface.
During the next two months, mission controllers will tweak the orbit until the climate orbiter is in a nearly circular orbit that passes from pole to pole.
Data from the Mars Global Surveyor, which has been orbiting Mars for two years, will be used to support the Mars Climate Orbiter as it aerobrakes. Mission controllers have to watch out for dust storms that can double the density of the atmosphere at a given altitude, said Richard Zurek, project scientist for the Surveyor '98 mission.
When storms sweep dust into the air and suspended it in the atmosphere, the atmosphere heats up, which changes its density, especially at high altitudes. During its mapping mission, the Mars Global Surveyor will be on watch for storms as they develop.
"It takes a couple days to for these storms to really get going underway," Zurek said. "If we see such a large storm beginning, then we'll do a small propulsive maneuver on the Mars Climate Orbiter that raises its periapsis -- its closest approach to the planet -- just a few kilometers because that's enough to compensate for that increase in density.
"What you're doing is you're trying to fly to a certain altitude where you expect a particular density that's safe for your spacecraft and yet big enough to slow you down and get you into orbit," he said. "If the atmosphere expands, you fly higher and if the atmosphere contracts after the dust falls out of the atmosphere, and the atmosphere is colder, then you just fly at lower altitudes."
By early November, the orbiter should have slowed enough that controllers can raise its closest-approach altitude to a level where it will not encounter any atmospheric interference as it orbits Mars. The craft will then have cut its velocity by nearly as much as the 16-minute main-engine burn did when the orbiter first arrived at Mars, and the orbiter will be ready for its mission.
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