Mission to Mars

After a six-month, 286-million-mile voyage, NASA’s newest Mars spacecraft is within a week of reaching the Red Planet. The hardest part of the journey is just ahead.

“It is a nail-biting time. A tense time,” said Bob Mase, a Jet Propulsion Laboratory engineer who serves as the mission’s lead navigator. “You only get one shot at it.”

Mars is not a small planet, but in the distant reaches of space, it makes a small target. Navigating to reach the planet required planning that started four years before the launching. And a lot of intensive baby-sitting.

Now, the task will be to insert the spacecraft into the turbulent Martian atmosphere and ease it into a finely tuned orbit.

As it flies toward Mars, the spacecraft is being tracked around the clock by two radio antennas on Earth, one in Australia and one in California. A new system that simultaneously tracks both the Odyssey and a distant quasar provides independent verification of the spacecraft’s speed, course and position.

“We look at the trajectory every day,” Mase said. “It looks great.”

The mission is being closely watched at every step in part because of the back-to-back failures of two Mars missions in 1999. The first, Mars Climate Orbiter, failed when the time came to place it into orbit because engineers confused English and metric measurements. The second mission, the Mars Polar Lander, is thought to have crashed into the planet.

This time, engineers so far have conducted four minor “burns” to correct Odyssey’s trajectory but have needed very little fuel to do so.

Launch rockets are aimed slightly away from Mars during launching so that pieces of the rocket, flying off into space, won’t crash into the planet.

This means that the trajectory of the spacecraft has to be corrected in flight. The flight path could have been off by 300,000 miles. Instead, it was off by less than 50,000 miles--and required very little fuel to correct.

“That was just getting lucky off the launch vehicle,” said Mase.

The fact that little fuel was used up is important because the spacecraft carries a very limited supply of propellant to tweak its path along the way.

The spacecraft weighed just 731 pounds and carried 779 pounds of fuel when it was launched atop a Boeing Delta II rocket. Carrying more fuel would have required using a larger launch vehicle, like an Atlas, and “would have cost tens of millions more,” said Dave Spencer, the mission’s manager. The total mission cost is $297 million, which included $50 million for the launch.

All the preparations will reach payoff Oct. 23 when the spacecraft will make its first pass around Mars, skimming just 252 miles over the planet’s north pole. As it nears the planet, the spacecraft will fire its main engines for the first and only time, speeding up so it can catch up to Mars and be captured into the planet’s orbit.

The mission team will be able to track the spacecraft for 10 minutes after its arrival. Then they will lose the signal for an agonizing 20 minutes as the spacecraft passes around the planet’s far side.

Once it emerges, the engineers will begin the painstaking process of aerobraking--slowing the spacecraft and gradually changing the wild, elliptical paths it will first take around the planet into the precise, circular orbits needed for the craft to take pictures of the planet and instrument readings of its surface.

The mission’s goals are to search for traces of water or ice and possible hot springs beneath the surface and to scout for geological areas worth studying with landers that are scheduled to travel to Mars in 2003.

Aerobraking is a fuel-conserving technique that is relatively new and was first tested in 1994 by the Magellan spacecraft at the end of its mission to Venus. It is tricky business that involves using the drag from a planet’s atmosphere to slow a spacecraft. As the craft slows, long, elliptical orbits get shorter and shorter, until a circular orbit is reached.

Even the thin atmosphere far above a planet’s surface provides enough resistance to create drag. As a spacecraft dips lower and lower, it encounters an increasingly thick atmosphere and more drag to act as a brake. The closest the spacecraft will come to Mars during the process is 62 miles; the farthest is 17,400 miles. The final orbit will be 250 miles above the surface, well above the Martian atmosphere.

The process requires hundreds of “drag passes”--dips into the Martian atmosphere--that slow the craft, but also can heat it to dangerous levels. So the spacecraft is never left unattended by controllers on Earth. If the spacecraft warms too much, engineers can make adjustments to save it.

The spacecraft’s solar array, which acts as a parachute, takes the brunt of the heat. During the coldest parts of the mission, temperatures around the spacecraft drop to 220 degrees below zero Fahrenheit. But the solar panels can heat up to 374 degrees during aerobraking.

“The aerobraking process is really a race around the clock. It’s a 24-hour, seven-day-a-week activity we expect to do for 78 to 85 days,” Spencer said. “Because it’s so human-intensive, it’s the most risky. It’s the thing I’m most worried about.”

The task may sound grueling, but it’s not as tough as what engineers steering the last successful Mars orbiter, the Mars Global Surveyor, faced in 1997. Because of a weakened solar panel, that spacecraft could survive only a very gentle, and therefore greatly extended, aerobraking process.

“MGS did this for a year and a half and survived, so we can do this for three months,” Spencer said. All indications are that Odyssey is healthy and ready for the rugged task ahead. “We can barrel our way through,” he added.

Since the Martian atmosphere is not well understood and can be wildly variable, engineers cannot be sure exactly where the spacecraft should travel. “We toe-dip our way in, sort of the way a swimmer gets into the water, to test the atmosphere,” Spencer said. “We want to find, essentially, the sweet spot.”

The massive dust storms swirling around Mars that scientists recently detected may actually make the aerobraking process easier. “We don’t have to go to as low an altitude to get the amount of drag we need,” said Mase.

Everyone working on Mars Odyssey is feeling pressure to succeed. “This is the most reviewed project JPL has flown in recent years--or ever,” Spencer said.

But from the time the journey started in Denver, where the spacecraft was manufactured by Lockheed Martin Astronautics, through its flawless launching from Cape Canaveral, Fla., Odyssey has led something of a charmed life. That good fate continued during its interplanetary cruise, Mase said.

“A million things have to go right to get to Mars,” he added. “So far, a lot of them have.”