The next U.S. spacecraft to inspect Mars is completing the last few days of its seven-month journey to the red planet and is scheduled to enter orbit Friday. Traveling the nearly 500 million kilometers apparently has been the easier phase of the effort. The U.S. space agency NASA says settling into orbit is the most suspenseful part of the ordeal, a maneuver that has failed on several previous missions. Mission controllers will be working hard to prevent such a mishap.
The Mars Reconnaissance Orbiter carries a suite of instruments that will make it the most powerful imager ever sent to another world. But the NASA official managing the spacecraft, James Graf, says they will all be useless if it fails to perform the crucial series of maneuvers that will slow its rush and let Martian gravity capture it.
"We are getting into the dangerous portion of the mission," said James Graf. "The cruise has not been easy, but now we are starting to enter into the realm where we have lost two spacecraft in the last 15 years."
Indeed, the U.S. success rate for getting spacecraft to orbit Mars since the 1960s is surprisingly low, according to the official in charge of the U.S. Mars exploration program, Douglas McCuistion.
"Really, orbiters seem like they might be the easiest thing to do," said Douglas McCuistion. "But in reality, we have only about a 65 percent rate of getting orbiters into orbit, whereas landers, we have about an 80 percent success rate of getting them on the surface."
McCuistion says the orbiting success rate at Mars is only 40 percent if Russian and European failures are averaged in.
The most recent U.S. debacle came in 1999 when the Mars Climate Orbiter broke up in Mars atmosphere because two control teams used different units of measurement - metric and Imperial - to calculate the craft's path and speed.
The procedure is always the same. The orbiter's thruster engines burn for almost 30 minutes to reduce the probe's speed from more than 10,000 kilometers per hour to just two-thirds that rate so the red planet's gravity can pull it in. Otherwise, the fate of the Mars Reconnaissance Orbiter could be the same as the failed mission seven years ago.
James Graf says it will be a tense period for mission controllers, especially since the spacecraft will duck behind the planet for half of the time it is slowing down, causing a temporary communications blackout.
"So we're going to be sitting in the control center for about 15 minutes just holding our breath and squeezing - being white knuckled - and waiting to get the information back," said James Graf.
If the Mars Reconnaissance Orbiter succeeds in entering orbit, its path will be extremely elongated, 35,000 kilometers high at its furthest point and a mere 200 kilometers at its closest, taking 35 hours to make one loop. The goal is to change this to a circular two-hour orbit of about 300 kilometers altitude. To do this, the spacecraft will spend half a year adjusting its orbit with an adventurous process called aerobraking. This will use hundreds of carefully calculated dips into the upper atmosphere - deep enough to slow the spacecraft by atmospheric friction, but not too deep into the thicker part of the atmosphere to overheat the orbiter.
Graf says this will be another trying period for mission controllers.
"We have an atmosphere we don't fully understand," he said. "If there's a bloom and there's a dust storm and the atmosphere heats up, then we could be going in at a particular altitude, envisioning one particular type of heating rate when we get a different one. So we have to be very attentive and there's a lot of people who lose a lot of sleep."
To lower the risk, NASA says three older orbiters currently circling Mars will provide a daily watch of the lower atmosphere.
Only after that drawn-out phase ends in October will the Mars Reconnaissance Orbiter be ready to take up its main business of returning torrents of new data about Earth's nearest planetary neighbor.
