Aerobraking Mars Orbiter Surprised Scientists

The Mars Reconnaissance Orbiter (MRO) has completed the intricate job of aerobraking and its primary science phase will soon begin in earnest. MRO’s Project Scientist and members of the Navigation Team discussed the intricacies and challenges of aerobraking in Mars’ ever-changing atmosphere.

Aerobraking is a technique that was first used by the Magellan mission to Venus in 1993, and also used on two other Mars missions, the Mars Global Surveyor (MGS) in 1997 and Mars Odyssey (2001). Aerobraking uses repeated dips into the atmosphere to gradually slow the spacecraft and reduce the size of the orbit. While aerobraking takes time, it saves on the amount fuel required, as in MRO’s case, by 600 kilograms (1,300 pounds).

At the start of MRO’s aerobraking in early April 2006 the most distant point of the orbit, or apoapsis, was about 45,000 km (28,000 miles) above the surface. During aerobraking, the closest point of the orbit, called periapsis, ranged from 98 to 110 km (61 to 69 miles). The latitude of this closest approach started near 65 degrees south, moved southward past the winter pole and was almost back to the equator when an exit maneuver on August 30 ended aerobraking. Two subsequent propulsive orbit adjustment maneuvers then established MRO’s current orbit at 316 km (196 miles) by 250 km (155 miles) above the surface, with periapsis and apoapsis frozen over the south and north poles, respectively.

During aerobraking, the goal is to control the spacecraft within a “corridor” of the atmosphere at each periapsis passage that will provide enough drag to slow the spacecraft down without overheating any of the components.

To aid in the aerobraking process, the navigation team employs an atmospheric model called the Mars-GRAM (Global Reference Atmospheric Model), a computer database of information from what previous missions have encountered, combined with a mathematical model that attempts to simulate Mars’ atmospheric dynamics. This provides a prediction of the density of Mars’ atmosphere, giving the navigators an estimate of how far down into the atmosphere the spacecraft should go.

But the atmospheric density that MRO actually experienced was much different than what was predicted by the Mars GRAM.

“At some points in the atmosphere, we saw a difference in the atmospheric density by a factor of 1.3, which means it was 30% higher than the model,” said Han You, Navigation Team Chief for MRO. “That’s quite a bit, but around the south pole we saw an even larger scale factor of up to 4.5, so that means it was 350% off of the Mars GRAM model.”

“When we first started out at a somewhat higher altitude, the Mars GRAM model was doing pretty well,” said Richard Zurek, Project Scientist for MRO. “When we got to the lower altitude the scale factor to which it was off was larger and it became even larger as periapsis moved toward the south pole.”

Zurek said some variations in the atmospheric density were expected, and they anticipated that adjustments would have to be made to the model. But they were a little surprised at the amount of the variations. “We thought after our earlier experience we should have the poles down a little better,” he said.

Zurek explained the cause of the differences. “One of the variations is where you are on the planet,” he said. “The polar region is different than the low latitudes, the tropics and the equator. So even though we were in the atmosphere aerobraking on previous missions, it wasn’t exactly the same season, it wasn’t exactly the same latitude at that season, and finally it was at a different time of day.”

“Essentially what was happening,” Zurek continued, “was that the model was predicting that the poles were colder than what they actually must have been so that the density decreased less steeply as you went toward the pole than what was predicted. Those were the kinds of factors that we had to compensate for during aerobraking.”

To make things even more challenging, the atmospheric density on Mars can vary widely from day to day, and even orbit to orbit.

“When you’re flying to a particular altitude during an aero-pass you can experience a different density even though you go to the same altitude repeatedly,” said Zurek. “For MRO, we were at 100-105 km for quite a long period of time, for several months and yet we saw quite a variation.”

Zurek explained that the daily variations in the atmosphere are quite large, in part because there is no ocean, which serves as large heat storage capacity on the surface. The ground warms up quickly during the day and cools off equally as quick at night. Daily temperature variations of 100 C (180 F) are common, and that cycle of heating and cooling is reflected in atmospheric variations. “That energy propagates up, and when integrated to the high altitudes, it can make a big difference from day to night in the densities that we saw at a given altitude,” said Zurek.

Also, the air pressure on Mars is drastically lower than on Earth – 6 millibars compared to 1000. The main effect of low surface pressure is that the atmosphere reacts very quickly to changes in temperature.

“It’s hard to anticipate what the density will be at a given altitude high in the Martian atmosphere,” said Zurek. “That’s because it depends on all the things that are happening at lower altitudes, sometimes all the way down to the ground.”

Even Earth meteorologists admit that they can’t always predict the state of our atmosphere at a specific location at any given time, so forecasting the atmospheric conditions on another planet is quite complex. Mick Kjar, a meteorologist for KVLY-TV in Fargo, North Dakota who has also trained as a pilot, expressed admiration for his counterparts who are attempting to predict Martian weather.

“I’m sure they are learning as they go,” said Kjar. “Early aviators used to call it flying by the seat of your pants; you just had to pull on the stick based on how it felt. But that’s a huge challenge when you’re digitally flying a spacecraft from millions of miles away.”

To say that the MRO navigators were ‘flying by the seat of their pants’ may be an exaggeration, but the aerobraking process did require constant vigilance by the navigation team. “Its all about checking, checking, checking, and sometimes using your instinct from previous experience,” said Neil Mottinger, a member of the MRO navigation team. “That’s why we were there around the clock.”

Another problem that previous Mars missions haven’t had to deal with, until recently, is the possibility of two orbiting spacecraft colliding. The probability is very small, but as Han You pointed out, it’s definitely a situation that all the missions want to avoid.

“It’s starting to get a little crowded at Mars, and all the missions like a similar type of sun-synchronous polar orbit,” said You. The science team for MRO chose 3:00 pm as the ascending node for its local (mean) solar time because it provides the best viewing conditions overall for its different cameras and spectrometers. Furthermore, its polar orbit lets MRO view nearly any place on the planet over the span of a couple of weeks.

Collision avoidance (COLA) analysis is used extensively for the shuttle and other Earth orbiting satellites, but only recently has been developed for Mars. “This is something new, very recent, and we’ve put a lot of effort into that area,” said You. “MGS and Odyssey started monitoring this regularly with ESA’s (European Space Agency’s) Mars Express. We realized that it’s something that needs to be done carefully on our side. The other spacecraft are very stable in their trajectory but there was a great uncertainty in our trajectory during aerobraking, mainly due to the fluctuation of the Martian atmosphere from orbit to orbit, which made the COLA monitoring very difficult and challenging.”

MRO performed 27 aerobraking maneuvers, six of which were executed to avoid a possible collision with the other spacecraft, accounting for more than 20% of the total aerobraking maneuvers. This will still need to be monitored during the science phase of the mission, as Mars Express occasionally crosses the other spacecrafts’ orbital paths.

MRO’s orbit is lower than the other Mars orbiters in order to provide the best science results for the instruments on board. With this lower orbit Han You believes the spacecraft could still be affected by the atmosphere. “If we are going to sense atmosphere during the primary science orbit that means our timing is going to be off by several seconds and that’s not good enough,” he said. “We do have a very stringent accuracy requirement to satisfy the MRO science team. Any type of uncertainty decreases our capability of telling the scientists on the project exactly where the spacecraft will be. So, I think our work is just beginning with the primary science phase coming up, but this is a very fun mission to work with.”

Measurements of densities in the upper atmosphere derived from tracking data and from the onboard accelerometers during aerobraking will be used to improve predictions for future missions, as well as to address scientific questions such as how water vapor and other gases are lost to space. With measurements that will be made during a full Martian year by the science instruments on board MRO, Zurek hopes that some of the atmospheric variation issues can be addressed for future missions that use aerobraking, or even aero-capture. MARCI (Mars Color Imager) will monitor the lower atmosphere, and Mars Climate Sounder will profile the temperature up to about 80 km above the planet’s surface.

“The MARCI camera looks from horizon to horizon across the track as the spacecraft flies along,” said Zurek, “and you take those swaths from pole to pole on the daylight side, put them together and you’ll get a daily image of the global weather on the planet. What we’re looking for is, where are the clouds, where are the storms, where do they go. Mars does have storm tracks and jet streams just like Earth does.”

The Mars Climate Sounder is an instrument that looks at heat radiation to determine the temperature of the atmosphere, how much water vapor it contains, as well as how much dust is in the atmosphere and how that dust is distributed with altitude and distance over the planet. “Those are the key things we need to know to be able to produce better models of the atmosphere,” Zurek said.

Over the next few weeks, MRO’s science payloads will be deployed and tested. However, because of Mars position relative to the sun, the main science observation phase will not start until early November.

“Just as we get done with aerobraking and are ready to go, then we have to wait because of Solar Conjunction, where Mars and the spacecraft are on the other side of the sun,” said Zurek. “Communications aren’t reliable during that period because the radio waves have to transit across the sun’s corona, which could cause outages and dropouts. We’ll officially start our primary science phase on the 8th of November. And we are looking forward to that.”

By Nancy Atkinson