 |
 |
||
 |
|
||
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
||||||||
 |
Planetary News: Mars (2006)Mars Reconnaissance Orbiter Begins AerobrakingBy A.J.S. RaylApril 3, 2006
The Mars Reconnaissance Orbiter (MRO) began the six-month aerobraking phase with rehearsals or drag passes last Friday and tomorrow night the spacecraft will dip down for its first "taste" of the Red Planet's atmosphere. "We're putting the spacecraft through its paces," said Peter Xaypraseuth, MRO flight engineer in an interview with The Planetary Society earlier today. "We're telling it to do the exact things it would be doing for each upcoming aerobraking pass, except that we're actually not going to be touching or sensing the atmosphere. It's kind of a mock rehearsal." In other words, everything is real, he said, "except the altitude." MRO arrived at Mars and successfully entered into a highly elliptical orbit on March 10. The orbiter has since been flying about 426 kilometers (265 miles) above the Red Planet's surface at the nearest point of each loop, then swinging more than 43,000 kilometers (27,000 miles) away before heading in again in, making one orbit every 35 hours. However, in order for the spacecraft's science instruments to collect optimal data and its cameras to get the best possible images of Mars, MRO must move in closer to the planet. By aerobraking -- essentially "grazing" the atmosphere each orbit and allowing the friction of the atmosphere against the spacecraft to slow it down -- the spacecraft will reduce and reshape its highly-elliptical, 35-hour orbit into a near-circular, two-hour orbit over the next 5 to 6 months. That sounds easy enough, but aerobraking is “like a high-wire act in open air,” as Jim Graf, MRO project manager, has described it. To get where the scientists want it, MRO must make some 550 carefully calculated ‘dips’ into Mars' upper atmosphere – just deep enough to slow the spacecraft by atmospheric drag but not deep enough to overheat the orbiter. Using aerobraking to get the spacecraft's orbit to the desired shape, instead of doing the whole job with rockets or thruster firings, reduces how much fuel a spacecraft needs to carry when launched from Earth. "It allows you to fly more science payload to Mars instead of more fuel," pointed out Daniel Kubitschek, deputy leader for the aerobraking phase of the mission at JPL. At first, with the dips being some 35 hours apart, the team will have plenty of time to analyze the data from each orbital burn and send new engine commands if needed, but by August the orbits will be every 6 hours and eventually by late October every 2 hours. "During these early, long periods we have time to build up history with the spacecraft so we can have more confidence in what it can handle," offered Xaypraseuth. "In the later orbital periods, 5 months from now, after the spacecraft comes out of a drag pass, it will take about a half-hour for us to receive that telemetry on the ground leaving us only 1.5 hours before it [dips] again." As a result, MRO was designed to be "smart" enough to be able to tolerate multiple orbits and determine on its own, determine if a drag pass was too hot or too cold, and adjust accordingly. The aerobraking rehearsal maneuvers last Friday and over the weekend went without a hitch and the initial part of the aerobraking phase, known as the "walk-in," has gone smoothly, according to Xaypraseuth. Everything on the spacecraft has been checked and reviewed, and is set up correctly and functioning properly, he said, and flight team members are getting to know their ship. Tonight and into tomorrow, however, it gets real. As MRO heads into apoapsis #17 – the 17th time it's been at this, the farthest distance in its orbit around the planet -- it will once again fire its engines. "The burn is expected to happen at 11:53 pm Pacific Daylight Time (PDT) [06:53 UTC] tonight into tomorrow," said Xaypraseuth. "After we execute that maneuver, we will be lowering ourselves to 147 kilometers (91 miles) at periapsis (the point in an orbit closest to the body being orbited), which will give us a small taste of the atmosphere, and at apoapsis #18, we'll lower it a little bit more to put us formally into what we consider the corridor for our aerobraking regime. Normal aerobraking altitudes will leave us at about 100 kilometers (62 miles) above the surface at periapsis and right now we're at about 230 kilometers (143 miles) above the surface so we still need to go another 130 kilometers (81 miles) down before we can actually sense the atmosphere, start to get drag from the atmosphere, and actually begin the aerobraking," he explained. "The biggest challenge [of the aerobraking process] is the variability of the atmosphere," noted Kubitschek. MRO is equipped with accelerometers that take readings during the passes through the atmosphere that provide the spacecraft with information about upward swelling of the atmosphere due to heating. Additionally, instruments on the NASA orbiters already at Mars -- the Thermal Emission Imaging Spectrometer (THEMIS) on board Mars Odyssey and the Thermal Emission Spectrometer (TES) onboard Mars Global Surveyor (MGS) -- will provide a daily watch of the lower atmosphere. While preparing for aerobraking, the flight team tested several instruments, obtaining the orbiter's first Mars pictures with the High Resolution Imaging Science Experiment (HiRISE) camera and demonstrating the ability of its Mars Climate Sounder (MCS), the weather satellite, to track the atmosphere's dust, water vapor and temperatures. Since MCS also has the capability to monitor changes in temperature that would affect Mars' atmosphere's thickness, it could be used to provide climate input for MRO if Odyssey or MGS were unable for whatever reasons to deliver the goods. "We demonstrated that we're ready to support aerobraking, should we be needed," said JPL's Mars scientist Daniel McCleese, principal investigator for the MCS. At the present time, however, all science instruments on MRO are turned off. The data from Odyssey's THEMIS and MGS' TES, however, are expected to be the main sources of information to the advisory team of atmospheric scientists providing day-to-day assistance to the aerobraking navigators and engineers. "There is risk every time we enter the atmosphere, and we are fortunate to have Mars Global Surveyor and Mars Odyssey with their daily global coverage helping us watch for changes that could increase the risk," said Graf. MRO should be ready to begin science operations in November if all goes as planned. The mission is expected to return more information about the planet than all previous Mars missions combined through a veritable communications pipeline, designed for deluge of data. Beyond helping researchers better understand this alien planet, MRO will aid future missions to the surface of Mars by examining potential landing sites and providing a high-data-rate communications relay. |
||||||
 |
 |
||||||
[ what is this? ]