Around Mars the space traffic really isn’t all that bad – five spacecraft vying for hundreds of miles or open cosmos around the planet – but serious space traffic control is still necessary to prevent a collision.
NASA’s Jet Propulsion Laboratory (JPL) which controls the airspace around the red planet this week said it implemented formal collision-avoidance technology that will keep the current and future orbiters a safe distance from each other and warn the scientists if two orbiters approach each other too closely.
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In a paper written by JPL scientists that detailed the collision avoidance software and algorithms the researchers said that the possibility of a collision around Mars (or the moon which uses similar avoidance technology) is minuscule.
“However, the consequences of collision are catastrophically high: millions to billions of dollars/euros/yen/etc. in lost tax revenue investment, irreparable loss of science data, and the creation of a debris environment in otherwise pristine orbital environments are three obvious consequences. The international repercussions of spacecraft from two different nations/agencies colliding would also very likely be undesirable. Given world economy, spacecraft collisions and the resultant waste of tax revenue can potentially lead to reduced popular support for the world's space agencies, the many benefits of space exploration notwithstanding.”
NASA said the new formal collision-avoidance process for Mars is part of what’s known as the Multi-Mission Automated Deep-Space Conjunction Assessment Process. “A side benefit of it is that information about when two orbiters will be near each other -- though safely apart -- could be used for planning coordinated science observations. The pair could look at some part of Mars or its atmosphere from essentially the same point of view simultaneously with complementary instruments,” NASA said.
The spacecraft in operation around Mars include NASA’s Odyssey, Mars Reconnaissance Orbiter (MRO) and Mars Atmosphere and Volatile Evolution (MAVEN), India’s Mars Orbiter Mission and the European Space Agency’s Mars Express. The system also tracks NASA's Mars Global Surveyor, a 1997 orbiter that is no longer working.
The need for a centralized control is needed when you take into considerations the different types of orbits the spacecraft follow. NASA says MAVEN studies the upper atmosphere and flies an elongated orbit, sometimes farther from Mars than NASA's other orbiters and sometimes closer to Mars, so it crosses altitudes occupied by other orbiters.
"Previously, collision avoidance was coordinated between the Odyssey and MRO navigation teams," said Robert Shotwell, Mars Program chief engineer at NASA's Jet Propulsion Laboratory, Pasadena, California in a statement. "There was less of a possibility of an issue. MAVEN's highly elliptical orbit, crossing the altitudes of other orbits, changes the probability that someone will need to do a collision-avoidance maneuver. We track all the orbiters much more closely now. There's still a low probability of needing a maneuver, but it's something we need to manage."
Trajectory information from all Mars spacecraft is sent to JPL over NASA’s Deep Space Network and gathered by JPL engineers who run computer projections of future trajectories out to a few weeks.
“The amount of uncertainty in the predicted location of a Mars orbiter a few days ahead is more than a mile (more than two kilometers). Calculating projections for weeks ahead multiplies the uncertainty to dozens of miles, or kilometers. In most cases when a collision cannot be ruled out from projections two weeks ahead, improved precision in the forecasting as the date gets closer will rule out a collision with no need for avoidance action. Mission teams for the relevant orbiters are notified in advance when projections indicate a collision is possible, even if the possibility will likely disappear in subsequent projections. This situation occurred on New Year's weekend, 2015, NASA said.
According to NASA, on Jan. 3, automated monitoring determined that two weeks later, MAVEN and MRO could come within about two miles (three kilometers) of each other, with large uncertainties remaining in the exact passing distance. Automatic messages were sent to the teams operating the orbiters. In this case, before the timeline got short enough to need to plan an avoidance maneuver, the uncertainties shrank, and that ruled out the chance of the two spacecraft coming too near each other, NASA said.
According to NASA, “if preparations for an avoidance maneuver were called for, spacecraft commands would be written, tested and approved for readiness, but such commands would not be sent to a spacecraft unless projections a day or two ahead showed probability of a hazardous conjunction. The amount of uncertainty about each spacecraft's exact location varies, so the proximity considered unsafe also varies. For some situations, a day-ahead projection of two craft coming within about 100 yards (100 meters) of each other could trigger a maneuver.”
Space traffic management at Mars is much less complex than in Earth orbit, where more than 1,000 active orbiters plus additional pieces of inactive hardware create a challenging avoidance environment.
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