The planet Mars is fairly modest, as spaceports go. There are only five active satellites in orbit there.
But that doesn’t mean collisions are impossible. And because Mars missions are so massively expensive, NASA doesn’t want to take any unnecessary chances with the orbiters that have managed to get there, and survive.
So, in a lovely piece of forward-thinking mathematical prudence, air traffic control at the red planet is being re-thought and upgraded, in anticipation of busier times to come.
“As long as we’ve had multiple assets in orbit there, we’ve always kept track between the different navigation teams of where they are, and if there’s any proximity issues,” said Robert Shotwell, chief engineer of the Mars program for NASA and the Jet Propulsion Laboratory (JPL).
“As we get more and more spacecraft there, the overall density goes up.”
Two new craft entered Mars' orbit in the past year – both taking non-typical elliptical orbits which can and do cross the paths of satellites taking more traditional, circular routes. Any time orbits intersect, catastrophic collisions are possible.
Of particular note is MAVEN, NASA’s Mars Atmosphere and Volatile Evolution probe.
“MAVEN is in a highly elliptical orbit, and at higher inclination as well,” Shotwell explained.
“That’s intended to allow the opportunity for the periapse (low point of the orbit) to go down into the atmosphere. Many of its instruments actually sample the atmosphere."
It ducks into the Martian air, takes a sniff, and rises back out. But every time it does this, its speed and orbit are altered. And there’s no way to predict my how much.
“If we get a projection of satellites coming within a couple of kilometers, we look at both proximity potential and timing potential,” Shotwell said.
“The orbits could cross at exactly the same point, but if one orbiter gets there ten minutes in front of the other one, then there’s no possibility of a collision.”
Another problem is that, even travelling at the speed of light, a navigation command signal from Earth can take up to 20 minutes to reach the red planet. That makes it vitally important to plan ahead.
“If there’s a ‘red event’ – where they intersect – then we start tracking them more closely," he said,
"If we get a week or so away and it looks like it’s getting even tighter, then we might start discussions about doing a manoeuvre."
Shotwell concedes that with so few satellites orbiting such a large planet, the chances of collision are slim. But he stresses that this won’t last.
“There’s a lot of countries that are proposing missions to Mars,” he noted.
“There’s a lot of commercial interest in going to Mars. There is the NASA plan to send humans, and all the precursor missions that go with it. We could have dozens of assets active at Mars in the next 20 or 30 years. We’re trying to just build up a process, so that as all these future missions happen, we’ll be able to easily plug them in to our process, and make sure that everybody’s part of our larger community when we get to Mars.”