Space. The very word conjures vastness and emptiness. In fact, space is becoming more crowded all the time, especially near Earth. With the recent proliferation of small satellites, there’s an increasing need to control space traffic. Fortunately, Roberto Armellin and Laura Pirovano of the University of Auckland’s Space Institute – Te Pūnaha Ātea – are working on solutions.

Armellin is a professor and Pirovano is a postdoctoral research fellow, both in the Department of Mechanical Engineering. They are experts in astrodynamics, the study of the motion of man-made objects in space.

 Armellin first got into the field to understand the motion of asteroids and whether they might be dangerous for the Earth. Work he did with the European Space Agency helped determine that an asteroid due to approach Earth in 2029 wouldn’t pose a danger – and neither would its return in 2036. With no immediate need to save humanity from the fate of the dinosaurs, Armellin turned to a much more credible danger: space debris.

Space is admittedly a lot emptier than a city. But just as there’s more traffic on some roads than others, there are more man-made objects in some regions of space than others.

Low Earth orbit is the “downtown” of space because it’s close to Earth. This means it’s cheaper and easier to get satellites and astronauts up there.

The next busiest region is geostationary orbit. This is much further out but at a specific altitude: 35,786 km from Earth. Satellites in this orbit travelling at three kilometres per second spin at the same rate as the Earth, allowing them to stay in an apparently fixed position in the sky. This is useful for telecommunications satellites, because an antenna on Earth can always remain pointed at that satellite.

A major part of Pirovano and Armellin’s work is cataloguing space objects.

“You can’t regulate traffic if you don’t know where the cars are,” Armellin says.

The difficulty for scientists is that though it may be possible to observe unknown objects, they’re observed for such a short time that it’s hard to know where they’re going or if two observations scattered over time are of the same object.

The catalogue must also be maintained because objects may move from their previously known orbits. For that reason, part of Pirovano’s research is about detecting satellite manoeuvres.

The catalogue Pirovano and Armellin have been working on with international colleagues now contains about 37,000 objects of ten centimetres and above in size.

“If you go down to a centimetre, though, there are at least 700,000 objects in space,” says Armellin. “Problem is, the catalogue is not complete. Considering we now have a lot of cube satellites of ten cubic centimetres, a bullet of one centimetre would completely destroy it.”

For a satellite to avoid a collision, it has to know one is likely. Several private companies now popping up internationally propose to help governmental efforts, says Armellin.

“Satellite operators receive messages that tell you, ‘Your spacecraft is going to have a conjunction on this date, at this distance, with a collision probability of this value.’”

One challenge is the aforementioned problem of knowing what’s out there. Another is accurately determining the trajectory of space objects.

Once collisions can be predicted with a higher degree of accuracy, the next challenge becomes figuring out what to do about them. If the predicted collision is with a piece of space debris, the satellite has to move over. If it’s with an active satellite, though, the two satellites have to coordinate a response.

The problem is, projected collisions could occur with little notice at any time of day or night, potentially between two operators who aren’t in contact or who don’t speak the same language. 

The solution? Autonomous manoeuvring. 

These days, satellites need to have an end-of-life plan. For satellites in geostationary orbit, the accepted solution is to move further away from Earth into a graveyard orbit. 

“The problem is that a graveyard orbit doesn’t remove the problem,” says Armellin. “In the future, these regions of space may become of interest to other missions and then the space junk out there may interfere.”

Satellites in low Earth orbit are supposed to have enough fuel at the end of their lives that they can be pulled into Earth’s atmosphere, where small satellites burn up and larger ones can be manoeuvred to crash into the ocean. 

“However, operators may tend not to implement end-of-life manoeuvres because basically, you’re wasting propellant to kill the spacecraft,” says Armellin. Armellin and some of his colleagues have been working on a solution that exploits satellites’ orbit perturbations to reduce the change in velocity necessary to make a spacecraft re-enter the atmosphere.