皇家华人

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鈥檚 an increasing need to control space traffic. Fortunately, and of the University of Auckland鈥檚 鈥� 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鈥檛 pose a danger 鈥� 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鈥檚 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 鈥渄owntown鈥� of space because it鈥檚 close to Earth. This means it鈥檚 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鈥檚 work is cataloguing space objects.

鈥淵ou can鈥檛 regulate traffic if you don鈥檛 know where the cars are,鈥� Armellin says.

The difficulty for scientists is that though it may be possible to observe unknown objects, they鈥檙e observed for such a short time that it鈥檚 hard to know where they鈥檙e 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鈥檚 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.

鈥淚f you go down to a centimetre, though, there are at least 700,000 objects in space,鈥� says Armellin. 鈥淧roblem 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.鈥�

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.

鈥淪atellite operators receive messages that tell you, 鈥榊our spacecraft is going to have a conjunction on this date, at this distance, with a collision probability of this value.鈥欌��

One challenge is the aforementioned problem of knowing what鈥檚 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鈥檚 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鈥檛 in contact or who don鈥檛 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. 

鈥淭he problem is that a graveyard orbit doesn鈥檛 remove the problem,鈥� says Armellin. 鈥淚n the future, these regions of space may become of interest to other missions and then the space junk out there may interfere.鈥�

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

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