In mid-February, the Swiss Space Centre announced a plan to start removing space debris from orbit. Their proposal involved using a satellite, called CleanSpace One, to approach, grapple and then “de-orbit” a debris object.
But how realistic a mission is this? And how much of a danger does space junk really pose?
Space junk is the byproduct of over 50 years of space exploration, and comprises everything from spent rocket stages, to now-defunct satellites, to fragments from explosions and collisions. Space junk poses a real threat to future space operations due to a considerable collision risk to operational spacecraft.
These dangers became abundantly clear on 10 February 2009 with the accidental collision between the inoperative Russian satellite Cosmos 2251 and the operational US satellite Iridium 33. This collision event resulted in the creation of thousands of debris fragments larger than 1cm and the loss of an operational satellite.
While this collision is the most prominent example yet of the dangers posed by space junk, the problem has really been known since the idea of “collisional cascading” was first introduced by NASA scientists Donald J. Kessler and Burton G. Cour-Palais in 1978.
Kessler and Cour-Palais suggested space junk fragments generated by collisions are becoming the dominating source of debris in Earth orbit, with more fragments being created through every collision. The number of objects increases uncontrollably as collisions occur, and will continue to do so until the population of intact objects is reduced.
This is the aim of the Swiss CleanSpace One mission – to reduce the number of intact objects in orbit. But it’s not that simple.
Computer simulations have revealed an unsettling fact about space junk in some regions of low-Earth orbit (generally defined to be altitudes below 2,000km). Namely, the amount of space junk in low-Earth orbit has reached a level where, even if no new objects were added to the region, the number of objects greater than 10cm in diameter would still increase over the next 200 years.
In reality, the situation would be worse since new objects (such as satellites) would continue to be launched in the meantime. This shows the critical nature of the problem and the need for action.
So what can be done?
For a start, it’s worth mentioning that post-mission disposal methods – such as performing manoeuvres to lower a satellite so it deorbits within 25 years – are insufficient to make significant inroads on the problem.
The space junk problem has reached a point where uncontrolled debris objects need to be removed actively or collisions (including debris-debris collisions) averted in some way.
Active debris removal studies have shown that the low-Earth orbit region could be stabilised by removing five specific objects per year. That said, there’s every chance that missions to remove these objects could contribute to the debris problem if they go wrong.
According to the Swiss Space Centre, CleanSpace One will be tested within three to five years.
The test mission, at an altitude of between 630 and 750km, will target one of two Swiss satellites ready to be decommissioned: SwissCube or TIsat-1. After attaching to the target satellite, CleanSpace One and its target will be destroyed as they start to re-enter the earth’s atmosphere.
Even this test mission will involve some non-trivial problems. For a start, “docking” with an object travelling at roughly 28,000km/h requires extremely accurate predictions of the target object’s position. This involves complicated orbital dynamics and the consideration of forces acting on the object.
CleanSpace One will also need to perform numerous orbital corrections in order to grapple the object without colliding and contributing to the space junk problem. These difficulties are enhanced if the target object is uncontrolled and tumbling.
In the case of the Iridium-Cosmos collision of 2009 (mentioned above) both objects were tracked with the Satellite Orbital Conjunction Reports Assessing Threatening Encounters in Space system (SOCRATES), which predicted the two objects would pass within 584 metres of each other. This error shows the uncertainty in some position prediction methods.
Due to these uncertainties, performing an orbital manoeuvre to avoid a collision may actually result in moving the satellite into the path of another object. The fundamental problem here is the ability to accurately predict the orbits of thousands of objects. If we know where an object will be then we have a better chance of a proactive remediation being successful.
Laser tracking of debris objects – such as that undertaken at EOS Space Systems – is able to determine an object’s orbit extremely accurately. Better orbit determination enables better orbit predictions, leading to the accuracy required for on-orbit collision avoidance.
The Satellite Position for Atmosphere, Climate and Environment (SPACE) Research Centre at RMIT University – of which I’m a member – is partly focused on “situational awareness” in space to study the orbital debris problem.
We are working in collaboration with EOS Space Systems to improve models of the earth’s atmosphere, enabling better orbit predictions at low altitudes in the low-Earth orbit environment.
One other method being proposed to avoid debris-debris collisions is the use of a ground-based laser. The laser wouldn’t be used to destroy the objects; rather, it would move one debris object after it is deemed to be on a collision course with another.
EOS is investigating this method and performing a feasibility study. If a sufficient number of collisions are avoided, this may help stabilise the low-Earth orbit environment and potentially avoid the need for difficult and dangerous active debris removal missions.
That said, technological advancements are required before this technique could become a reality – not to mention the political, legal and military issues involved.
The space junk problem is a side-effect of our ongoing use of space and unless we do something soon, the problem is only going to get worse. But if ongoing efforts – including the CleanSpace One mission – are successful we could well be heading in the right direction.