‘Sling-Sat’ Could Remove Space Junk on the Cheap
Image:Sling-Sat is a space sweeper idea designed to remove orbital debris from Earth orbit.
Jonathan Missel/Texas A&M
A proposed space-junk removal system would hop from one piece of debris to the next without burning much fuel, potentially making a de-clutter mission economically feasible with current technologiy.
The TAMU Space Sweeper with Sling-Sat, or 4S for short, would harness the momentum imparted by capturing and ejecting one object to slingshot on to the next chunk of space junk, its developers say.
“The goal of this mission is to remove as many pieces of debris with the minimum amount of fuel,” said Daniele Mortari of Texas A&M University.
A few months ago I did some research space debris removal technologies. The TAMU sweeper/sling sat was one of them. It is a beautifull exercise in momentum exchange, and in a 2D environment the theory works wonderfull. There are however a number of problems which will be very hard to solve in my opinion.
First of all, most of the basic theory has been worked out in 2 Dimensions. This ignores possible of axis components to the forces involved in the capture. If an object’s centre of gravity does not impact on the axis of the arm, they will induce a rotation about another axis then the primary axis of rotation. The properties of the debris objects are often unknown and often they spin (slowly) about multiple axis.
Second of all, The debris object needs to be captured with a velocity difference. It will be next to impossible to foresee the exact energy exchange between the objects. In order to gain more momentum, a higher velocity difference is needed, increasing the changes of damage to the debris object. If the debris object is damaged during retrieval, it will be very hard to predict the orbit of the (now) several debris objects. Possible, the number of debris objects in orbit will have increased after release. Note however that the theory behind the AGI sweeper assume a plastic collision. Practically this will be very hard to achieve, with a significant momentum exchange. If the capture has a velocity difference which is too low, the momentum exchange might not be enough to reach the next object.
Third of all, the momentum exchange is dependant on the mass difference between the sweeper and the debris object. If you mostly want to move the sweeper around, it should be a lot lighter than the debris object, however the debris object will hardly change its orbit in this case. The opposite happens when the debris object is a lot lighter, making it hard to reach the next object.
Last of all, and this is an important point for all debris removal missions. The largest and heaviest objects in the busiest orbits pose the biggest threats. I think this concept is made for the more common smaller debris objects, seeing as well that it probably needs to be heavier than the debris it removes. A collision between 2 massive objects can easily produce hundreds or thousands of new debris objects. If this concept removes a few hundred objects, which I see as optimistic, all that work could be nullified by one other collision. Removing these massive high risk objects should be the priority, for the smaller objects it seems that there are less complicated options available.
As an additional note I would like to point out that there are a lot of concepts available for debris removal. A lot of these are technically feasible, what is the better method depends more on the requirements that you set. The biggest obstacles to make these missions happen are political and economical. There are some laws in space, and these mostly relate to liability and who owns what in space. Both of these topics relate to space debris. The other part is that the return of a removal mission is not clear to many financiers and I am afraid they will only see it if it is already too late.
The Kessler syndrome is real. We can only stop further collisions of debris objects with proper post mission disposal, active removal of a number of objects every year and making sure no more upper stages explode.
A number of articles and publications on the AGI sweeper and Sling Sat can be found on the website of the inventor, Dr. Daniele Mortari: link.