Flirting with Kessler: Why the Physics of Space Debris Makes It Such an Orbital Pain

Imagine a large parking lot with 131 million cars in it. Now imagine that they are spread throughout the entire inhabited areas of the Earth. While still a large number, it definitely pales in comparison to the approximately 1.47 billion cars registered and in use today, with room to spare for homes, parks and more. The 131 million represents the total number known and estimated space debris objects in orbit with a size of 1 mm and larger, according to the European Space Agency. This is in addition to the approximately 13,200 satellites still in orbit, of which 10,200 are still functional.

Now imagine that most of these 131 million cars from the past are 4 inches or smaller in size. Spread over the entire surface of the earth you would not be able to see more than one at most. Above the Earth’s surface there are many orbital planes and no pesky oceans to prevent millimeter and centimeter sized cars from being out there. This gives a rough idea of ​​how incredibly empty Earth’s orbital planes are and why you rarely notice such space debris from the International Space Station until a small piece crashes into a solar panel or something equally funny.

Cleaning up space debris seems rather unnecessary from this perspective, except that even the smallest piece of debris travels at an orbital speed of several kilometers per second and still has kinetic energy left. Hence your task: to track down debris less than 4 inches across hundreds of miles of largely empty orbital planes as it speeds along with destructive intent. This can’t be that hard with lasers on the ISS or something, right?

Orbital Delta V

When it comes to getting and keeping a job, speed is everything. Go too fast (escape velocity) and you’ll fly off track into the darkness of space. If you go too slowly, you will find yourself becoming well acquainted with the intricacies of atmospheric plasma formation. This means that space debris can only become a problem if it has to get the right initial velocity relative to Earth from somewhere, which usually happens as a result of a rocket launch throwing away pieces that remain in Earth orbit, a catastrophic satellite or missile jamming. a dropped tool by an astronaut, or even the use of anti-satellite weapons (ASAT) that create a shower of particles that may or may not enter orbit.

Once in orbit, altitude determines how stable that orbit is, with atmospheric drag being one of the most important factors orbital decay. For debris in relatively low orbits close to the Karman Line (~100 km altitude), atmospheric effects are quite noticeable and debris in these orbits will decay quickly, sometimes burning up within hours to weeks. Other orbits will experience some atmospheric drag, but only so slight that the decay period is measured in years or decades. For the International Space Station (ISS), the altitude is maintained between 370 and 460 km, with atmospheric drag reducing the altitude by about 2 km per month.

Being the largest object currently in orbit, the ISS’s atmospheric drag is obviously quite high. The upper stage of Japan’s H-2A rocket that launched the GOSAT satellite into space in 2009 has since remained passively orbiting the Earth at an altitude far above that of the ISS. While this type of object could one day reenter Earth’s atmosphere, this would be far in the future, with every active mission doing its utmost to avoid being hit by the thing.

Meanwhile, there is a lot of debris less than 10 cm in size floating around in Earth orbit, whose decay in orbit would be insignificant due to their small size and their exact position uncertain.

Schedule a meeting

For something large, like the upper stage of a rocket, we can track the objects using observations on the ground and in space. This knowledge recently allowed a company called Astroscale to bring a spacecraft within about 50 meters of the upper stage of the GOSAT mission as part of the ADRAS-J space debris mission. Even this required careful orbital mechanics as the exploration spacecraft was maneuvered closer to its unsuspecting target. In future missions, this approach should theoretically result in the prey being pushed to a fiery demise in the atmosphere.

An important aspect to note here is that in all cases of orbital rendezvousit’s a nerve-wracking experience even when you’re controlling all aspects of both spacecraft, such as when a spacecraft docks with the ISS. Since they all appear to be essentially motionless relative to each other, it seems like an easy task, just like getting closer to another person on the Earth’s surface. Instead, the experience is closer to trying to meet another person while you’re both skydiving. Even the slightest change in your trajectory can cause you to bump into the other person, far away from him or her, or to start spinning uncontrollably.

Small needles in a big haystack

As the above graph makes clear, our ability to detect space debris is highly dependent on its size and height, while our ability to detect smaller debris is quite limited. For anything smaller than something like an intact rocket stage, we rely heavily on statistics to predict how many such objects are likely to be in orbit. This means most orbital debris management relies on passive defenselike the Whipple shield that provides ballistic armor to dissipate the energy of a collision.

As the number of objects in orbit and thus the debris increases, such defense mechanisms will be increasingly tested, and parts of spacecraft that cannot be protected – such as solar panels – will increasingly be hit by that debris. This is where we enter the territory of the infamous Kessler syndrome. Imagine these increasing attacks causing more damage, destroying parts of spacecraft and producing more debris, which in turn will damage and destroy additional active orbital objects, producing more debris ad nauseam.

The point here is not that Earth’s orbits will be ‘full’, but rather that it would turn the orbital planes above the Earth’s surface into the equivalent of walking into a large room that appears empty, but out of seemingly nowhere a few pieces metal and perhaps a bolt of lightning will suddenly crash into your body at a few km/s. Knowing this to be the case, the more likely this becomes, the less likely people will be to step foot in that room.

In one Review article from 2022 by Barış Can Yalçın et al. in Frontiers in space technology the issue of space debris is explored, along with a range of methods being explored as possible methods of debris removal. These include ways of “pushing” the target object in various ways, others actively try to capture the target with a harpoon, net, foam, etc. There is also the idea of ​​using lasers to destroy the target, which in many ways encounters. practical issues, not least the amount of energy required for a usable laser system.

Damn lasers on space stations

The use of laser systems has produced a number of proposed systems, including some that would be mounted on the ISS. The wildest idea here was to use a ground-based laser that would heat orbital debris to change its orbital path as a so-called laser broom. Although many such projects have achieved a certain level of funding and planned implementation dates, this has remained a purely theoretical issue to this day. With the disposal of rocket stages and defunct satellites being much better regulated today than a few decades ago, the sense of urgency seems to have diminished along with it.

The fact remains, however, that orbital debris remains a danger. You only have to observe the impacts on the ISS to get an impression of the debris density in low Earth orbit. For a sense of scale, even a relatively small object weighing 50 grams colliding with a relative speed of 15 km/s gives the equivalent energy of 1 kg TNT. The generally much smaller debris that regularly hits the ISS isn’t as destructive, but its marks are quite distinctive, from holes in the solar panel to visible impacts in the windows.

There appear to be three different approaches to orbital debris: one is based on preventing and clearing large debris, while another focuses on active defense, such as equipping a space station with sensors and (laser) domes to remove debris to remove. The third would be to move the track quite randomly, to detect and neutralize active or passive debris, especially the type that is generally not tracked today.

What is clear is that we don’t lack options for dealing with orbital debris, but the complications of orbital mechanics and low debris density make it a fun game of finding needles in haystacks. Except these needles are super small and can seriously draw blood.