via Hackaday: Flirting With Kessler: why Space Debris Physics Make it Such an Orbital Pain

Picture in your mind a big parking lot with 131 million cars on it. Now imagine that they are spread out over the entire Earth’s inhabited areas. Although still a large number, it is absolutely dwarfed by the approximately 1.47 billion cars registered and in use today, with room to spare for houses, parks and much more. The 131 million represents the total number of known and estimated space debris objects in Earth orbit sized 1 mm and up, as per the European Space Agency. This comes on top of the approximately 13,200 satellites still in Earth orbit of which 10,200 are still functional.

Now imagine that most of these 131 million cars of earlier are sized 10 cm or smaller. Spaced out across the Earth’s entire surface you’d not be able to see more than at most one. Above the Earth’s surface there are many orbital planes and no pesky oceans to prevent millimeter and centimeter-sized cars from being spaced out there. This gives a rough idea of just how incredibly empty Earth’s orbital planes are and why from the International Space Station you rarely notice any such space debris until a small bit slams into a solar panel or something equally not amusing.

Cleaning up space debris seems rather unnecessary in this perspective, except that even the tiniest chunk travels at orbital velocities of multiple kilometers per second with kinetic energy to spare. Hence your task: to chase down sub-10 cm debris in hundreds of kilometers of mostly empty orbital planes as it zips along with destructive intent. Surely this cannot be so difficult with lasers on on the ISS or something?

Orbital Delta V

When it comes to achieving and maintaining an orbit, velocity is everything. Go too fast (escape velocity) and you will zip out of the orbit into the darkness of space. Go too slow and you’ll find yourself becoming intimately familiar with the intricacies of atmospheric plasma formation. This means that for space debris to become a problem, it needs to get this correct amount of initial velocity relative to Earth from somewhere, which generally happens as a result of a rocket launch discarding pieces that remain in orbit, catastrophic satellite or rocket failure, a dropped tool by an astronaut, or even the use of anti-satellite (ASAT) weapons creating a shower of particles that may or may not achieve orbit.

Once in orbit, the altitude determines just how stable that orbit is, with atmospheric drag being one of the prime factors in orbital decay. For debris in fairly low orbits close to the Karman line (~100 km altitude) atmospheric effects are quite noticeable and debris in these orbits will decay rapidly, sometimes burning up in as little as 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), its altitude is maintained between 370 and 460 km, with atmospheric drag decreasing its altitude by about 2 km per month.

As currently the largest object in Earth orbit, the ISS’ atmospheric drag is of course rather significant. The Japanese H-2A rocket’s upper stage that launched the GOSAT satellite into space in 2009 has continued to passively orbit Earth since then at an altitude far above that of the ISS. Although this type of object may one day re-enter the Earth’s atmosphere, it would be far in the future, with every active mission doing their utmost to not get hit by the thing.

Meanwhile there is a lot of sub-10 cm debris zipping around in orbit, for which their orbital decay would be insignificant due to their small size, and their exact position uncertain.

Scheduling A Meeting

For something large like a rocket upper stage, we are able to track the objects using ground- and space-based observations. This knowledge is what recently allowed a company called Astroscale to recently get a spacecraft to a distance of about 50 meters from the GOSAT mission’s upper stage as part of the ADRAS-J space debris mission. Even this took careful orbital mechanics, as the reconnaissance spacecraft was maneuvered closer to its unsuspecting target. In future missions this approach should theoretically end up with the prey being pushed to a  fiery demise in the atmosphere.

An important aspect to note here is that with all cases of orbital rendezvous, it’s a nerve-wrecking experience even when you control all aspects of both spacecraft, such as when a spacecraft docks with the ISS. Since each will appear to be effectively motionless relative to each other it seems an easy task, as with moving closer to another person on the surface of the Earth. Instead the experience is closer to trying to meet up with another person while both of you are skydiving. Even the slightest change in your trajectory can send you careening into the other person, far away from them, or have you begin to spin uncontrollably.

Tiny Needles In A Big Haystack

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

As the number of objects in orbit and with it debris increases, such defenses will be increasingly tested, and parts of spacecraft which cannot be protected – such as solar panels – will increasingly get hit by said debris. This is where we enter the territory of the infamous Kessler syndrome. Imagine these increasing strikes doing more damage, which causes parts of spacecraft to be destroyed and produce more debris, which in turn will damage and destroy additional active orbital objects, which will produce more debris, ad nauseam.

The point here is not that Earth’s orbits will be ‘full’, but rather it would turn the orbital planes above Earth’s surface into the equivalent of walking into a large room that seems empty, but out of seemingly nowhere a few specks of metal and maybe a bolt will suddenly impact into your body with a few km/s. If you know that this is the case, the higher this chance gets, the fewer people will be inclined to set foot into said room.

In a 2022 review article by Barış Can Yalçın et al. in Frontiers in Space Technologies the issues of space debris is examined, along with a series of methods that are being investigated as potential debris removal methods. These include ways to ‘nudge’ the target object in a variety of ways, others actively seek to capture the target with a harpoon, net, foam, etc. There is also the idea to use lasers to destroy the target, which runs into many practical concerns, not the least of which is the amount of energy required for a usable laser system.

Friggin’ Lasers On Spacestations

Using laser systems has seen a number of proposed systems, including a few that would be mounted on the ISS. The wildest idea here has been to use a ground-based laser that would heat orbital debris to change its orbital trajectory as a so-called laser broom. Although a lot of such projects have seen some level of funding and intended deployment dates, to this day it has remained a purely theoretical matter. With the disposal of rocket stages and defunct satellites much better regulated these days than a few decades ago, it seems the sense of urgency has deflated along with it.

Fact of the matter remains that orbital debris remains a hazard, however. One only has to observe the impacts on the ISS to get a sampling of the debris density in just low Earth orbit. For a sense of scale, even a relatively small object with a weight of 50 grams impacting with a relative speed of 15 km/s imparts the equivalent energy of a 1 kg of TNT. The generally much smaller debris that hits the ISS on a regular basis is not quite as destructive, but its traces are quite distinctive, from holes in the solar panel to visible impacts in the windows.

There would seem to be three distinct approaches to orbital debris: one is based around prevention and disposal of large debris, while another focuses on active defense, such as equipping a space station with sensors and (laser) turrets to take out debris. The third would be the fairly random sweeping of orbit, to actively or passively track down and neutralize debris, especially the type that is generally not tracked today.

What is clear is that we do not lack options to deal with orbital debris, but the complications of orbital mechanics and the low debris density makes for a fun game of finding needles in haystacks. Except that these needles are super small and can draw serious blood.

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