Space Debris and the Kessler Syndrome

One of the YouTube channels that I am subscribed to is SpaceX. But it's gotten very boring. Every few days, a new launch of 40-50 Starlink satellites takes place, sometimes from the East coast, sometimes from the West. There are close to 3,000 Starlink satellites up there now. Here's a recent launch (although they all look much the same):

Mobile telephony to date has mostly been based around terrestrial basestations, the ones you see beside freeways (when they are not trying to disguise them as trees or cacti). The hot buzzword for that technology is 5G, the fifth generation of communication standards.

The one big early attempt to do something using satellites was Iridium. But nine months after the system went live, it went bankrupt. The company was bought out of bankruptcy for $25M and is still operational. There are at least three new satellite-based systems. Starlink, obviously. But also Amazon's Kuiper and the UK's OneWeb.

Space Debris

One challenge that Starlink—and other low earth orbit (LEO) constellations—face is that there is a lot of space debris in LEO where Starlink operates, and so the satellites have to maneuver around all of them. Every so often, someone else launches a rocket, and the satellites may have to get out of its path, too—and on re-entry.

Where did all this space junk come from?

A small amount is from the early days of the various space programs when people had not yet realized that it would be a problem, and so everything from boosters to tools. In 2016, the International Space Station (ISS) was struck by a fast-moving flake of paint that chipped its window. It is 50 years since the start of the space programs, and even at the leisurely (compared to SpaceX) launch rates back then, that is enough time to accumulate a lot of stuff. There are two areas of the biggest concern. One is the geostationary orbit above the equator, where satellites don't move relative to the earth, so they can operate with fixed antennas on both the satellite and earth. This is quite high, at around 26,000 miles. Since these satellites need to be both above the equator and at a given altitude, the narrow ring is crowded. These days, geostationary satellites are boosted to higher altitudes when they become obsolete, leaving space for new satellites.

The other area of concern is low earth orbit where many other satellites operate that have a track that passes over the earth for photography, communication (Starlink and its competitors), and military uses. And also where the ISS is to be found. 

In 2021, NASA summarized the problem in Space Debris and Human Spacecraft:

More than 27,000 pieces of orbital debris, or “space junk,” are tracked by the Department of Defense’s global Space Surveillance Network (SSN) sensors. Much more debris -- too small to be tracked, but large enough to threaten human spaceflight and robotic missions -- exists in the near-Earth space environment.  Since both the debris and spacecraft are traveling at extremely high speeds (approximately 15,700 mph in low Earth orbit), an impact of even a tiny piece of orbital debris with a spacecraft could create big problems.
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There are approximately 23,000 pieces of debris larger than a softball orbiting the Earth. They travel at speeds up to 17,500 mph, fast enough for a relatively small piece of orbital debris to damage a satellite or a spacecraft. There are half a million pieces of debris the size of a marble or larger (up to 0.4 inches, or 1 centimeter) or larger, and approximately 100 million pieces of debris about .04 inches (or one millimeter) and larger. There is even more smaller micrometer-sized (0.000039 of an inch in diameter) debris.

Satellite Collisions and Destruction

A lot of this debris comes from the destruction of satellite testing. Anti-satellite (ASAT) weapons. Some comes from accidental collisions of satellites, which at least isn't deliberate. NASA again:

On Feb. 10, 2009, a defunct Russian spacecraft collided with and destroyed a functioning U.S. Iridium commercial spacecraft. The collision added more than 2,300 pieces of large, trackable debris and many more smaller debris to the inventory of space junk.

China's 2007 anti-satellite test, which used a missile to destroy an old weather satellite, added more than 3,500 pieces of large, trackable debris and many more smaller debris to the debris problem.

If you do the math, those two incidents created about a fifth of all the space junk.

India also did an ASAT test, but at least it used a specially launched satellite in a special low-altitude orbit to minimize the amount of space junk that would be created (and not almost immediately burn up in the atmosphere).

Kessler Syndrome

You probably have at least a vague idea of how a nuclear reactor works. A neutron hits an atom of (usually) Uranium 235. This releases more neutrons and thus leads to a self-sustaining reaction. It also generates heat that can be used to power steam turbines and so generate electricity. This is how nuclear power stations and nuclear-powered submarines work.

Donald Kessler was a NASA scientist, and in 1979, in the paper Collision Frequency of Artificial Satellites: The Creation of a Debris Belt, discussed a scenario where the density of debris in low earth orbit could reach a point where a similar self-sustaining "reaction" could occur, where collisions between existing space debris cause more space debris, which causes more collisions. Potentially whole orbits could become unusable for satellites for decades. 

At the end of the 2008 paper, The Kessler Syndrome: Implications for Future Space Operations, Kessler and his co-authors conclude:

The conclusion is reached that while popular use of the term may have exaggerated and distorted the conclusions of the 1978 paper, the result of all research to date confirms that we are now entering a time when the orbital debris environment will increasingly be controlled by random collisions. Without adequate collision avoidance capabilities, control of the future environment requires that we fully implement current mitigation guidelines by not leaving future payloads and rocket bodies in orbit after their useful life. In addition, we will likely be required to return some objects already in orbit.

What to Do About It?

There are a number of programs now in existence to remove obsolete satellites and space debris. One is ClearSpace (based in Switzerland) which has an $86M contract with the European Space Agency (ESA). Here's a one-minute video about it (the second half has the best stuff):

There is also a Japanese company, Astroscale, with its "Active Debris Removal" or ADR program. But most of Astroscale seems focused on servicing satellites rather than removing them.

Gravity

I've not seen the 2013 movie Gravity, but the space.com piece Kessler Syndrome and the Space Debris Problem describes how the movie uses the Kessler syndrome to kick off the plot:

Consider this scenario: The destruction of a dead spy satellite spawns a swarm of debris in Earth orbit, which wreaks ever-increasing havoc as it zooms around our planet. The cloud destroys a number of communications satellites, generating more and more debris with every violent collision. It takes out the iconic Hubble Space Telescope and a NASA space shuttle, killing several crewmembers aboard the winged vehicle. It then lines the International Space Station (ISS) up in its crosshairs, destroying the $100 billion orbiting lab with a hail of fast-flying shrapnel.

Here's the movie trailer. Let's hope that this remains fiction.

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