Thursday, May 23, 2019

Might satellite constellations learn to avoid debris with sensors on satellites?

There were no artificial satellites before Sputnik in 1957.
Today there are about 5,000 with plans for thousands more.
Space debris is problematical.

The European Space Agency (ESA) reported that as of January 2019 there were about 5,000 satellites in space and 1,950 of them are still functioning. Hopefully, those functioning satellites have fuel and thrusters that will enable them to de-orbit and (mostly) burn up in the atmosphere when their useful life is finished. The remaining 3,050 are slowly drifting, along with a lot of debris.

The ESA estimates that there have been over 500 break-ups, explosions, collisions, or anomalous events resulting in fragmentation and they estimate that there are 34,000 debris objects >10 cm, 900,000 from 1 to 10 cm and 128 million from 1 mm to 1 cm. NASA says there are more than 20,000 pieces of debris larger than a softball, 500,000 the size of a marble or larger and many millions so small they can’t be tracked. (watch: NASA's Animation Shows Massive Space Junk Around Earth)

In low-earth orbit (LEO), debris circles the Earth at speeds of about 7 to 8 km/s. However, the average impact speed of orbital debris with another space object is approximately 10 km/s and can be up to about 15 km/s, which is more than 10 times the speed of a bullet. At those speeds, a collision with a small object can do significant damage. This sounds like a disaster waiting to happen and the current and planned proliferation of LEO satellites increases the likelihood of a Kessler Syndrome event -- a cascade of collisions between satellites and the ensuing debris.

As Kessler says "The cascade process can be more accurately thought of as continuous and as already started, where each collision or explosion in orbit slowly results in an increase in the frequency of future collisions." If you aren't worried yet, watch the following short video or read Kessler's 1978 paper.

Kessler's warning was taken seriously and NASA and others have been working on debris mitigation policy and technology for years, but the silver bullet has not been found. The Space Surveillance Network tracks approximately 23,000 relatively large objects and you can query the database here, but what about the millions of objects that are too small to track?

The SpaceX press release for their Starlink Mission hinted at their collision-avoidance strategy, saying that
Each spacecraft is equipped with a Startracker navigation system that allows SpaceX to point the satellites with precision. Importantly, Starlink satellites are capable of tracking on-orbit debris and autonomously avoiding a collision.
That sounds promising, but autonomously resolving and recognizing a marble-sized object that is approaching at up to 15 km/s, computing its trajectory and firing thrusters to avoid a collision can't be done -- even by Elon Musk.

Relatively few debris objects can be tracked terrestrially, but a satellite might be able to recognize a piece of debris and transmit its characteristics to a terrestrial processor, greatly expanding the tracking database. SpaceX may be approaching this as a machine-learning problem in which the entire constellation, not individual satellites, is learning to avoid collisions.

That is pure speculation, but it was triggered by a few thoughts.

For a start, at the end of 2017, SpaceX delivered a space debris sensor (SDS) to the International Space Station. As shown in the following short video, the SDS is capable of monitoring the size, speed, direction, and density of small particles that impact it.

Elon Musk also has a strong interest in machine learning -- he was a co-founder of openAI and his Tesla cars act as sensors uploading driving data that is used for training autonomous vehicles.

Going out to the very end of the limb -- Musk is a fan of science fiction and speculation on the possibility of a swarm of man-made objects learning about existential risks is reminiscent of emergent intelligence in Asimov's fictional planet Gaia or Teilhard de Chardin's noosphere).
Musk opened his Tesla patents and, if SpaceX demonstrates the feasibility of this approach to debris avoidance (and perhaps one-day removal), I expect that he would share this technology with competitors like OneWeb and Telesat and the space agencies of all nations.

Like global warming, space debris is an example of a tragedy of the commons and is a threat to all nations. As the cartoon character Pogo said, "We have met the enemy and he is us." Ironically, global tragedies of commons can unite us.

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