Saving Fermi: NASA’s system for avoiding collisions with space junk

Ars Technica » Scientific Method 2013-05-08

In late March last year, the people operating the Fermi Gamma-ray Space Telescope got a bit of a scare. Their hardware was one week away from a close encounter with a defunct Russian spy satellite. A week might seem like short notice for one-of-a-kind hardware like Fermi, but in some ways the team was lucky to have any warning at all. Prior to 2007, NASA didn't even have a policy in place to identify threats to unmanned hardware.

That has now changed. Thanks to cooperation between the military and the Goddard Space Flight Center, everything that can possibly get out of the way of space junk is regularly tracked for potential collision risks. That system is what alerted Fermi's controllers to the danger and allowed them to use on-board thrusters for something they were never designed to do: move the satellite safely out of the way.

Tracking threats

The technical term for the risk identification process is "conjunction assessment," or CA. This process involves taking the latest tracking information, calculating orbits, and figuring out whether two objects are likely to be in the same place at the same time. This may seem like an easy problem—after all, it's just math—but there are a lot of potential complications. One is simply the ever-expanding catalog of junk in the relevant orbits (primarily low-Earth and geostationary), some of it caused by collisions between two existing pieces of junk. Another is the fact that these orbits aren't stable. The atmosphere may be sparse at these altitudes, but it's still there, and it creates varying amounts of drag on things in orbit.

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