Wednesday, August 7, 2013

Space Junk 1: Orbital Debris


Still on that Mars Curiosity high! It's interesting how exciting I am each time we as a species launch something into space. It's stunning to hear how things zing around above our heads studying various properties of Earth, how far probes travel, or how hardware land successfully on other celestial bodies due to precise calculations and engineering.

All these amazing feats are so rare and so special, proposals have recently emerged to protect them as national treasures. However, it then occurs to me just how much hardware we have released into our solar system, on both successful and unsuccessful missions. We value many -- but others are pure hazards. Then I remember: space debris, as a byproduct of exploration, is a genuine hazard.

Orbital Debris in Low Earth Orbit
Artist's Concept of Objects in Low Earth Orbit

At this time, pieces of orbital debris in Low Earth Orbit (LEO) and Geosynchronous Orbit (GSO) number in the tens of millions. About 20,000 of these objects are larger than 10cm. Particles between 1-10cm are estimated at about half a million. The rest are less than 1cm.

Orbital Debris is defined as a human-made object in orbit around the Earth which no longer serves a useful purpose, such as launch vehicle upper stages, spent payload carriers, derelict spacecraft, pieces resulting from explosions or collisions, and even tiny paint flecks released by impacts.

Most orbital debris reside within 2,000 km of the Earth's surface, or are in LEO.  How do we estimate the numbers and placements? Ground-based radars can detect objects as small as 3mm, and space-based detection systems can detect things as far as 40,000km out.

Orbital Debris in Geosynchronous Orbit
Artist's Concept of Objects in GeoSynchronous Orbit (GSO)

The US Space Surveillance Network tracks all orbital debris larger than 10cm.  An average of one catalogued piece of debris falls back to Earth each day, and this has been the case for the past four decades.  In LEO and below, orbital debris circles at around 7-8 kilometers per second, or up to 18,000mph.

As a result, the International Space Station (ISS) is the most heavily shielded spacecraft ever flown. Nodes holding human crews and pressure tanks on the structure are able to withstand impact of debris as large as 1cm. The ISS can also maneuver to avoid tracked objects.

The higher the altitude, the longer the debris will remain in orbit. Debris left below 600km fall back to Earth within a few years, though precious little survives the super-heated re-entry through Earth's atmosphere. Things higher than 800km take decades to return. Above 1,000km or more? Those may circle for a century or more.

Geosynchronous Orbit as seen from Polar View
GSO Polar View

Most telecommunications and meteorological satellites operate at the 36,000km altitude in geostationary orbit, where the problem or orbital debris is less severe. Which is not to say... harmless.

If you happen to have nothing to be indignant about this week yet, you can read about how NASA handles Orbital Debris Re-Entry. Or if you want to be part of all future re-entry dangers, you can sign up for NASA's "Orbital Debris Quarterly" newsletter.