Originally posted Mar 4, 2016
Here we come to save the day!
“Space” said author Douglas Adams, “is big. Really big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist, but that’s just peanuts to space.”*
Low Earth orbit (LEO) is a significant chunk of space, but its bigness wasn’t enough to save the working satellite Iridium 33 from a 2009 collision with Russia’s defunct Kosmos 2251 communications satellite. The impact at an altitude of about 500 miles was the first time two satellites had collided at hypervelocity, and the results were not pretty.
Yellow dots represent estimated debris from the 2009 collision of two LEO satellites. This graphic from Wikimedia Commons shows the debris location about 50 minutes after the collision. The structure of the debris field is an excellent example of conservation of momentum.
The danger of collisions such as this is highest in LEO, which spans altitudes of 100 – 1200 miles. The danger is a function of the volume of that space, the large number of objects there, and their widely varying orbits. The intersections of these orbital paths provide countless opportunities for destructive high-velocity collisions.
It’s estimated that the 2009 collision alone produced 1000 pieces of debris four inches or larger in size, and countless smaller fragments. Because objects as small as a pea can disable a satellite, and because larger ones can turn a satellite into another cloud of impactors, the danger to vital resources in LEO is clear.
This chain reaction hazard or “debris cascade” was detailed as far back as 1978 by NASA’s Donald Kessler in a paper that led to the scary term Kessler syndrome.
The concept is scary because there’s no simple way to avoid the problem. What’s worse, our existing tools aren’t fully up to the task of identifying objects and accurately predicting collisions. The earlier 1961-vintage ground-based VHF radar system could track only those objects bigger than a large beach ball, and accuracy was not sufficient to allow the Iridium satellite to move out of danger.
Cue the RF/microwave engineering cavalry: With their skill and the aid of signal analyzers, signal generators,network analyzers, and the rest of the gear we’re so fond of, they have created a new space fence. Operating in the S-band, this large-scale phased-array radar will have a wide field of view and the ability to track hundreds of thousands of objects as small as a marble with the accuracy required to predict collisions.
Alas, predicting collisions is most of what we can do to avoid a Kessler catastrophe. Though the company designing and building the fence mentions “cleaning up the stratosphere,” it’s Mother Nature and the very faint traces of atmosphere in LEO that will do most of the job. Depending on altitude, mass, and cross-section, the cleaning process can take decades or longer.
In the meantime, we’ll have to make the most of our new tools, avoid creating new debris, and perhaps de-orbit a few big potential offenders such as Envistat.
There may be another opportunity for the engineering cavalry to save the day. There are proposals for powerful lasers, aimed with unbelievable precision, to blast one side of orbiting debris, creating a pulse of vapor that will aim objects to atmospheric destruction and render them mostly harmless.* I’m looking forward to the RF/microwave designs for tracking and aiming that will make that possible.
* From the book The Hitchhiker’s Guide to the Galaxy