Russia recently did another anti-satellite test and destroyed one of its satellites in orbit. This has caused a backlash of international censure since it supposedly has put both the international space station and the new Chinese space station in danger of debris hitting it.
My question is, how big is the threat, actually? I mean, countries keep doing these tests periodically, I guess to demonstrate they have the capability. They obviously deem to reward of getting to show off their ability to be greater than the risk. So, my question is…how big is the actual risk? Also, why are countries testing this capability on this range of targets (i.e. satellites in similar orbits to things like space stations) instead of the ones at much lower orbits that, presumably if hit would degrade faster and burn up in the atmosphere?
It depends in part on where the satellite was and the debris is
That’s Low Earth Orbit (LEO), which is typically less than 2400 km from the surface. Lots of satellites there, along with the ISS. That’s why this is of concern.
Anything orbiting at 200 km or less will not last all that long, and will fall pretty rapidly. The bulk of this debris is above that point.
Here’s a guide to typical orbital lifetimes (for circular orbits)
Satellite Altitude
Lifetime
200 km
1 day
300 km
1 month
400 km
1 year
500 km
10 years
700 km
100 years
900 km
1000 years
Left to themselves, the orbital detritus left from this explosion will be up there for the next 1-10 years.
A lot of things are in much higher orbit – geosynch is 36,000 km. GPS satellites are at 20,000 km typically. So some things are effectively out of reach. But lots more is closer down, and vulnerable.
The last anti-satellite weapon test in orbit was over 12 years ago (the 2008 interception of the failed deployment of NROL-21/USA-193 by a modified SM-3 missile, ostensibly because of hazard posed by the frozen monomethylhydrazine propellant upon reentry, but realistically to protect information about the NROL-21 satellite and demonstrate the ASAT capability of the Standard Missile system) and the year before that the interception of a defunct Chinese weather satellite by their system. Prior to that, the last deliberate kinetic interception of an object in orbit was the 1985 ASM-135 test of an F-15 launched interceptor against the Solwind P78-1 stellar observatory. There have been a handful of unintentional collisions, the most notable of which was the 2009 collision of Kosmos-2251 with an Iridium telecom satellite. So, this is scarcely a common event.
The concern isn’t just the debris that this latest test created but the potential for the debris to impact other objects in orbit, producing a progressive cascade of debris that could make entire orbital altitudes uninhabitable for decades, threaten the safety of crew and the ultimate integrity of the ISS, and perhaps even prevent launching objects through any affected orbit. The common term for this is a Kessler cascade (or more dramatically “Kessler Syndrome”, although that term was only later added to the popsci lexicon), named for the NASA astrophysicist Donald Kessler (and co-author Burton Cour-Palais)who first proposed it over forty years ago. This is especially a concern for the increasing commercialization of space and in particular at altitudes of >500 km where orbital decay of debris can take decades depending upon the effects of solar activity upon the thermosphere and the relative momentum of the bodies at impact.
This is also the altitude at which SpaceX’s Starlink system and several other proposed LEO telecommunications systems plan to operate enormous fleets of satellites, and while the claim is made that such satellites will be able to detect and dynamically evade debris (although the “how” of that claim is never explicated much less authoritatively demonstrated) it would only take a handful of such impacts to thoroughly contaminate a wide array of orbital azimuths such that nothing could survive up there for very long, essentially requiring abandoning the orbit for decades while debris density dropped back down to pre-impact levels. You can also imagine what would occur if a large (and by “large”, something the size of a baseball”) struct a structural member of the ISS and caused it to fail, resulting in the station becoming unusable and producing debris. The only choice in that case would be to deorbit the station, assuming that propulsion still works, and wait for the orbit to clear out.
It was irresponsible for China to conduct an ASAT test, it was even more reckless for the US to conduct an ASAT test, and it is incomprehensibly wrong for Russia do have done so recently. Orbital space is a precious shared resource, and like most natural resources, one that cannot be readily remediated by any existing or plausibly near-term capability.
I thought India also recently (2019? Maybe earlier?) anti-satellite test as well. Maybe I’m misremembering if the last one was in 2008. I was thinking they were more common than I guess they are or have been.
Ok, so according to this Wiki page, there have been several such tests, though sometimes the country doing them have said it was actually something else.
Here is the one from India I recalled:
“On 27 March 2019, India successfully conducted an ASAT test called Mission Shakti.”
Some of these tests didn’t actually shoot down a satellite, just demonstrated the capability. Now I’m really curious…why would Russia do something like this at this time? I mean, doesn’t this put their own cosmonauts in danger who are on the ISS??
That test was conducted at a very marginal orbit of 300 km altitude; most of the debris, and especially anything of significant size, wouldn’t last more than a few months. There are a bunch of nanosatellites (CubeSats and similar forms) running around this orbit, too, with no propulsive capability or ability to monitor anything “above the horizon”, so they can’t avoid collisions, and this is accepted because debris won’t persist in orbit and is designed to not pose a hazard of ground impact.
Note that orbital lifetimes also depend on size (or, more precisely, sectional density, but that typically scales linearly with size). The small pieces of debris that a satellite is blasted into will have their orbits decay more quickly than the original satellite.
I recall reading that one of the Space Shuttles was hit by a small chip of paint which gouged the windshield. The concern is that a small particle - moving without wind resistance - can reach terrific speeds and have the effect of a much larger particle.
That is more or less true in the thermosphere (up to about 600-700 km altitude depending on solar activity) where interactions can be treated as in a rarified gas but less so above the thermopause where the mean free path of atoms and molecules is so long that they don’t interact with one another. That is why you see what is essentially logarithmic scale on the table posted by @CalMeacham. (My references are slightly different but in the same general range; if you wanted to make actual predictions you’d actually reference the current NASA Global Atmospheric Reference Model (GRAM) because upper atmospheric density varies by location and time.
And it is even worse than that; nearly everything in orbit is moving at such high differential speeds that an impact doesn’t just produce a narrow stream of debris moving along the original vector but instead a wide spread of debris fanning out along the original vectors of the impacting objects. Even worse, if these two objects are both in orbits that periodically intersect one another, there is a good chance of progressive impacts which produce even more debris that gets spread out as time goes on.
And while you’ll find speculative ideas such as “laser brooms” or using aerogel to increase drag, there is really no practical way to deploy such technologies and power them to the degree necessary to effect a cleanup of contamination. No one has yet even made a proof-of-concept demonstration of proposed debris mitigation methods, and the logistics to support clearing an entire orbit are beyond daunting, essentially requiring an in-space infrastructure to be able to maintain it.
It must be true regardless of how rarified the molecules are. You’re going to have some collisions with stray molecules, each of which will impart some distribution of impulses. The number of such impacts, and hence the total impulse, will be proportional to the cross-sectional area. The total delta-v will be the total impulse divided by the object’s mass, so delta-v will be inversely proportional to sectional density. And ultimately, it’s delta-v that causes an orbit to decay.
Yes, the orbital lifetime will depend strongly on altitude: We already know that. But at any given altitude where any orbit decays at all, the orbit of a smaller object will decay quicker than that of a larger one.