Posted by
Duck Archer on Friday, February 27, 2009 10:30:40 AM
A defunct Russian satellite has collided with a (formerly) operational Irridium satellite. It’s the first actual accidental orbital collision –the first in which ‘space junk’ (other than a paint chip) has hit a functioning satellite. The 30+ years of fear & hype have finally come true.
Note the other category, for a moment. A year ago, China hit a satellite on purpose. A few months later, the USA did similar. Both created debris clouds. Differences:
- China used ‘might’ as it used a purpose-built anti-satellite rocket to advertise its arrival on the Anti-Satellite Scene; USA used ‘finesse’ as it quickly reconfigured operational Air Defense assets for an ASAT mission
- USA aimed at hitting a satellite at a point in the orbit where all debris would quickly de-orbit due to atmospheric frictional drag; most chunks would burn up in the atmosphere, the larger ones (if any) were aimed for an empty “broad ocean area”. China’s debris will be in orbit for a long, long time. Most media outlets have forgotten this simple orbital fact. [Most media personnel understand space issues less than they understand their own bias.]
Back to main topic.
The fact of ‘low earth orbit’ (LEO) becoming ‘junked up’ is long-hyped, but still not quite true.
1. Let’s be clear on the realm.
- LEO extends from the upper atmospheric fringes to about a thousand miles: 200-1,000 miles above mean sea level (MSL)
- - some are in normally inclined orbits, like the International Space Station and (when flying) the space shuttle. Manned missions are the heaviest, so don’t generally go higher than they have to. ISL & space shuttle orbit at about 250 miles above MSL. Space Shuttle is literally incapable of getting higher than 800 miles ASL
- - some are in polar orbits, such as the low-altitude high-resolution weather satellites NOAA operates. These orbits allow overflight of all (or nearly all) of Earth's surface, with greater or lesser frequency depending upon altitude and inclination.
- - a few are in retrograde, such as from Israel. Retrograde orbits are those going in directions opposite of Earth's rotation. One example is Israel, which has found it desirable to launch westward (over the long axis of the Mediterranean Sea) to avoid overflying hostile territory. Cost: orbital speed remains the same, but retrograde launches cannot use Earth's rotational velocity as a starting boost.
- - Orbital speeds. The speed a satellite needs depends on altitude. Whether at the equator or at considerable northern latitudes, Earth's speed is a huge advantage towards orbit-sustaining speed of 17,000mph (LEO) or 7,000mph (GEO) -- or even towards the 25,000mph Earth Escape Velocity (such as for missions to Mars). At the equator, the boost from Earth's rotation is about 1,000 miles/hour (mph). At 45 Degrees North (such as Twin Cities MN, Venice Italy, and Baikonur Cosmodrome Kazakhstan), orbital speed is still 'way up there', at about 700 mph. Conversely, Earth's speed is a considerable disadvantage when launching to orbital speeds in the retrograde direction. Incidentally, the Moon's orbital speed is a bit over 2,000mph; at a distance 10x GEO distance (above sea level), Moon moves at about one-third the GEO speed.
- GPS satellites, and now a few other satellites too, are in the ‘medium Earth orbit’ (MEO): 8,000-12,000 miles above MSL
- Most communications satellites, aside from Irridium, are geo-synchronous (GEO): 23,000 miles above MSL.
- - Most of the geosynchronous satellites are geostationary too: 23,000 miles above MSL and right over the equator. Arthur C. Clarke was the first scientist to postulate a satellite could ‘hang’ in one place in the sky like GEO sats do in “the Clarke belt”
- - The ‘weather satellite images’ we’re used to seeing come from the 5 international-agreement GEO satellites that are weather watchers, not comms satellites.
- - for those who have direct-to-home satellite TV, the Dish Network and DirecTV satellites are in the Clarke Belt.
2. The danger.
It’s only the LEO satellites that are at risk from debris of the Russian-Irridium collision. Likewise, from the Chinese debris.
But remember that the Russian-Irridium collission was a little below 500 miles altitude. Satellites above and below are in minimal danger. Debris bits flung above & below the altitude of the colliding satellites basically go into unstable orbits that quickly decay -- that debris re-enters Earth's atmosphere & burns up.
Nevertheless, for possibly the first time in the space age, there is an accumulation of debris that *may* have reached dangerous levels. Why? Earth has a ‘vacuum cleaner’, at least for LEO debris. Every 11 years, the Sun has a ‘solar maximum’, pumping its maximum amount of energy outwards, including to Earth. This energy collides with the Atmosphere, exciting molecules and causing Earth’s atmosphere to expand a bit. Atmospheric expansion functions to slow down functioning satellites & orbiting debris in a much faster timespan than satellites normally face.
Most people don’t realize that the International Space Station requires, even during Solar Minimums, periodic boostings back to a higher orbit. Reason: Earth’s tenuous atmosphere, even at 250 miles altitude ASL, slows ISS down, and so ISS continuously loses altitude in deference to gravity’s inexorable pull.
The “solar max vac”, in fact, provides periodic disappointments as favorite satellites’ orbits decay faster than desired. Solar maximums were responsible, along with budgetary decisions to not intervene, in bringing down Skylab (1979) a mere several years after launch. There had been plans for the Space Shuttle to visit Skylab, rehabilitate it, and shuttle crews to it. The country didn’t feel the treasure expended in Skylab was worth saving. Instead, we later had to start all over; penny-wise, pound-foolish. Likewise, the Russian nuclear-powered satellite (Cosmos 954, 1978) and MIR (2001) were de-orbited during solar maximums. MIR was intentional, to control it’s impact location. Cosmos 954 and Skylab was completely uncontrolled. They are only 3 examples of deorbits during ‘solar max’.
The last solar maximum was several years ago, and we are currently in a rather extended solar minimum that *may* now be showing signs of giving way to the start of the next solar cycle. Old Sol is just taking his time in starting the new solar cycle. Our ‘spacecraft junk vacuum cleaner’ won’t kick in again for at least a few more years – it all depends on the timing & intensity of the solar maximum.
3. Conclusion.
I suppose there are mathematicians & physicists out there who can quickly figure out the “volume of space & rarified atmosphere” (in multiple hundreds of square miles) between altitudes of 200 & 1,000 miles above MSL. I’ll leave it for now, though, with, “there’s a lot of space, in space”, even low earth orbit.
We have yet another ramification of “It’s the Sun, Stupid!”