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Comment by harperlee | Hacker News Reader

I was wondering if it really was a good idea to spread so many little things in orbit, and was subsequently very happy to read this in the paper linked by jimsojim:

    Due to their extremely low ballistic coefficient, 
    the Sprites are Expected to remain in orbit for only 
    a few days before reentering and burning up in the 
    atmosphere, alleviating debris concerns.

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This is something that is designed into pretty much all cubesat/nanosat missions, for exactly this concern.

Even the ones deployed from ISS (at around 400km altitude) have decaying orbits. I believe the hard rule is no greater than a 25-year expected life before re-entry.

It's amazing to think that the long-term plan for something is for it to move slower and slower around the earth and then just burn up in the atmosphere.

Really elegant design pattern that is highly reliable (gravity) as long as these things don't get pushed out into space accidentally (which I'm guessing is highly unlikely and would take a sustained force).

EDIT: but now I'm wondering about all of the space 'trash' I've read about. Why doesn't that stuff descend back into the atmosphere also? Or does it and it just takes a while so things get a bit crowded up there?

Now I barely know much about this, but I recall reading something that the space debris isn't actually that big of a deal. Since most satellites go in one direction (with the exception of israel, which has to launch retrograde orbits for political reasons), the actual speed delta between any 2 objects which are close to each other is fairly small. You really only need a bit of forethought about how to safely "deorbit" your crafts, and some working together to avoid trying to put 2 things in the same spot.

Combine that with the fact that space is still REALLY big (the surface area of any single orbit is still larger than the surface area of the earth), means that it's not as much of a problem as I thought it would be.

I don't mean to downplay it, it's still rocket science after all, but I think the worry of a massive wall of debris which makes it impossible for any craft to get past is closer to science fiction than it is to real life.

Interesting.

Just looked up why Israel launches them retrograde (counter to the Earth's rotation which costs more energy to launch). It's so launch debris doesn't land in populated areas, it lands in the unpopulated Mediterranean instead. Make sense.

Also these fully orbit the earth in 90 minutes, so theoretically you could get more time-resolution out of retrograde orbiting satellites (though I'm guessing there are other benefits of going with the Earth's rotation).

The energy requirement is the massive reason why launches are primarily prograde.

Even the time resolution you mentioned isn't much of and advantage of retrograde because a prograde satellite can get the same period at a lower altitude.

Certain classes of satellite (like remote sensing) use sun synchronous orbits which are slightly retograde. It allows the orbit to precess slightly to keep tracking the sun as the earth rotates on its axis.

Orbital debris do have decaying orbits, but they can take quite a while depending on altitude (with estimates for a existing systems in LEO anywhere from several months to several centuries):

http://www.nasa.gov/news/debris_faq.html

> I was wondering if it really was a good idea to spread so many little things in orbit

Something I've been wondering: why are all small cubesats designed as stand-alone satellites that are detached from a launcher? Why not instead build one big satellite, with shared solar panels and a deorbit engine, and sell "shelf space" for numerous small scientific modules that remain attached to it?

https://www.astrobotic.com/ is doing it for the moon.

Most of the small sat stuff isn't for science work. Commercial entities want to reorient on demand, manage their own coms and antenna direction, etc.

What you describe is a good idea, and how most older sats work (just not modularized).

Even with this rapid decay, I'd still be concerned about the probability of a collision with something - depends how congested that part of LEO is

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