The ISS and Orbital Decay

[sammoe]

I was reading on Wikipedia about the American Skylab and how it reentered over Australia (I think) due to orbital decay. First question: how is it possible for something in a vacuum to slow down and fall back to Earth while in a stable orbit? Am I reading the article on Wikipedia wrong? Second question: could this happen to the ISS? Isn't it to high up for there to be any chance of its apogee/perigee dipping into the atmosphere?

-Thanks

[Kerbface]

I was reading on Wikipedia about the American Skylab and how it reentered over Australia (I think) due to orbital decay. First question: how is it possible for something in a vacuum to slow down and fall back to Earth while in a stable orbit? Am I reading the article on Wikipedia wrong? Second question: could this happen to the ISS? Isn't it to high up for there to be any chance of its apogee/perigee dipping into the atmosphere?

-Thanks

Pretty sure the reason for the decay is that it IS in the atmosphere. High up in a low density part of it, sure, but it still has some atmospheric drag. It needs to be boosted now and then to stop it from reentering.

[stibbons]

I was reading on Wikipedia about the American Skylab and how it reentered over Australia (I think)

Yep. Fell over Western Australia. The Shire of Esperence, which copped the majority of the debris, famously issued NASA a $400 fine for littering.

First question: how is it possible for something in a vacuum to slow down and fall back to Earth while in a stable orbit?

As Kerbface said, it isn't in complete vacuum, there's still a small amount of atmosphere.

Second question: could this happen to the ISS? Isn't it to high up for there to be any chance of its apogee/perigee dipping into the atmosphere?

It is happening to the ISS - it regularly has to make adjustment burns. But an active station has a lot more to worry about than just air friction. Vibrations from machinery and momentum of people moving around, as well as docking maneuvers, can affect orbits as well.

EDIT: I don't actually have a citation for saying that vibration affects the orbit. It came from some Cmdr Hadfield tweets talking about the vibration isolation treadmill, but he might have only been talking about disturbing experiments on board. So, uh, sorry if I was wrong.

Edited June 20, 2013 by stibbons

[Nibb31]

First question: how is it possible for something in a vacuum to slow down and fall back to Earth while in a stable orbit? Am I reading the article on Wikipedia wrong?

The same happens to most LEO satellites after a while. There are still small particles of atmosphere in LEO which are going to cause some drag. The density of the upper atmosphere varies with solar activity, but it will be enough to pull you down after a while.

The other factor is gravity variations. Gravity is not uniform around the Earth like it is in KSP. The Moon also introduces a bit of instability. This means that orbits are going to vary slightly and that you need some sort of station-keeping to maintain orbit over long periods.

Note that the extent of the decay phenomenon wasn't well known at the time of Skylab, which is why NASA thought that Skylab would remain on orbit until an early Shuttle flight could reboost it. As you know, it reentered before the Shuttle could visit it, if that capability was ever actually maintained.

Second question: could this happen to the ISS? Isn't it to high up for there to be any chance of its apogee/perigee dipping into the atmosphere?

It would definitely happen if the ISS was left unattended. This is why reboost burns are done on a regular basis to raise the orbit of the ISS. These reboosts are usually performed by a visiting Progress or ATV.

Note that the orbit was raised quite a lot after the last Shuttle flight. The reason for its low altitude was mainly so that the Shuttles could reach it. A higher altitude minimizes reboost requirements, which saves fuel.

Edited June 20, 2013 by Nibb31

[Bunsen]

You can see the ISS's orbital decay and re-boosts here. The ISS was built in a low orbit to make it accessible with a minimum of fuel (or rather to maximize the payload capacity of vehicles with a fixed amount of fuel), but that also means that it's in relatively thick atmosphere for a satellite. Without those periodic burns, it would deorbit within a few years.

[PakledHostage]

I had read elsewhere that the ISS actually reorients its solar arrays to minimise aerodynamic drag while on the night side of the Earth. A quick bit of online research (a.k.a. Google search) reveals that the procedure is called "Night Glider Mode". Supposedly this reduces the average drag on the station's solar arrays by 30% and saves about 1,000 kg of orbital-maintenance propellant per year.

[DerekL1963]

Note that the extent of the decay phenomenon wasn't well known at the time of Skylab, which is why NASA thought that Skylab would remain on orbit until an early Shuttle flight could reboost it.

Nit: The decay phenomenon was quite well understood. What brought Skylab down was greater than predicted solar activity, which heated and expanded Earth's outer atmosphere.

[Scotius]

First space-worthy VASIMR engine will be (is supposed to) mounted on ISS to help with station keeping. ISS solar panels do not produce enough electricity for 200kW engine, but station does produce its own hydrogen For now it's vented to space, in the future it will be used as a propellant for plasma drive

[Tommygun]

I had read elsewhere that the ISS actually reorients its solar arrays to minimise aerodynamic drag while on the night side of the Earth. A quick bit of online research (a.k.a. Google search) reveals that the procedure is called "Night Glider Mode". Supposedly this reduces the average drag on the station's solar arrays by 30% and saves about 1,000 kg of orbital-maintenance propellant per year.

That's interesting, well if they are paying $10,000 per Kilo to launch that propellant, I guess they are saving about ten million a year, depending on how much it is to use a Progress.

[jwenting]

Nit: The decay phenomenon was quite well understood. What brought Skylab down was greater than predicted solar activity, which heated and expanded Earth's outer atmosphere.

And put pressure on the solar array and station skin, pushing it down ever so slowly (yes, light and solar wind do impart momentum, the principle behind solar sails, now THAT would be an interesting thing to investigate, does KSP model light pressure...).

[Kryten]

...no, it just expanded the extreme upper atmosphere through increased temperature. With something the mass-to-surface-area ratio of skylab, light pressure is negligible.

[GJames]

What I wonder is if they'll de-orbit the ISS once it's "life" is over, or if they'll boost it's altitude into a more permanently stable orbit. As much as I love de-orbiting stuff in KSP, having a load of semi-usable equipment in space could be useful in an emergency, but the station would have to be drained of fuel and other volatile liquids to make sure any tanks don't explode...

[Nibb31]

The plan is to deorbit the ISS at its end of life. Boosting it to a higher orbit will only postpone the day when it comes down uncontrollably, and to keep it on orbit leaves it at the risk of being hit by MMOD, which would create even more debris.

[GJames]

The plan is to deorbit the ISS at its end of life. Boosting it to a higher orbit will only postpone the day when it comes down uncontrollably, and to keep it on orbit leaves it at the risk of being hit by MMOD, which would create even more debris.

Not looking forward to that day...

[Nibb31]

This PDF provides a brief study of the possible options for deorbiting the ISS:

http://www.nasa.gov/pdf/578543main_asap_eol_plan_2010_101020.pdf

[DerekL1963]

As much as I love de-orbiting stuff in KSP, having a load of semi-usable equipment in space could be useful in an emergency

Not really... keeping the ISS around for an 'emergency' is like living in New York and stockpiling worn out parts for your car at the South Pole. Not only are plane changes expensive, the ISS is in an orbit that's not terribly useful for anything except being accessible to the Russians. The Russians can only launch to so low an inclination without dropping stages on China (obviously very undesirable) or using very expensive (in terms of Delta-V) dogleg orbits. So, ultimately, the orbit was chosen based on the (relatively low) performance of the Soyuz booster carrying a Soyuz capsule or Progress vehicle.

Incidentally, this is what caused the costs of the US side of the station to balloon and the capabilities to be slashed... that orbit is very difficult for the Shuttle to reach, and slashes it's cargo capacity by nearly half. So not only did the station have to be redesigned (for the umpteenth time*), the assembly schedule had to be reworked and spread out over a greater number of Shuttle flights. Some modules that formerly were to be launched fully kitted out had to be launched partially stripped and later outfitted via MPLM flights... (The MPLM itself has an interesting history... it was originally the "Mini-Pressurized Logistics Module", and something of a backup that could be carried on crew exchange flights. When the Russians came onboard and orbit was changed, it was quietly promoted to a prime role as the Multi-Purpose Logistics Module.)

This is one of the many reasons more thoughtful members of the space community are seriously skeptical of calls for international cooperation - there's very little evidence that it makes things simpler, or cheaper, or any other positive outcome.

*It was this particular change that caused a friend of mine to quit his job developing and designing the station's ECLSS and electrical systems and go to law school... He's now a small town lawyer outside of Nashville, and much *much* happier.