3 ms·

Satellites are pretty much bound to their orbit once they are launched. They fire their engines only to restore speed (and thus height), and to control rotation

by andor 12y ago

Satellites are pretty much bound to their orbit once they are launched. They fire their engines only to restore speed (and thus height), and to control rotation. Their speed is such that centrifugal force is in equilibrium with gravity, and most of the kinetic energy is from launching the satellite (with a large rocket). They don't carry enough fuel for arbitrary maneuvers a la Star Trek.

ckozlowski 12y ago
I disagree. Certainly, there's no "arbitrary" maneuvers, "set course", etc. But surveillance satellites will carry maneuvering propellant for a number of reasons: - Station keeping (as you listed) to maintain altitude. LEO satellites will be affected by atmospheric drag. - Assignments - Radar and imagery satellites cannot view everything via a change in rotation alone, and orbital adjustments can be made to the inclination and eccentricity in order to produce an ground track over a target that it didn't follow before. the importance of the target must justify the expenditure in fuel. - Defense - ASATs are a real concern, consisting either of missiles or another satellite steered onto an intercept. Placing one object deliberately onto the path of another is hard enough, surveillance and military satellites will carry propulsion to perform evasion (again, eccentricity, inclination, or altitude adjustments) and mess up a solution. There are no satellites (to my knowladge) that are carrying anything approaching the delta-v required to break orbit, but radical changes in orbit are feasible, and especially for surveillance and reconnaissance satellites, essential.
mikeash 12y ago
That's not really true. Most satellites will only carry out those maneuvers because anything else is expensive and unnecessary. Plane changes are generally pointless for a civilian satellite because they're launched into the appropriate plane to begin with. Spy satellites are different because the nature of the mission means you can't really guarantee you'll pick the appropriate plane in advance. If something interesting starts happening somewhere in the world, you may want satellites over it sooner than would occur in their current orbits. Spy satellites can and do maneuver to accomplish this. The amount of maneuvering is pretty limited by basic physics and the size of the satellites, but it is done. Here's a page that discusses one: http://fas.org/spp/military/program/imint/kh-12.htm http://fas.org/spp/military/program/imint/kh-12.htm "The primary difference between the 28,000 pound KH-11 and the heavier KH-12 is that the additional amount of maneuvering propellant carried on the IMPROVED CRYSTAL. This additional propellant can be used to prolong the operating life of the IMPROVED CRYSTAL, to maneuver to improve coverage of areas on the Earth of particular interest, and to maneuver to evade Soviet anti-satellite interceptors." A quick calculation: taking the numbers in the article to mean there's 7 tons of fuel on board and 11 tons of non-fuel stuff, assuming the fuel is hydrazine giving a specific impulse of about 220, that gives the satellite a delta-v of about 1km/s, which is fairly substantial.
- darkmighty 12y ago
  A quick back of the envolope calculation tells me a plane change of D degrees requires a delta-v of D/60*v for small D, where v is orbital velocity. Not that much.
  - mikeash 12y ago
    So 1000m/s would get you about 7-8 degrees of plane change in low orbit? Sounds about right. That's not a whole lot, but it should be plenty to dodge sky watchers for a while if you have the ability to hide for a bit and change course while doing it.

by andor 12y ago

ckozlowski 12y ago

I disagree. Certainly, there's no "arbitrary" maneuvers, "set course", etc. But surveillance satellites will carry maneuvering propellant for a number of reasons: - Station keeping (as you listed) to maintain altitude. LEO satellites will be affected by atmospheric drag. - Assignments - Radar and imagery satellites cannot view everything via a change in rotation alone, and orbital adjustments can be made to the inclination and eccentricity in order to produce an ground track over a target that it didn't follow before. the importance of the target must justify the expenditure in fuel. - Defense - ASATs are a real concern, consisting either of missiles or another satellite steered onto an intercept. Placing one object deliberately onto the path of another is hard enough, surveillance and military satellites will carry propulsion to perform evasion (again, eccentricity, inclination, or altitude adjustments) and mess up a solution. There are no satellites (to my knowladge) that are carrying anything approaching the delta-v required to break orbit, but radical changes in orbit are feasible, and especially for surveillance and reconnaissance satellites, essential.

mikeash 12y ago

That's not really true. Most satellites will only carry out those maneuvers because anything else is expensive and unnecessary. Plane changes are generally pointless for a civilian satellite because they're launched into the appropriate plane to begin with. Spy satellites are different because the nature of the mission means you can't really guarantee you'll pick the appropriate plane in advance. If something interesting starts happening somewhere in the world, you may want satellites over it sooner than would occur in their current orbits. Spy satellites can and do maneuver to accomplish this. The amount of maneuvering is pretty limited by basic physics and the size of the satellites, but it is done. Here's a page that discusses one: http://fas.org/spp/military/program/imint/kh-12.htm http://fas.org/spp/military/program/imint/kh-12.htm "The primary difference between the 28,000 pound KH-11 and the heavier KH-12 is that the additional amount of maneuvering propellant carried on the IMPROVED CRYSTAL. This additional propellant can be used to prolong the operating life of the IMPROVED CRYSTAL, to maneuver to improve coverage of areas on the Earth of particular interest, and to maneuver to evade Soviet anti-satellite interceptors." A quick calculation: taking the numbers in the article to mean there's 7 tons of fuel on board and 11 tons of non-fuel stuff, assuming the fuel is hydrazine giving a specific impulse of about 220, that gives the satellite a delta-v of about 1km/s, which is fairly substantial.

darkmighty 12y ago
A quick back of the envolope calculation tells me a plane change of D degrees requires a delta-v of D/60*v for small D, where v is orbital velocity. Not that much.
- mikeash 12y ago
  So 1000m/s would get you about 7-8 degrees of plane change in low orbit? Sounds about right. That's not a whole lot, but it should be plenty to dodge sky watchers for a while if you have the ability to hide for a bit and change course while doing it.