Satellites Collide in Space

[Marshall]

A privately owned U.S. communications satellite collided with a defunct Russian satellite in the first such collision in space, a U.S. military spokesman said on Wednesday.

The collision, which took place on Tuesday in low-earth orbit, involved a spacecraft of privately owned Iridium Satellite LLC and a "non-operational" Russian communications satellite, said Air Force Lieutenant Colonel Les Kodlick of the U.S. Strategic Command.

"We believe it's the first time that two satellites have collided in orbit," he said.

Those damn Rooskies!
This must be something like two bullets fired from opposite ends of a football field colliding. I smell conspiracy.
news.yahoo.com/s/nm/20090211/ts_nm/us_space_collision_2

[tarrant]

Sounds like incompetance on the part of Iridium.

If the Russian satellite is defunct, it is almost certainly quite old. It's orbit will be entirely predictable.

Iridium should have comensated for it.

[Charles Martel]

You beat me to posting it here, Marshall!

Yeah...given how big space is, and (relatively speaking) how few objects humans have sent up there, it's quite surprising to hear about a collision.

[avieder]

Can somebody explain why a certain altitude (780 Km) is "preferable"?
From physics I studied a few decades ago, the altitude is the result of the initial speed and the mass of the satalite. Therefore there should be infinite number of orbit radii.
Add to that the fact that the direction of rotation could be at any angle and that the place of the satalite in its orbit could be anywhere you get 1/infinity chances of collision.

[avieder]

Can somebody explain why a certain altitude (780 Km) is "preferable"?
From physics I studied a few decades ago, the altitude is the result of the initial speed and the mass of the satalite. Therefore there should be infinite number of orbit radii.
Add to that the fact that the direction of rotation could be at any angle and that the place of the satalite in its orbit could be anywhere you get 1/infinity chances of collision.

[Marshall]

Certain altitudes are preferable for geosynchronous orbit (meaning the satellite rotates with the earth and so always stays above the same point).

I'm not sure how Iridium should have compensated - these don't normally have boosters, do they?

[Charles Martel]

Geosynchronous orbit (if above the earth's equator) has the advantage of pointing at the same spot all the time, but a geostationary satellite has to be some 40,000 km above the center of the earth, which is nearly seven times the earth's radius. This could be a disadvantage because in the far field (see Friis' Equation), the amount of power you need to transmit to obtain a fixed power density at the earth's surface goes up as the square of distance (just as the surface area of the enclosing sphere). So a lower altitude is preferred for communications satellites. Secondly, less energy is required to place a satellite at a lower orbit (analogous to the potential energy of an electron orbiting an atomic nucleus).

[Charles Martel]

Feb 12, 2009 2:03:32 GMT Marshall said:

Certain altitudes are preferable for geosynchronous orbit (meaning the satellite rotates with the earth and so always stays above the same point).

I'm not sure how Iridium should have compensated - these don't normally have boosters, do they?

It depends on how the satellite was designed. Generally, once you parked a satellite in orbit, that's it, gravity keeps it in place. If you want to move it around, you need to provide thrust in the form of small rocket motors.

[Charles Martel]

Feb 12, 2009 2:02:20 GMT avieder said:

Can somebody explain why a certain altitude (780 Km) is "preferable"?
From physics I studied a few decades ago, the altitude is the result of the initial speed and the mass of the satalite. Therefore there should be infinite number of orbit radii.
Add to that the fact that the direction of rotation could be at any angle and that the place of the satalite in its orbit could be anywhere you get 1/infinity chances of collision.

Yes, the orbit could be at any altitude above the surface (with the caveat you want to be well above the atmosphere), so long as the plane of orbit passes through the center of the earth. If two satellites are at the same altitude (assumed to be circular) but rotating at different inclinations with respect to the equator, their paths will intersect. But they still won't hit each other because they go around the earth with the same period. In practice, an orbit will always be somewhat elliptical. So that would be a mechanism for collision. If a satellite was changing orbits, then collisions become more likely. Still, for two satellites to hit each other is like two people firing different rifles, and then discovering their bullets have collided. It's such an unlikely event!

[flatandy]

Feb 12, 2009 6:27:11 GMT Charles Martel said:

Geosynchronous orbit (if above the earth's equator) has the advantage of pointing at the same spot all the time, but a geostationary satellite has to be some 40,000 km above the center of the earth, which is nearly seven times the earth's radius. This could be a disadvantage because in the far field (see Friis' Equation), the amount of power you need to transmit to obtain a fixed power density at the earth's surface goes up as the square of distance (just as the surface area of the enclosing sphere). So a lower altitude is preferred for communications satellites. Secondly, less energy is required to place a satellite at a lower orbit (analogous to the potential energy of an electron orbiting an atomic nucleus).

That's geostationary, dude, not geosynchronous.

Geosynchronous orbits are judt ones where the orbit repeats.

[tarrant]

It's occured to me that the iridium satallite sounds suspiciously like some sort of spy satallite.

In which case, the Russians probably took steps to destroy it and who can blame them?