Re: X-37B Orbit


Message posted by Griffon_314 on April 17, 2012 at 7:34:06 PST:

Another way to think of it:

Imagine you're sitting on a skateboard, getting towed behind a Ferrari sports car. The Ferrari gets you up to 150 mph, then you release the tow rope.

Now, as you're screaming along at high speed, you have an aerosol can in your pocket. You can point that aerosol can in any direction you like, and spray - creating some thrust.

You can increase or decrease your speed by a very tiny amount, by spraying with the can pointed ahead or behind, or even steer a tiny bit left or right by pointing the can to the side. But you've only got a limited amount of spray pressure in the can.

And there's no bloody way you're going to be able to "steer" your way through a set of cones in an obstacle course.

The ferrari is the Atlas booster, and the aerosol can is the maneuvering thrusters on the X-37B.

Practically speaking, low-earth orbit vehicles can adjust their orbit height (add some thrust at apogee to increase orbit altitude a little, apply counter-thrust at perigee to reduce orbit altitude a little, etc).

But a change in orbit inclination takes tremendous amounts of delta-v (acceleration - effectively thrust). Not even the shuttle, with it's massive engines, could effect much of an inclination change. It's just spitting in the wind compared to the thrust that got you into orbit.

When you hear about analysts estimating an orbital vehicle's mission based on its orbit, that doesn't mean the vehicle is "steering" over specific points on the earth...what they are looking at is the inclination of the orbit (which you can tell from the original launch track), it's apogee and perigee (optical recon sats often have elliptical orbits to provide opportunities to take "close up" images at perigee), and to what extent the orbit is sun-synchronized (optical sats obviously benefit from synchronizing their orbits with daylight on earth, as much as possible, whereas radar sats don't need daylight).

They can also deduce the type of sensor by other means sometimes - amateur telescopes are starting to become powerful enough to resolve the shape of low-earth satellites, and if the satellite orbits with one or more "partner" vehicles in close formation, we know it's likely performing triangulation of radio signals, etc. The US NOSS formations are famously good targets for amateur observing, as triads or pairs of vehicles easily visible to the naked eye. They triangulate the position of ships at sea from their radio emissions.

Even the famous KH-11 series of US optical recon satellites, which do maneuver from time to time, don't do so very often...and when they do, it is usually to boost their orbit back up after it has been lowered too much by drag (there is drag at low-earth orbit altitudes - not much, but enough to cause satellites orbits to decay over time).

The one exception to "satellites don't steer over specific points on the earth" is geosynchronous orbits, which match their orbital period to the earth's rotation...these orbits effectively "park" their payload over a specific spot on the earth. But they cannot easily move from one spot to another - at least, not very quickly. And these orbits are *much* higher - 22,000 miles instead of the < 1000 mile altitudes of low-earth satellites like the X-37B. I've attached an image that illustrates this pretty well.

Geosynchronous orbits are good for some types of classified missions - eavesdropping on radio and cellular traffic, for instance - but lousy for most classified imaging missions (optical, radar, etc).

I hope this helps explain why "hey I heard this classified sat is steering over Iran" type posts, are silly.

Attached link: Diagram illustrating orbital altitudes

In Reply to: Re: X-37B Orbit posted by Brian L on April 16, 2012 at 11:58:55 PST:

Replies:



[ Discussion Forum Index ] [ FAQ ]