At what speed can satellites cross the sky?

Last weekend, before the Perseids, in a cloudless night, my friends and I saw a bright star-like object cross the sky. It took about three minutes from horizon to horizon.

I live in part of the flight path for Heathrow airport, so my buddies were claiming that it was just a plane. However, there were planes in the sky as well that night, and they were clearly planes - red flashing lights that altered in brightness, sometimes landing lights on, and jet noise. This thing was completely silent and did not flash nor dim from one part of the sky to another.

I’ve seen satellites before, including Skylab, but they were all considerably slower. Haven’t been able to google anything, so I’m wondering how fast a satellite could cross the sky, as far as a ground-based observer would be concerned?

I once saw what I took to be a satellite but which could have been an *extremely * high-flying plane cross from horizon to horizon in about that length of time. Never did find out what it was. It was just before sunset, and the sky was still mostly blue, so I never did convince myself it was a satellite.

The Space Shuttle has an orbital altitude of between 150 and 200 miles and orbits the earth in about 90 minutes.

For a ground based observer, without my handy calculator, I would think it would cross the sky in 3-4 mins.

I’m not suggesting for a minute that what you saw was the Shuttle but it’s quite possible you saw an earth launched satellite.

Try for the locations, times, durations etc of satellites appearing above your location.

This page states that the International Space Station takes between 1 and 4 minutes to traverse any bit of sky. If you mean this weekend just past that’s also going to have the Shuttle docked, so will be especially noticeable. I’m guessing that the ISS is in about as low an orbit as anything up there, and so will be the fastest across the sky.

It would be really cool if that’s what it was. I would estimate that we saw it between 10pm and 11pm GMT+1 on August 6th. I haven’t been able to work anything out from bcoz I am 2 stoopdi.

I had no idea they would go past so quickly.

This is the visibility of it on August 6th from my location in Scotland:

Mag : Start : Highest : End

-1.5 : 22:57:52 : 23:00:30 : 23:01:50

Given that’s me looking south towards you, I’m betting that’s what you saw. A magnitude of -1.5 is very bright too.

Ooh. I’m excited. I’m camping this weekend in the wilderness. Looks like the ISS will be passing almost overhead on Sunday night. Twice.

Hopefully the shuttle will be docked to it.

jjimm, I noticed that you are in Oxford, so here hopefully is a link to where you were at. Pretty much the same times, but even brighter.

ETA, Ooooh and here is the ground tracki at the time.

What an excellent, excellent website!

I think satellites move at a velocity of Mach 20 to Mach 25, depending on the orbital altitude.

Aren’t satellites well outside of the atmosphere? How does a Mach number make sense?

Well good grief Struan, this is pretty much the direction it travelled too. I now think there’s a fairly high likelihood that that was it. Fantastic!

Keep in mind that the Shuttle/ISS do not produce their own light (well not enough to be visible from the ground) and so are only visible if they pass overhead near enough to the dawn or dusk to reflect light from the sun.

Did I just get wooshed or is that like asking, “How does a miles per hour number make sense, we use kilometers here?”

Isn’t a Mach number just a way of expressing speed? The way AUs express distance even if you’re not measuring that distance right here in our solar system?

It’s just a way of expressing speed relative to the speed of sound in a fluid media.
Mach numbers become useless in vacuum, where there is no media for sound propagation.

Yes, but it’s relative to the speed of sound in the medium through which you’re travelling. Mach 1 at sea level is actually considerably faster than Mach 1 at several thousand feet, because sound travels faster in the warmer, higher pressure air conditions at sea level.

So the reason “Mach 1” is meaningless in space is because sound doesn’t travel through a vacuum, so there’s no such thing as the speed of sound there, and thus no way to measure your velocity relative to it.

Preview: aw, pipped to the post.

Well, okay then. Ignorance fought. :slight_smile:

Just under 4 min. if it were near perigee and 4.5 min. near apogee

jjimm, if you were near Oxford at the time, check this link for a list of recent visible passes of the ISS. If you remember when you saw it, see if any of those date/time combos matches up.

There’s nothing that says for sure that you saw the space station, but since you said it was bright, it’s a likely candidate. You can see satellites nearly all the time in the sky if you’re looking in the right place, Look up on a clear night for five minutes and watch for motion, you’re bound to see one or two. But most satellites appear no brighter than a average-to-dim star. But they do typically traverse the sky over the course of a few minutes.

However, if what you saw was as bright as the brightest stars in the sky, or even as bright as Jupiter (which is very prominent right now between sunset and midnight, and the brightest thing in the night sky other than the moon around that time), the ISS is one of the only “moving” things that would be a candidate. So I’ll guess that’s what you saw.

It could have also been the Space Shuttle if it was since August 8. But Heavens Above’s past dates for Shuttle passes are wacky, because they’re calculated from current orbital parameters. So it shows visible passes before the Shuttle even launched, and they always match up with ISS passes since they’re currently docked.

ETA: I shoudl also mention that you can see from those pass predictions on HA how long the ISS remains visible. And you can also use the altitude and azimuth numbers to figure out where it starts and ends. From those, you can see that it typically takes about 5 minutes to make it from one side of the sky to the other.

I’ve always thought that it would have been really cool if the Wright Brothers could have got their flier up to sub-orbital altitude. At their paltry 30mph, they could have simultaneously been the first to fly and also break the sound barrier.

Of course there are other problems in the way of pulling that off :slight_smile:

Don’t sweat it. When briefing Air Force officers on the speed of certain space objects, I was routinely asked to characterize those objects’ velocities in Mach numbers, because the officers were familiar with that speed scale. I got into a science argument with the first one who asked me that, but the next time I briefed I had an answer. Something like “25 times the speed of sound at sea level, but at its own altitude more like Mach 20. Remember, though, that at these altitudes Mach numbers are almost meaningless.”