A question about that falling satellite

[J-P L]

I did not want to highjack his thread http://boards.straightdope.com/sdmb/showthread.php?t=625059

Does any one know how that satellite is expected to come down whereever it hits.

Will it be basically coming straight down or more say at some angle where it would skid on the ground some distance, that is obviously assuming that it hits on land.

[Boyo Jim]

It will not fall straight down as it is traveling nearly horizontally at thousands of miles per hour, It is extremely unlikely to land in one piece. It will most likely break up into hundreds or thousands of small pieces, most of which will vaporize as they heat up while re-entering the atmosphere. NASA usually has a pretty decent handle on which parts are most likely to survive -- the largest and heaviest components that are not likely to completely melt down.

[Mangosteen]

I did not want to highjack his thread http://boards.straightdope.com/sdmb/showthread.php?t=625059

Does any one know how that satellite is expected to come down whereever it hits.

Will it be basically coming straight down or more say at some angle where it would skid on the ground some distance, that is obviously assuming that it hits on land.

I was on a beach one night in the southern Philippines years ago and saw one come down. It was low enough that I could see its rotation. It was coming down only at a very slight angle. It might have actually hit the water 100 miles from where I was.

The island I was on didn't have electricity let alone a newspaper or local radio station, so I never heard anything about it. The fisherman (who fish only at night) were pretty excited the next day saying that they had never seen anything like it.

[Carl Pham]

It depends a bit on the scale on which you are looking. If you're thinking of, say, the satellite before it disintegrates, or its center of mass, then, yes, this hits the ground at a shallow angle, because the satellite is traveling downward fairly slowly, no more than a few hundred miles per hour, while its horizontal velocity is very substantial, starting off at 15,000+ miles per hour, and slowing considerably as it enters the atmosphere.

On the other hand, the satellite is expected to break up, and each individual piece will pursue its own trajectory. The pieces will likely initially have very similar horizontal and vertical velocities as the center of mass. However, it is likely that, particularly for the smaller pieces, the horizontal velocity will decrease much faster than the vertical velocity (because air resistance grows as a strong power of velocity, e.g. the air resistance at 5,000 MPH is much more than 5 times as much as the air resistance at 1,000 MPH, probably 25-200 times as much).

If the horizontal velocity is shrinking faster than the vertical velocity, then the angle of the fall relative to the ground will steepen. If there is enough time, meaning the piece is small enough, then the horizontal velocity will disappear entirely, eliminated by air resistance. The vertical velocity remains of course, since it is being sustained by gravity, and in this case the piece will finally hit the ground vertically.

In short, the angle at which pieces hit the ground depends generally on their size, and the smaller the pieces, the more vertical the impact.

In case you're wondering why air resistance acts more strongly on the smaller pieces, the reason is that the magnitude of the air resistance force goes as the surface area of the piece, which goes like the diameter d square, d^2. However, the mass of the piece goes like the volume, i.e. the diameter cubed, d^3.

Newton's law saws the acceleration a = F/m, or in this case like d^2/d^3 = d^-1. That is, the deceleration of the piece due to air resistance is inversely proportional to the size of the object: smaller objects decelerate faster.

[StrangerThanFiction]

Awesome post/username combo - need answer fast?

[Yllaria]

The last NASA article I read on it said they expected about half a ton (out of six tons) to make it to the ground in various sized fragments.

[Chronos]

(because air resistance grows as a strong power of velocity, e.g. the air resistance at 5,000 MPH is much more than 5 times as much as the air resistance at 1,000 MPH, probably 25-200 times as much).Air resistance scales as the velocity squared, so the 5000 MPH fragment will have 25 times as much drag as the 1000 MPH one, all else being equal.

[mutantmoose]

Can I sue NASA if it lands on my head?

[Zakalwe]

Can I sue NASA if it lands on my head?Actually, I think that in this country dead people can't sue anybody.

[Rysdad]

I wonder if there are any of those Skylab Warning Helmets still laying around?

[automagic]

According to NASA

http://www.nasa.gov/mission_pages/uars/index.html

it will fall somewhere between Canada and Canada. Apparently its orbit has it going over Canada, The Atlantic, Africa, The Indian Ocean, Australia, the Pacific and back over Canada. I tried visualizing it and my brain started to smoke so I opened Google Earth and gave it a spin, which gave me hope that I might see it from So Cal. It should be falling anywhere from the next 2 hours to 5 hours so I will be pulling out the lawn chair soon.

[ZenBeam]

If you look at the bottom of this page (http://www.bbc.co.uk/news/science-environment-15034073), they show orbital tracks for the "possible reentry area", but they've had the same tracks for days now, as the expected reentry area has varied around.

Does anyone have a site with the satellite's ground track Vs. time?

[Darth Panda]

Can I sue NASA if it lands on my head?

I'm assuming that if your head injured their satellite, they could sue you.

[Colophon]

If you look at the bottom of this page (http://www.bbc.co.uk/news/science-environment-15034073), they show orbital tracks for the "possible reentry area", but they've had the same tracks for days now, as the expected reentry area has varied around.

Does anyone have a site with the satellite's ground track Vs. time?
Not quite up to date but try here: http://www.n2yo.com/

I did read rumours that it had crashed already, a couple of hours ago, over Saskatchewan, which fits with that track, but that is so far just internet hearsay.

[Colophon]

Or there's this (http://www.ustream.tv/channel/theweatherspace-news) which claims to be live, but I can't confirm...

[Boyo Jim]

The Washington Post is quoting NASA (http://www.washingtonpost.com/national/health-science/nasa-satellite-debris-might-hit-us/2011/09/23/gIQA5VmiqK_story.html?hpid=z2&wpisrc=al_national) as saying it's down, though they're not sure where yet.

[Leaffan]

So, where did it come down?

[ZenBeam]

North Pacific, apparently. But that's not important now...[/Leslie Nielsen]

I was thinking about this today. One of the links above, or maybe from one of the articles, had a plot of the apogee and perigee with respect to time. The perigee was much lower than the apogee after the maneuver to make its orbit decay. It took something like a couple years (from memory) after the maneuver for the satellite to come down. The apogee and perigee both decreased over that time.

Anyway, wouldn't the Latitude of the perigee stay pretty much fixed as the orbit decayed? And then wouldn't the satellite be most likely to come down near or shortly following perigee, so that the probability of where it would come down wouldn't be random, but peak near the latitude of the perigee following the de-orbit maneuver?

[Boyo Jim]

I don't think there was a de-orbit maneuver. As I read it, the satellite has been drifting uncontrolled for some weeks now, and that's the main reason the crash was so unpredictable. No one could tell the exact time or place where there would finally be enough friction to, er, derail, the orbital path.

[ZenBeam]

I don't think there was a de-orbit maneuver.Found it (http://planetary.org/blog/article/00003192/). The "maneuver to disposal orbit" was way back in December 2005.

The apogee and perigee stay separate in that plot, but it was becoming more circular. Maybe it does circularize before final descent.

[Boyo Jim]

That must have been the worst de-orbit maneuver in the history of the world.

[Jack Dash]

I understood that if re-entry is at the wrong angle, objects could easily bounce off especially if travelling at speed which I assume all satellites do. How come these things tend to enter without too much trouble?

And if the chances of hitting someone is 32,000 to 1, is the chances of hitting a particular individual then (6 billion x 32,000) to 1?

[standingwave]

North Pacific, apparently. But that's not important now...[/Leslie Nielsen]So bummed that it couldn't have stayed up for another half hour or so because it was supposed to have been visible from our location in WA. We had azimuth and elevation. We had cameras ready. And nothing. :confused: At first we thought that perhaps we had incorrect ephemeris data but then we heard that it had reentered over the Pacific. Ah well, c'est la vie.