Starship Asterisk*

In today's APOD (http://antwrp.gsfc.nasa.gov/apod/ap081016.html), it shows TIROS2 and the ISS. I am trying to understand why the TIROS2 weather satellite has not fallen to earth in the 48 years it has been in space. It is my understanding that eventually orbits decay (at least ones short of geosynchronous) and these things fall back to earth. Or at least that is the case if there is nothing to keep it in orbit (like some kind of on-board propulsion) I am remembering spacelab and other satellites that have burnt up in the atmosphere.

If you could help me clear up my confusion, I would greatly appreciate it.

Thanks!
--Tom

In theory, once an object achieves orbit, it should stay there forever. In practice, molecules of Earth's atmosphere slow down satellites just enough for them to lose orbital velocity and fall to Earth. These molecules extend hundreds of miles into space, but at some point (I don't know how high) space is enough of a vacuum for decay not to occur. The ISS is within this decay limit, so periodically it gets a boost from a Progress ship or the Shuttle to raise its orbit to a safe height.

Seeing the image of TIROS 2 really takes me back. I remember when newspapers printed these ghostly, obscure, black and white images of storms. Only the experts could decipher them, but we were all amazed at the feat.

Yes, that makes sense that atmospheric drag causes objects in low earth orbit (LEO) to lose altitude. This is shown for the ISS at:

http://www.heavens-above.com/issheight.aspx

So, why hasn't TIROS2 succumb to this drag over its 48 years in orbit? I assume that like aerodynamic drag on the earth, frontal area makes a difference. Since TIROS2 is relatively small compared to the ISS or modern satellites, I would guess it experiences less drag and thus less loss of altitude.

Maybe this combined with a higher starting altitude (see below) means that it really has maintained orbit for 48 years.

Does this logic make sense?


Some interesting data from: http://www.islandnet.com/~see/weather/h ... tiros1.htm

The TIROS I satellite was launched from Cape Canaveral (now Cape Kennedy), Florida on a three-stage Thor-Able rocket system. It achieved a nearly circular, prograde orbit at an angle of 48 degrees to the equatorial plane. TIROS I orbited the Earth every 99.19 minutes at between 796 km (495 statute miles) at perigee and 867 km (539 miles) at apogee.

BTW, thanks for your thoughts Earthbelow!

tbirchar wrote:So, why hasn't TIROS2 succumb to this drag over its 48 years in orbit?

Compare the orbits - TIROS is about 350 km higher than the ISS, even given the lower orbit of 700-750 km reported by Wikipedia. For back-of-the-envelope purposes, assume a scale height of 10 km for the atmosphere. Thus the density at TIROS heights should be about e^-35 ~ 10^-15 of what it is around the ISS. I have no good idea how the speed of orbital decay varies with atmospheric density, but let's be conservative and imagine it's goes as the fifth root. Even then, the TIROS satelite should be able to orbit for about 1000 times as long as the ISS between needing artificial boosts. I.e., about a century!

(Actually, one can probably not extend the exponential scaling law that high without significant correction, but any way you slice it there'll be many orders of magnitude of difference between 350km and 700km).

Henning Makholm wrote:

tbirchar wrote:So, why hasn't TIROS2 succumb to this drag over its 48 years in orbit?

Compare the orbits - TIROS is about 350 km higher than the ISS, even given the lower orbit of 700-750 km reported by Wikipedia. For back-of-the-envelope purposes, assume a scale height of 10 km for the atmosphere. Thus the density at TIROS heights should be about e^-35 ~ 10^-15 of what it is around the ISS. I have no good idea how the speed of orbital decay varies with atmospheric density, but let's be conservative and imagine it's goes as the fifth root. Even then, the TIROS satelite should be able to orbit for about 1000 times as long as the ISS between needing artificial boosts. I.e., about a century!

(Actually, one can probably not extend the exponential scaling law that high without significant correction, but any way you slice it there'll be many orders of magnitude of difference between 350km and 700km).

Indeed:
The exosphere hydrogen gas scale height ~ 500 km. ~ 4 x 14 x 8.5 km.
http://apod.nasa.gov/apod/ap000701.html
------------------------------------------------
scale height: H = kT/Mg
with:
T(exosphere) ~ 1200 ~ 4 x room temperature
M(hydrogen) ~ 1 ~ 1/14 x Nitrogen
------------------------------------------------
http://upload.wikimedia.org/wikipedia/c ... sphere.svg
<<The exosphere is the uppermost layer of the atmosphere. On Earth, its lower boundary at the edge of the thermosphere is estimated to be 500 km to 1000 km above the Earth's surface, and its upper boundary at about 10,000 km. It is only from the exosphere that atmospheric gases, atoms, and molecules can, to any appreciable extent, escape into space. The main gases within the exosphere are the lightest gases, mainly hydrogen, with some helium, carbon dioxide, and atomic oxygen near the exobase. The exosphere is the last layer before space.>>
-----------------------------------------------------------
With such large exosphere hydrogen gas scale heights
the real comparison here is not between Tiros & the ISS
but between both such heavy/dense exosphere satellites
and the "fluff ball" that was Echo 2
which lasted 5+ years in the exosphere:



<<Echo 2, a 41.1 m diameter metallized PET film balloon with an improved inflation system to improve the balloon's smoothness and sphericity, was launched January 25, 1964 on a Thor Agena rocket. It was used for more passive communications experiments, and also to investigate the dynamics of large spacecraft and for global geometric geodesy. NASA abandoned passive communications systems in favor of active satellites following Echo 2. Echo 2 reentered on June 7, 1969.>>

Nice find neufer. Even more impressive than the 135 foot diameter of the balloon (giving it the really high drag/momentum that brought it down quickly), however, is that it is utterly and completely dwarfed by the hangar that it's in. Those old blimp hangars were huge.

The photographer who took this APOD shared it a couple days ago on another space site. It's awesome to see it show up here, too.