Thursday, October 04, 2007

Sputnik, fifty years later

The small dots denoted by the arrows on this photo are 34 out of about 240 geostationary satellites, orbiting Earth at an altitude of 35,786 km above ground, where the orbital period is 24 hours and a satellite appears to be fixed at a spot above Earth's equator. The image shows a strip of roughly 30° along the celestial equator. During the 8.5 hours of exposure at Kitt Peak, Arizona, the stars in view moved along their usual paths in the sky, and left the bright trails.
(Credits: Dave Dooling, National Solar Observatory, Tucson, Arizona; from Earth Science Picture of the Day (EPOD) from September 4, 2007)
On October 4, 1957, a team of Russian engineers and scientists successfully put the the first man-made satellite into an orbit around Earth. Sputnik 1, the "fellow traveller", transmitted a simple radio signal while circling the Earth once every 96 minutes at a height between 215 and 940 kilometres. Sputnik was visible through binoculars in the sky: TIME magazine told its readers on October 14 that "the satellite's orbit shifts around the earth at 4° per day. This will bring it over the U.S. at twilight on about Oct. 20, when it should be visible through small telescopes or binoculars."

50 years later, the sky is full of satellites, for communication, navigation, observation of the weather, military reconnaissance, and the transmission of more television channels than I ever wanted to see. And satellites in low orbits are not difficult to see by the naked eye:

I was surprised, when spotting Perseids in August this year, that I could see nearly as many satellites as meteors. On a clear night about two hours after sunset, just watch out for faint, steady lights travelling on straight paths among the stars. If the light does not blink, chances are high that it's a satellite, not an airplane, and if the light fades away when reaching the Eastern parts of the sky, you can be sure it was a satellite: it has reached a point where even at a height of a few hundred kilometres above ground, it has eventually entered the Earth's shadow that is the night.

I was even more surprised when I came across the photo on the left, showing 34 satellites at a height of 35,786 km above ground in geostationary orbits. While these satellites are quite bright at radio frequencies, and are observed on a regular basis by hundreds of millions of radio dishes all around the world, they are a hundred to ten thousand times dimmer than the faintest stars visible to the naked eye, so that it takes a telescope and quite a long exposure to catch them on film.

Including the geostationary satellites and the low-orbiting ones visible to the naked eye after nightfall, about 850 active satellites are in orbit today. The article Space debris by David Wright in the October issue of Physics Today (doi: 10.1063/1.2800252) gives an interesting breakdown of the current satellite population according to usage, orbital altitude, and country. More than half of the satellites currently in use are from the US, and from the total number of satellites, 15% are used for science, and 66% for communication. But the main intention of the paper is to point us to the huge number of debris orbiting the Earth: scraps of defunct satellites, rocket parts, gloves of astronauts - more than 700,000 pieces larger than 1 centimetre are rushing around, and may pose a serious risk to satellites or the space station.

But already when looking at the satellites currently in operation, the sky looks quite crowded: The NASA Satellite Tracking website features a Live 3D Java Tracking Display (as the name implies: Java required), which provides an interactive 3D visualisation of satellites and their orbits around Earth. I didn't know the the GPS satellites actually have quite high-lying orbits, at altitudes of about 20,000 kilometres above ground. In case you now the exact name of a satellite, detailed information about the orbit are available also from the Select Satellite form at Heaves Above.

50 years after Sputnik, Space below the geostationary orbit has become quite crowded. Fortunately, it is still wide enough.



  1. Thanks to the Badastronomy blog I learned today about Iridium flares (search for 'Satellite flare' at Wikipedia), coming from a set of satellites which can have up to -8 magnitude and be seen in the day. If you go to you can find out when these will be visible from where you are positioned.

  2. If mankind were to abruptly end itself the most persistent markers of its past existence would be landfills and orbital debris - garbage. It's not much of an epitaph.

  3. It's interesting to read Sir Arthur C. Clark's reactions to the fifty years of space exploration. He remains optimistic of space exploration's future despite some of the disappointments of the last half century.
    Read the Q&A at

  4. Hi Jonathan,

    thanks for reminding me of the Iridium flares!

    I've checked out flare events already several times at heavens above (you have to make sure that your location is correct, since this is crucial to get correct direction and brightness etimates), but then always missed to have a look at the sky at the decisive moment ;-), so I really should keep this on my "I'd like to do" list :-)

    Hi Josh,

    thank you for the link to the Arthur C. Clarke interview! Indeed, besides 2001 and all that, he is the guy who propagated the concept of the geostationary orbit!

    Best, Stefan

  5. Dear Stefan, That Physics Today article was nice, but I wished it would have mentioned ESA's historically long involvement in monitoring space debris. One of the difficulties of the NASA studies is that their databases are classified (ESA's are not), so that poses an extra difficulty for people (my colleagues and I), who wish to have more data. You might say that a significant number of planetary dust scientists are also interested in, and/or who have worked on debris. The Physics Today article also focused on LEO, but you know the number of satellites orbiting at GEO is pretty high! The best data for small debris in GEO comes from the GORID dust detector, which was a copy of the Ulysses and Galileo dust detectors, that was riding piggyback on a Russian telecommunications satellite for 5 years until 2002. The results from that experiment are not all analyzed yet, and is the main topic of an unfinished PhD thesis by my colleague, but you can read some results in the links below.

    The debris that I'm most interested in is the small particles the size of the smallest interplanetary and interstellar dust: 10 nm to 1 micron. When a rocket boosts a satellite from LEO to GEO, there are two typical products: 10 nm Al2O3 spheres (this charming sphere is attached to a natural micrometeroid), released at high velocity from the rocket exhaust (usually the spheres charge up and accelerate and leave the Earth's magnetosphere and some our solar system(!)), and 'slag' particles that are the sticky gooey junk that sticks to the nozzle of the rocket, and are later sloughed off, released at much lower velocities. Both kinds of particles are charged up due to the plasma environment in the Earth's magnetosphere, and they have particular orbital and/or clustering characteristics that can be used to separate their population from natural micrometeroids.

  6. for 5 years until 2002.

    Scusi', I meant 7 years (it's a fantastic dataset).

  7. Stefan - thanks for the interesting post. I watched the Aurigids in 1-Sept of this year for an hour and saw four satellites pass by (this is early a couple of hours before sunrise)!

    I have a subscription to Physics Today, but I have not yet read the mentioned article.

    With respect to the interesting picture you posted. Sigh - I helped to put Galaxy 16 in its proper geostationary orbit and all we see now is just a tiny point of light!

    Amara, what you posted about dust was very interesting, thanks. I'l' read your arXive preprint with great interest.

    If you want to get orbit elements of nearly any s/c, visit



  8. Lastly, I wanted to emphasize: the launch of Sputnik was truly a revolutionary event in the history of humanity: slowly but surely we're starting to leave the cradle.


  9. changcho: That paper is on the short side (it was written to conform to this Conference proceedings. We are in the beginning of preparing a much longer paper of results. That work is one of the many output from one ESA contract: 16272/02/NL/EC, "Processing, Analysis and Interpretation of Data from Impact Detectors". Some of the other results were presented on this day:

    I think that one of the most interesting results from that ESA contract was a different study : "Debris Cloud Evolution" by K.D. Bunte, who used a debris population database called: "MASTER 2001". The ESA MASTER debris database consists of:

    fragments (0.1 mm - 10 m) resulting from spacecraft explosions

    NaK droplets (2 mm - 4 cm) coolant droplets released by Russian RORSATs

    solid rocket motor (SRM) slag particles (0.1 mm - 3 cm) larage particles released during the final phase of solid rocket motor firings

    SRM Al2O3 dust (10 nm - 80 micron) small particles released during solid rocket motor firings

    paint flakes (2 micron - 0.2 mm) resulting from surface degradation

    ejecta (1 micron - 5 mm) resulting from meteoroid and debris impacts on exposed surfaces.

    With this, one can simulate debris clouds and follow their orbital evolution, and see how long those debris clouds remain in orbit. Using these populations, debris clouds were generated and followed. Bunte chose some representative cases:

    − Object 25942 in LEO Sun-synchronous orbit 2000-03-11
    This is the second Long March 4 to break-up in only four missions. The first break-up (flight 2) occurred on 4 Oct 1990, one month after launch. Long March 4 missions did not resume until 1999, when two more were flown. This break-up involved the second 1999 mission (flight 4) and occurred five months after launch. This event has created more trackable debris than the 1990 break-up.

    Object 22925 in semi-synchronous transfer orbit (or similar) 2000-09-07
    This is the 22nd break-up event for an object of this class, and the first of the year 2000. The break-ups are assessed to be caused by residual propellants. Russian officials have been aware of the problem since 1992 and have made design changes, although the date of full implementation is unknown. The environmental consequence of the break-up will be short-lived; the object is in catastrophic decay from a Geosynchronous transfer orbit. Although the initial orbit of this object is a GTO, it can be regarded as an example for a semi-synchronous transfer orbit, since the orbital elements are similar.

    Object 19773 in GTO (Ariane upper stage) 2001-01-01
    This is the first break-up of an Ariane 2 third stage officially recognized. One Ariane 3 third stage (same as Ariane 2) is known to have broken-up within a few days of launch in 1987. Both vehicles were launched before passivation measures were incorporated with Ariane third stages. Ariane third stage passivation was introduced in January 1990 and has been employed on all Ariane missions since October 1993. The age of the Ariane 2 third stage at the time of the break-up was nearly 12 years.

    Object 16235 in inclined high eccentric orbit 2001-04-29
    Cosmos 1701 was in the final stage of catastrophic decay from a highly elliptical orbit. Break-up was reported near perigee, which was less than 100 km. Assessed cause of the break-up is aerodynamic forces encountered during perigee passage. The parent satellite and break-up debris will decay rapidly and pose no long-term environmental impact.

    With conclusions:

  10. (K.D. Bunte conclusions)
    The orbital lifetimes of the larger cloud particles (SRM slag, fragments) is greater than the orbital lifetimes of small particles (SRM dust).

    SRM firing clouds of semi-synchronous orbit insertion burns do not survive the first orbit.

    SRM firing clouds on GTO or GEO insertion burns generate long living large debris clouds.

    LEO clouds build up a "debris ring" a few days after the event.

    Fragmentation clouds with few objects show complete dispersion after one month

    LEO fragmentation cloud dispersion occurs a few months after the event.

    Dispersion of GTO fragmentation clouds needs more than one year.

    As expected LEO cloud dispersion is faster than GTO or GEO cloud dispersion.

    Other studies can be found here.

    This Meteoroids and Debris Website might be useful to you too.

  11. I was skygazing the other night and saw a couple of bright flashes, like flashbulbs, and thought "what the Hell?" They were probably those iridium flares or something similar. Glad to know it wasn't aliens taking pictures. ;-)

  12. Dear all,

    coming back to Clarke and the geostationary orbit - here is a link to Arthur C. Clarke's paper "Extra Terrestrial Relays" in Wireless World, October 1945, pages 305-308... I came across it by chance on this page about Observing Geostationary Satellites.

    What I actually wanted to find out is, how many (active and defunct) satellites are there in geostationary orbits?

    There should be some catalogue somewhere? Does someone know a link? My registration to does not work yet... And does anyone know a number?

    From the photo, which covers 10 percent of the equator and shows 34 satellite, one can extrapolate to roughly 350, or one at every degree of longitude, but that's probably only a rough estimate.

    Thank you, Stefan

  13. Dear Amara,

    thank you for the many informative details about space debris and dust, and for the links to the ESA site!

    These aluminium oxide nanospheres, that's funny. How comes that they are so round? And are these exhaust fume particles from the fuel? Aluminium in solid fuel? Or does the aluminium come form parts of the rocket engine?

    And this dusty debris you are interested in most is not a danger for other satellites, or the space station, I would guess? There is probably much more natural dust around at that size?

    This interesting ESA page that you hinted me at, MICROMETEOROIDS AND SPACE DEBRIS, says that at any moment, there is 200 kg of meteoroid mass within 2000 km of the Earth's surface, while the estimated mass of man-made orbiting objects within 2000 km of the earth's surface is about 3000000 kg.

    That sounds as if the man-made stuff is fantastically more abundant - but most of the mass is concentrated in large chunks, I guess, different from the mass distribution in meteoroids?

    You see, I'm trying to exploit your expertise ;-)

    Best, Stefan

  14. Hi changcho,

    I watched the Aurigids in 1-Sept of this year for an hour and saw four satellites pass by (this is early a couple of hours before sunrise)!

    Yeah, that was then similar to my experience, which was in the evening. I was really surprised that there were so many artifical lights up there ;-)

    I helped to put Galaxy 16 in its proper geostationary orbit and all we see now is just a tiny point of light!

    Really? The third small point of light from the bottom of the photo? What a funny coincidence - and what interesting readers we have :-)

    May I ask, what was your involvement in the launch? And from this report, I understand that the satellite was brought into orbit by Sea Launch, this James-Bond like company that launches rockets from revamped oil rigs? All that sounds very fancy!

    Best, Stefan

    PS: Thanks for the pointer to - my regstration is awaiting approval...

  15. Hi -

    Amara, thanks for the extra info. I haven't gotten to reading your paper yet but I'll do so when I have a bit of time.

    Stefan - As far as the total number of geostationary satellites is concerned, I don't know off the top of my head but I'm sure there's a catalogue somewhere; I'll try to find out. Also, thanks for the link to the original A Clarke paper; I've never read it before, though of course I've heard about it.

    One thing to also keep in mind is that once geostationary satellites run out of fuel, it is considered "proper form" to raise their semi-major axes a few hundred km above the geostationary arc to empty that slot.

    With respect to G16, I helped plan the orbit raising manuevers to go from the geostationary-transfer orbit (where the Sea Launch rocket left it) to GEO proper.



  16. "What I actually wanted to find out is, how many (active and defunct) satellites are there in geostationary orbits?

    Sorry, I forgot about this. The current number I have of operational GEO satellites is 350, exactly the number you estimated! The density of geo satellites is not uniform; as expected there are more over populated areas (i.e. over Asia, Europe and CONUS) and fewer over, say, the Pacific ocean, but on average it's slightly less than 1 per degree. Here's the list:


  17. Hi changcho,

    thank you for the link - that's exctly what I had been looking for :-)

    In the meantime, I've located the confirmation of my registration to space-track in my junkmail folder and can accesse all the fancy TLEs - not that they are useful to me, but it's cool ;-).

    But they don't have such a nice list of the "claims" along the equator, organized along longitude at space-track, so this one is really great!

    Best, Stefan

  18. These aluminium oxide nanospheres, that's funny. How comes that they are so round? And are these exhaust fume particles from the fuel? Aluminium in solid fuel? Or does the aluminium come form parts of the rocket engine?

    Sorry for the long delay in answering this. I should know by heart the answers, especially since I was preparing for, and I made early this week, a short UK trip to work with someone on our Earth dust/debris stuff. But I don't know precise answers for your questions regarding the spheres.

    What I know for the reasons of the Al2O3 shape is what I read on the shuttle Challenger report: the solid fuel propellant contains about 70% ammonium perchlorate, 16% aluminum powder, 14% binder and curing agent, and 0.1% iron oxide. During combustion, the aluminum is converted to a specific crystalline state from the high temperature of the combustion process. The spheres probably come from the fine aluminum powders in the mixture. This fuel mixture might be common too, because I learned when I visited Kiruna six years ago that they were a common by-product of the rocket launches there; i.e., the technicians were/are cleaning up the Al2O3 spheres from the ground after every launch.

    And this dusty debris you are interested in most is not a danger for other satellites, or the space station, I would guess?

    A single particle would not be a problem, but long-term exposure to that flux of small particles (the size distribution of the Al2O3 spheres peaks at 10^{-7} m, their density is 3.2 gr/cm^{-3}, [1]) would be a problem for the optics and other exposed surfaces of sensitive instrumentation. In GTO, the geotransfer orbit location at low altitude where the satellites are boosted to GEO, about 20% of the smallest particles have stable orbits with lifetimes of many years [1]. In GEO, the lifetime for the smallest (less than 10^{-7} m) particles are shorter; they are usually ejected out of the Earth's magnetosphere (and some can escape the solar system), according to Juhasz [1], in times of hours to days. The 'larger' debris particles (say radius between 10^{-5} -- 10^{-3} m stay in GTO and GEO for greater than 1 year. Particles of larger than 1 cm could seriously damage a satellite and particles of size 10 cm in LEO could destroy a satellite immediately (and they are tracked). [2]

    [1] Juhasz, Antal and Horanyi, Mihaly (1997), "Dynamics of charged space debris in the Earth's plasma environment", Journal of Geophysics Research, vol. 102, No. A4, Pages 7237-7246, April 1, 1997.

    [2] Flury, W. (1998), "The Space Debris Environment of the Earth- Amounts and Growth", Perserving the Astronomical Windows, ASP Conference Series, Vol. 139, pg. 49.

    There is probably much more natural dust around at that size?

    No, the opposite: the debris flux is about 20 times higher than the (natural) micrometeoroid flux. See page six of my short paper.

    That sounds as if the man-made stuff is fantastically more abundant - but most of the mass is concentrated in large chunks, I guess, different from the mass distribution in meteoroids?.

    I think yes.. but I don't know enough about the mass distribution of the large stuff.. I always think of the smallest particles...

    You see, I'm trying to exploit your expertise ;-)

    I know.... everything in my life, including my expertise, is going into boxes and these days I am constantly needing to find workarounds for things I unrealistically expect to work. :-(

    More information to browse at your leisure is: The Orbital Quarterly News.

    And something fun: My Life as a Solid Rocket Booster.

    Ciao, Amara

  19. Dear Amara,

    thank you very much for getting back to my questions, and for the detailed answers and links. That "Life of the rocket booster" video is really great!

    Concerning the aluminium oxide spheres, I've searched around a bit and found a page on Solid rocket propellants and their properties, from which I learn that the flame temperature of the Shuttle solid rocket booster is about 3500 K, which is above the boiling point of Al2O3 at 3250 K. This means that the about 700 tons of Al2O3 produced in each shuttle launch condense from the gas phase and form droplets, which then solidify. The spheres are, so to say, Al2O3 hail from the exhaust gases... The first two google hits for Al2O3 droplet link to abstracts on the ADS server for a Survey of recent Al2O3 droplet size data in solid rocket chambers, nozzles, and plumes and The Al and Al2O3 droplet cloud in solid rocket motors... so I guess that's indeed the origin of these spheres...

    OK, and I see that the amount of this stuff is really high, and that you do not want an anti-corrosive aluminum oxide coating deposited on sensitive optical satellite equipment ;-)...

    Thanks again, and good look finding workarounds whereever needed,
    Cheers, Stefan



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