Artificial Satellites

While perhaps not as photogenic as their natural cousins, artificial satellites do have a fascination of their own due to their very rapid motion across the sky. Watching a bright dot move swiftly and silently among the stars is something that mankind has been able to do for less than 70 years, and nowadays the sheer number of such travellers visible on a dark night reminds us of the great progress that has been made in space science in a little over half a century.

However, most satellites are quite dim objects because although they are very near to us they are also extremely small by planetary standards. This means they have limited observational interest, so I did not bother with them until my attention was drawn, some years ago now, to an upcoming series of near-overhead passes by the much-enlarged International Space Station (ISS). I was aware this was visible from the UK on occasion but with the addition of a series of solar-cell arrays its brightness had increased many-fold, turning it into a really dramatic object able to out-shine Sirius, the brightest star in the sky, by a factor of almost 7 times. Not only that, its closeness to the Earth (just 210mls up when first constructed, raised to about 250mls in 2012) means it moves across the sky quite quickly. I just had to get the camera out! For more information on the ISS click here.

During the period when I was making my observations, the ISS was regularly visited by two types of Russian spacecraft, Soyuz TMA and Progress, versions of which are still currently in service. Both are quite small and so are difficult to observe. It was also re-supplied every 14 months or so by Automated Transfer Vehicles. Finally, when they were in service there were also many visits by vessels from the Space Shuttle fleet which were large enough to be easily seen from Earth. However, because the parts of the missions when the Shuttle is in independent flight were quite short, it was fairly unusual to be able to see them from a specific location, such as the UK.

A Progress freighter The ATV

The Progress freighter is derived from the Soyuz manned spacecraft and, like the Soyuz, has three main elements:- A pressurized forward module. This carries the supplies for the crew such as scientific equipment, clothes, pre-packaged and fresh food, and letters from home. The docking drogue is similar to that of the Soyuz but features ducting for the fuel and oxidiser. A fuel compartment. The re-entry module of the Soyuz was replaced with an unpressurized propellant and refuelling compartment with ducting along the outside of the spacecraft. This meant that if a leak occurred, the poisonous gas would not enter the station's atmosphere. The fuel is carried in two tanks. A propulsion module, at the rear of the spacecraft, which remained unchanged from Soyuz and contains the orientation engines used for the automatic docking. It may be used to boost the orbit of the station once docked. The ATV was an expendable, unmanned, re-supply spacecraft developed by the European Space Agency (ESA). ATVs were designed to supply the ISS with propellant, water, air, payloads, and experiments. ATVs could also reboost the station into a higher orbit. The ATV was designed to complement the Progress spacecraft, having three times its capacity. Like the Progress, it carried both bulk liquids and relatively fragile freight which was stored in a cargo hold kept in a pressurized shirt-sleeve environment so that astronauts could have access to it without putting on a spacesuit. The ATV pressurized cargo section was based on the Italian-built Multi-Purpose Logistics Module (MPLM), which was a Shuttle-carried 'space barge' transporting equipment to & from the Station.

While it is sufficiently well-known to not really need a great deal of description, for the record I shall add that the Space Shuttle (or Space Transportation System) was a crewed, partially reusable low Earth orbital spacecraft operated by the U.S. National Aeronautics and Space Administration (NASA). The first of four orbital test flights occurred in 1981, leading to operational flights beginning in 1982. It was used on a total of 135 missions from 1981 to 2011, launched from the Kennedy Space Center (KSC) in Florida. Operational missions launched numerous satellites, interplanetary probes, and the Hubble Space Telescope (HST); conducted science experiments in orbit; and participated in construction and servicing of the ISS. The first orbiter, Enterprise, was built purely for Approach and Landing Tests and had no capability to fly into orbit. Four fully operational orbiters were initially built: Columbia, Challenger, Discovery, and Atlantis. Of these, Challenger and Columbia were lost in mission accidents in 1986 and 2003 respectively, in which a total of fourteen astronauts were killed. A fifth operational orbiter, Endeavour, was built in 1991 to replace Challenger. The Space Shuttle was retired from service upon the conclusion of Atlantis' final flight on July 21, 2011.

  Pictures of the ISS and its visiting spacecraft

Given the brief visibility of the ISS (and the other craft which visit it), knowing when to look is of course rather important! Daily predictions for any location are given on several Internet websites, such as heavens-above.com, and I soon became interested in the other satellite phenomena also listed there. Photographically, Iridium flares looked the most intriguing as they were bright (sometimes extremely so!) and short-lived thus providing images that could be more than just a uniform faint streak. But what, I hear you say, is an Iridium flare? It was a reflection of sunlight from one of the many first generation satellites providing communications services for the Iridium Corporation. I say "first generation" because they were eventually replaced by newer versions, which (unfortunately for sky watchers) do not flare in such a dramatic way. Any satellite can reflect sunlight of course but Iridium satellites were particularly likely to do so - to find out why, click here.

  Pictures of Iridium flares

Further information on the visibility of the ISS and some calculations concerning the several different periodicities of Iridium flares may be found in the Artificial Satellites article of the Astronomical Theory section of this website. For a quick link to this section, click here - the section index will open in a new tab.