chi and h site logo

{ Practical astronomy | Astronomy | Artificial satellites }

Artificial satellites

Since 1957, humans have put their best minds to it to fill the space around the Earth with junk. Some – a few thousand – of the objects are functioning satellites that still have some fuel left to control their orbit and that still receive orders from their control centre on Earth.

International Space Station (Zarya)
ISS (Zarya), 2010-05-08/09.

Very few of these satellites are staffed by human beings. These are the largest and brightest satellites. The first part of the International Space Station was launched in 1998 by a Russian rocket and had the name Zarya – Russian for "dawn". The space station is still officially listed as "ISS (Zarya)".

Physical parameters:

Taking pictures of satellites is made difficult by the rapid movement of these objects across the sky. The satellite illuminates a given pixel in the camera only for a very short time; even a bright satellite has at best only a moderate impact on the pixels along its path. In addition, satellite passes occur mostly during twilight, when the sky in general limits how high the ISO rating and how long the exposure can be. The fast movement of satellites drives us toward high ISO ratings, but the brightness of the twilight sky then limits how short the exposure must be and how short an arc of the satellite track can be recorded. An option is to take several frames in rapid succession and to layer them into a single image making each output pixel as bright as its maximum brightness across the sequence of frames ("brighten only" layers in the Gimp). All pieces of the satellite track will then come through at full brightness, while the unchanging sky background will come through roughly at its average brightness. In general one should use a wide field to catch a long arc of the satellite's path, perhaps including its ingress into or egress from the Earth's shadow.

The image shows a pass of the International Space Station in ten frames of 10 s exposure. The track is broken due to the time between frames when the shutter was closed. The track passes close to the bright star Spica, but the brightest "star" in the image is the planet Saturn.

Image parameters:

Iridium flare
Iridium flare, 2004-10-04/05.

The Iridium satellites are a fleet of about 70 satellites that orbit the Earth to support satellite phones. Each of these has three sizable, roughly flat, shiny aerials. The orientation of these antennas is known and it can be predicted which points on Earth receive sunlight reflected by them. The illuminated spots move rapidly across the Earth, and an observer in a suitable location will see only a brief flare lasting on the order of 10 s. The brightest flares can reach −8 mag.

Physical parameters:

The flares of Iridium satellites can occur in a quite dark sky, they can be very bright, and they cover only a shorter arc during the flare. This makes them somewhat easier targets than regular satellites.

Image parameters: