The crash of another large satellite is imminent once more. Fraunhofer researchers predict that the defective Mars probe Phobos-Grunt will plunge back to Earth between January 15th and 17th, 2012. As with the demise of the ROSAT satellite, Fraunhofer researchers are using the unique TIRA space observation radar, which was developed by the Fraunhofer Institute for High Frequency Physics and Radar Techniques.

Following the favorable outcomes of the UARS and ROSAT re-entries in the past year, the entire world is now awaiting the re-entry of the Russian Mars probe Phobos-Grunt. Around the world, research and aerospace organizations are working to provide a precise crash forecast. Also involved is the "Radar for Space Observation" team at Fraunhofer FHR, which is following the orbit of Phobos-Grunt over Europe with the TIRA space observation radar.

Dr. Ludger Leushacke, head of the "Radar for Space Observation" department, reports about the danger that might arise from fuel contained in the probe's tank: "There are 11 tons of the hydrazine fuel on board, which is extremely toxic and highly flammable." It is presumed that the liquid hydrazine will burn up upon entry into the Earth's atmosphere and will therefore not present a danger to people. However, given the size and the weight of 13 tons, it is expected that individual parts of the probe will collide with the surface of the Earth.

This is why it is so important to determine the time of the crash and the crash location as precisely as possible. Three factors are important for the re-entry forecast:

• High-precision orbital data
• Mass and approach surface of the object
• Density of the upper atmosphere (which is decisively influenced by solar activity)

The researchers of Fraunhofer FHR can determine the orbit and the surface of approach with the help of the TIRA space observation radar. At 16 revolutions per day, the object enters into the visibility of the antenna three to five times per day. Two different radar systems, which are integrated into the space observation system, take readings to provide a measurement. The target tracking radar precisely measures the movement of the object in its orbit, while the target imaging radar images the object. On the basis of these series of images, the researchers can determine whether parts have broken off or whether the orbiter is moving in a whirling pattern, and can thereby calculate the approach surface. According to current estimates of Fraunhofer FHR, Phobos-Grunt will enter the Earth's atmosphere between the 15th and 17th of January. However, the crash site remains uncertain. It lies somewhere between the 51st parallel north (Essen-Kassel-Leipzig line) and the 51st parallel south. A more precise designation is only possible some hours prior to the actual crash.

Nonetheless, the danger to people is only minimal. With a probability of 1:100,000,000,000 of being hit by space debris the average person is, for example, a million times more likely to die due to an accident in his or her own home.

The last satellite fuelled with hydrazine that threatened to crash on the Earth was the American satellite USA 193. This was shot down by the USA on 21 February 2008. The reason given for this at the time was that there was a danger arising from the hydrazine fuel, which had frozen solid after the power supply failed.