Saturday, October 22, 2011

The German ROSAT satellite has fallen back to earth - The satellite re-entered the atmosphere between 9:45 and 10:15 pm ET (01:45 and 02:15 GMT). Up to 3,750 pounds of the decaying satellite could survive re-entry.
The defunct satellite is hurtling towards the atmosphere and satellite pieces expected to hit earth within hours - "According to the data we currently have, we expect it not to hit over Europe, Africa or Australia." It is not clear whether any pieces have yet reached the Earth's surface.
Most parts of the minivan-sized satellite were expected to burn up during re-entry into the atmosphere but up to 30 fragments weighing 1.7 metric tons could crash into earth at speeds up to 450km/h. The satellite orbits every 90 minutes and it could hit almost anywhere along its path - a vast swath between 53-degrees north and 53-degrees south that comprises much of the planet outside the poles, including parts of North America, South America, Europe, Africa and Asia. Fluctuations in solar activity and the fact that scientists are no longer able to communicate with the dead satellite render predictions of where and when it will come down yet more difficult. The largest single fragment of ROSAT that could hit into the earth is the telescope's heat-resistant mirror.
The German space agency puts the odds of somebody somewhere on Earth being hurt by its satellite at 1-in-2000 - a slightly higher level of risk than was calculated for the NASA satellite. But any one individual's odds of being struck are 1-in-14 trillion, given there are 7 billion people on the planet. Any Rosat wreckage is strongly tipped to hit the ocean, given that so much of the Earth's surface is covered by water.
The dead NASA satellite fell into the southern Pacific Ocean last month, causing no damage, despite fears it would hit a populated area and cause damage or kill people. Experts believe about two dozen metal pieces from the bus-sized satellite fell over an 800km span of uninhabited portion of the world. The NASA climate research satellite entered earth's atmosphere generally above American Samoa. But falling debris as it broke apart did not start hitting the water for another 480 kilometres to the northeast, southwest of Christmas Island. Earlier, scientists had said it was possible some pieces could have reached northwestern Canada.
Destructive design - Future spacecraft sent into orbit may have to meet stricter guidelines that limit the amount of debris likely to fall back on to the planet, but these rules are still some way from being introduced. "Up until now we've designed satellites to survive the harsh environment of space, and we haven't given much thought to designing them for destructive re-entry. But in future, we will have to consider whether we have got this balance right, and perhaps satellites should be designed in such a way that we can ensure more of what comes down is destroyed in the atmosphere and doesn't hit the surface. Unfortunately, there is a whole legacy of spacecraft - 50 years of satellites - and we are going to have to put up with this situation for quite some time, I'm afraid." The U.S. Space Surveillance Network has catalogued 16 thousand things in Earth orbit, many of them are quite small pieces of debris, but about 7,000 of those are large objects, spacecraft and large rocket bodies.
Rosat will start to tumble rapidly when it engages the thicker parts of the atmosphere, about 80km up. Mechanical forces will first rip off its flimsiest structures, such as its solar arrays and antennas. The heating the satellite then experiences as it plunges downwards will deform and melt low-temperature materials and vaporise them. Only high-temperature metals such as stainless steel and titanium will put up much resistance.
Tracking stations will typically witness the uncontrolled return of at least one piece of space debris every day; and on average, one intact defunct spacecraft or old rocket body will come back into the atmosphere every week. Something the size of Nasa's UARS satellite is seen to enter uncontrolled perhaps once a year. Much larger objects such as space station cargo ships return from orbit several times a year, but they are equipped with thrusters capable of guiding their dive into a remote part of the Southern Ocean.