Monday, October 17, 2011

Comet Elenin Missed Earth But German Satellite Won't

Last look at Comet Elenin?
Photo courtesy of

'Doomsday Comet' Elenin came closest to Earth, early on Sunday morning (Oct. 16), without losing a single snowball to Earth's gravity but a defunct German satellite will find it's way back home this week... well, sort of.

Dubbed the Doomsday Comet, Elenin met up with a solar blast from the sun on September 10, 2011, that reduced the icy ball into a pile of strewn rubble.  That rubble was all that was left of the comet as it passed within 35,000 km of Earth.  It won't be back for another 12,000 years. 

Doomsday has been postponed.

Bye, Elenin.  Say hello to 'Herr ROSAT'!


600 ROSAT Satellite


Deutsche deja vu: Kaput satellite in Earth plunge

( A defunct German satellite is expected to plunge to Earth this week, but exactly when and where the satellite will fall remains a mystery.

The massive German Roentgen Satellite, or ROSAT, is expected to plummet to Earth on Saturday or Sunday (Oct. 22 or 23), though German space officials have also offered a wider re-entry window of between Oct. 21 and Oct. 25. This latest falling satellite comes about a month after a dead NASA climate satellite, called the Upper Atmosphere Research Satellite (UARS), plunged into the Pacific Ocean in late September.

The 2.4-ton X-ray space observatory is expected to break up as it travels through Earth's atmosphere, but some large pieces will likely make it through the intense heat of re-entry. According to German aerospace officials, approximately 1.6 tons of satellite debris, consisting primarily of up to 30 large glass and ceramic fragments, could survive the journey through the atmosphere and reach the Earth's surface.

"We don't expect big parts to re-enter, except the mirror and the glass and ceramic parts," Jan Woerner, head of the executive board of the Deutsches Zentrum für Luft- und Raumfahrt (DLR), Germany's space agency, told "Usually during re-entry, you have rather clear burning of all the elements, but glass and ceramics may survive and may come down in bigger pieces."

There is a 1-in-2.000 chance a piece of ROSAT could strike someone on Earth, DLR officials have said. That's a slightly higher risk than the 1-in-3,200 chance of a debris hit NASA gave for the UARS satellite fall.

German aerospace officials are actively tracking ROSAT, but they will not be able to determine precisely when and where the satellite will fall until roughly two hours before it impacts Earth.

ROSAT's orbit extends from the latitudes of 53 degrees north and south, which essentially covers a huge swath of the planet. This means the satellite could fall anywhere stretching from Canada to South America.

Officials at Germany's space agency calculated a 1-in-2,000 chance that someone on Earth will be hit by ROSAT debris, but the risk of serious injury from such an event remains extremely remote.

Originally, the dead satellite was projected to fall to Earth in November, but refined estimates show that the spacecraft will likely make its fiery descent through the atmosphere later this week -- earlier than mission controllers previously thought.

"With satellites like ROSAT, you depend on external circumstances," Woerner said. "For instance, solar wind and changes in the atmosphere may change the time of re-entry. We just have to wait and observe."

ROSAT was launched in June 1990 as a joint venture between Germany, the United States, and the United Kingdom. In 1998, the satellite's star tracker failed, which caused its onboard camera to be directly pointed at the sun. This permanently damaged the spacecraft, and ROSAT was officially decommissioned in February 1999.

Since the satellite does not have a propulsion system, and there is no fuel left onboard, the satellite will make an uncontrolled return to Earth.

"We expect public attention because it's a satellite coming down, but in history, we have had much bigger debris fall," Woerner said.

On Sept. 24, NASA's UARS satellite fell uncontrolled toward Earth and plunged into the Pacific Ocean. The event captured the attention of people around the world, and ROSAT's impending re-entry will mark the second time a large spacecraft makes a publicized fall from space within two months.

Officials at Germany's space agency studied NASA and the U.S. Air Force's response to the falling UARS satellite in order to prepare for ROSAT's final return to Earth.

"For us, it was an advantage that UARS fell before," Woerner said. "We know now a little better how to interpret all the data and use the global network. It was an advantage that the satellite came down before so that now we can look at how to deal with ROSAT and how we deal with this in the future."



Falling ROSAT satellite to make reentry between Oct. 21 - 25

Achtung: A satellite strapped to a 1.7-ton telescope will soon be descending in a flaming heap back to earth, possibly as soon as Friday. Because this out-of-control spacecraft, known as the Roentgen Satellite or ROSAT, was built with the finest German engineering, the "very heat resistant" optical device is expected to crash to the ground relatively unscathed.

The ROSAT satellite was launched by NASA in 1990 and managed by the Germans until its fuel ran out and its mission ended in the late '90s. People who bought killer-satellite insurance after the UARS reentry in late September might be feeling smug right now. But although the hunk of expensive junk could make an impromptu landing pad out of basically any major city, the chances of a telescope-on-noggin impact are still abysmally low. Consider that 71 percent of the earth's surface is ocean, and that 369 satellites made uncontrolled reentries in 2010 without any known harm (although this woman was hit by part of a Delta II rocket in the '90s, if you're the type who can't stop fretting about things).

The window of the ROSAT reentry, which the German Aerospace Center puts at Oct. 21 to 25 (though the dates could change slightly), should not be a time for finger-biting but for reflection about the probe's accomplishments. Namely, ROSAT gave the universe a thorough X-ray scan shortly after its launch in 1990. This "all-sky" observation recorded 80,000 things out in the black that are spewing out Roentgens, a finding that among other things helped scientists understand the nature of nearby stars and supernovae. The satellite even caught the moon and comets emitting X-rays!

NASA praised the vehicle's discoveries in 2001:

Using these data, for the first time, astronomers could see in full the large X-ray structures in the Milky Way Galaxy, and in other galaxies; could get a nearly complete measurement of bright X-ray sources, including stars in all stages of evolution, and neutron stars and black holes too; could see the shape and brightness of the "diffuse X-ray background", the high energy emission which seems to surround us... and could use the shape of the X-ray emission to trace the hidden material making up most of the known Universe.

In other words, it was much cooler than what you'll see at the dentist's office. The visual results of ROSAT's penetrating scan are wonderful to behold:

The "all-sky" map of X-ray sources in the nearby universe. Large version. (ROSAT / DLR)

So why the uncertainly about the satellite's reentry? The sun's recent, mercurial outbursts have been heating and cooling the atmosphere, altering the drag that ROSAT experiences while traveling over the planet. Here's the latest from the German space agency's ROSAT page, a good source to check if you want to keep up with this device's death plunge:

Currently, the re-entry date can only be calculated to within plus/minus three days. This time slot of uncertainty will be reduced as the date of re-entry approaches. However, even one day before re-entry, the estimate will only be accurate to within plus/minus five hours. All areas under the orbit of ROSAT, which extends to 53 degrees northern and southern latitude could be affected by its re-entry.

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