Read out The "Erdbahnkreuzer" in the sights. In space lurks death. But so far only a few astronomers keep track of the chunks that can collide with the earth. The "Project Spaceguard" would be a life insurance against the ultimate disaster. On March 22, 1989 flew the over 200 -meter Earth orbit cruiser 4581 Asclepius (1989 FC) in the double lunar distance to Earth over. If he had hit our planet, one of the greatest tragedies in human history would have happened. The encounter did not go unnoticed at the time and was reconstructed only after the orbit had been recalculated after the object was discovered by astronomers at the Mount Palomar Observatory on March 31st.

But Asclepius is just a cosmic vagabond among many who can be dangerous to us. "Earth is in a swarm of planetoids, " warned the acclaimed American geologist Eugene Shoemaker, who died last year during Australia's search for craters. That's no exaggeration. It is estimated that around 2000 kilometers of planetoids drift through the area close to the earth without any knowledge of it for a long time. So far, only about ten percent of them have been discovered and not even every thousandth of the chunks below 100 meters in diameter.

The data to date suggest that the Earth's orbit is crossed by more than a billion planetoids and comets over ten meters in diameter, and also by about a million over 100 meters and perhaps 10 000 between 0.5 and 5 kilometers. As a rule of thumb, a 100-meter near-earth object (NEO) hits Earth every 10 000 years on average, a body of one kilometer in diameter every one million kilometers, and every 100 million years at one in ten kilometers.

Shoemaker, who in 1960 was the first to prove the impact nature of the Nördlinger Ries, which was then interpreted by many colleagues as a volcanic crater, belonged to the "Near Earth Objects Search Committee". It was established in 1994 on the collision of the comet Shoemaker-Levy 9 with Jupiter, which he had discovered, by the US Congress. Objective: to develop a cheap but effective space surveillance proposal to capture as many orb cruisers as possible over the next ten years. display

However, most telescopes are not available for search and are usually unsuitable because they are designed for the observation of distant, faint galaxies with high magnification. For the search for the Erdbahnkreuzern, however, only medium-sized instruments are needed, but must have a special design to quickly scan extended sky regions for objects up to the 20th brightness class. Such objects are a million times fainter than the stars, barely visible to the naked eye, over a large city on a moonless night.

The improbability of an impact in the next few years is no argument for inaction. Because larger objects on a collision course could demand a huge number of dead, the risk of a human being being killed by an extraterrestrial celestial being converted over a period of one year is not so small. It is about the same risk of being killed in a plane crash in normal line traffic. The chance is about 1 to 20, 000 per year and passenger. Considering that over 100 million dollars are spent on aviation safety worldwide each year, similarly costly precautionary measures against space bombs - primarily observation programs - are not a luxury.

As late as April 1997, Shoemaker had suggested in a hearing before the US Congress to build at least two 2-meter-class telescopes and equip the unused 1-meter Air Force telescopes with sensitive detectors. It would also be necessary to have an improved infrastructure to track new NEOs in a targeted manner and to be able to determine their course quickly. This would also reduce the risk of an irritating false alarm, as triggered by orbit calculations of the minor planet 1997 XF11 last spring.

With this slimmed down Spaceguard project, 90 percent of the one-kilometer NEOs and about 10 percent of NEOs with diameters between 100 meters and one kilometer could be detected within 15 years. With the support of the Air Force, which is actually responsible for such dangers, and through international cooperation, this could even be achieved within ten years. Cost: $ 24 million for the first five years, then $ 3.5 million a year, for a total of about $ 60 million. David Morrison of NASA's Ames Research Center: "The entire Spaceguard program would cost less than either Deep Impact or Armageddon."

=== Rüdiger Vaas

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