Read out neutron bombs and sun mirrors to protect our planet. The bombardment from space is life-threatening. But we can protect ourselves. Scientists have already calculated the emergency. What can we do if one day an Earth orbit cruiser is on a collision course with Earth? Accurate orbital determinations and predictions could rob a small meteorite of much of its horror. Because as with hurricane and tidal wave warnings, a timely evacuation would be possible.

However, no one can escape from a larger orbit cruiser. At most, some people could fly into space with a rocket, as the science fiction film "When Worlds Collide" in 1951 has shown. A better way would be to preempt the disaster with a preemptive strike - if you recognize the danger early enough. A "war on the stars" could save us. "There are four options, " says Derrick Pitts of the Franklin Institute in Philadelphia. "We could slow down the celestial body, accelerate it, get it off course or blow it up."

The experts agree: A rail change threatening NEOs is the most effective defense and technically relatively easy to implement. But this "Star War" requires nuclear weapons for over a kilometer objects - a double-edged thing: "We could bring more danger into space than there already lurk, " fears Tom Gehrels of the University of Arizona. His colleague Jay Melosh, working at the same university, also warns: "The threat of gigatron bombs in orbit or on Earth appears to me much greater than the threat of planetoids."

But it is also not necessary to develop concerted defense batteries already. Because the probability of discovering a terrestrial cruiser immediately on a collision course, is much lower than that, in the near future, there is even one. display

In addition, there are defense strategies that are not based on nuclear warheads and therefore far less dangerous, but currently also difficult to implement. One strategy could be to attach thrusters to the NEO and steer it out of alignment with such an outboard motor. The energy source can be chemical, electrical, nuclear or solar drives. The main difficulty is anchoring the thrusters firmly enough to the NEO.

Another idea came from Jay Melosh and Ivan Nemchinov from the Institute of Geosphere Dynamics in Moscow. A large, lightweight mirror, which is brought into space with a space shuttle or a rocket and unfolded there, is intended to distract the NEO. This mirror would have to fly to the comet or planetoid some years before the impending impact. For this he could be equipped with small thrusters. It would be even more efficient to use it as a sunshade and allow the sun's radiation pressure to drive it to its destination like a sailing ship. Such a sailing trip is readily feasible.

Near the NEO, the approximately 500-meter mirror would have to be aligned so that it bundles the sunlight and directs it to the "bomb". Their surface should be heated to around 2000 degrees. This selective evaporation would have the same effect as a rocket propulsion system: the dust and gas masses would gradually bring the dangerous object out of the collision course.

=== Rüdiger Vaas

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