Anyone who worries that the fate of the dinosaurs could one day befall us if the earth is hit by another large asteroid is usually quickly reassured.

According to NASA, for example, almost all of the large rocks that could pose a threat to the earth are known.

It already has more than 90 percent of those objects that will come close to Earth in the future (“Near Earth Objects” – NEO) and have a size of more than one kilometer in view.

She is still working on achieving this quota for the objects that are more than 140 meters in size.

Sibylle Anderl

Editor in the feuilleton, responsible for the "Nature and Science" department.

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30,445 asteroids whose orbits cross the earth's orbit are currently listed in catalogues, 857 of them are larger than one kilometer.

For comparison: the size of the asteroid that struck around 66 million years ago and caused a worldwide mass extinction is estimated at 10 to 15 kilometers.

All over the world, many telescopes, operated by amateurs and professional astronomers alike, keep an eye out for other objects.

However, one should not feel too safe.

Because the observation programs have a blind spot.

And that it needs to be taken seriously is shown by a discovery by astronomers announced this week that it is "the largest NEO potentially dangerous to Earth found in years."

The blind spot in the search for asteroids arises from the fact that objects in the sky that are close to the Sun are difficult to observe.

Earthly telescopes can only be aimed at this area during twilight.

But then the light has to traverse a large area of ​​the atmosphere interfering with the observations, and the sky is still very bright.

Compared to nighttime observations, these circumstances increase the noise in the data: it is extremely difficult to detect asteroids using such observations.

You need large telescopes and a lot of patience, because you can only observe twice a day for about ten minutes.

Dangerous objects near the sun

This is exactly what astronomers have now done with the four-meter Chilean Blanco telescope and published their first results in the "Astronomical Journal".

Attached to this telescope is the Dark Energy Camera, a large-scale survey of the sky whose main task is to unravel the cosmological mystery of dark energy.

Since 2019, the telescope has also been used to search for objects orbiting within the orbits of Earth and Venus.

Two asteroids were discovered within Earth's orbit.

The third, dubbed "2022 AP7", which is crossing Earth's orbit and was discovered on January 13, 2022, is the potentially dangerous asteroid, which is around one kilometer in diameter.

However, it does not pose an immediate danger, at best this could change in a few thousand years.

However, its existence underscores once again how important it is to also keep an eye on objects that are close to the sun.

Normally, when observable at night, AP7 is in fact at a great distance from Earth.

When he's close to us, he hides near the sun.

AP7 probably has this behavior in common with most of the large NEOs that are still unknown to us.

Knowing the populations of asteroids in the interior of our solar system is important to understand how these objects have been transported through the inner solar system over time.

So far, only 25 asteroids are known to have orbits entirely within Earth's orbit - while tens of thousands have been observed outside.

Two more have now been added inside: "2021 PH27" was discovered in August 2021, has the shortest orbital period of all known asteroids at 113 days and is also about one kilometer in size.

With a probability of 0.7 percent it will collide with Venus in the next million years, on its way around the sun its surface reaches temperatures of 500 degrees Celsius.

"2021 LJ4", on the other hand, takes a good six months to orbit the sun and is 300 to 400 meters tall.

The astronomers are surprised that the three newly discovered asteroids are mostly larger than one kilometer, although the survey should be particularly sensitive to small asteroids.

A possible explanation would be that small asteroids have less stable orbits or that they suffer particularly from the extreme gravity and radiation close to the sun.

According to the astronomers, whether smaller asteroids are actually rarer in the vicinity of the sun must first be investigated with the help of further observations.

There are already corresponding plans: At the end of the decade, NASA's NEO Surveyor Mission will set new standards in the search for potentially dangerous asteroids.