For months, the asteroid dubbed “2021 QM1” topped risk lists around the world. For all we knew, there was a real chance that the space rock would hit Earth on April 2, 2052. But after a series of scientifically impressive observations and calculations, the asteroid team from the European Space Agency (ESA) worked with experts from the European Observatory (ESO) to remove the asteroid from the risk list. These observations include the analysis of “the faintest asteroid ever observed” with one of the most sensitive telescopes on Earth.
2021 QM1 was first discovered on August 28, 2021 by the Mount Lemmon Observatory in Tucson, Arizona. Initially, the discovery was nothing special, as about a dozen new near-Earth asteroids are discovered every dark night. But the following routine observations, obtained from telescopes around the world, began to tell a more worrisome story.
“These early observations gave us more information about the asteroid’s path, which we then projected into the future. We could see its future paths around the sun and in 2052 it could get dangerously close to Earth. The more the asteroid was observed, the greater that risk became,” said Richard Moissl, ESA’s Head of Planetary Defense, in a press statement.
It should be noted, however, that orbit calculations based on just a few nights of observations carry uncertainties. This is why asteroids often appear on ESA’s risk list shortly after they are discovered and are later removed. Once more data is collected, the uncertainties narrow and the asteroid has mostly proven to be safe. But on this occasion that was not possible.
Coincidentally, just as the asteroid’s risk seemed to increase, a cosmic alignment was in the works and got in the way of observation: the asteroid’s path brought it closer to the sun as seen from Earth, which meant it was impossible to see for months due to the brightness of the sun.
“We just had to wait. But to top it all off, we knew that the 2021 QM1 in its current orbit was also moving away from Earth – meaning that by the time it faded from the sun’s glare, it could be too faint to detect.” explains Marco Micheli, astronomer at ESA. Near-Earth Object Coordination Center (NEOCC), in a press statement. But they were preparing while they waited.
ESO’s Very Large Telescope (VLT), located in the Atacama Desert in northern Chile, was ready for observation. The VLT would aim its 8-meter mirror at the disappearing 50-meter-long asteroid as soon as it came out of sunlight, weather conditions permitting.
“We had a short time to spot our risky asteroid. To make matters worse, it passed through part of the sky with the Milky Way just behind it. Our small, faint, receding asteroid should be found against a backdrop of thousands of stars. These would turn out to be some of the trickiest asteroid observations we’ve ever made,” said Olivier Hainaut, an astronomer at ESO, in a press statement.
On the night of May 24, VLT took a series of new images and once the data came in, Olivier and Marco began processing them by stacking successive observations and removing background stars in a time-consuming process. This resulted in the positive detection of the faintest asteroid ever observed.
At the time of observation, 2021 QM1 was 250 million times fainter than the faintest stars visible to the naked eye from a dark spot. Olivier was sure that this tiny spot was an asteroid. Marco could go one step further and confirm that given its location, it was the asteroid they were looking for.
These new observations were used to refine the projected path of the high-risk asteroid, ruling out an impact in 2052, and 2021 removed QM1 from ESA’s risk list, leaving only 1,377 other asteroids on the list.
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