The asteroid 2024 YR4, estimated to be about 40 to 90 metres in diameter, was discovered in late December last year on an orbit that could cause it to collide with Earth on 22 December 2032. Because of its size and likelihood of impact, the asteroid quickly rose to the top of the European Space Agency's (ESA) risk list, a catalogue of all space rocks with any chance of impacting Earth.

ESO's VLT was used to observe 2024 YR4 in mid-January, giving astronomers the crucial data they needed to more precisely calculate its orbit. Combined with data from other observatories, the very precise measurements from the VLT improved our knowledge of the asteroid's orbit, leading to an impact probability exceeding 1% -- a key threshold to trigger disaster mitigation. More observations were triggered and the International Asteroid Warning Network issued a potential asteroid impact notification, alerting planetary defence groups, including the Space Mission Planning Advisory Group, about the possible impact.

With multiple telescopes around the world observing the asteroid, and astronomers modelling its orbit, the impact probability rose to around 3% on 18 February, the highest impact probability ever recorded for an asteroid larger than 30 metres. However, just the next day, new observations made with ESO's VLT cut the impact risk in half.

This rise and fall of the asteroid's impact probability follows an expected and understood pattern. To know where the asteroid will be in 2032, astronomers extrapolate from the small bit of the orbit measured thus far. ESO Astronomer Olivier Hainaut makes an analogy: "Because of the uncertainties, the orbit of the asteroid is like the beam of a flashlight: getting broader and broader and fuzzier in the distance. As we observe more, the beam becomes sharper and narrower. Earth was getting more illuminated by this beam: the probability of impact increased."

The new VLT observations, together with data from other observatories, have allowed astronomers to constrain the orbit enough to all but rule out an impact with Earth in 2032. "The narrower beam is now moving away from Earth," Hainaut says. At the time of writing, the impact probability reported by ESA's Near-Earth Objects Coordination Centre is around 0.001% and the asteroid no longer tops ESA's risk list.

As 2024 YR4 is moving away from Earth, it has become increasingly faint and difficult to observe it with all but the largest telescopes. ESO's VLT has been instrumental in observations of this asteroid because of its mirror size and superb sensitivity, as well as the excellent dark skies at ESO's Paranal Observatory in Chile, where the telescope is located. This makes it ideal to track faint objects such as 2024 YR4 and other potentially dangerous asteroids.

Unfortunately, the same Paranal's pristine dark skies that made these crucial measurements possible are currently under threat by the industrial megaproject INNA by AES Andes, a subsidiary of the US power company AES Corporation. The project is planned to cover an area similar in size to that of a small city and be located, at the closest point, about 11 km from the VLT. Due to its size and proximity, INNA would have devastating effects on the quality of the skies at Paranal, especially due to light pollution from its industrial facilities. With a brighter sky, telescopes like the VLT will lose their ability to detect some of the faintest cosmic targets.

Hainaut warns: "With that brighter sky, the VLT would lose the faint 2024 YR4 about one month earlier, which would make a huge difference in our capability to predict an impact, and prepare mitigation measures to protect Earth."

• RELATED TOPICS
  • Space & Time
    • Space Exploration
    • Space Telescopes
    • NASA
    • Space Missions
    • Asteroids, Comets and Meteors
    • Astronomy
    • Space Station
    • Solar System

• RELATED TERMS
  • European Southern Observatory
  • Asteroid belt
  • Space observatory
  • Compton Gamma Ray Observatory
  • Definition of planet
  • Active optics
  • Edwin Hubble
  • Near-Earth object