YR4: the asteroid with a tiny chance of hitting Earth 
2025-02-04
Category: news

UN planetary defence organisations are closely monitoring an asteroid that has a tiny risk of hitting the Earth.The European Space Agency (ESA) has said that it has an almost 99% chance of safely passing Earth on 22 December 2032, but a possible impact "cannot yet be entirely ruled out".The probability that the asteroid, called 2024 YR4, may impact Earth on 22 December 2032 is currently estimated to be 1.3%.Dr Robert Massey of the Royal Astronomical Society, says that he is "not panicking or losing sleep over it"."There is no need for alarm," he said. "The thing about this kind of event is that historically they tend to go away when the calculations are refined."We need to be aware alert and we need to give astronomers the resources they need to track these kinds of threats so that we can take action as soon as possible." YR4 was detected on 27 December 2024. Astronomers calculated that it was between 40m and 90m across. This would have the power of a nuclear bomb were it to hit the Earth and cause severe damage if the impact was in a populated area. But it is much more likely that YR4 would fall into the ocean or a remote part of the planet. It is too far away from Earth and there are too many uncertainties at this stage to determine where a potential impact could occur in the unlikely event of a collision.Since early January, astronomers have been using telescopes to calculate the asteroid's size and trajectory more precisely. YR4 is now rated at level 3 out of 10 on the Torino Impact Hazard Scale: "a close encounter that warrants attention from astronomers and the public". A collision is only certain when it reaches 8, 9 or 10, with the numbers rising in line with the damage likely caused.
When asteroids are initially calculated to have a small probability of hitting the Earth, that impact probability usually drops to zero after additional observations. This happened in 2004 when an asteroid called Apophis was calculated to have a 2.7% chance of striking Earth in 2029; further observations ruled out an impact.Any object that may be more than 50m wide and has a greater than 1% chance of hitting the Earth, triggers a set of precautionary measures. These are to ensure that the threat, however tiny, is closely monitored and, if necessary, steps are taken to nullify it. The first stage is to activate two UN-endorsed asteroid reaction groups: the International Asteroid Warning Network (IAWN), chaired by Nasa, and the Space Mission Planning Advisory Group (SMPAG), chaired by the European Space Agency.The SMPAG is having a series of meetings this week to determine its next steps.  It has already concluded that it is too early to take immediate action but said that it would "monitor the evolution of impact threat and possible knowledge about the size closely". Another meeting to take decisions on future activities will be held towards the end of April or early May, or earlier, "if the evolution of the threat merits".If the asteroid's impact probability remains above the 1% threshold, SMPAG will provide recommendations to the UN and may begin to evaluate options.In the unlikely event YR4 were headed our way, one option would be to divert it by hitting it with a robotic spacecraft, as was successfully tested out with Nasa's Dart mission in 2022. That changed the course of an asteroid that was not on a collision course with the Earth."Nasa's Dart mission showed that we have the means to divert an asteroid, but only if we spot it early enough," says Dr Massey.YR4 is currently moving away from Earth in almost a straight line, making it difficult to accurately determine its precise orbit before it returns towards Earth.Over the next few months, the asteroid will begin to fade from view, after which  it will be monitored by ground and space telescopes.According to ESA "It is possible that asteroid 2024 YR4 will fade from view before we are able to entirely rule out any chance of impact in 2032. In this case, the asteroid will likely remain on ESA's risk list until it becomes observable again in 2028."