YR4: the asteroid with a tiny chance of hitting Earth
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.”
