Telescopes around the world are currently trained on a building-sized asteroid hurtling in Earth’s direction, in an effort to try to understand whether it might hit us. Our current best guess is that this object, called 2024 YR4, has a 1-in-43 chance of striking in 2032, the highest risk of any known asteroid. But what does that number really mean, and when should we start panicking?
Astronomers are continuously tracking thousands of asteroids as they move through our solar system. This information is then collated at the Minor Planet Center, based at the Smithsonian Astrophysical Observatory in Massachusetts, from which space agencies can calculate the trajectory and possible impacts with Earth.
But this is a messy process. Because no observation is perfect, the data will have errors and uncertainties that can lead to a wide range of different projected orbits. The proportion of these orbits that intersect with Earth is how the chance of an impact is calculated, says at the European Space Agency – similar to how meteorologists use multiple weather forecasts to estimate the probability of rain.
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With more data, this “cloud of uncertainty” shrinks and astronomers can say with greater certainty whether an asteroid will strike. The chance of 2024 YR4 hitting Earth, for instance, has been upgraded from 1-in-83 since it was initially spotted on 27 December, first to 1-in-67, then to 1-in-53 and now . It will probably change again with more observations.
If the asteroid is on a collision course with Earth, then this number will slowly approach 100 per cent, but the scenario in which it narrowly misses Earth will also look similar until astronomers get enough data on the trajectory to be able to call it a close shave. “This uncertainty is going to be reducing, the impact probability is going to be increasing, until the moment when the Earth falls out of that uncertainty region,” says Cano.
Along with the risk of collision, what makes 2024 YR4 particularly concerning is its estimated diameter of 40 to 70 metres, which is larger than the so-called Chelyabinsk meteor that hit Russia in 2013, injuring people and damaging buildings. That space rock was estimated to be nearly 20 metres across.
Combining the impact risk and size gives a score on the 10-point Torino rating system that reflects the likely consequences. 2024 YR4 is currently rated 3, high enough to trigger several international bodies to take further action. The United Nations-affiliated International Asteroid Warning Network, which coordinates telescopes around the world, will study the asteroid more closely, while the UN-endorsed Space Mission Planning Advisory Group (SMPAG) will begin considering whether to draw up an asteroid deflection mission, similar to NASA’s Double Asteroid Redirection Test that successfully knocked a 160-metre-wide moonlet called Dimorphos off course in 2022.
Such plans would need to kick into gear surprisingly soon. In April, 2024 YR4 will fly beyond the view of most telescopes on Earth and won’t be bright enough to see again until 2028 – before disappearing once more until just before a possible impact in 2032. That means groups like the SMPAG need to make a decision based on the available evidence within a couple of months, given the amount of time it takes to plan a mission.
Even this timeframe would be tricky, however, says Cano. “Eight years would be very, very tight. We typically need between three and five years to construct and build a spacecraft in order to do this job, and then we would also need between half a year and a year to fly to the object.” A further wrinkle is that SMPAG’s rules mean that it can only recommend a deflection mission if an asteroid is bigger than 50 metres, but the current uncertainty range for 2024 YR4 straddles that figure, says Cano. For now, all we can do is wait for more data.
