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Scientists comment on asteroid 2024 YR4 which may hit Earth in 2032.
Prof Martin Ward, Emeritus Temple Chevallier Professor of Astronomy, Durham University, said:
“The way these percentages for the chance of impact are calculated are based on extrapolations of the asteroid trajectory and the position of the Earth when the asteroid arrives. They get more accurate with time as we get more data. Think of it as a circle in the sky (much bigger than the asteroid itself). The direction of future travel plus the position of the Earth could take any line out of the circle towards the Earth. Most lines are projected to miss, but a few would hit. As our data on the asteroid trajectory and Earth’s position when it arrives get better by tracking it with time the circle becomes smaller, and the number of possible future lines decreases. If the lines that previously hit the Earth are now outside the improved circle, then the chances go down (and maybe vanish). But if the smaller circle still includes the lines that hit the Earth, then the chances of hitting us go up correspondingly. So, as time goes on we will get either good news, that eventually the future direction of travel misses us, or bad news, that the ratio of lines that hit us to those that miss us, goes up. If that trend continues, then duck… Before that happens, it will be possible to predict the path it would take through the atmosphere, and hope for the best. The Earth is 70% covered by water, and of the remaining land mass, 33% is desert. So the question will be… “do you feel lucky?”
Prof Danny Steeghs, Professor and Head if Astronomy & Astrophysics Group, Department of Physics, University of Warwick, said:
What is the situation?
“We monitor many asteroid orbits, but this object was not previously known. It was identified in December 2024, near its closest approach, when it was bright and moving through the sky fast. Routine is to model the orbit of newly discovered asteroids including projecting forwards in time to see if there are any concerns around close approaches with the Earth. Almost always the answer is no. In this case we could not, and still cannot, fully rule that out. This is rare, it has been many years since we have had an asteroid with this level of impact probability.”
How strong is the science behind a potential earth impact?
“We understand the dynamics of solar system orbits well. It’s about measuring the orbit with better precision and characterising the nature/size of the object. The Earth is a small target on the scale of the solar system, but orbit calculations have a finite precision since they are based on measurements of the position of the asteroid at different times since its discovery. These measurements have a finite precision, and we do not have many yet, and we can accurately translate this into a probability of impact during future passes. At the moment this is a few 2%, which is considered high enough to warrant further and urgent observations.”
How worrying/expected is this?
“It is not really worrying, expectations are that as we refine the orbit of the asteroid, the probability of impact will reduce. Its better maybe to think of it as, we cannot fully rule out that it may impact, rather than expecting it to impact. The next steps are clear – securing more data and improving our precision/confidence.”
How is the risk of collision calculated, and could it change?
“The measurements that feed into these calculations are collected by a number of telescopes and observatories. These are globally shared so that a number of teams can calculate orbit projections. Each time new data is added, the calculations can be refined. This is why the number is changing and in the early stages it can change more erratically. As precision improves, it will tend to settle down to the point where we can be very confident. This could take some time, as the asteroid is getting faint quickly, and it may not be fully settled until it passes again in 2028.”
What could we do to stop it / prepare?
“We do not yet accurately know the size of the object, and what any impact might do and how we could mitigate it depends on that. We have tested deflecting an asteroid by sending a probe to hit it from the right angle, just to change its orbit a little. That is the preferred intervention route, but what is involved depends on the mass, size and orbit.”
Dr Darren Baskill, Physics & Astronomy Lecturer, University of Sussex, said:
“An asteroid passes by the Earth, within the orbit of the Moon, typically every month – which is very close in cosmic terms. The last time this happened was when a 26m asteroid passed us by on the 4th of February 2025.
“Watching an asteroid approach is a bit like watching a long pot in snooker – you don’t know if the ball will go in a pocket until the very last moment, and there is always a good chance of the ball rattling in the jaws of the pocket and missing. This is why there is so much uncertainty surrounding this asteroid. Of course, the distances involved in astronomy are much, much larger, making accurate long-term predictions difficult.
“It is worth noting that the predicted size of asteroid 2024 YR4 is 40-70m, which is only around 3 times larger than the asteroid that famously struck Russia in 2013. While there were large amounts of minor damage in that event, including large numbers of windows being blown out in the area due to the resulting shockwave, there were no fatalities, nor any major damage recorded.
“At the moment, while there are teams around the world who are able to detect asteroid threats better than ever before, there are no systems in place that could prevent an asteroid strike should one be found to be on a collision course. Hopefully, that will change over the next 7-and-a-half years, just in case!”
Dr James O’Donoghue, planetary scientist at the University of Reading, said:
“Scientists are getting better at tracking the asteroids that could hit Earth, with the result that we have become accustomed to asteroids having incredibly low impact probabilities, typically in the order of one in tens of thousands. That’s why a 1-in-49 chance is making us sit up and notice.
“This level of risk demands serious attention, and it’s clear that it is being taken seriously, as seen by NASA’s response and the high-level notifications that have been triggered. The recent diversion of the James Webb Space Telescope to observe this object is particularly noteworthy. Understanding its precise size is crucial – an 80-meter asteroid would impact with eight times more energy than a 40-meter one, as doubling the diameter increases the volume and mass by a factor of eight. To put this in perspective, a 40-meter asteroid carries the energy of a few megatons of TNT, comparable to a nuclear warhead, while a 90-meter asteroid exceeds 50 megatons – roughly equivalent to the Tsar Bomba, the most powerful nuclear device ever detonated.
“The fact that we can predict this potential impact many years in advance is a testament to how well humanity is advancing in planetary defence through astronomy. It’s also a reminder than we have discovered less than half the total number of similarly sized asteroids so far, so there is much more work to be done.”
Dr Olivier R. Hainaut, Astronomer, European Southern Observatory, said:
What is the situation?
“Observatories around the world (including ESO) are working to refine the orbit. This takes time, as one has to wait for the asteroid to move to perform more measurements. We can fairly easily observe YR4 for another month with large telescopes, then it will become harder and harder. By early April it will be out of reach of most telescopes. Hopefully the orbit will be refined enough to completely rule out an impact.
How is the risk of collision calculated and could it change?
“Orbital calculation and celestial mechanics… You measure the position of the asteroid in front of the background stars over as many nights as possible, then solve for the orbit, which is the 3D trajectory of the asteroid in space. The longer you can measure, the better you know the orbit. Currently, we could observe YR4 for a little less than 2 months over its 4y orbit, so we need to extrapolate a lot, hence the large uncertainty. As we will observe more, the orbit will be refined, and we will know better where the asteroid will be in 2032. Eventually, the probability of impact will converge towards one of two possible values: 0% or 100%… Hopefully 0%, and hopefully by the end of this visibility period. Note that the probability is likely to go up even if it ends up going down to 0% -that’s normal.”
“Measuring and computing the orbit is very robust- celestial mechanics is very well known, and demonstrated by the navigation of spacecrafts all around the solar system.
“Measuring the size and composition of the asteroid is not scientifically complicated, but is tricky in this case because the object is very faint.”
How worrying/expected is this?
“How worrying: currently, the probability of impact is ~2%. It is typical to start preparing for disaster relief when the probability goes above 1% (astronomers did not make that number up – it is what is customarily used for all kind of disasters). IF the probability stays above 1% at the end of the visibility period (~April), it will be time to start possible mitigation space mission. Keep in mind that IF these mission would fail and IF the asteroid does impact, it is not a large one. Possibly a few times larger than the one that blasted above Chelyabinsk in 2013, but ~150x SMALLER than the Chicxulub dinosaur killer.
“How expected: look at the Moon. It is covered with craters, and the Earth is just next to it. Collisions with asteroids are expected, which is why we try to discover as many as possible (and all the “large” ones). The question is not “if”, but “when” and “how big”.”
What could we do to stop it / prepare?
“Short answer: first, refine the orbit to determine whether it will hit or not. If it will, then nudge it to slightly change its orbit so that it misses the Earth. The sooner the better, as a later nudge will need to be larger. To nudge it, refer eg to the DART mission, which did it as a test on an asteroid about 2x the size of YR4.
“DON’T break the asteroid (refer to a series of bad SciFi movies): you would end up with many fragments to deal with.”
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