Since December, astronomers have been carefully studying whether an asteroid between 130 and 300 feet long will impact the Earth in just under eight years. And the odds, overall, seem to be rising.
On Jan. 29, the chances of this asteroid (named 2024 YR4) striking our planet on Dec. 22, 2032, were 1.3 percent. Then they rose to 1.7 percent on Feb. 1, before dropping the next day to 1.4 percent.
Then on Thursday, they leaped to 2.3 percent, before slipping slightly to 2.2 percent on Friday. That’s a one-in-45 chance of an impact (but also a 44-in-45 chance of a miss).
To many, this feels unsettling. But what appears scary is, in fact, typical when it comes to newly discovered near-Earth asteroids.
“It is true that the probability of impact has doubled recently, but that doesn’t mean that it will keep doing so,” said Davide Farnocchia, a navigation engineer at the Center for Near-Earth Object Studies at NASA’s Jet Propulsion Laboratory in California who is involved in overseeing the programs that make these orbital calculations. “What matters is that the probability of impact is very small, and that it is likely to drop to zero as we keep observing 2024 YR4.”
Two key organizations are involved in calculating these impact odds. They are the NASA center Dr. Farnocchia works at, and the Near-Earth Object Coordination Centre in Italy, which is part of the European Space Agency. These groups are the cartographers of near-Earth space, looking out for parts of the cosmic map where they can mark “here be dragons” — in this case, potentially hazardous asteroids or comets.
When an asteroid (or a comet) is discovered, both centers use their automated orbital dynamics software (Scout and Sentry for NASA, and Meerkat and Aegis for the European center) to consider the available observations of the object.
When the asteroid’s many possible future orbits are plotted out, some may result in an Earth impact. But many of these orbits will shift away from Earth, so the probability of an impact will be low. It’s as if the asteroid has a wide spotlight that’s beaming out ahead of it. Earth is initially caught in the beam, but so is a lot of the space around it.
Then, more observations come in. The spotlight of those possible orbits shrinks. The outliers are gone. But Earth is still in the spotlight and now takes up proportionally more space in it. “Earth now covers a larger fraction of the uncertainty, and so the probability of impact has gone up,” Dr. Farnocchia said.
This can happen for some time as observations continue. “That’s why the impact probability rises,” said Juan Luis Cano, an aerospace engineer with the Near-Earth Object Coordination Centre. “Little by little, it grows.” And it explains what’s been happening with 2024 YR4’s odds.
Sometimes, as has been the case for 2024 YR4, the odds can fluctuate slightly. This is because the quality of some observations can be better or worse than others, which can move the cluster of anticipated orbits around a bit. “All this is expected,” Dr. Farnocchia said.
Normally, additional observations significantly reduce the orbital uncertainty, and Earth falls out of that trajectory — dropping the impact odds to zero. Humanity will have to see whether the same outcome awaits 2024 YR4.
Telescopes can observe 2024 YR4 until April, after which time it will be too distant and faint to see until another Earth flyby in 2028. By April, it’s likely that astronomers will have enough observations of the asteroid, spread across several months, to know its orbit precisely, and they will ultimately determine that no impact will occur in 2032. “People should not be worried at this point,” Mr. Cano said.
Nevertheless, 2024 YR4 is being taken seriously by NASA and ESA. “Even though the probability of impact is small, it is larger than we usually find for other asteroids,” Dr. Farnocchia said.
If this asteroid were to hit Earth, it would unleash a destructive force similar to a nuclear bomb. And the current uncertainty over its future orbit extends to its possible impact locations, which include a mix of uninhabited, sparsely populated and densely populated areas: the eastern Pacific Ocean, northern South America, the Atlantic Ocean, parts of Africa, the Arabian Sea and South Asia.
2024 YR4 is unlikely to be on a collision course. But “we don’t get to choose when the next significant asteroid impact will be,” Dr. Farnocchia said. “We just don’t want to take any chances, and so we will keep tracking 2024 YR4.”
And if it does become a problem, it may be time for Earth to rally anti-asteroid defenses.
Robin George Andrews is the author of “How to Kill an Asteroid,” a book about the science of planetary defense.
