The asteroid Apophis, which scientists feared 20 years ago, is no longer as dangerous as the space rock 2024 YR4. Experts are evaluating the potential damage, although the danger is still minimal.
On Tuesday, astronomers declared that the asteroid 2024 YR4 was now the most likely large spacerock to strike Earth in history. First discovered in December, the object is between 130 and 300 feet long and is predicted to pass very close to the Earth in 2032. As of tow, its chances of striking Earth on December 22 of that year are 3.1 percent.
That surpasses the danger previously posed by the considerably larger asteroid Apophis, which was found in 2004. Its initial odds of striking Earth in 2029 were estimated by astronomers to be 2.7 percent. The likelihood of an impact at any point during the following century was eliminated by additional observations of Apophis. But for a while, the idea was unnerving.
Even though 2024 YR4 is far smaller than Apophis, it can still cause a great deal of destruction. Where it would enter the Earth's atmosphere would determine a lot.
Although 2024 YR4 would not wipe out a country, a direct hit might scar or destroy a city. And there is a slight probability that it will. Much of the object's expected path goes over the water, although several potential impact sites are close to major cities such as Bogotá, Lagos, and Mumbai.
The kinetic energy of an asteroid indicates how catastrophic its impact might be. And, because asteroids typically move at the same speed — approximately 38,000 miles per hour — the significant variable is their mass.
Astronomers can only estimate the mass of 2024 YR4 based on a small number of observations. "We don't know how dense or porous it is, so its mass, and thus the energy it would release if it hit the Earth's surface or exploded in the atmosphere, is unknown," said Mark Boslough, a physicist at the Los Alamos National Laboratory.
However, "the bigger it is, the worse it is," said Gareth Collins, an asteroid impact scientist at Imperial College London. Small gains in size lead to massive jumps in destructive capacity. The rule of thumb is that if an asteroid's radius doubles, it has eight times the kinetic energy; a 300-foot asteroid will cause significantly more damage than a 130-foot one.
Composition is equally crucial. An asteroid composed primarily of iron, for example, would penetrate deeper into the atmosphere and deliver a more devastating blow to the Earth. However, 2024 YR4 is statistically more likely to be a stony asteroid, which is more prone to fragmentation when it heats up during its atmospheric descent.
However, even an asteroid's aerial immolation, known as an airburst, can be exceedingly powerful.
If 2024 YR4 is stony and on the smaller end of forecasts (130 feet), the chances of an airburst are considerable, according to Kathryn Kumamoto, the leader of the planetary defense program at the Lawrence Livermore National Laboratory.
"The main comparison point we have for a stony asteroid impact of this magnitude is Tunguska," says Dr. Kumamoto. In 1908, an asteroid equal in size to 2024 YR4 exploded above a sparsely populated area of Siberia. It produced a blast wave of approximately 12 megatons, similar to that of a nuclear weapon, which leveled a forest more than twice the area of New York City.
A 130-foot boulder erupting above the open ocean, or even closer to shore, would be less worrying because it "would be unlikely to cause a significant tsunami," according to Lorien Wheeler, an expert at NASA's Ames Research Center in California.
An airburst above a metropolis would be more uncomfortable. Windows would explode inward, spraying glass like a shotgun, causing massive damage to buildings. Some injuries may be life-threatening.
The asteroid's entry angle into the atmosphere is important. If it comes in straight down, it may get closer to the earth before bursting, potentially causing much more damage. A more gradual entry angle may result in an explosion at a significantly higher height.
If 2024 YR4 is 300 feet long, its impact "could cause more severe damage," according to Michael Aftosmis, an analyst with the Asteroid Threat Assessment Project.
Such an asteroid "is more likely to make it through the atmosphere, particularly if we are unlucky and the entry angle is steep," added Dr. Kumamoto. "A portion could make it to Earth's surface relatively intact."
According to Dr. Kumamoto, an impact in a remote ocean far from land would represent much less risk, with big waves that would soon shrink before reaching land. However, a splashdown on a shoreline could result in a tsunami capable of flooding neighboring territory.
If this larger version of the 2024 YR4 collides with solid ground, it might create a crater about two-thirds of a mile in diameter.
"The asteroid would create an enormous explosion," Dr. Boslough added. And the blast wave would be extremely powerful. Multistory buildings surrounding the crater would buckle and crumple, bridges would collapse, and cars, trees, and people would be tossed everywhere. Dr. Boslough also discussed the possibility of a "hot jet of asteroid vapor that would descend to the surface and incinerate everything."
People near Ground Zero would most likely perish, he claimed. Even those tens of miles away would be struck by a thunderous, expanding blast wave. "People in the local area would be at risk of serious injury," Dr. Kumamoto warned.
According to experts, it is still unlikely that 2024 YR4 will impact Earth in 2032. However, the wide range of impact possibilities is precisely why planetary defenders are treating this asteroid as lethal.
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