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Bennu is a slow-spinning, fast-flying asteroid, and it's coming our way.

Bennu zips by Earth every six years. Experts have been keeping close tabs on the lumpy space rock since discovering it in 1999. They are watching it because Bennu measures about 1.6 kilometers (1 mile) around. That is big enough to pack a wallop if it slams into Earth.

If Bennu does hit us, and there is a chance that someday it could, the asteroid would hurtle into the atmosphere at nearly four times the speed of sound. The collision would release enough energy to cause widespread devastation across the globe. But take heart: Life probably would not disappear.

“We want to know where Bennu is going and what’s the likelihood that it will hit the Earth,” he says.

Just like Earth and the other planets, asteroids orbit the sun. Bennu is one of nearly 1,500 “potentially hazardous asteroids” that scientists are tracking with particular attention. Each of these asteroids could make a threatening and close approach to Earth. For an asteroid, “close” means it must come within 7.5 million kilometers (4.8 million miles) of Earth. But don't lose any sleep: For now, Bennu’s closest predicted brush with us won’t happen until 2135, when it will pass within about 300,000 kilometers (186,000 miles) of Earth.

This image maps the orbits of the nearly 1,500 potentially hazardous asteroids that make close approaches to Earth. The sun is the yellow dot at center. The orbits of Mercury, Venus, Earth and Mars are shown in white.

By studying an asteroid’s movements, scientists can calculate the chance of a collision. Of all the potentially hazardous asteroids, Bennu is one of the most likely to smash into Earth.

Scientists who track asteroids have a simple goal: Find any Earth-bound megarocks before they find Earth. These researchers are designing devices to capture asteroids. Other scientists are working to knock them far enough off course to avoid a deadly collision. Still other scientists, including Lauretta, are planning space missions to visit asteroids — or even bring them back to Earth. In the process, these experts are learning more about where asteroids came from and where they’re going.

Shaped by collisions

Shortly after sunrise on February 15, 2013, people in central Russia watched a ball of fire careen across the sky. The blaze came from a meteor. A meteor is an object from space that falls into Earth's atmosphere. In this case, the space rock measured 20 meters (66 feet) across when it entered the atmosphere at 67,000 kilometers (41,600 miles) per hour. Friction with Earth’s air heated the meteor. It exploded high in the atmosphere, creating a flash brighter than the sun and a thunderous boom that shattered windows. No one died, although some 1,500 people suffered cuts and other injuries. The explosion also showered the ground with meteorites, which are small chunks of rocky debris that survive the fall and land on Earth.

The meteor likely broke off an asteroid during a collision hundreds of millions of years ago. That’s what researchers reported in a May study published in Scientific Reports, after studying the meteor's flight and the meteorites recovered from the ground.

Asteroids are as old as the planets. Over their long lifetimes, ancient and powerful collisions have shaped them. Scientists have seen evidence of that firsthand. In 2003, Japanese scientists sent a spacecraft called Hayabusa to land on an asteroid. Called Itokawa, the asteroid looks a little like a giant space cucumber. Planetary scientist Tomoki Nakamura at Tohoku University in Japan led the study of the samples of the asteroid brought back to Earth.

When Nakamura opened the canister containing the samples, it looked empty. But by studying its walls, he found particles of dust. He used an electron microscope to take a close look at the grit. This tool fires a beam of electrons at a sample. Some electrons bounce off the particles, and some are absorbed, generating X-rays with different energies. By studying these X-rays, scientists can identify the chemical makeup of a sample.

A Japanese spacecraft captured this image of Itokawa in 2005 as it approached the asteroid. Japanese researchers used samples later returned by the spacecraft to determine the asteroid had broken off a much larger space rock in the distant past.

That's what Nakamura and his team did. And they found minerals. One of them, called troilite, is very rare on Earth but common in asteroids. That finding told the scientists that Hayabusa's grit really did originate in an asteroid.

In fact, Nakamura says, the particles told an exciting story. His probing suggests Itokawa used to be 100 times bigger than it is today. The asteroid likely broke apart during some long ago impact with another asteroid. That impact did not just break up the rocks. It also fractured the particles that Nakamura studied.

The microscopic surfaces of the particles revealed to scientists the presence of tiny craters created during high-impact collisions. Researchers reported the discovery, two years ago, in the Proceedings of the National Academy of Sciences. Eizo Nakamura, a planetary scientist at Okayama University in Japan, led the research team.

Large craters, visible to the naked eye and scattered across the surface of Earth, show that asteroid impacts are a natural part of history. Collisions, with rocks bigger than Bennu, have been catastrophic. For instance, 65 million years ago, an asteroid smashed into the Yucatán Peninsula of what is now Mexico. The dust and debris kicked up during the collision changed Earth’s climate so dramatically that it is believed to have caused the extinction of the dinosaurs (or at least shoved them in that direction).

But don’t despair: There’s hope for humankind. Scientists predict that an “extinction-level” asteroid, measuring 5 kilometers (3 miles) across, crosses paths with Earth only about once every 1 billion years. That means the scientists working hard to avoid such an event should have plenty of time to prepare.

Source from : https://student.societyforscience.org/sciencenews-students