NASA has landed a spacecraft on an asteroid with an uneven, rocky surface, NPR reported. The van-sized spacecraft had been orbiting the asteroid for two years before touching down on it. What about near-Earth asteroids?
According to NPR, NASA just made a noteworthy accomplishment in the name of space exploration. “A NASA spacecraft successfully touched down on a skyscraper-sized asteroid 200 million miles away, in order to collect a small amount of rock and dust that can then be returned to Earth,” the article said. “The probe, called OSIRIS-REx, is about as big as a 15-passenger van, and it was aiming for a specific spot inside a boulder-strewn crater.
“The maneuver was tricky and fraught with peril, as the spacecraft had to reach a safe area that’s only the size of a few parking spaces.”
Two hundred million miles is just over two astronomical units (au) away. One au is the distance from the Earth to the Sun—approximately 93 million miles, or 150 million kilometers. Some asteroids pass by us much more closely.
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Outer space is lousy with asteroids. They sail through the cosmos at impossible speeds. Some fly close to Earth, hence the somewhat unimaginative name “near-Earth asteroids” (NEAs).
“These asteroids have orbits that travel partly or fully inside a distance of 1.3 astronomical units from the Sun,” said Dr. Sabine Stanley, Bloomberg Distinguished Professor in the Morton K. Blaustein Department of Earth and Planetary Sciences at Johns Hopkins University. “That’s already pretty close to Earth’s orbit, at one astronomical unit from the Sun. But there is a subset of near-Earth asteroids known as potentially hazardous asteroids that are even closer.”
Dr. Stanley said that near-Earth asteroids must meet two qualifications to be considered “potentially hazardous.” First, their orbits must be within plus or minus 0.05 au of Earth. Dr. Stanley said that is about 4.6 million miles or “20 times the Earth-Moon distance.” The second qualification is that they must be larger than 120 meters, which is about 395 feet; whereas, a smaller asteroid generally burns up in the atmosphere.
Several asteroids have struck Earth, or burned up in the atmosphere still causing significant damage. The best-known incident is the asteroid that killed most life on Earth, which struck during the age of the dinosaurs. Dr. Stanley said it was 10 to 15 kilometers in size and created the Chicxulub Impact Crater in Mexico, which she said is 150 kilometers wide. More recent incidents have rocked the Earth as well—without reaching the 120-meter qualification of a potentially hazardous asteroid.
“In February 2013, the city of Chelyabinsk in Russia experienced the biggest such event in over a century when an asteroid of about 20 meters in extent entered Earth’s atmosphere and exploded 30 kilometers high in the atmosphere with the energy of 40 Hiroshima bombs,” Dr. Stanley said.
“The flash of light was briefly brighter than the Sun and even caused some severe cases of sunburn. Shockwaves from the blast spread throughout the city, damaging buildings—especially windows—and injuring 1,500 people.”
Finally, in 1908, the Tunguska event in Eastern Siberia flattened 2,000 square kilometers of forest, downing approximately 80 million trees. Dr. Stanley said that an asteroid 36 meters in diameter disintegrated somewhere between five and 10 kilometers up in the atmosphere.
“The energy released from the disintegration of the asteroid was equivalent to 2.8 million tons of TNT,” she said. “That’s about 185 times greater than the energy of the atomic bomb dropped on Hiroshima.”
Dr. Sabine Stanley, PhD, contributed to this article. Dr. Stanley is a Bloomberg Distinguished Professor in the Morton K. Blaustein Department of Earth and Planetary Sciences at Johns Hopkins University. She received a HBSc degree in Physics and Astronomy from the University of Toronto and then completed MA and PhD degrees in Geophysics from Harvard University.