One of the most anticipated findings has to do with the Yarkovsky effect on the asteroid. Because asteroids rotate like planets and moons, one side always is either heating in the sun or cooling in the shade. The heat thrown back into space from the surface acts as a thruster. An asteroid's gravity is so weak that even this tiny force has a measurable effect on the body's orbit around the sun. Lack of knowledge about the effect adds uncertainty about the precise orbit of a potentially hazardous object and whether it will hit Earth.
Six close flybys of Bennu are planned to map the perfect touchdown spot. When NASA experts sign off on the landing zone, OSIRIS-REx will descend toward the surface for its grand performance.
Bennu's gravity is so weak that walking on it would be impossible even if the surface were compact enough for a stroll. A single step would launch an astronaut out of the asteroid's orbital influence and into deep space.
Using tiny bursts from its rocket thrusters, the spacecraft is to descend to the asteroid's surface.
The plan is for the end of the robot arm to touch the asteroid for about five seconds. A gust of nitrogen gas will stir up gravel and suck it into a small chamber, to be deposited into a return capsule. When the spacecraft approaches Earth's atmosphere a few years after contact with Bennu, the capsule would be ejected, fall to Earth, and get picked up by scientists. After ejecting the capsule, the spacecraft is to permanently orbit the sun.
Researchers can't wait to get their hands on what will be the first sample brought back from an asteroid, composed mostly of carbon-based material rather than metals or rock. A Japanese spacecraft, the Hayabusa, returned dust collected from stony asteroid Itokawa (which has no risk of Earth collision) in 2010.
Analyzing the sample is where the "spectral interpretation" part of the spacecraft's name comes in. There are up to 800,000 known asteroids, but most will be studied only with a telescope, of course. Bennu has been studied more than most.
The up-close imaging and direct sampling of the small rocks, gravel, and dust (the regolith) on the asteroid's surface will be compared with information already gleaned from Bennu by telescopes. This telescope data will be "ground-truthed" by what minerals are found in the sample, meaning "we've improved our ability to interpret the spectra of asteroids across the solar system," Lauretta says.
Thanks to a 1978 made-for-TV movie called A Fire in the Sky, we more easily can imagine what a comet strike on Phoenix would be like.
A promo clip on YouTube includes a scene of the comet hitting what looks to be lower west side of the city, followed by the destruction of the Hyatt Regency building.
"Phoenix lay gutted and dead under the immense desert sky, all its splendors ruined, all its towers of glass and steel crushed and devastated," reads a passage near the end of the movie's novelization, released the same year.
The likelihood of any particular city getting destroyed by an impact from space is slim. Water covers more than two-thirds of Earth's surface, making an ocean strike a better bet. Yet because a large impact would be so destructive, some experts believe the odds of any one of us dying in a comet strike can be compared to other causes of death.
Estimates vary hugely: One estimate on a Tulane University website gives lower odds of 1 in 3,000 for each person on Earth (making death by drowning less likely) to 1 in 250,000, still far less than death by botulism or shark attack. Last year, the economist.com article stated that impact death is a long shot, with 75 million-to-1 odds, though that still beats the chances of winning a Powerball jackpot.
"An impact by an object a kilometer or two in diameter could kill about a billion people and happen once in a million years on average," writes Donald Yeomans in the 2013 book Near-Earth Objects: Finding Them Before They Find Us.
Yeomans, manager of NASA's Near-Earth Object Program Office, believes that a large impact would "have the capacity to reduce large regions of the Earth's surface to ashes and create another extinction event. We cannot afford to play the odds when civilization is at stake."
About 50,000 years ago, the mile-wide Meteor Crater near Winslow was carved out in 10 seconds by a nickel-iron asteroid about 150 feet long, evidence shows. It's impossible for anyone who's seen movies like A Fire in the Sky to stand on its windy rim and not think about how the crater would look in the middle of a city.