asteroid Bennu
© NASA/Goddard/University of Arizona
An image of the asteroid Bennu taken by the Osiris-Rex spacecraft Nov. 16.

The spacecraft now begins a close study of the primitive space rock, seeking clues to the early solar system.

Launched two years ago, NASA's Osiris-Rex spacecraft pulled alongside the asteroid Bennu on Monday. Its mission is to survey the asteroid ahead of retrieving pristine bits of the solar system from the rock's surface and then bringing them back to Earth in the years ahead.

With a short engine burn, the spacecraft matched the speed and direction of Bennu.

A few minutes after noon, Javier Cerna, a communications systems engineer at Lockheed Martin, which built and operates the spacecraft, announced, "We have arrived."

What does NASA mean by "arrive?"

Osiris-Rex's arrival at Bennu was not like the landing of NASA's InSight spacecraft in one piece on the surface of Mars last Monday. (Happily, it landed flawlessly.)

By contrast, Osiris-Rex pulled in at a modest speed, and the moment of arrival was somewhat arbitrary. The spacecraft started the approach phase of its mission in August when it was 1.2 million miles from Bennu. On Monday, it was just 12 miles away, although still too far away to orbit the asteroid.

There was no drama, just a smooth transition to the next phase of the mission.

What happens next?

Osiris-Rex will make a series of passes over the asteroid at a range of 4.3 miles for an initial survey to better determine its mass, rate of spin and shape.

In January, the spacecraft will get closer to Bennu, between 0.9 and 1.2 miles, and be drawn into orbit around the asteroid, which will be the smallest object ever to be orbited by a spacecraft. Osiris-Rex will then spend more than a year performing reconnaissance of Bennu, before attempting to bounce off the surface and collect a sample of the asteroid in mid-2020.

What can you tell me about Bennu?

Bennu, discovered in 1999, is a carbon-rich, almost black asteroid, about 1,600 feet wide. (That compares to the Empire State Building, which is 1,454 feet tall including the antenna at the top.) Scientists believe that it is a conglomeration of leftovers from the formation of the solar system, largely unchanged over the last 4.5 billion years.

Bennu is categorized as a near-Earth asteroid, and scientists say there is a small chance it could slam into Earth, but not until the 22nd century if it happens at all. (It is not large enough to cause planet-wide extinctions, but it would be catastrophic at the point of impact.)

What are scientists hoping to learn from Bennu?

By studying a primitive asteroid, scientists hope to get a better idea of what was around in the solar system's earliest days.

Dante Lauretta, the principal investigator of Osiris-Rex, said he was particularly interested in gleaning information about organic molecules like amino acids, the building blocks of proteins found on Earth that are also known to exist in interstellar space. One question is whether Bennu contains higher concentrations of the 20 amino acids used by forms of life on Earth. That would suggest the universe favors these amino acids and not dozens of others, even in nonbiological chemical reactions.

Life on Earth also exclusively uses so-called "left-handed" amino acids and not the mirror, right-handed versions. Study of the Bennu material could help explain whether nonbiological chemical reactions in space pushed life toward left-handed molecules or whether that shift occurred later when life arose.

How will Osiris-Rex grab a piece of Bennu?

In July 2020, the spacecraft, about the size of a sport utility vehicle, is scheduled to slowly descend and bounce off the surface like a pogo stick at a gentle pace of a quarter-mile per hour. A sampling head, which looks like an automobile air filter, will shoot a burst of nitrogen to kick up dirt and small rocks during the three to five seconds it is in contact with the surface.

The goal is to collect at least a couple of ounces of material and possibly as much as 4.4 pounds. The spacecraft carries enough nitrogen to attempt to extract material three times if necessary.

Why does it take so long to collect a sample and return to Earth?

A quicker mission would have required a larger spacecraft carrying more fuel - more expensive than the $800 million cost of Osiris-Rex. Instead, the spacecraft took a more efficient but longer trajectory, taking advantage of a flyby of Earth last year to fling it on a path to intersect Bennu. Scientists also want to study Bennu is as much detail as they can before heading back to Earth.

After departing Bennu in 2021, Osiris-Rex will pass by Earth in September 2023, dropping off a capsule with the samples that will land via parachute in a Utah desert.

Haven't other spacecraft explored asteroids already? What's so special about Osiris-Rex?

Quite a few spacecraft have made flybys of asteroids, beginning with NASA's Galileo spacecraft, which passed within 1,000 miles of the asteroid Gaspra in 1991 en route to Jupiter. NASA's NEAR Shoemaker (NEAR is short for Near Earth Asteroid Rendezvous) went into orbit around the near-Earth asteroid Eros in 2000. Even though it was not designed to land on the asteroid, NEAR Shoemaker did just that in 2001 and continued operating for two weeks from the surface of Eros.

The Japanese spacecraft Hayabusa collected some dust samples from an asteroid and returned them in Earth in 2005. A follow-up mission, Hayabusa2 is, like Osiris-Rex, headed to a carbon-rich asteroid and is scheduled to bring its samples back to Earth in 2020.

However, not all asteroids are alike, and Osiris-Rex is expected to bring much more rock and dirt back than the Japanese missions. Comparing data from different asteroids will create a fuller picture of what the early solar system was like.

Is this the most contorted name of a NASA mission ever?


Dr. Lauretta said that as he jotted some themes of the scientific objectives - origins, spectroscopy, resources and security - he had most of the letters of Osiris, the Egyptian god who was the lord of the underworld but who also represented the seeds of regeneration to new life.

"It was the dual nature of the Osiris myth," Dr. Lauretta said a couple of years ago before launch. "I had to buy a couple of vowels."

With some finagling, he came up with Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer.

Kenneth Chang has been at The Times since 2000, writing about physics, geology, chemistry, and the planets. Before becoming a science writer, he was a graduate student whose research involved the control of chaos. @kchangnyt