Two years after launch from Cape Canaveral, NASA's OSIRIS-REx spacecraft finally reached its target Monday, slowly closing to within about four miles of the asteroid Bennu for a year-and-a-half of close-range study before attempting to collect soil and rock samples for return to Earth.

"We have arrived!" an engineer at Lockheed Martin's flight control center in Colorado called out, watching telemetry stream in from 76 million miles away that confirmed the completion of a critical rocket firing

OSIRIS-REx will now make five close-range mapping runs before slipping into orbit around the asteroid at the end of the year.

This series of images taken by the OSIRIS-REx spacecraft shows the asteroid Bennu in one full rotation from a distance of around 50 miles (80 km). NASA's Goddard Space Flight Center/University of Arizona

The OSIRIS-REx mission and NASA's New Horizons spacecraft, now closing in on a denizen of the Kuiper Belt a billion miles past Pluto, are both aimed at studying primitive bodies left over from the birth of the solar system 4.6 billion years ago.

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New Horizons will fly past its target, a frozen, unaltered body known as Ultima Thule, on New Year's day, a few hours after OSIRIS-REx slips into a gravitationally-bound orbit around Bennu.

The goal of both missions is to learn more about how the original solar nebula coalesced to form the sun and its retinue of planets, asteroids and comets, giving scientists a better understanding of the raw materials that went into Earth's construction and even the eventual development of life.

OSIRIS-REx — the convoluted acronym stands for Origins, Spectral Interpretation, Resource Identification and Security-Regolith Explorer — is equipped with three cameras, two spectrometers, a laser altimeter and an X-ray imaging system developed by college students.

It also is equipped with a "touch-and-go-sample acquisition mechanism," or TAGSAM, on the end of a 10-foot-long robot arm. In July 2020, after a year and a half of detailed mapping and rehearsals, OSIRIS-REx will briefly push TAGSAM, shaped somewhat like an inverted pie dish, into contact with the surface of Bennu.

During the five-second maneuver, compressed nitrogen gas will shoot into the collector, stirring up and lifting small rocks and soil. Some of that material, at least two ounces and up to 4.4 pounds, will be trapped in filters for return to Earth.

After backing away, OSIRIS-REx will direct the robot arm to place the collector inside an aerodynamic sample return capsule. At that point, in situ science will come to an end and OSIRIS-REx will simply wait for Earth and Bennu to reach the proper positions in their orbits to begin the long trip home.

"We can't leave Bennu before March of 2021, just because we've got to wait for the orbital mechanics to line up," Dante Lauretta, the principal investigator, said in an earlier interview. "So that's over two-and-a-half years of operations at the asteroid. We only touch the asteroid for that brief, five-second sampling contact. We are going to do everything we can to make sure we get the sample on the first attempt."

OSIRIS-REx will return to Earth in September 2023, dropping the sample container off for a high-speed plunge to the Utah Test and Training Range about 80 miles west of Salt Lake City in Utah. Recovery crews will deliver the capsule to the Astromaterials Acquisition and Curation Office at the Johnson Space Center in Houston where Apollo moon rocks are stored.

Some of Bennu sample will be held in reserve while the rest is subjected to two years of detailed analysis.