NASA's OSIRIS-REx mission just made history as the first U.S. mission to deliver a piece of an asteroid to Earth. The mission, led by the University of Arizona, launched in 2016 and has traveled over 4 billion miles before returning home. Now its next journey begins. 

After jettisoning its sample return capsule in the Utah desert, OSIRIS-REx fired its thrusters to put the spacecraft on course to rendezvous with another potentially hazardous near-Earth asteroid, called Apophis. NASA renamed the extended mission OSIRIS-APEX, short for OSIRIS-APophis EXplorer.

While the spacecraft's mechanisms are no longer in place to collect another sample, the UA-led science team will study and map the asteroid in great detail. What the team learns will further enrich our understanding of the early solar system and potentially hazardous asteroids.

Planetary sciences assistant professor and OSIRIS-REx deputy principal investigator Dani DellaGiustina will serve as principal investigator for OSIRIS-APEX. DellaGiustina became OSIRIS-REx's deputy principal investigator in 2021.

"All we learned during our time at Bennu will inform the questions we ask of Apophis," DellaGiustina said. "I'm looking forward to this next chapter and am excited to carry on the legacy of this history-making mission."

Regents Professor of Planetary Sciences at the Lunar and Planetary Laboratory Dante Lauretta will remain principal investigator of OSIRIS-REx through the remaining two-year sample analysis phase of the mission.

"I am so proud to see the long-awaited and much-anticipated sample delivery, which so many University of Arizona community members have tirelessly worked on these past many years. The OSIRIS-REx mission has already surprised us in so many ways. I know that it will continue to do so as our scientists dig into the sample and work to reveal more about the origins of the solar system and potentially life itself," said University of Arizona President  Robert C. Robbins. "The OSIRIS-APEX mission will only compound the success of the OSIRIS-REx mission and continue to demonstrate to the world – especially students who want to study what lies beyond our planet – that the University of Arizona is a leader in space sciences."

After nearly six more years of travel and several laps around the sun, OSIRIS-APEX will begin snapping pictures of Apophis in early April 2029 – just a few days before the asteroid's rare close encounter with Earth, coming to within less than 20,000 miles, or one-tenth the distance between the Earth and moon. Scientists will then spend the next 18 months studying the asteroid up close.

Apophis is an "infamous" asteroid, DellaGiustina said, and while many other targets were considered, it's the perfect target for this mission.

When Apophis was first discovered in 2004, scientists studying its orbit thought it would impact Earth in 2029, but later observation ruled out that possibility. Then, scientists again thought it would strike in 2036. That also was ruled out. Further observation and tracking have now shown that Earth is safe from Apophis for at least 100 years.

While Apophis won't hit Earth any time soon, it will come very close, DellaGiustina said, and this is exactly what makes it such a rich target of study.

The mission science team plans to study how Earth's gravitational influence during this close approach changes the asteroid by disturbing the rotation rate and surface, potentially revealing what lies just beneath the surface and more about its material properties. Instruments onboard the spacecraft will snap pictures and collect data as it travels to the asteroid and will continue to monitor any changes after the close approach.

Scientists are also interested in learning more about the composition of this asteroid. OSIRIS-REx's target asteroid, Bennu, is a carbonaceous asteroid, meaning it is relatively rich in carbon-bearing materials such as organic molecules. Apophis, on the other hand, is expected to be poor in carbon-bearing materials and water and is slightly smaller than Bennu. Both are about 1,500 feet at their widest points.

The knowledge gleaned from the Apophis rendezvous will inform scientific understanding of potentially hazardous asteroids and how to protect against future collisions with these solar system wanderers.