OSIRIS-REx flies on as OSIRIS-APEX to explore its second asteroid

After seven years in space and over 4 billion miles traveled, NASA's OSIRIS-REx mission successfully collected and delivered the first U.S. sample from a near-Earth asteroid. Yet, after all this time and travel, the spacecraft will not retire.

Instead, NASA extended the University of Arizona-led mission so that the spacecraft can be used to study another near-Earth asteroid named Apophis. The mission was renamed OSIRIS-APEX, short for OSIRIS-APophis EXplorer. An overview of the mission was published in The Planetary Science Journal.

OSIRIS-REx deputy principal investigator Dani DellaGiustina is now the principal investigator for the OSIRIS-APEX mission.

Twenty minutes after dropping the sample high above Earth's atmosphere on Sept. 24, the spacecraft fired its thrusters to put it on course to rendezvous with Apophis in 5½ years—just after Apophis makes its own close approach to Earth. This path includes three Earth gravity assists and several nail-bitingly close laps around the sun.

By April 2, 2029, the spacecraft cameras will begin collecting data as it approaches the asteroid. Apophis will also be closely observed by Earth-based telescopes. But in the hours after the close encounter, Apophis will appear too near the sun in the sky to be observed by Earth-based optical telescopes. This means any changes triggered by the close encounter will be best detected by spacecraft.

The spacecraft will catch up to the asteroid on April 13, 2029, as the asteroid whizzes 20,000 miles above Earth's surface. Scientists will then spend the next 18 months studying the asteroid in detail. They'll also disturb the material on the surface with the spacecraft to reveal what lies just beneath.

"Apophis is an infamous asteroid," said DellaGiustina, who is an assistant professor of planetary sciences at the UArizona Lunar and Planetary Laboratory.

"When it was discovered in 2004, there was a scare that it was going to impact the Earth in 2029, but that risk was retired. Then there was another scare that it was going to impact the Earth exactly seven years later, in 2036, but observations combined with modeling now show that Apophis doesn't pose a risk for at least the next one hundred years. Despite this, Apophis still has this role in the psyche of all of us who study these things. While it's not going to impact the Earth in 2029, however, it does get very close."

The 340-meter-wide Apophis is a stony, or S-type, asteroid made of silicate materials and nickel-iron, which are different than C-type asteroids like OSIRIS-REx's first target, Bennu, which are rich in carbonaceous material.

Apophis likely formed from a collision of a parent body in the asteroid belt that knocked it toward Earth's neighborhood. The asteroid will pass closer than some of Earth's orbiting satellites and one-tenth the distance to the moon. This is the closest approach by an asteroid of this size in modern history, and it will be visible to the naked eye in the Eastern Hemisphere.

An asteroid of this size coming so close to Earth is rare, occurring roughly once every 7,500 years.