NASA's OSIRIS-REx mission was a smashing success—probably. Technicians are still struggling to open the sample canister that was delivered to Earth in October, but the spacecraft has a whole new mission to complete while NASA puzzles over those stuck screws. NASA flipped the switch from OSIRIS-REx to OSIRIS-APEX. This endeavor will end with a visit to another asteroid, this one fresh off a close encounter with Earth.
The target of this revived mission is a space rock known as 99942 Apophis, an S-type asteroid with a diameter of about a quarter mile (370 meters). Apophis is composed of silicate materials and deposits of nickel-iron, making it much different from the C-type asteroid Bennu that the mission sampled back in 2021.
Setting aside the amazing feat of returning an asteroid sample to Earth, OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification and Security – Regolith Explorer) was also an incredible logistical accomplishment. After traveling millions of miles and exploring the surface of an asteroid, the spacecraft still has about a quarter of its fuel remaining. NASA will use this to get OSIRIS-APEX (Origins, Spectral Interpretation, Resource Identification, and Security – Apophis Explorer) into position to study Apophis during and after its upcoming encounter with Earth.
Apophis seems like a pretty ordinary asteroid, save for the fact that it gets so very close to Earth. In 2029, NASA predicts Apophis will pass within 20,000 miles of Earth, well inside the orbit of the moon and even closer than some satellite orbits. Scientists estimate a space rock of this size gets this close to the surface only once every 7,500 years.
OSIRIS-APEX needs to build up some speed before the 2029 encounter. The spacecraft has now departed Earth after dropping off the Bennu sample; its next milestone is the first of six slingshots around the sun. Add in three gravity assists from Earth, and OSIRIS-APEX will be ready to match course with Apophis in April 2029. Astronomers predict Apophis will make its closest approach on April 13, 2029 (a Friday the thirteenth, as it happens).
Scientists were surprised to find Bennu was very "loosely packed." The probe detected small pebbles being flung into space, and the surface almost swallowed the spacecraft when it collected its samples. While Apophis has a different composition, scientists predict interacting with Earth's gravity could cause quakes, landslides, and other geological upheaval on Apophis. OSIRIS-APEX will use the same instruments it used to characterize Bennu to understand these changes.
While the probe has already parted with its sampling hardware (it's currently at NASA's Johnson Space Center), it will perform an encore of the descent to Bennu. By approaching the surface and firing its thrusters, NASA hopes to stir up the surface and uncover the materials below. If all goes as planned, we probably won't hear much from OSIRIS-APEX over the next few years as it maneuvers into position for this operation.
