OSIRIS-APEX to fly closer to Sun to reach Asteroid Apophis

OSIRIS-APEX to fly closer to Sun to reach Asteroid Apophis
by Lonnie Shekhtman for GSFC News
Greenbelt MD (SPX) Dec 04, 2023
Reinvented as NASA's OSIRIS-APEX, the spacecraft formerly known as OSIRIS-REx is about to face the first major test of its mission to asteroid Apophis: it will fly closer to the Sun than ever before, exposing its components to higher temperatures than they were designed to endure.

At its closest approach (known as its perihelion) on Jan. 2, 2024, OSIRIS-APEX will be about 46.5 million miles away from the Sun - roughly half the distance between Earth and the Sun, and well inside the orbit of Venus. However, this is 25 million miles closer to the Sun than the spacecraft was designed to operate, as the Apophis rendezvous wasn't part of the original mission plan to fly to asteroid Bennu to collect a sample.

When OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security - Regolith Explorer) left Bennu in May 2021 with a sample aboard, it had a quarter of its fuel left and its instruments were in great condition. So instead of shutting down the spacecraft after it delivered its Bennu sample to Earth on Sept. 24, 2023, and sending it into a forever orbit around the Sun, the team proposed to dispatch it on a bonus mission to Apophis. NASA agreed and OSIRIS-APEX (Origins, Spectral Interpretation, Resource Identification, and Security - Apophis Explorer) was born.

The upcoming perihelion is the first of six close Sun passes - along with three Earth gravity assists - OSIRIS-APEX will undertake to reach Apophis in April 2029. To keep its critical components safe during these passes, engineers at mission partner Lockheed Martin Space developed a creative spacecraft configuration.

"We are most creative when the spacecraft is in flight and we're pushing boundaries to meet mission needs," said Sandy Freund, OSIRIS-APEX program manager at Lockheed Martin Space in Littleton, Colorado, who had the same role on OSIRIS-REx.

To prevent overheating, the spacecraft will maintain a fixed orientation with respect to the Sun and reposition one of its solar arrays to shade its most sensitive components. Thermal models indicate this configuration will keep the spacecraft safe.

"We've done a lot of modeling to ensure the spacecraft will be safe," said Dani Mendoza DellaGiustina, principal investigator for OSIRIS-APEX at University of Arizona, Tucson. "But any time you take a piece of space flight hardware beyond the design criteria you incur risk," said DellaGiustina, who is also the deputy principal investigator of OSIRIS-REx.

The spacecraft is executing commands this week to tuck in one of its two solar arrays; the second will remain extended to produce power.

During this time, the spacecraft will be inactive, with only critical systems, such as telemetry, turned on. It will communicate with Earth at low data rates, so the mission team will receive only limited information about spacecraft status. Once OSIRIS-APEX is farther from the Sun in March and April, engineers will turn the instruments on to test them.