Author: Erin Morton

NASA’s OSIRIS-APEX (Origins, Spectral Interpretation, Resource Identification and Security – Apophis Explorer) spacecraft kept its cool again this past fall after passing within 46.5 million miles of the Sun, between the orbits of Mercury and Venus, and surpassing its designed heat tolerances.

On Jan. 23 the mission team completed its review of all the data recorded by the spacecraft and its instruments during the solar pass. “There were no surprises, and the spacecraft is operating well,” said Mike Moreau, OSIRIS-APEX deputy project manager at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

As planned, on Sept. 2, 2024, OSIRIS-APEX passed through perihelion — the phase of its orbit closest to the Sun. The trajectory to Apophis takes the spacecraft much closer to the Sun than it was originally designed for. Between Aug. 1 and Oct. 13, the spacecraft was configured in a special orientation that uses one of the solar arrays to shade the most heat-sensitive components, keeping them within safe operating temperatures.

While in perihelion configuration, communication with the spacecraft is only possible through one of the spacecraft’s low-gain antennas, thus only very limited data is available to monitor its systems. During this most recent perihelion, there was also a period of several days when no communications were possible while the spacecraft was on the other side of the Sun from Earth.

On Oct. 13 the spacecraft exited the perihelion configuration and methodically returned to full operating status, allowing flight engineers to downlink and analyze spacecraft telemetry and assess the health of the system. In November 2024, the spacecraft executed routine checkouts of instrumentation on the spacecraft. OSIRIS-APEX appears to be healthy after the second of six close perihelion passages on its six-year journey to rendezvous with asteroid Apophis.

OSIRIS-APEX successfully passed through its first perihelion earlier in 2024. To reach asteroid Apophis in 2029, the spacecraft must survive four more close encounters with the Sun, the next of which will occur in May 2025.

Preliminary telemetry indicates that NASA’s OSIRIS-APEX spacecraft successfully completed a two-month operation that brought it 25 million miles closer to the Sun than it was designed to function. The spacecraft continues its journey to reach asteroid Apophis in April 2029.

The OSIRIS-APEX spacecraft, formerly known as OSIRIS-REx, repositioned one of its two solar arrays to shade critical components from the Sun’s heat during its Jan. 2 close pass, or “perihelion.” When it returned to a thermally safe distance of about 60 million miles from the Sun in early February, OSIRIS-APEX rotated its array back into place to help generate power.

The limited telemetry available suggests that OSIRIS-APEX performed as predicted during the perihelion. It will take a few months for the mission team to conduct a full assessment of the spacecraft’s performance, during which it flew to roughly half the distance between Earth and the Sun.

Since early December 2023, engineers have had limited information about spacecraft status, as the spacecraft was configured for its safety. This included orienting its main antenna such that high-speed communication with Earth wasn’t possible.

But, in mid-March, the team finally will be able to point the spacecraft’s powerful high-gain antenna toward Earth and download the data OSIRIS-APEX collected during perihelion.

Engineers also plan to turn on and test the spacecraft’s instruments in early April and will provide a spacecraft health assessment in May to determine any possible degradation to surfaces or components caused by the extreme heat it endured. Until then, the team will carefully monitor the spacecraft to ensure it continues to function as expected.

The January perihelion was the first of seven perihelions the spacecraft will complete to reach Apophis, six of which will bring OSIRIS-APEX within 46.5 million miles of the Sun. The second perihelion is scheduled for Sept. 1, 2024.

By Lonnie Shekhtman
NASA’s Goddard Space Flight Center, Greenbelt, Md.

UPDATE AS OF JAN. 4, 2024: NASA’s OSIRIS-APEX spacecraft continues to transmit a telemetry signal and operate normally in its new configuration. Once OSIRIS-APEX is farther from the Sun in March, the mission team will re-orient the spacecraft to point its high-gain antenna toward Earth to allow for high-speed communication. At that point, the team will download and analyze telemetry data to determine how the spacecraft performed during its close encounter with the Sun.”

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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.

By Lonnie Shekhtman
NASA’s Goddard Space Flight Center, Greenbelt, Md.