The black hole appeared as a bright speck, a glowing spot against a cosmic backdrop, but one that easily could have been missed altogether — especially because astronomers weren't even looking for it.
Students from the Massachusetts Institute of Technology and Harvard University and scientists associated with NASA's OSIRIS-REx mission — an expedition to robotically retrieve a sample from a near-Earth asteroid — were conducting routine observations when they unexpectedly spotted the distant black hole belching out X-rays, the agency announced Friday.
What they had stumbled upon was special; it is the first time that such an X-ray outburst has been glimpsed from interplanetary space, according to NASA.
The cosmic phenomenon was captured by the Regolith X-Ray Imaging Spectrometer (REXIS), a shoebox-size instrument aboard the OSIRIS-REx spacecraft, which is in orbit around the asteroid Bennu. The spectrometer, which is jointly operated by scientists and students at the two schools, is designed to measure the X-rays Bennu emits as it absorbs solar radiation.
The fortuitous observation provides a nice confidence boost for those involved with the student experiment.
"Detecting this X-ray burst is a proud moment for the REXIS team," Madeline Lambert, an MIT graduate student who designed the instrument's command sequences that revealed the black hole, said in a statement. "It means our instrument is performing as expected and to the level required of NASA science instruments."
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The "flaring" black hole was spotted Nov. 11, appearing as a luminous object just off the asteroid's edge.
X-ray outbursts can occur when a black hole pulls in matter from a star in its vicinity, according to NASA. As stellar debris interacts with the disk of material that surrounds a black hole, intense bursts of energy can be unleashed. But these types of eruptions can be observed from space, because Earth's atmosphere protects the planet from X-rays and other forms of high-energy radiation.
The black hole's outburst was separately confirmed by two telescopes installed at the International Space Station in low-Earth orbit.
The surprise detection is a valuable example of how many scientific discoveries can happen in unanticipated ways, said Richard Binzel, a professor of planetary sciences at MIT who is one of the scientists working with the REXIS instrument.
"We set out to train students how to build and operate space instruments," he said in a statement. "It turns out, the greatest lesson is to always be open to discovering the unexpected."
The OSIRIS-REx mission's robotic probe, which reached Bennu in December 2018, is scheduled to return to Earth in 2023.
Bennu is thought to be a relic from the early days of the solar system. The diamond-shaped space rock measures about one-third of a mile at its equator and orbits at an average distance of about 100 million miles from the sun.
According to NASA, the mission could help astronomers understand how planets and other cosmic objects formed in the solar system, and it could help researchers study near-Earth asteroids that pose a threat to the planet.