But the direct hit on a mirror caught NASA by surprise and is still being analyzed. Details of the micrometeoroid strike were revealed by NASA in a blog post dedicated to the Webb.
“Between May 23 and 25, NASA’s James Webb Space Telescope sustained an impact to one of its primary mirror segments,” the NASA Webb blog stated. “After initial assessments, the team found the telescope is still performing at a level that exceeds all mission requirements despite a marginally detectable effect in the data.”
The 18 segments of the mirror can be individually adjusted in response to meteoroid impacts such as this one, NASA said.
“By adjusting the position of the affected segment, engineers can cancel out a portion of the distortion … although not all of the degradation can be cancelled out this way,” the NASA blog stated. “Engineers have already performed a first such adjustment for the recently affected segment … and additional planned mirror adjustments will continue to fine-tune this correction.”
The exact size of micrometeoroid is not known. It may have been no bigger than a grain of sand, said Heidi Hammel, a planetary astronomer who has long been involved with the telescope and will use it to study our solar system. Even something that small can cause damage because of the tremendous speed at which the telescope orbits the sun and periodically slams into a random particle.
This was a known hazard, because although it’s lonely out in space it’s not as empty as it looks.
“There’s no loss of science at all from this event. … This telescope is out there in space — we knew there would be tiny impacts on it. We were just surprised that one hit so soon,” Hammel said.
She said scientists had anticipated such an impact every five years or so, on average.
This extraordinarily complex observatory, heralded as the long-awaited successor to the still-functional Hubble Space Telescope, is orbiting the sun in a position that keeps it roughly 1 million miles from Earth. It is too far away for astronauts to visit, and it’s not designed to be fixed or to have instruments swapped out.
The Webb has been going through a “commissioning” phase for months as its instruments are calibrated and the 18 gold-plated, hexagonal mirrors are brought into alignment to function as a single massive mirror about 21 feet in diameter.
So far, NASA has reported nothing but success.
“Astronomers are giddy with how well things are going (but also nervous not to jinx it, yes we can be superstitious too) and anxious to start doing science!” astrophysicist Michael Turner of the University of Chicago said in an email.
The telescope, folded upon itself at launch last year, flowered over the course of many days as its sprawling sun shield opened up and the mirrors deployed. The telescope traveled for 29 days to reach its outpost, an orbital position known as L2 where other telescopes have operated safely and offered scientists data on the frequency of micrometeoroids.
“While the telescope was being built, engineers used a mixture of simulations and actual test impacts on mirror samples to get a clearer idea of how to fortify the observatory for operation in orbit. This most recent impact was larger than was modeled, and beyond what the team could have tested on the ground,” the NASA Webb blog stated.
The Webb is different from most telescopes: It’s wide open, with the mirrors exposed rather than tucked into a tube. The telescope is designed to observe the universe at infrared wavelengths that are outside the detection range of the Hubble.
This requires mirrors and instruments that are extremely cold, which is why the mirrors face away from the Earth and sun at all times. NASA has announced that the “first light” images will be released July 12, but has not said what those will show.
Already, though, it has produced an image of a star, used for focusing the mirrors. In the background of that image are numerous galaxies whose light was emitted billions of years ago, and that has thrilled astronomers who expect that the Webb will see deeper into space (and into the past) than the Hubble, launched in 1990.
The Webb has multiple goals, including studying the earliest light in the universe, emitted a few hundred million years after the big bang. It will also look at the evolution of galaxies, and study objects in our own solar system, including small, icy bodies that orbit the sun far beyond the orbit of Neptune.