Astronomers celebrate performance of JWST

SEATTLE — Six months after the release of the first science images from the James Webb Space Telescope, astronomers and NASA officials say the observatory continues to exceed expectations while providing lessons for future space telescopes.

Talks and other presentations at the 241st Meeting of the American Astronomical Society (AAS) here Jan. 9 highlighted both the science produced by JWST since it started regular operations last summer as well as the status of the telescope itself.

The conference, the largest annual gathering of astronomers, was the first since the July 12 release of the first science images that generated worldwide headlines. It came after years of conferences where astronomers instead heard about the potential of the telescope, as well as its delays and technical problems.

“At this AAS, we’re really taking a victory lap and celebrating JWST,” said Mark Clampin, director of NASA’s astrophysics division, during an agency town hall meeting at the conference.

There was, project officials said, a lot to celebrate. The telescope continues to meet or exceed its specifications on aspects ranging from sensitivity and stability to resolution. “It really is better than we expected across the board,” Jane Rigby, operations project scientist for JWST at NASA’s Goddard Space Flight Center, said in a plenary talk at the conference.

There have been a few minor issues with the telescope, notably micrometeoroid impacts on its mirrors. One such impact, in May 2021, was significantly larger than expected and is now thought to be an impact that happens only once every five years. There have been 21 impacts to date. “All together they’ve had only a very small effect” on the quality of images, said Jonathan Gardner, deputy senior project scientist for JWST at NASA Goddard, at a separate town hall meeting about the mission.

To mitigate the risk of future impacts, JWST will change how it observes later this year when a new round of observations, called Cycle 2, begins. The mission will implement an “avoidance zone” where the flux of micrometeoroids, in the direction of the spacecraft’s motion, is the greatest, and not point the mirrors in that direction. “We can always go back at a different time and look at the same part of the sky,” he said.

Gardner said spacecraft controllers had resolved a concern with one mode of the spacecraft’s Mid-Infrared Instrument (MIRI) when a mechanism showed increased friction. Controllers now use a different movement pattern for the affected grating wheel that is working well. A software fault in an attitude control system triggered a safe mode Dec. 7, caused by what he called an “unusual sequence of events” that has since been resolved. That was the only safe mode since science operations began.

Astronomers and engineers are still testing the limits of JWST’s capabilities. The spacecraft was designed to be able to track moving objects, like bodies in the solar system, at a rate of up to 30 milliarcseconds per second. (There are 3.6 million milliarcseconds in a degree.) However, for observations of the near Earth asteroid Didymos during the collision of NASA’s DART spacecraft with the asteroid’s moon Dimorphos in September 2022, JWST was able to track the asteroid at nearly four times that rate, 110 milliarcseconds per second.

The only other issue with JWST is an external one. Controllers use the Deep Space Network (DSN) to communicate with the spacecraft, downloading an average of 57 gigabytes of data per day. JWST uses the DSN eight hours out of every 24, but Rigby said at the JWST town hall that there some challenges using the DSN, which can be oversubscribed by other missions, including the recent Artemis 1 Orion test flight.

“When Artemis was up, we had to scrap our plans for a week and move in observations with very low data rates” to compensate for the limited DSN time, she said. That could be an issue on future Artemis missions, she added, since those missions will have priority on the DSN.

In her plenary, she credited JWST’s performance to the efforts of thousands of people involved in its development. “JWST is artisanal. It’s really made by hand,” she said. “That science performance, better than expectations, is because of a tremendous amount of work by 20,000 people around the world.”

NASA is starting to fold in lessons learned from JWST’s development into future missions, including the Habitable Worlds Observatory, the name NASA is giving to a large space telescope recommended by the Astro2020 decadal survey in 2021 to operate at near-infrared, optical and ultraviolet wavelengths.

Clampin said that the Habitable Worlds Observatory will leverage the segmented mirror technology used for JWST. It will also likely feature a design change to address the micrometeoroid impacts seen by JWST, whose primary mirror is exposed. “We need to have a telescope in a tube.”