Even as the James Webb Space Telescope is allowing astronomers to see inside vast, distant galaxies, it's also studying some tiny, nearby objects — albeit inadvertently.
These are micrometeoroids, tiny mysteries zipping through the solar system at lightning speed. They're far too small for scientists to observe directly in deep space, but they shouldn't be ignored: Micrometeoroids can pack quite a punch, as NASA's James Webb Space Telescope (JWST or Webb) can attest. Since JWST's Christmas 2021 launch, engineers have detected more than 20 micrometeoroid impacts to the telescope; only one noticeably hurt the observatory. The mission is adjusting its operations to reduce the frequency of micrometeoroid hits, but still, the impacts themselves are perhaps the least expected data from the powerhouse new observatory.
"It is essentially a meteoroid flux detector, although not intentionally," Margaret Campbell-Brown, a meteor physicist at the University of Western Ontario in Canada, told Space.com. "Although, of course, we're sad for them when their mirror gets hit by meteoroids."
The JWST team briefly worried that it had underestimated the threat from these tiny particles in May 2022, when scientists saw a relatively large micrometeoroid hit on the observatory's massive golden mirror even before normal science observations had begun.
But by the time the observatory marked the first anniversary of its Christmas 2021 launch, the team's confidence had returned: Scientists had determined the worrying micrometeoroid was large enough that they wouldn't expect to encounter such an object more than about once a year, and engineers had determined that the particle managed to hit a particularly vulnerable location.
"For the most part, we've been getting about one to two a month that we can actually detect," Lee Feinberg, optical telescope element manager for JWST at NASA's Goddard Space Flight Center in Maryland, told Space.com of the impacts. "At this point, it's really been a very minor thing."
However, JWST is now targeting perhaps two decades of operations, so the team has decided to play it safe, adapting its observation strategy to limit the amount of time the telescope will be vulnerable to the most energetic impacts. "We want those pictures of the Carina Nebula to look just as beautiful 20 years from now," Feinberg said.
And that means understanding micrometeoroids.
An unusual observatory
JWST is in a unique situation. The $10 billion observatory is perched at what scientists call the Earth-sun Lagrange point 2 (L2), which is about 1 million miles (1.5 million kilometers) away from Earth in the direction opposite the sun. L2 is one of the pockets of the solar system where gravitational tugs balance out, making it a relatively cheap outpost to occupy fuel-wise, and it's perfect for the telescope's high-power infrared optics that need protection from the sun.
But scientists have sent only a few spacecraft to L2, and none of them had the vulnerability of JWST: The telescope's massive mirror is bared to space, and engineers keep an eye on its smoothness to help scientists understand their data. Compare that design with an observatory like the Hubble Space Telescope, which is sheathed by a tube that absorbs impacts with no visible scars.
"We're actually able to monitor this stuff at a level of detail that nobody's ever been able to do before," Feinberg said.
Despite the flurry of concern in May, engineers working on JWST knew all along that micrometeoroids would hit the observatory. "If you put anything out in space long enough, it's gonna get hit by something," Bill Cooke, head of NASA's Meteoroid Environments Office at Marshall Space Flight Center in Alabama, told Space.com. "ISS [the International Space Station], Chandra, Hubble — you name a vehicle that's been up there for years, they've all been hit. Most of the hits are not significant to mission operations, but they do get hit."
Early in the JWST design process, mission personnel simulated micrometeoroid impacts on a mirror, although Feinberg noted that engineers don't have a way of accelerating tiny particles all the way to the speeds they reach in the solar system, so the experiments can't really mimic the power of an impact. Scientists also used the models they had at the time to get a sense of how many hits the observatory might experience during its planned five-year lifespan.
"That's kind of how we dealt with it from the point of view of developing JWST," Feinberg said. "And then, honestly, I don't know that I thought much about micrometeoroids and our mirrors until we were actually in space."
But while Feinberg and countless colleagues were making JWST a reality, meteor scientists were busy as well, honing their understanding of space around us.
Scientists have determined that only about 10% of micrometeoroids are connected to the meteors we're most familiar with, those in streams that cause specific meteor showers like the Perseids or Leonids. The other 90% of micrometeoroids are what scientists call sporadics, which travel alone, zipping through the solar system on random orbits, which can make them more challenging to understand.
"It's a little more work to observe sporadics than meteor showers, because they're not nicely organized into events," Althea Moorhead, a meteor scientist at NASA Marshall, told Space.com.
(Feinberg said that the JWST team believes the impacts the observatory is detecting have come from sporadics.
Scientists also know what type of bodies micrometeoroids come from: about 90% from comets and 10% from asteroids, either the rare active asteroids or debris from a collision between space rocks. And a micrometeoroid's origin shapes its impact. "Of course it makes a big difference if your spacecraft gets hit by a solid rock as opposed to kind of a fluffy aggregate of little grains," Campbell-Brown said. "One's like being sandblasted, and the other one's like being shot."
Because micrometeoroids are far too small for any telescope to see, they're tricky to study, so scientists have combined three main approaches.
First, scientists can study nearby meteoroids thanks to their interactions with Earth's atmosphere. As each meteoroid travels through the atmosphere, its edges warm and erode, leaving what scientists call an ionization trail, which specially tuned radar systems can detect.
"The little tiny particles themselves are much too small for the radar to see," Campbell-Brown said of the meteoroids. But the trails they leave are much larger. "All of those electrons in the atmosphere have a scattering cross-section the size of an aircraft carrier, so there we can get a really good signal even off of these tiny, tiny little particles."
And these trails offer scientists a trove of data. The observatory Campbell-Brown uses, Canadian Meteor Orbit Radar in Ontario, catches thousands of meteor trails each day, she said, and that's enough information to calculate each object's orbit. "So we get thousands and thousands of meteor orbits every day, which really helps us to build up a picture of where these small particles are coming from," Campbell-Brown said.
Second, scientists can consult data from two key missions. NASA lofted three Pegasus spacecraft during the 1960s and '70s; each sported massive wings designed to catch meteoroids and soared to the altitudes Apollo astronauts would reach. Pegasus was followed in the 1980s by the Long Duration Exposure Facility, which the space shuttle program left in orbit for nearly six years and then returned to Earth, letting scientists directly study meteoroid impact scars.
With just four objects that never left Earth's orbit, the spacecraft data is limited, but still useful. "The vast majority of our data is looking at meteors, but it's nice to have some other form of detection to help us tease out some of the ambiguities," Moorhead said.
Help from computers
But that's basically all scientists have in the way of observations, so the final technique is modeling.
Scientists can use computers to simulate the solar system's smallest debris, both its formation and its path; they can smash asteroids to smithereens, create artificial comets and watch them dribble material through the neighborhood, and test how Jupiter's massive gravity might shape meteors' paths.
These days, the models are powerful enough to include what direction particles are coming from. "Our models have advanced to the point where we can tell you which are the riskiest directions to look, whereas the older models were more smeared out, if you will," Cooke said. That's particularly important information for JWST, since head-on impacts are more energetic and so cause more damage.
Still, figuring out what's going on around JWST is a tricky business, since both sources of direct observation come from Earth's neighborhood and there's no guarantee the two regions are identical when it comes to micrometeoroids.
"The problem we have is that the only meteors that we observe generally are close to the Earth, because you're going to look at them in the Earth's atmosphere or you have some impact on the satellites," Auriane Egal, a science advisor at Planetarium Rio Tinto Alcan in Canada who works on modeling meteor streams, told Space.com.
"You can never say, like, 'I'm sure that at L2 this is what's happening,'" she added. "But you're using the Earth and every impact that has occurred on spacecraft in the past to confirm your theoretical models and your numerical models and use that as a basis to predict what's going to occur elsewhere in the solar system."
And so far, JWST's experiences suggest that scientists have been on track with their estimations of the environment at L2. Still, observatory personnel are tweaking their approach, limiting the amount of time the telescope's mirror can point forward, when it's vulnerable to the most energetic — and therefore most damaging — impacts.
No one expects that micrometeoroids will take top billing as scientists consider JWST's legacy years from now. But it's likely not the last telescope we'll send to L2, nor is it likely to be the last observatory with a bare mirror. It's worth knowing what's happening out there.