Webb telescope has a new mission: surveillance of a threatening asteroid

The most powerful space telescope ever built is peering into some of the deepest realms of the universe.

It will also now peer at a big asteroid that has low but still concerning odds — around 2 percent as of Feb. 11 — of hitting Earth in 2032. " Currently, no other known large asteroids have an impact probability above 1 percent," NASA explained.

The director of the Space Telescope Science Institute, which runs the James Webb Space Telescope, has afforded planetary defense scientists a special, unplanned use of the space observatory, called the Director’s Discretionary Time. That's because Webb can do what Earth-based telescopes haven't yet been able to accomplish: Refine the size estimates of asteroid 2024 YR4, currently estimated to be between 130 to 300 feet wide, which is big enough to be dubbed a "city-killer" asteroid — if it indeed hit a city.

Knowing such an object's mass is critical. There's a big difference between a 130- and 300-foot-wide asteroid.

"The destructive power of an asteroid is closely correlated with its size," Andrew Rivkin, a planetary astronomer at Johns Hopkins University Applied Physics Laboratory, told Mashable. "If we can get a good estimate of an asteroid's size, we can get a good idea of what would happen if it hit Earth." Rivkin is leading the Webb observation of asteroid 2024 YR4.

For reference, the asteroid that hit Arizona 50,000 years ago and created the 600-foot-deep "Meteor Crater" was 100 to 170 feet, or 30 to 50 meters, across. "A similar-size impact event today could destroy a city the size of Kansas City," David Kring, an impact cratering expert at the Lunar and Planetary Institute, explained in a NASA blog. That's why you may have read that this asteroid is a potential "city-killer."

The Webb telescope is well-suited to identify asteroid 2024 YR4's size. Webb views a type of light we can't see with the naked eye, called infrared. Infrared light is largely heat energy, allowing Webb to observe the heat from distant objects, whether they're planets or distant galaxies.

Yet the size estimates of many asteroids are based on visible light observations, based on the sunlight reflected off of the space rocks. These measurements, while hugely valuable, can be imprecise. For example, a small but lighter asteroid may reflect lots of light, suggesting it may be larger than it actually is; conversely, a darker asteroid may appear smaller than it truly is.

But the infrared glow from an object's heat provides a much better sense of its size, Rivkin explained. And the Webb telescope — with mirrors over 21 feet across and six times the light collecting area of the Hubble Space Telescope — will be able to observe this glow from a relatively small object speeding through the vast solar system.

"JWST is the most powerful telescope that we have," Rivkin marveled. "It's amazingly capable."

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An image captured by the Asteroid Terrestrial-impact Last Alert System (ATLAS) when it discovered asteroid 2024 YR4. Credit: ATLAS / NASA

The asteroid will become observable by Webb beginning on March 8, and actual observations are likely to begin soon after. The telescope has a tennis-court-size sunshield, which blocks out light from the sun, Earth, and the moon, allowing Webb to detect extremely faint infrared signals from profoundly distant cosmic objects. But that intentional view-blockage also means the Webb-viewing team will need to wait for the object to fly into sight.

In the coming months, the odds of asteroid 2024 YR4 impacting Earth are likely to go up before they go down and perhaps vanish. (If it becomes clear the asteroid will miss Earth, Rivkin noted that perhaps these Webb observations won't be prioritized any longer, as there is extremely high demand for Webb viewing time.) That's because as observations increase, the asteroid's solar system trajectory become clearer, and the area of potential hazard will shrink. But while Earth is still in that shrinking area of hazard uncertainty, it takes up a large percentage of that region, so the impact probability increases.

Most times, however, this zone of uncertainty moves off from Earth. "It’s a funny thing about homing in on an asteroid and calculating its path, future position, and probability of impacting Earth — it will often appear risky during initial observations, get riskier, and then suddenly become entirely safe," the European Space Agency noted.

The different components of the James Webb Space Telescope, including its huge sunshield. Credit: NASA

But until that happens, planetary defense experts will keep working to understand the risk the asteroid poses to Earth. If it were indeed headed to a populated region, you'd have plenty of notice. Space agencies like NASA and the European Space Agency, along with organizations like the International Asteroid Warning Network (IAWN), would vigilantly monitor the menacing object. If necessary, NASA would issue its first-ever asteroid warning. People could be evacuated from vulnerable regions.

"The odds are very much in favor of it missing," said Rivkin. "But we have to do our due diligence."