Asteroid hunting at dusk brings up space rocks we wouldn’t normally see

For decades, the standard way to search for asteroids in our solar system has been to scan the night sky for fast-moving points of light – but a new method of hunting for these space rocks at dusk is proving also fruitful. That’s much harder to achieve, but by scanning parts of the sky at dusk, astronomers have been able to find key asteroids they otherwise wouldn’t have seen.

The two largest asteroid detectors today are the Pan-STARRS observatory in Hawaii and the Catalina Sky Survey, which operates several telescopes out of Arizona. Over the past decade, these two programs have been the premier hunters for near-Earth asteroids. But they mainly search the sky at night, looking away from the Sun. This limits the parts of the sky they can observe to the area just around Earth and the outer solar system.

Recently, asteroid hunters turned their telescopes towards the Sun just after it sets or just before it rises. The sky is hazy at this time but still clear enough to complicate the search. But by braving the twilight, asteroid hunters were able to find many asteroids crossing Earth’s orbit and some circulating inside the solar system. Observing at dusk, scientists working with the four-meter Blanco Telescope in Chile found the first known asteroid that orbits closer to the Sun than Venus and the largest potentially Earth-hazardous asteroid discovered in recent years. (Don’t worry, he won’t cross paths with the planet.)

“We find things that other people basically can’t find,” Scott Sheppard, an astronomer at the Carnegie Institution for Science, who detailed this twilight method in an article for Sciencerecount The edge. “And so it’s always, I think, a good thing to observe areas that other people don’t observe.”

Hunting for asteroids is hard enough, even when searching at night. Near-Earth asteroids appear as very blurry, faint points of light moving across the sky. Asteroids do not emit light on their own but reflect light from the Sun, so it is easier to see these small dots at night. But we can only see part of the sky in the dark. “Daytime covers half the sky and nighttime covers half the sky,” Sheppard explains. “So if you’re only looking at night, you’re pretty much only looking at half the sky.” Many asteroids that spend most of their time inside the solar system never really appear at night; they can only be found during the day, which is far too bright to spot.

Searching at dusk can help reveal some of these mysterious objects, but it makes the asteroid hunting process even more difficult. Asteroid hunters are particularly interested in a specific time of twilight, around 10 to 15 minutes just before sunrise and 10 to 15 minutes just after sunset, according to Sheppard. That doesn’t give astronomers a lot of time to find those fuzzy bright spots, and then, if they spot one, they have to re-observe it in the same short amount of time to confirm its position.

The biggest headache of all is sun glare. “When you take an image, your background is much brighter, so an object doesn’t pop out as easily when you have a very noisy background,” Sheppard says. To this difficulty is added the fact that telescopes point almost towards the horizon to observe the sky normally surrounding the Sun. This means that telescopes are actually looking through even more of Earth’s atmosphere than usual, far more air than if the telescope is pointed straight up and out. This makes blurry points of light even blurrier. On top of all that, the angle at which these asteroids are relative to the Sun only makes them partially illuminated.

Despite all this, astronomers have in the past used much smaller telescopes – around a meter in diameter – to search for asteroids at dusk. But since last summer, Sheppard and his team have used a special camera called the Dark Energy Camera on the National Science Foundation’s four-meter Blanco Telescope. Their search uncovered three remarkable new asteroids, including the potentially dangerous asteroid 2022 AP7. It measures about a kilometer and crosses Earth’s orbit, according to Sheppard, although it is not supposed to approach the planet. Its size and trajectory technically put it in the “potentially hazardous” category, which is a category reserved for asteroids of a specific luminosity that are within a certain distance of Earth. Most of these asteroids have already been spotted as astronomers are eager to find them due to their potential to wreak havoc on Earth if they hit us.

In addition to the four-meter Blanco telescope, astronomers also used the 48-inch Zwicky Transient Facility telescope, located in California, to find asteroids at dusk, where they successfully detected space rocks. While finding more asteroids is obviously a boon for planetary defense, Sheppard says it’s also about better understanding how asteroids move through our cosmic neighborhood. Many asteroids are believed to originate from the main asteroid belt between Mars and Jupiter, but astronomers are curious if there are any unknown reservoirs of space rocks that contribute asteroids elsewhere. And searching at dusk might help answer that question.

“Our main goal for the survey is to understand the population of these very interesting asteroids to give us a comprehensive view of where they came from and how they move through the solar system,” Sheppard explains.

Updated July 25 at 12:45 p.m. ET: This story has been updated to clarify when astronomers search for asteroids at dusk.

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