For decades, the standard way to look for asteroids in our solar system has been to scan the night sky for fast-moving blobs of light — but a new way to hunt down those space rocks at dusk is also proving fruitful. It’s a lot harder to pull off, but by scanning parts of the sky at dusk, astronomers have been able to find important asteroids they wouldn’t otherwise have seen.
The two currently largest asteroid seekers are the Pan-STARRS observatory in Hawaii and the Catalina Sky Survey, which operates several telescopes in Arizona. Over the past decade, these two programs have been the leading hunters of near-Earth asteroids. But they mainly scan the sky at night, looking away from the sun. This limits the parts of the sky you can observe to around Earth and the outer solar system.
Recently, asteroid hunters have rotated their telescopes to the sun just after sunset or just before sunrise. The sky is hazy at this time, but still bright enough to make the search difficult. But by braving the twilight, asteroid hunters have been able to find many asteroids that cross Earth’s orbit and some orbiting the interior of the solar system. Through twilight observations, scientists working with the four-meter Blanco telescope in Chile have found the first known asteroid orbiting closer to the Sun than Venus, and the largest potentially dangerous asteroid for Earth in recent years years was found. (Don’t worry, it won’t intersect with the planet.)
“We find things that other people basically can’t find,” Scott Sheppard, an astronomer at the Carnegie Institution for Science, describes this twilight method in an article for Sciencehow The edge. “So I think it’s always a great thing to watch areas that other people don’t watch.”
Asteroid hunting is already quite difficult, even if you’re searching at night. Near-Earth asteroids appear as very fuzzy, faint points of light darting through the sky. Asteroids don’t emit light of their own, but instead reflect light coming from the sun, making these small dots easier to see at night. But we can only see part of the sky in the dark. “Day covers half the sky and night covers half the sky,” says Sheppard. “So if you’re just looking at night, you’re basically only looking at half the sky.” Many of the asteroids that spend most of their time in the interior of the solar system never actually show up at night; They can only be found during the day, which is far too bright to actually see them.
Searching in the twilight can help reveal some of these mysterious objects, but it makes asteroid hunting even more difficult. Twilight covers a period of about 10 to 15 minutes just before sunrise and 10 to 15 minutes just after sunset. That doesn’t give astronomers much time to find those fuzzy points of light, and when they do spot one, they have to observe it again in the same short amount of time to confirm its position.
The biggest headache of all is the glare from the sun. “When you take a picture, your background is a lot lighter, so if you have a very noisy background, an object doesn’t stand out as easily,” says Sheppard. Add to this the fact that the telescopes point almost to the horizon to observe the sky that normally surrounds the sun. This means the telescopes are actually observing through even more of Earth’s atmosphere than usual, much more air than if the telescope were pointed straight up and out. This makes the blurred points of light even more blurred. In addition, the angle at which these asteroids are positioned relative to the Sun means that they are only partially illuminated.
Despite all this, astronomers have historically used much smaller telescopes – about a meter in diameter – to look for asteroids at dusk. But since last summer, Sheppard and his team have been using a special camera called the Dark Energy Camera on the National Science Foundation’s four-meter Blanco telescope. Their search has uncovered three new notable asteroids, including the potentially dangerous asteroid 2022 AP7. It’s about a kilometer across and is crossing Earth’s orbit, according to Sheppard, although it’s not supposed to come close to the planet. Its size and trajectory technically place it in the “potentially dangerous” category, a category reserved for asteroids of a certain brightness that are a certain distance from Earth. Most of these asteroids have already been sighted as astronomers are eager to find them as they could potentially wreak havoc on Earth if they hit us.
In addition to the four-meter Blanco telescope, astronomers have also used the 48-inch Zwicky Transient Facility telescope in California to find asteroids in the twilight, where they have successfully revealed some space rocks. While finding more asteroids is obviously a boon to planetary defenses, Sheppard says it’s also about better understanding how asteroids move in our cosmic neighborhood. Many asteroids are thought to originate from the main asteroid belt between Mars and Jupiter, but astronomers are curious if there are unknown reservoirs of space rocks that contribute to asteroids elsewhere. And searching in the twilight could help answer that question.
“Our main goal for the survey is to understand the population of these very interesting asteroids to give us a global view of where they come from and how they move in the solar system,” says Sheppard.
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