For decades, the standard way to search for asteroids in our Solar System has been to scan the night sky for fast-moving specks of light — but a new method of hunting for these space rocks at twilight is also proving fruitful. It’s much harder to pull off, but by scanning parts of the sky at dusk, astronomers have been able to find key asteroids they wouldn’t have seen otherwise.
The two most important asteroid-finders at present are Pan-STARRS out of Hawaii, and Catalina Sky Survey out of Arizona. They operate multiple telescopes. For the last decade, these two programs have been the premier hunters of near-Earth asteroids. But they primarily search the sky at night, looking away from the Sun. This limits their ability to see the entire sky, except the region around Earth and in the outer Solar System.
Recently, asteroid hunters have been turning their telescopes toward the Sun just after it sets or just before it rises. The sky is hazy at that time but still bright enough to add difficulty to the search. But by braving twilight, asteroid hunters have been able to find plenty of asteroids that cross Earth’s orbit and some circulating in the Solar System’s interior. Scientists using the Blanco 4-meter telescope in Chile discovered the first known asteroid orbiting closer to Venus than the Sun. It also made it the most dangerous asteroid ever found to Earth in recent years. It won’t cross paths with Earth, but it’s not worrying. )
“We are finding things that other people can’t find, basically,” Scott Sheppard, an astronomer at the Carnegie Institution for Science who detailed this twilight method in an article for Science, tells The Verge. “And so it’s always, I think, a great thing to observe areas that other people aren’t observing.”
Asteroid hunting is already fairly difficult, even when you are searching at night. Near-Earth asteroids are very faint, fuzzy points of light that zoom through the skies. Because they don’t emit any light, but rather reflect the Sun’s light, it is easier to spot these tiny dots at night. But we can only see part of the sky in the darkness. Sheppard says that daytime only covers half of the sky, while night covers the other half. Sheppard says, “Since night covers half of the sky, and daytime covers half of it, then you are only seeing about half of the sky.” Most asteroids, such as those that live in the inner Solar System, can be seen only during daylight, when they will not appear at all.
Searching at twilight can help reveal some of these mysterious objects, but it does make the process of asteroid hunting even harder. Twilight encompasses a period of time about 10 to 15 minutes just before sunrise and 10 to 15 minutes just after sunset. Astronomers don’t have a lot of time to locate these points of light. They also need to observe it again in the same time frame to verify its location.
The biggest headache of all is the glare of the Sun. Sheppard says that a bright background makes it harder to see objects in an image. Adding to that difficulty is the fact that the telescopes are pointing almost at the horizon in order to observe the sky normally surrounding the Sun. That means the telescopes are actually observing through even more of Earth’s atmosphere than usual, much more air than if the telescope is pointed straight up and out. This makes it even more difficult to see the light fuzzier. These asteroids can only be partially illuminated because of the Sun’s angle to them.
Despite all this, astronomers have used much smaller telescopes in the past — about one meter in diameter — to look for asteroids at twilight. But starting last summer, Sheppard and his team have used a special camera called the Dark Energy Camera on the National Science Foundation’s Blanco four-meter telescope. Their search has turned up three new asteroids of note, including the potentially hazardous asteroid 2022 AP7. It’s about one kilometer in size and crosses Earth’s orbit, according to Sheppard, though it’s not supposed to come near the planet. Its size and path technically put it in the category of “potentially hazardous,” which is a category reserved for asteroids of a specific brightness that come within a certain distance of Earth. Since astronomers have already seen most of these asteroids, they are keen to locate them. They fear that they could cause havoc and damage to Earth.
Along with the Blanco four-meter telescope, astronomers have also been using the 48-inch Zwicky Transient Facility telescope, located in California, to find asteroids at twilight, where they’ve successfully turned up some space rocks. While finding more asteroids is obviously a boon to planetary defense, Sheppard says it’s also about better understanding just how asteroids move around our cosmic neighborhood. Plenty of asteroids are thought to stem 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 at twilight could help answer that question.
” Our main objective for this survey is to comprehend the population of these extremely interesting asteroids in order to provide us with a global view on where and how they travel around the Solar System,” Sheppard says.
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