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 largest asteroid finders at the moment are the Pan-STARRS observatory out of Hawaii and the Catalina Sky Survey, which operates multiple telescopes out of Arizona. 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. That limits the parts of the sky they can observe to the area right around Earth and 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. By observing at twilight, scientists working with the Blanco four-meter telescope in Chile have found the first known asteroid that orbits closer to the Sun than Venus and the largest potentially hazardous asteroid to Earth that’s been found in the last several years. (Don’t worry, it won’t be crossing paths with the planet.)
“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 Sciencetell 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 appear as very fuzzy, faint points of light zooming through the sky. Asteroids don’t emit light on their own but reflect light coming from the Sun, so it’s easier to see these little dots at night. But we can only see part of the sky in the darkness. “Daytime covers half the sky and nighttime covers half the sky,” says Sheppard. “So if you’re only looking at night, you’re only observing basically half the sky.” Many of the asteroids that spend most of their time in the interior of the Solar System never really show up at night; they can only be found during the day, which is far too bright to actually spot them.
Searching at twilight can help reveal some of these mysterious objects, but it does make the process of asteroid hunting even harder. Asteroid hunters are particularly interested in a specific period of twilight, roughly 10 to 15 minutes just before sunrise and 10 to 15 minutes just after sunset, according to Sheppard. That doesn’t give astronomers a ton of time to find these fuzzy points of light, and then, if they do spot one, they have to reobserve it in the same short timeframe to confirm its position.
The biggest headache of all is the glare of the Sun. “When you take an image, your background is much brighter, so an object doesn’t stand out as easily when you have a really high noisy background,” says Sheppard. 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. That makes the fuzzy points of light even fuzzier. On top of all that, the angle at which these asteroids are in relation to the Sun makes them only partially illuminated.
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 White 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, although 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 comes within a certain distance of Earth. Most of those asteroids have already been spotted since astronomers are eager to find them because of their potential to wreak havoc on Earth if they did hit us.
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 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 around the Solar System,” says Sheppard.
Update July 25th, 12:45PM ET: This story was updated to clarify the time when astronomers are looking for asteroids at twilight.