"We haven't found them all."


An asteroid in the glare from the sun.

Credit: DOE / FNAL / DECam / CTIO / NOIRLab / NSF / AURA / J. da Silva / Spaceengine


图片来源:DOE / FNAL / DECam / CTIO / NOIRLab / NSF / AURA / J. da Silva / Spaceengine

Astronomers are vigilantly scanning the skies for asteroids that veer into Earth's solar system neighborhood. But, some are hidden.


Most asteroids — leftover rubble from our solar system's formation some 4.6 billion years ago — orbit the sun between Mars and Jupiter. Millions exist out there. But in the inner solar system, asteroids are obscured by the blinding glare from the sun.


Now, a new survey of space rocks in regions around the orbits of Venus and Mercury have spotted sizable asteroids in this elusive zone of space. One is nearly a mile wide, the type of "planet-killer" rock that would decimate life on Earth. Fortunately, these rocks don't currently pose any danger to our planet, nor will they for the foreseeable future — though over centuries, or much longer, one of the asteroids' orbits may change and potentially pose a threat.


To find these rocks, scientists must scour the sky at twilight (at dawn and dusk). They get just 10 minutes. At dusk, for example, they have the narrow viewing time after the sun has dimmed, but before this sun-facing sky disappears under the horizon.


"You don't have a lot of time," Scott Sheppard, an astronomer at the Carnegie Institution for Science, told Mashable. Sheppard led the new research about these twilight asteroid discoveries, which was recently published in The Astronomical Journal.


A powerful telescope is needed to find these rocks. At 7,200 feet up in Chile, scientists attached a camera, called the Dark Energy Camera, to a 4-meter (13-foot) wide telescope. It's the biggest camera on a telescope of such a large size, explained Sheppard, and it can view great swathes of the sky. (Previously, astronomers used it to look for truly deep space objects, beyond Pluto.) In just one image, scientists can view a region of sky encompassing about 11 full moons, as opposed to their previous twilight-viewing capabilities of around two full moons.


So far, they've spotted three "near-Earth asteroids," or NEAs. It doesn't actually mean they're literally "near" Earth, like the moon. It means they're relatively near — because space is huge. These are rocks whose orbit can at times pass within some 30 million miles of Earth's orbit around the sun (not necessarily Earth itself), explains NASA.


The Víctor M. Blanco 4-meter Telescope is used to detect asteroids at twilight.

Credit: CTIO / NOIRLab / NSF/ AURA / D. Munizaga

Víctor M. Blanco 4米望远镜用于探测黄昏时的小行星。

来源:CTIO / NOIRLab / NSF/ AURA / D. Munizaga

The designation also doesn't mean they're a threat. No known asteroid over 460 feet across will threaten Earth in the next century or so. Crucially, the chances of a major impact in our lifetimes is, as far as we know, extremely small, Eric Christensen, the director of the near asteroid-seeking Catalina Sky Survey in Arizona, told Mashable earlier this year. As Mashable reported:

这一称号也不意味着它们是威胁。下个世纪左右,没有直径超过460英尺的小行星会威胁到地球。关键是,据我们所知,在我们的一生中发生重大撞击的可能性非常小,埃里克·克里斯滕森(Eric Christensen)今年早些时候对Mashable说,他是亚利桑那州寻找近小行星的卡特琳娜巡天计划的负责人。据Mashable报道:

Impacts by objects around 460 feet in diameter occur every 10,000 to 20,000 years, and a "dinosaur-killing" impact from a rock perhaps a half-mile across or larger happens on 100-million-year timescales. Though something smaller could certainly surprise us, like the unexpected football-field-sized asteroid that swung just 40,000 miles from Earth in 2019. That's why watching is critical. We might not be able to nudge an approaching rock away from our planet — that's an ambitious space endeavor that takes years of planning — but we can prepare for an impact and move people out of the way.


Two of the three newly revealed asteroids, however, are indeed of "planet-killer" size. They likely came from the main asteroid belt, where most asteroids live. The near-Earth asteroids are on eccentric orbits that are stable for a million years or so, explained Sheppard. But as they interact with the gravity of nearby planets, their orbits can shift. Eventually, they'll most likely be ejected to the outer solar system.


"There's no danger."


The largest of these objects, 2022 AP7, is expected to pass extremely close to Venus — within a few thousand miles — in the next 1,000 years. It could potentially hit Venus, though that probability remains low. "It's unlikely to happen, but you never know," noted Sheppard. One day in the future, 2022 AP7 may also travel into the path of Earth's orbit, too. That's why this colossal rock also earns the designation of "Potentially Hazardous Asteroid" (meaning it's bigger than 460 feet wide and its orbit passes within 4.6 million miles of Earth's orbit, or path, around the sun.)

这些天体中最大的一个,2022 AP7,预计将在未来1000年里非常接近金星——在几千英里之内。它有可能撞击金星,尽管这种可能性仍然很低。“这不大可能发生,但你永远不知道,”谢帕德说。在未来的某一天,2022年的AP7也可能会进入地球轨道。这就是为什么这颗巨大的岩石还获得了“潜在危险小行星”的称号(这意味着它的宽度超过460英尺,其轨道在地球绕太阳轨道460万英里的范围内。)

But that day, if it ever occurs, isn't any time soon. That possibility is on the order of centuries or millennia away. "There's no danger," Sheppard emphasized. "There are no interactions with Earth in the foreseeable future."


Unfortunately, this reality won't stop some news sites from publishing misleading and scaremongering headlines about the newly found space rocks, such as, I kid you not: "Huge ‘planet killer’ asteroid discovered – and it’s heading our way." That's rubbish. In fact, any time a news site or entity on social media warns of an asteroid "headed our way," ignore it. These egregious efforts are just seeking your clicks. These dubious stories are published weekly. Yet, NASA has literally never even issued a warning about a menacing, incoming asteroid. If a space rock ever does become a threat, NASA, the White House, and political leaders will be involved.


To protect humanity from asteroid strikes, astronomers and planetary defense experts want to know where most of the near-Earth asteroids are headed. If one is projected to one day — perhaps in decades or centuries — come unsettlingly close to Earth, then we can do something about it. "You need to know what's coming, when it's coming, and how hard it's going to hit," Christensen told Mashable. Even a smaller asteroid, some 100- to 170-feet across, could destroy a place like Kansas City, home to half a million people. So the surveys for rocks large and "small" are vital.


A graph showing near-Earth asteroid discoveries

Credit: NASA / Center for Near Earth Object Studies


图片来源:NASA /近地天体研究中心

With enough time (years), NASA recently has proven that we have the ability to slightly change an asteroid's trajectory, and thereby save Earth from a potential disaster. The space agency recently crashed a vending machine-sized spacecraft in an asteroid the size of a stadium. The sci-fi-like endeavor, called DART, or Double Asteroid Redirection Test, was a successful mission to see how civilization could alter the path of a menacing asteroid, should one be on a collision course with our planet.


Of the largest class of near-Earth space rocks — those 1 km wide or larger — astronomers estimate they've found around 95 percent of them. They are non-threatening. As of November 2022, they've located 857. That means there are some 20 to 50 of those "planet-killer" asteroids still out there. Astronomers would like to find more, just to be safe. That means peering into the twilight sky.


"We haven't found them all," Sheppard said.