Asteroids with ‘unstable orbits' hide around Venus—do they threaten Earth?

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Venus has groupies—a family of asteroids that share its orbit, either trailing it or leading it as the planet revolves around the sun. Researchers have known that such stealthy space rocks might exist for years, but now, a pair of papers (one published in a journal, and one a pre-print undergoing peer-review) conclude that some might develop unstable orbits and, over a very long period of time, arch toward Earth.
But despite what several histrionic headlines have claimed, Earth is not at risk of one of these asteroids suddenly sneaking up on us and vaporizing a city. While some of these asteroids could be large enough to cause this sort of damage, there is no evidence whatsoever suggesting any of these Venus-pursuing asteroids are currently heading our way.
'I wouldn't say that these objects are not dangerous,' says Valerio Carruba, an asteroid dynamicist at the São Paulo State University in Brazil and a co-author of both studies. 'But I don't think there is any reason to panic.'
These studies simply highlight that asteroids near Venus have the potential to fly our way on sometime in the next few thousand years or so. 'The likelihood of one colliding with Earth any time soon is extremely low,' says Scott Sheppard, an astronomer at the Carnegie Institution for Science in Washington, D.C. who was not involved with the new research. 'There isn't too much to be worried about here.'
The real problem, though, is that asteroids like this are remarkably difficult to find, and you can't protect yourself against a danger you cannot see. Fortunately, in the next few years, two of the most advanced observatories ever built are coming online. And together, they will find more asteroids—including those hiding near Venus—than the sum total already identified by the world's telescopes.
While the Japanese and European space agencies mostly request time on busy telescopes to search for these space rocks, NASA leads the pack: It funds a network of observatories solely dedicated to finding sketchy-looking asteroids.
Planetary defenders are chiefly concerned about near-Earth asteroids. As the name suggests, these have orbits that hew close to Earth's own. Many of these asteroids were removed from the largely stable belt between Mars and Jupiter, either through the chaotic gravitational pull of the planets (often Jupiter, as it's the most massive) or through asteroid-on-asteroid collisions.
If one gets within 4.6 million miles of Earth's orbit, there's a chance that, over time, both orbits cross and a collision becomes possible. And if that asteroid is 460 feet long, it's big enough to plunge through the atmosphere and (with a direct hit) destroy a city. Combined, these characteristics describe 'potentially hazardous asteroids'—and finding them is of paramount importance.
Asteroids are first found because of the sunlight they reflect. That works well for most, but there are known to be asteroids hiding interior to Earth's orbit, toward the direction of the sun. And that's a problem.
Astronomers seeking out these asteroids cannot just point their telescopes directly at the sun: It would be like trying to see a lit match in front of a nuclear explosion. Instead, they look in the vicinity of the sun in the few minutes just after sunset, or just before sunrise. Not only are these surveys severely time-limited, but by aiming close to the horizon, they are peering through more of the Earth's atmosphere, which distorts what they are looking at.
'All of these factors make it hard to search for and discover asteroids near Venus' orbit,' says Sheppard.
(Here's how researchers track asteroids that might hit Earth.)
Asteroids have occasionally been spotted in this sun-bleached corner of space. And twenty of them have been found scooting along the same orbital highway Venus uses to orbit the sun. These are known as co-orbital asteroids; similar rocks can be found either following or trailing other planets, most notably Jupiter.
Co-orbiting asteroids tend to cluster around several gravitationally stable sections, known as Lagrange points, along the planet's orbital path. But over a timescale of about 12,000 years or so, it's thought that the Venus co-orbital asteroids can dramatically alter their orbits. They remain on the same orbital path as Venus, but instead of maintaining a circular orbit, they get creative: Some migrate to a different Lagrange point, while others zip about in a horseshoe pattern around several Lagrange points.
Some of these new, exotic orbits become quite stretched-out and elliptical—and, in some cases, these orbits can eventually bring these asteroids closer to Earth. When they do, 'there is a higher chance of a collision,' says Carruba.
In their first study, published in the journal Icarus earlier this year, Carruba and his team looked at the 20 known co-orbital asteroids of Venus. Their simulations forecast how their orbits would evolve over time and show that three of the space rocks—each between 1,000 and 1,300 feet or so—could approach within 46,500 miles of Earth's orbit. (For reference, the moon is an average of 240,000 miles from our planet.)
That proximity may make them potentially hazardous asteroids. But there's no need to worry—it can take as long as 12,000 years for an asteroid to end up on an elliptical, near-Earth orbit. Perhaps they will be a problem for our very, very distant descendants.
The team's latest study, uploaded to the pre-print server arXiv last month, delves into how easy it might be for any of Venus' co-orbital asteroids—including those astronomers have yet to find—to end up on these precarious orbits. To find out, they created virtual asteroids and simulated their many potential orbital voyages 36,000 years into the future.
Many things could perturb the orbits of asteroids over that many years, so any truly accurate predictions are impossible. But the simulations came to some broad conclusions. The first is that a Venus co-orbital asteroid is more likely to approach Earth if it switches from a circular to a considerably elongated orbit—it's zooming over a larger patch of the inner solar system, including our own planet's neighborhood.
The second, more surprising thing, is that some asteroids still manage to reach near-Earth space even they start out with only a mildly stretched-out orbit. It seems that their chaotic journeys through space, filled with gravitational disturbances, can still end up throwing them our way.
But to be clear, these potentially worrisome orbits develop over the course of many millennia. 'This is not something to be alarmed about, as these asteroids are still relatively dynamically stable on human timescales,' says Sheppard.
(These five asteroids pose the highest risk to Earth.)
For Marco Fenucci, a near-Earth object dynamicist at the European Space Agency, the paper raises awareness about these relatively mysterious asteroids in Venus' orbit. And that is a good point to make, he adds: We don't know much about these asteroids, including their population size, their dimensions, and their orbits, because we struggle to find them with today's telescopes.
Two upcoming facilities are about to make this task considerably easier. The first, the U.S.-owned Vera C. Rubin Observatory in Chile is set to officially come online in the next few weeks. With a huge field-of-view, it can see huge swathes of the night sky at once, and its giant nest of mirrors can gather so much starlight than even the smallest, faintest objects can be seen.
In just three to six months, the observatory could find as many as a million new asteroids, effectively doubling the current total. Meg Schwamb, a planetary scientist at Queen's University Belfast who was not involved with the new research, explains that Rubin will also conduct its own twilight surveys, the very sort used today to search for near-Venus asteroids.
If these surveys are conducted over the next decade, 'Rubin could find as many as 40 to 50 percent of all objects larger than about [1,150 feet] in the interior-to-Venus-orbit population,' says Mario Jurić, an astronomer at the University of Washington and who was not involved with the new research. But, as with all ground-based optical telescopes, Rubin will still have the sun's glare, and Earth's atmosphere, to contend with.
As long as the federal government decides to continue to fund the mission—something that is not guaranteed—NASA will also launch a dedicated asteroid-hunting space observatory, the Near-Earth Object (NEO) Surveyor, in the next few years. Unobstructed by Earth's atmosphere, it will seek out space rocks by viewing them through a highly-sensitive infrared scope, meaning it can see those hidden by the luminous sun.
Even those asteroids sneaking around near Venus won't be able to hide from NEO Surveyor. And, finally, says Carruba, 'we can see if the impact threat is real, or not.'