We could get an up-close look at our solar system's latest interstellar interloper a quarter century from now if we so desired, a new study suggests.
Late last month, Russian amateur astronomer Gennady Borisov spotted a comet that appears to be visiting from very far away. The odd trajectory and tremendous speed of the object, known as Comet C/2019 Q4 (Borisov), strongly suggest that it was born around another star rather than our own sun, scientists say.
This comet is the third potential interstellar visitor we know about — and all of them were spotted in just the past two years.
Related: Interstellar Comet: Here's Why It's Got Scientists So Pumped Up
The first was 'Oumuamua, which zoomed through the inner solar system in the fall of 2017. 'Oumuamua is intriguing for many reasons beyond its pioneer status (and the fact that it's the only one of the three that's a confirmed interstellar object). For example, 'Oumuamua is oddly shaped — its contours resemble those of a pancake or a cigar — and it displayed nongravitational acceleration during its trek through Earth's backyard. (That is, it moved in ways unrelated to the tugs of the sun, Jupiter or other bodies.)
Such movement can be caused by cometary outgassing, but astronomers never saw a tail streaming from 'Oumuamua, or any other obvious comet characteristics. So, Harvard University astronomers Shmuel Bialy and Avi Loeb have proposed that 'Oumuamua might actually be an alien spacecraft — specifically, one equipped with a lightsail, whose harnessing of sunlight pressure could explain the object's odd movement.
Loeb was involved in spotting a second potential interstellar visitor: a small meteor that exploded in the sky near Papua New Guinea in January 2014. Loeb and Harvard undergraduate Amir Siraj announced this object's likely exotic origin earlier this year, in an unpublished preprint. They based their conclusion on the meteor's trajectory, which they reconstructed after studying the fireball database compiled by the Jet Propulsion Laboratory's Center for Near Earth Object Studies (CNEOS).
Like Comet C/2019 Q4 (Borisov), the 3-foot-wide (1 meter) meteor remains a potential, rather than confirmed, interstellar visitor for now. Siraj and Loeb haven't been able to publish their paper yet, primarily because the CNEOS database does not report measurement error. (This information is classified, because it could reveal details about the government sensors used to detect fireballs. Siraj and Loeb nailed down the error bars thanks to help from people with the proper security clearance; the journal they submitted the paper to is trying to find someone similar, Siraj told Space.com last month.)
Shortly after 'Oumuamua came on the scene, researchers around the world began investigating the feasibility of launching a probe to study the object up close. The information gathered by such a mission could reveal key insights about the formation and evolution of other solar systems, and perhaps tell us if objects such as 'Oumuamua commonly carry the building blocks of life, or maybe even life itself, from place to place, scientists have said.
One team that looked into the feasibility of an 'Oumuamua mission back then was led by Andreas Hein, of the Initiative for Interstellar Studies in England. Hein and his team determined that humans could indeed send a craft to fly by the object using current technology.
For example, a probe the size of NASA's New Horizons spacecraft could launch toward 'Oumuamua atop a SpaceX Falcon Heavy rocket in 2021 and get to 'Oumuamua in 2049, the researchers calculated. The journey would be circuitous, involving flybys of Jupiter and Saturn, as well as a maneuver close to the sun.
Now, Hein and some members of the same team have investigated the prospect of a Comet C/2019 Q4 (Borisov) probe. Their new study found that such a flyby mission is indeed feasible, but it would have to employ a much smaller spacecraft than a trip to 'Oumuamua would.
Related: 'Oumuamua: Our 1st Known Interstellar Visitor Explained in Photos
We could launch a 6.6-lb. (3 kilograms) cubesat toward the comet in 2030 using NASA's huge Space Launch System rocket, which is still in development, Hein and his colleagues reported in a paper they recently posted to the online preprint site arXiv.org. The little craft would get to its destination in 2045, after employing a Jupiter flyby and a maneuver near the sun.
Interestingly, we just missed an opportunity to send a much more capable craft toward Borisov. Orbital dynamics allow a 2-ton spacecraft to launch toward the comet atop a Falcon Heavy — if that launch had occurred in July 2018, the researchers determined.
It would be fascinating to see either interstellar object up close, of course. But Hein said that if he had to pick one flyby target, he'd go with 'Oumuamua.
"Given what we know, I would prefer 'Oumuamua at the moment," he told Space.com via email. "In terms of degree of oddness, 'Oumuamua currently definitely wins against C/2019 Q4 (Borisov). The strange cigar-like shape and its mysterious acceleration makes one curious to find out more."
"However, things might change once we have more data about C/2019 Q4 (Borisov) via telescope observations," Hein added. "During the next couple of days or weeks, we may discover oddities for this object as well. Another reason for catching 'Oumuamua is that it is a bit easier to reach than C/2019 Q4 (Borisov), as it travels slightly slower (26.3 km/s outside the solar system against about 30 km/s)."
We may never end up sending a spacecraft to either object. But Hein said knowing that we can do so is important, especially since Borisov is unlikely to be the last interstellar visitor we encounter. The Large Synoptic Survey Telescope, which is scheduled to come online next year, will likely find a significant number all by itself, for instance.
"We want to show to the space and science community that we have the technology to develop missions to interstellar objects if we want to," Hein said. "The fact that we were able to show the technical feasibility for sending a spacecraft to both discovered objects makes it very likely that the next one can also be reached."
And that's just using current technology. There are potential breakthroughs on the horizon that could make the exploration of 'Oumuamua and its kin a rather straightforward affair. If the Breakthrough Starshot interstellar flight project gets off the ground as planned, for example, humanity could start greeting interloping rocks a few decades from now with swarms of nanocraft that sail out to their destinations in just a few days at most.
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Mike Wall's book about the search for alien life, "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), is out now. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook.