Scientists are racing to design spacecraft to reach the enigmatic ‘Oumuamua — or be ready for the next one.

Key takeaways

  • In 2017, scientists found ‘Oumuamua, the first known object from outside our solar system, but it was moving too fast to study closely.
  • Reaching ‘Oumuamua requires a spacecraft that can travel at very high speeds, posing significant technical challenges.
  • ‘Oumuamua’s unusual shape and unexplained acceleration suggest it might not be a typical asteroid or comet, sparking theories of an artificial origin.
  • Scientists are working on mission ideas to intercept ‘Oumuamua or similar future interstellar objects, potentially within a few decades.
  • The upcoming Vera C. Rubin Observatory aims to discover more interstellar objects, making it easier to study these mysterious visitors.

1I/2017 U1 (‘Oumuamua) was discovered in October 2017, and was later proven to be the first object ever spotted inside the solar system that has come from beyond it. However, by the time its origins were discovered, the intergalactic invader had already rounded the Sun and was traveling at 85,700 mph (138,000 km/h). It was reported to be 1,300 feet (400 meters) broad and faded from view of even the most powerful telescopes within weeks.

The only way to gather additional information and discover its true nature would be to send a spaceship to investigate it closely. However, ambiguities in ‘Oumuamua’s precise trajectory, the difficulties of detecting its ever-diminished light, and its quick retreat render the idea of designing, building, and launching a mission in time to catch up to it seem utterly impossible.

Several scientific organizations have been striving to determine how to reach this increasingly distant asteroid. They have proposed a number of scenarios that could result in a close encounter with ‘Oumuamua within decades. If it doesn’t succeed, others are planning a mission to rendezvous with the next interstellar object that passes by.

One possible explanation for ‘Oumuamua’s shape is that it is not a natural object, as depicted at right, but an artificially created solar sail or alien probe, such as the device imagined at center.

Strange visitor

Why bother with such a mission? Because ‘Oumuamua—named after a Hawaiian word for “visitor” and designated 1I as the first interstellar object discovered—is unusual and tantalizing for a variety of reasons.

Astronomers noticed that its brightness fluctuated cyclically. The analysis of this pattern revealed that the object was not only tumbling, but also had a shape unlike anything ever seen in space. These brightness changes might be explained by either a long, narrow cylindrical object or a huge, flat, disk-like shape, but no known asteroids have such an extreme shape.

More significantly, once ‘Oumuamua departed the Sun, it accelerated in ways that could not be explained just by gravity. Outgassing, which occurs when the Sun’s heat boils out volatile components such as water ice and frozen carbon dioxide, can cause comets to accelerate their outbound voyage. However, this process creates a noticeable trail of gas and dust, known as the comet’s tail. ‘Oumuamua left no visible trace, hence its movement cannot be explained by such emissions.

Some astronomers argue that ‘Oumuamua is a block of frozen hydrogen or a hydrogen-helium mix, similar to an iceberg, whose evaporation might account for the object’s acceleration without leaving a discernible trace. Alternatively, it might be a loose agglomeration of particles or filaments that is so light that the solar wind’s pressure alone could rapidly accelerate it away without the requirement for outgassing.

Perhaps there’s another explanation. Astrophysicist Avi Loeb, former chair of Harvard University’s astronomy department, believes ‘Oumuamua’s behavior is so unusual that he has argued in a series of peer-reviewed publications and a popular science book that the object is most certainly a remnant of alien technology.
According to Loeb, natural theories for ‘Oumuamua all mention “rocks of a type we’ve never seen before” and lack a credible explanation for how they evolved. He contends that an artificial origin can better explain its odd characteristics. In addition to its unusual shape, if the object is composed of a thin, flat, lightweight material, it could function as a solar sail, driven outward by the force of sunlight. No such item could occur naturally, he says. It would have to be some structure built by a sophisticated alien civilization — if not a dedicated solar-propelled craft, then perhaps a piece of an alien Dyson sphere, a device to collect the energy from a star.

Many astronomers dispute Loeb’s claim, claiming that ‘Oumuamua is a normal asteroid on an unusual track. Karen Meech, an astrobiologist at the University of Hawai’i who led the study of the object following its discovery, stated that “its characteristics were consistent with a planetesimal with cometary activity.” She emphasizes that we only have roughly a week’s worth of data from its passage, stating that “expecting to have a complete understanding of anything with so little data is unrealistic.”

She continues, “The lack of gas detection was not a mystery. By the time we got to the object, it was too dim to do high-quality spectroscopy, which detects gas.” Follow-up observations were conducted using the Spitzer space telescope “not completely inconsistent” with limits predicted for the amount of outgassing needed to explain its motion, she says.

Nonetheless, Meech believes that close-up observations of ‘Oumuamua or other such objects might be beneficial. “If we had been able to have a close-up image of it, that would have been fabulous!” she laughs. “I believe an in situ mission would be great. Then we could acquire a good chemical characterisation of any gasses that come out — possibly even isotopic ratios — to better understand their creation and origin.

And if Loeb is correct, “if you can tell the composition, you can say that the composition is such that nature never produces such things,” he claims. “Alternatively, you can describe its structure and shape. If it resembles a spacecraft, it is clearly not a rock. So, what you need is detailed enough information—particularly if it has signatures of technology.”

The Vera C. Rubin Observatory nears completion in Chile in this December 2023 photo. Once operational, the observatory’s 8.4-meter telescope will image the entire southern sky every few nights, picking up numerous transient objects — including, astronomers believe, more interstellar interlopers like ‘Oumuamua and Borisov.

How Rubin will change our view

The nearly finished Vera C. Rubin Observatory holds the 8.4-meter Simonyi Survey Telescope, which is outfitted with a 3,200-megapixel CCD camera, the largest ever built. Once operational, it will survey the entire Southern Hemisphere sky every three to four days for a decade. This massive undertaking is known as the Legacy Survey of Space and Time (LSST).

In 2017, the 1.8-meter PanSTARRS1 telescope on Maui detected ‘Oumuamua. The object was spotted less than a week after its closest approach to Earth and more than a month after its closest pass of the Sun (known as perihelion). In a 2019 interview, astrobiologist Karen Meech told Astronomy that the Vera C. Rubin Observatory, if it was working at the time, should have observed ‘Oumuamua about three months before PanSTARRS1, before the object had passed perihelion.

The massive database that LSST will create—logging over 20 gigabytes of data per night for a total of 2 million photos over the course of a decade—will be excellent for detecting changes in the sky, including the motions of asteroids and comets. This includes approximately 6 million moving objects in our solar system, some of which will be temporary guests from other star systems. In a 2023 paper available on the arXiv preprint server and accepted for publication in the Planetary Science Journal, researchers estimated that LSST could find as many as 70 ‘Oumuamua-like interstellar objects each year, with sizes averaging 160 to 1,970 feet (50 to 600 m), though the number heavily depends on how common such travelers are through space, how fast they are moving, and how frequently they visit our solar system.

Catching up

Loeb believes that a trip to catch up with ‘Oumuamua is inconceivable, due to the high speed required and the difficulties of locating the little object in the immensity of space. Others, however, have presented a variety of realistic mission ideas that could reach ‘Oumuamua within a few decades.

The key to catching up to ‘Oumuamua is gaining enough speed to overtake the object, which is traveling at approximately 16 miles per second (26 km/s). Existing chemical rockets are incapable of achieving such speeds on their own. They may, however, gather speed and modify their course by performing slingshot flyby maneuvers, such as approaching the Sun, Jupiter, or other planets. This is a variation on the technology used to send interplanetary probes such as the Voyagers to the outer planets. A gravity assist or Oberth maneuver includes going close to a huge mass, such as a planet or the Sun, and then adding a rocket boost just as the spacecraft approaches its closest point, resulting in tremendous acceleration.

Adam Hibberd, an independent software engineer based in the United Kingdom who worked on the trajectory optimization software for the European Space Agency’s (ESA) Ariane 4 rocket, was already working on his own software for designing interplanetary trajectories when he learned of ‘Oumuamua’s discovery. He was testing the software by duplicating historical expeditions when he noticed the first interstellar object to be detected. I decided to solve missions to this object using my program, which seemed like a natural choice.” He soon discovered some workable paths.

Soon after, he joined Project Lyra, which had formed within months after ‘Oumuamua’s discovery. The crew has created and published a number of flight paths that could make the encounter possible. One such scenario, with a launch date of 2030, would be able to reach ‘Oumuamua in around 22 years, he claims.

Initial concepts included near encounters to the Sun, which would necessitate massive shielding to protect the vessel from the heat, imposing a significant penalty on payload mass. So Hibberd and others began developing similar maneuvers that avoided the Sun and instead relied on Jupiter’s mass.

The difficult thing is “you have to generate enough speed to get to Jupiter from Earth,” Hibberd explains. He and the Project Lyra team discovered two ways to accomplish this using a series of slingshots past the inner planets. Both approaches, he believes, would deliver a spacecraft near Jupiter with enough fuel left to do the gravity-assist maneuver, which would propel the probe into ‘Oumuamua. The journey would take around 31 years, nine years longer than trajectories that include a dangerous close approach to the Sun.

Marshall Eubanks, chief scientist at Space Initiatives Inc. and one of Project Lyra’s mission designers, believes it is inevitable that a mission will be sent to ‘Oumuamua, one way or another. Already, he points out, people are coming up with proposals to send missions to the planets of nearby stars, yet even a century from now ‘Oumuamua will still be vastly closer than such destinations.

“In a hundred years, it’ll only be about 150 AU away,” says the scientist. (One astronomical unit, or AU, is the average distance from Earth to the Sun. “That’s a huge distance, but it’s a lot closer than Proxima Centauri, so why not go there too?” he says. Proxima Centauri is the Sun’s nearest star, at approximately 4.24 light-years (or more than 268,000 AU).

Eubanks believes it is “certainly not physically impossible” to catch up with ‘Oumuamua. “It’s certainly not beyond our technical expertise.” He believes that a slingshot-based mission may be carried out using current rockets like SpaceX’s Falcon Heavy.

Some academics suggest that we should simply wait for the next such thing, Eubanks adds, because new telescopes, such as the Vera C. Rubin Observatory, which will begin operations next year, have the potential to uncover numerous such objects. However, with only two interstellar objects discovered thus far, we have no idea how rare such objects may be. We also don’t know if the others we locate will be as odd as ‘Oumuamua. Comet 2I/Borisov is the only other interstellar object we’ve discovered, and it appears to be a rather ordinary comet.

“If you really think [‘Oumuamua] is an object that’s a real outlier, you should want to go and investigate it,” according to Eubanks. “You’ll never solve it otherwise.”

This artist’s concept shows the three spacecraft of the Comet Interceptor mission — the larger, main craft in the foreground at left and the smaller probes in the distance at right — approaching their as-yet-unknown target for study.

Locking on target

Getting a rocket going fast enough is only half of the challenge. You also need to know where you’re going, which isn’t always easy. Because of the tiny number of observations and their poor precision, the uncertainty in ‘Oumuamua’s current position is comparable to the distance between Earth and the Moon. “You’ll have a difficult time finding it out there in the dark. “It is not a large object,” Eubanks explains.

One possibility is to send a reconnaissance mission to pinpoint the area before launching a larger spacecraft to collect measurements and observations. According to Eubanks, this precursor could have two different forms. It might be a single spacecraft with a big telescope, possibly half the size of Hubble, scanning the sky for the object. Alternatively, we may launch a swarm of hundreds or thousands of tiny spacecraft to spread out and cover the survey region. The Breakthrough Starshot project is now working on developing this technology (and has significant financial backing), with the goal of deploying such probe swarms to nearby star systems.
The tiny spacecraft would be thin disks, perhaps the size of frisbees, propelled by laser beams directed at them from anywhere on Earth, the Moon, or space. Even after this technology is established, reaching anywhere in the solar system will be considerably easier than reaching the nearest star, according to Eubanks. “And 1I is still in the solar system, even though it’s not of the solar system,” according to him. Such laser-propelled ship might theoretically reach ‘Oumuamua in a few of years.

Even these scout probes, he claims, would yield substantial results: “If one finds ‘Oumuamua and reports back its position, even with no further data, that would reveal whether the object had continued to experience non-gravitational acceleration after it was last seen, and if so, how much.”

Any follow-up mission would have to be much larger, at least comparable to NASA’s New Horizons probe, which explored Pluto closely and is still exploring the Kuiper Belt. Such missions cost approximately $1 billion. It’s also likely that no government, or even lone billionaire, would be willing to fund such a mission, which could either fail to reach its target or arrive and discover a relatively ordinary rock.

The next ‘Oumuamua

The most recent mission to visit a comet was ESA’s Rosetta, which gathered close-up images of Comet 67P/Churyumov–Gerasimenko. Comet Interceptor won’t stay in orbit around its target like Rosetta, but instead fly past it to create 3D images and take other measurements in conjunction with its two smaller probes.
Regardless of whether ‘Oumuamua ever gets visited by a spacecraft, there’s always next time. And there is one mission already fully approved and funded that may be ready when the next interstellar object comes along.
ESA’s Comet Interceptor is set to launch in 2029, with one main spacecraft and two smaller probes attached. The craft will go to the Earth-Sun system’s L2 Lagrange point, which is approximately 932,100 miles (1.5 million kilometers) beyond Earth, and wait for either a newly discovered interstellar object or a never-before-seen pristine comet from the outer solar system. It will then fly by the target, collecting photos, spectra, and other data. Approximately 200 people are actively working on the mission. “It’s the first implementation of a so-called rapid-response mission,” explains Comet Interceptor Project Scientist Michael Kueppers.
“We have a range of instruments,” explains Geraint Jones, the project’s transdisciplinary scientist. According to him, Comet Interceptor has three cameras — one on the main vessel and one on each of the two smaller probes — that provide three-dimensional views as they pass by. The mission will also capture infrared photographs and carry a mass spectrometer to determine the composition of gases that may be emitted by a comet.

The main craft is intended to pass within around 620 miles (1,000 kilometers) of the comet. According to Kueppers, at that distance, its main camera will have a resolution of around 26 feet (8 m) per pixel, which is sufficient to capture detailed images of objects such as ‘Oumuamua. The smaller probes would come even closer, with one approaching to 530 miles (850 km) and the other to 250 miles (400 km).

The craft will have around a five-year opportunity to find and select its target. If it is successful in locating a suitable interstellar object within that time frame, “that’s very tantalizing,” Jones says. “The two confirmed interstellar objects we’ve observed thus far were significantly different from one another. Comet Borisov was very similar to other solar system comets; if it hadn’t been on such an odd track, it might not have stood out as a particularly remarkable comet. On the other end of the spectrum is ‘Oumuamua, which had a very low activity level [no tail], and we know very little about its body other than suggestions of an odd form and color. And it would be amazing to witness one of these objects from another star system and compare and contrast it with what we know about comets in our solar system.”

Would Comet Interceptor be able to tell if an item was created artificially or naturally? “I don’t know what an interstellar spacecraft would look like,” Kueppers tells me. However, a really odd shape may make it evident. For example, “I guess it would be quite obvious,” he says, if the target is a long, thin, cylindrical object, which some have used to explain ‘Oumuamua’s brightness changes.

Whatever Comet Interceptor’s future target may be, getting up close and personal with an object from the depths of interstellar space will undoubtedly teach us new things about stars, planets, and the processes that create them. And if we ever get a close look at something made by sentient beings from another planet, it will be one of history’s most epochal discoveries, and a watershed moment in humanity’s understanding of its place in the universe.

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