Interstellar Visitor: The Strange Asteroid from a Faraway System

A staple of science fiction has always been aliens from other worlds. As science has revealed the inhospitableness of planets such as Mars and Venus, the source of these fictional visitors has increasingly moved to deep space. Now it has happened—the first visit from beyond. In late 2017, an interstellar asteroid named ‘Oumuamua swept through the solar system. Inevitably, it has stirred speculation that it could even be a spacecraft, a messenger from beyond our solar system.

This visitor was discovered on October 19, 2017, with the Pan-STARRS optical telescope on Haleakala, Hawaii, a part of the NASA-supported Spaceguard Survey of asteroids that come close to the Earth. At the time of its discovery, it was 33 million kilometers from Earth and had already passed its closest point to both the Sun and the Earth. As astronomers followed up on the discovery, it quickly became apparent that the orbit of ‘Oumuamua was unlike anything seen before. All members of the solar system are gravitationally bound to the Sun, with orbits that are ellipses. This object, however, was travelling far too fast to be part of the Sun’s family. Its orbit is a hyperbola, and when discovered it was already rapidly leaving the inner solar system, having passed close enough to the Sun to bend its orbit dramatically.

Although it was too distant for imaging by even large telescopes, its size and shape could be estimated from its brightness and large light fluctuations. Remarkably, ‘Oumuamua is highly elongated, with an approximately cylindrical shape. The nominal dimensions are about 200 meters in length and only thirty-five meters across, the most extreme of any natural object known. Large objects such as planets and moons are pulled by their own gravity into roughly spherical shape, and even small asteroids and comets (often described as “potato shaped”) rarely have irregularities of more than a factor of two. The extreme shape of ‘Oumuamua is as unique as its orbit.

An artist’s conception of the highly elongated interstellar asteroid. There is no telescopic image resolving the fast-moving object. Illustration: M. Kommesser, European Southern Observatory

‘Oumuamua is a Hawaiian word meaning “scout” or “first to reach out.” Note that the pictures widely published and posted on the Internet are artist’s impressions; no telescope has been able to resolve this object. All we know is the large amplitude of its light variation as it tumbles with a period of about eight hours. The most common such portrait makes it look like a shard of rock, but it could just as easily be presented as a broken obelisk or even a cylindrical spaceship.

In a way, the discovery of an interstellar asteroid or comet was not unexpected. Early in solar system history, before the planet orbits had sorted themselves into stable coplanar, nonintersecting paths, we estimate that quite a lot of mass was ejected, either whole planets or more numerous smaller fragments. Even today an occasional comet, coming in from the outer edges of the solar system, can have its orbit changed by gravitational interaction with Jupiter and the Sun. Some are captured into smaller orbits (called short period comets); others are ejected, departing on hyperbolic trajectories. As we have learned recently that exoplanetary systems are common, the question has arisen: Where are similar debris ejected from other planetary systems? Now we have found one, and improved surveys will probably soon add to this category.

‘Oumuamua was anticipated in another way—by the classic science fiction author Arthur C. Clarke, in his popular 1973 novel Rendezvous with Rama. In this novel, set in 2130, a fifty-kilometer-long cylinder was discovered entering the solar system on a hyperbolic orbit much like that of ‘Oumuamua. The asteroid was named “Rama.” Unlike ‘Oumuamua, however, Rama mysteriously decelerated as it neared the Sun and was captured into a temporary orbit. A team of astronauts rendezvoused with the spacecraft and entered its hollow interior, where they were baffled by the evidence of a highly advanced and mysterious technical civilization. Before they could complete their exploration or be joined by other teams, Rama, having apparently “recharged its batteries” near the Sun, began to accelerate out of the solar system, leaving myriad unanswered questions. Perhaps the discovery of ‘Oumuamua will energize the nascent efforts to make a film from Rendezvous with Rama. It has been reported that Neil deGrasse Tyson would like to play the science lead in the film.

There is another interesting connection between Rama and ‘Oumuamua. In the first chapter of Rendezvous with Rama, Clarke described a catastrophic asteroid impact in northern Italy. He suggested that the people of Earth created a Spaceguard program to ensure this never happened again by finding any threatening asteroid or comet well in advance. It is this Spaceguard that discovers Rama. In the early 1990s, when my colleagues and I were proposing an asteroid survey for purposes of planetary defense, we christened it the Spaceguard Survey, with the permission and encouragement of Clarke. It was one of these Spaceguard telescopes that discovered ‘Oumuamua.

Although few are suggesting that ‘Oumuamua was an interstellar spacecraft, we would like to learn more about it. It is already beyond the reach of our telescopes, but some have suggested that with enough financial resources we might be able to send a small spacecraft to catch up with ‘Oumuamua before it departs the solar system. Meanwhile, the SETI radio telescopes at Hat Creek in California and Green Bank in West Virginia did spend several days with multiband receivers pointed at ‘Oumuamua. They did not detect any transmissions from it. But even if it is simply an asteroid from a distant solar system, ‘Oumuamua would be fascinating to study up close to determine its composition, age, and geologic history. Until now, astronomers have been limited to studying the electromagnetic radiation alone from space beyond the solar system.

‘Oumuamua is gone, leaving behind more questions than answers. But it is surely just the first such visitor that will be discovered.

Meanwhile, asteroids that come close to the Earth are frequently in the news, particularly in tabloid articles and social media posts that state or imply they pose a threat of impact. NASA receives a steady influx of questions from the public about an imminent asteroid doomsday. It is a challenging communications problem to explain that while asteroid impacts are real and will happen someday, they are not a present danger. No known asteroid is on a collision course with Earth. Decades ago, astronomers and NASA decided to classify Near Earth Asteroids (NEAs) that can come very close to Earth as “Potentially Hazardous Asteroids (PHA).” Most asteroids in the inner solar system have orbits that, while they may cross the orbit of Earth, are not capable of coming very close or hitting us. “Potentially Hazardous Asteroid” or PHA is a technical description of the asteroid orbit, not an indication that there is any near-term danger of impact. But of course, it is easy to twist the meaning when NASA reports that a potentially hazardous asteroid is headed toward Earth. Some have jokingly suggested the usefulness of a new term for public communications, replacing PHA with PHABNTT: Potentially Hazardous Asteroid—But Not This Time.


A staple of science fiction has always been aliens from other worlds. As science has revealed the inhospitableness of planets such as Mars and Venus, the source of these fictional visitors has increasingly moved to deep space. Now it has happened—the first visit from beyond. In late 2017, an interstellar asteroid named ‘Oumuamua swept through …

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