WASHINGTON (Reuters) - The reddish cigar-shaped object called ‘Oumuamua spotted last year tumbling through space is a comet, scientists said on Wednesday, solving the mystery over how to classify the first interstellar object found passing through our solar system.

This artist’s impression shows the first interstellar asteroid, `Oumuamua as it passes through the solar system after its discovery in October 2017. European Southern Obervatory/M. Kornmesser/Handout via REUTERS/Files

Astronomers said they closely examined the trajectory of ‘Oumuamua, which measures about a half-mile (800 meters) long, as it speeds through our cosmic neighborhood after being evicted somehow from a distant star system.

They found that it is deviating slightly from a path that would be explained purely by the Sun’s gravitational pull because of what apparently is a very small emission of gas from its surface, indicative of a comet.

‘Oumuamua (pronounced oh-MOO-uh-MOO-uh) initially was pegged as a comet, but it lacks the tail of gas and dust characteristic of many comets, and some scientists argued that it was perhaps a dry asteroid.

“It does not display any tail in any observation we obtained,” said astronomer Marco Micheli of the European Space Agency’s SSA-NEO Coordination Centre in Italy, who led the study, published in the journal Nature.

“However, our analysis shows that the amount of emitted gas that is needed to generate this extra force we see would have been so small as to be invisible in our observations,” he said in an email.

This “extra force” acting on the object’s trajectory amounts to only about 0.1 percent of the Sun’s gravitational attraction.

‘Oumuamua was first detected last October by the University of Hawaii’s Pan-STARRS1 telescope. Its name refers in the native Hawaiian language to a messenger arriving from a great distance.

It previously slingshot past the Sun traveling at roughly 196,000 miles per hour (315,000 km per hour) and is heading out of the solar system in the direction of the constellation Pegasus. ‘Oumuamua as of last month was roughly the same distance from the Sun as Jupiter.

Astronomers suspect that more visitors from other star systems will be discovered passing through our solar system.

“The discovery of ‘Oumuamua is an absolute first in the field, and it provided our first opportunity to study an object coming from another star and planetary system,” Micheli said. “The existence of these objects was expected, but seeing one for the first time, and being able to study it in detail, is a unique chance to know more on these distant systems, and how they formed.”</s>It turns out that 'Oumuamua, our first known interstellar visitor, is a comet after all.

This cosmic object was first discovered in 2017 by researchers with the Panoramic Survey Telescope Rapid Response System (Pan-STARRS). Its strange, cigar-like shape, its lack of both a tail and a coma, has made it difficult to categorize — some even suggested it could hold extraterrestrial life. It has so far been classified as a comet, classified as an asteroid, and even put into a new "interstellar objects" class.

'Oumuamua, since its discovery, has been very difficult for researchers to study and understand because, since "this was actually a faint object … we had very little time to observe it," Karen Meech, an astronomer at the University of Hawaii and co-author on the new work, told Space.com. But now, in a paper published today (June 27) in the journal Nature, researchers seem to have concluded that the mysterious interstellar body is a comet. Scientists still don't know where the dark-red, 2,624-foot-long (800 meter) object came from, but at least part of the mystery has now been put to rest. ['Oumuamua: The Solar System's 1st Interstellar Visitor Explained in Photos]

Tumbling through our solar system, 'Oumuamua's movement and behavior along this journey has led researchers to their conclusion. They found that the arc that 'Oumuamua traveled along can't be entirely explained by the gravitational pull of the sun, planets and other solar system objects. So, as the researchers explained in the new paper, part of the object's acceleration isn't caused by gravity.

Classifying the object as a comet explains its arcing movement and nongravitational acceleration, as comets can be propelled by gas they release.

There are other possible explanations for this acceleration, like magnetic interaction with the solar wind, pressure from solar radiation, and forces of drag and friction. But the researchers ruled these out.

This leaves the remaining explanation that 'Oumuamua is propelled partially by gas, which would indicate that it is a comet.

Meech said that this study "conclusively ends" the mystery of what this object really is, while adding that "there's never a hundred percent guarantee on anything."

These researchers are "inferring that it is a comet based on all available evidence, but there is no other explanation," Meech said. She added that, if they wanted concrete proof that it is a comet, they would have needed to see "a tail of dust and gas, but that just wasn't possible in this case."

Still, 'Oumuamua is anything but a standard comet. It lacks a coma, which is the cloud of gas and dust that usually envelops a comet's core, and researchers didn't observe a classic comet tail. The object also has a noticeably unique appearance. So, with the knowledge that they have, the researchers found that 'Oumuamua must be a comet.

The main remaining mystery about 'Oumuamua is its origin. Researchers still don't know where the object came from and, according to Meech, knowing that the object has nongravitational motion will make it even more difficult to figure out its source.</s>The first known interstellar visitor to our Solar System, 'Oumuamua, which buzzed past the Sun last September, has proven evasive to define. First it was a comet. Then it was an asteroid. Then it was maybe both.

Now a new team of researchers led by the European Space Agency has delivered their verdict: 'Oumuamua, according to their analysis, is probably a comet after all.

If you have whiplash, you're not alone - but on the other hand, its interstellar origins notwithstanding, 'Oumuamua is unlike any object we've ever seen before.

It was first spotted on October 19 last year, 40 days after it had already made its closest approach to the Sun, and it immediately got scientists abuzz. Its velocity indicated that it had come from somewhere far outside our Solar System - and that it had been travelling for hundreds of millions of years.

It was also an unusual shape - up to 400 metres (0.25 miles) long, and distinctly cigar-shaped. It could be up to 10 times as long as it is wide - which was never before seen in an asteroid or comet.

Figuring out whether it's an asteroid or a comet, however, has proven extraordinarily tricky. At first, based on preliminary observations, astronomers thought it was a comet. But comets contain ice, the sublimation of which creates the tail as they zoom through space.

'Oumuamua showed none of the characteristics associated with comets. It was reddish in colour, and showed no signs of dust around it - indicating that it was composed of dense rock and metal, reddened by cosmic radiation, and contained no water or ice.

In December, a team of researchers announced that 'Oumuamua's organic-rich crust could conceal a comet-like icy core - concluding that the object is a planetesimal with a well-baked crust (it sounds like a delicious pie now).

But, according to ESA astronomer Marco Micheli and colleagues, there's one more aspect that seriously seems to indicate comet, and it hadn't yet been factored in.

The international team analysed both ground- and space-based observations of 'Oumuamua as it traversed our Solar System, and they found something strange. Its arc could not solely be explained by the effect of gravity exerted by the Sun or other objects. Which means it seems to be behaving… like a comet.

"The motion of all celestial bodies is governed mostly by gravity, but the trajectories of comets can also be affected by non-gravitational forces due to cometary outgassing," the researchers wrote in their paper.

They detected non-gravitational acceleration, mainly directed away from the Sun, in 'Oumuamua's trajectory. This may not have been caused by cometary outgassing, but it is consistent with the behaviour of comets.

Just to be sure, the team carefully examined other potential causes, including things like solar radiation pressure, various external forces, and interaction with solar wind. They even accounted for "geometric effects" which could happen if 'Oumuamua wasn't one solid object, but several chunks flying together.

After all that, they concluded "comet-like outgassing to be a physically viable explanation, provided that 'Oumuamua has thermal properties similar to comets."

As for the lack of dust cited as a reason for 'Oumuamua's asteroid designation, the team noted that this could be explained by an atypical dust-grain size distribution, a low dust-to-ice ratio, or changes to the object's surface sustained over its long journey from its mysterious origin point.

If it is a comet, that would have an impact on 'Oumuamua's hypothesised origins. A study earlier this year found that its most likely starting point was a binary star system, since that would be the most likely to eject an asteroid, as opposed to a comet, which apparently the Solar System flings about into interstellar space willy-nilly.

It may actually be impossible to tell whether 'Oumuamua is a comet or an asteroid based on our current observations. However, it's moving away from us at speed, and we've probably missed the boat to send a probe to study it in situ.

But, if there's a next time - another interstellar visitor in the future - our understanding could benefit greatly from knowing about it ahead of time, and having the infrastructure in place so that we could intercept it with a probe.

Whether such a visitor is similar to 'Oumuamua or vastly different, these observations could help us understand the physical properties of interstellar objects - and how much of 'Oumuamua's uniqueness is because of its long, lonely and possibly violent journey through the vastness of space.

So who votes for hedging our bets and calling it a cometsteroid?

The team's research has been published in the journal Nature.</s>An artist's illustration of the interstellar object 'Oumuamua' (ESA/Hubble, NASA, ESO, M. Kornmesser)

CAPE CANAVERAL, Fla. (AP) - Our first known visitor from another star system has now been identified as a comet.

A European-led team makes the case in Wednesday's edition of the journal Nature.

Telescopes first spotted the mysterious cigar-shaped object last October as it zipped through the inner solar system. Since then, astronomers have flip-flopped between comet and asteroid.

Neither a coma nor tail was spotted, hallmarks of an icy comet. But Italian astronomer Marco Micheli (Mee-KAY-lee) and his team report that the object's path and acceleration are best explained not just by gravity, but also gases shedding from a comet.

It's long gone, as are the chances of knowing conclusively what it was.</s>Astronomers around the world were positively tickled last year when we detected our first interstellar object inside the solar system. The object was named ‘Oumuamua, a term of Hawaiian origin meaning “a messenger from afar arriving first.” Telescopes around the world turned to watch as our visitor rocketed off into oblivion, and scientists eventually decided it was an asteroid. However, some new data calls that assessment into question. ‘Oumuamua may have been a comet after all.

The scientific community has long expected an interstellar visitor to show up, and ‘Oumuamua was undeniably from another star. It shot past us with such haste there was no way it could have started in our solar system — even a gravity boost from Jupiter wouldn’t have given a local object matching speed and trajectory.

So, ‘Oumuamua’s appearance wasn’t a shock, but the 800-meter cigar-shaped object still wasn’t exactly what we expected. Scientists believed the first interstellar object in our solar system would be a comet. If other solar systems are anything like ours, there should be a cloud of comets at the very edge. It wouldn’t take much for these objects to break away and sail through interstellar space. However, ‘Oumuamua looked more like an asteroid than a comet.

The hallmark of a comet is the way the icy body heats up as it passes close to the sun, leading to the formation of a “coma.” That’s why comets have tails and asteroids don’t. ‘Oumuamua didn’t have a visible tail. Researchers from the University of Hawaii Institute for Astronomy (where ‘Oumuamua was discovered) have kept watch on the object using the Hubble Space Telescope, and the latest data suggests something other than gravity has affected its path.

Astronomers have attempted to determine if the solar wind or radiation could have tweaked ‘Oumuamua’s trajectory using computer models, but nothing fit with the observations. The best explanation was outgassing. The process that produced a comet’s coma as it heats up. The release of gas can actually nudge the orbit. If that’s what caused the change to ‘Oumuamua’s orbit, it would have to be a comet.

There are still a lot of unanswered questions. We didn’t see a coma, so maybe a journey through interstellar space alters comets in some way. Or perhaps ‘Oumuamua is an asteroid, and there’s some mechanism at work that we’ve never seen before. We might never know — ‘Oumuamua is leaving, and it isn’t coming back.</s>WASHINGTON, June 27, 2018 /PRNewswire-USNewswire/ -- Using observations from NASA's Hubble Space Telescope and ground-based observatories, an international team of scientists have confirmed ′Oumuamua (oh-MOO-ah-MOO-ah), the first known interstellar object to travel through our solar system, got an unexpected boost in speed and shift in trajectory as it passed through the inner solar system last year.

"Our high-precision measurements of ′Oumuamua's position revealed that there was something affecting its motion other than the gravitational forces of the Sun and planets," said Marco Micheli of ESA's (European Space Agency) Space Situational Awareness Near-Earth Object Coordination Centre in Frascati, Italy, and lead author of a paper describing the team's findings.

Analyzing the trajectory of the interstellar visitor, co-author Davide Farnocchia of the Center for Near Earth Object Studies (CNEOS) at NASA's Jet Propulsion Laboratory (JPL) found that the speed boost was consistent with the behavior of a comet.

"This additional subtle force on ′Oumuamua likely is caused by jets of gaseous material expelled from its surface," said Farnocchia. "This same kind of outgassing affects the motion of many comets in our solar system."

Comets normally eject large amounts of dust and gas when warmed by the Sun. But according to team scientist Olivier Hainaut of the European Southern Observatory, "there were no visible signs of outgassing from ′Oumuamua, so these forces were not expected."

The team estimates that ′Oumuamua's outgassing may have produced a very small amount of dust particles – enough to give the object a little kick in speed, but not enough to be detected.

Karen Meech, an astronomer at the University of Hawaii's Institute of Astronomy and co-author of the study, speculated that small dust grains, present on the surface of most comets, eroded away during ′Oumuamua's long journey through interstellar space.

"The more we study ′Oumuamua, the more exciting it gets," Meech said. "I'm amazed at how much we have learned from a short, intense observing campaign. I can hardly wait for the next interstellar object!"

′Oumuamua, less than half a mile in length, now is farther away from our Sun than Jupiter and traveling away from the Sun at about 70,000 mph as it heads toward the outskirts of the solar system. In only another four years, it will pass Neptune's orbit on its way back into interstellar space.

Because ′Oumuamua is the first interstellar object ever observed in our solar system, researchers caution that it's difficult to draw general conclusions about this newly-discovered class of celestial bodies. However, observations point to the possibility that other star systems regularly eject small comet-like objects and there should be more of them drifting among the stars. Future ground- and space-based surveys could detect more of these interstellar vagabonds, providing a larger sample for scientists to analyze.

NASA will host a Reddit Ask Me Anything (AMA) about the ′Oumuamua observations from 4 to 6 p.m. EDT Thursday, June 28. To participate, go to:

The international team of astronomers used observations from Hubble, the Canada-France-Hawaii Telescope in Hawaii, and the Gemini South Telescope and European Southern Observatory's Very Large Telescope in Chile.

The paper with the team's findings will appear in the June 27 issue of the journal Nature.

JPL hosts CNEOS for the agency's Near-Earth Object Observations Program, an element of the Planetary Defense Coordination Office within the agency's Science Mission Directorate. Hubble is a project of international cooperation between NASA and ESA. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages Hubble. The Space Telescope Science Institute (STScI) in Baltimore conducts Hubble science operations.

Learn more about asteroids and near-Earth objects at:

For asteroid and comet news and updates, follow AsteroidWatch on Twitter at:

For more information from Hubble, go to:</s>An object that has captivated astronomers since its discovery in our solar system has raised many more questions than answers, at least until now.

With Asteroid Day (30 June) fast approaching – a day in which we make ourselves aware of the threat posed by interstellar objects hitting Earth – it seems fitting that a nearby asteroid has been found to not actually be an asteroid at all.

Since its discovery towards the end of last year, ‘Oumuamua was heralded as the first interstellar object to appear in our solar system, but confused many astronomers with its peculiar trajectory and the fact that it could potentially harbour some form of biological life.

During the many months since, attempts to classify it have flip-flopped, with it being changed from an asteroid to a comet and back again as time went on.

Now, new research published in Nature seems to finally determine that the object is indeed a comet and not an asteroid.

Using the Hubble Space Telescope and some of the most powerful Earth-based telescopes on the planet, an international team of astronomers revealed that the object is actually moving faster than we had predicted.

The measured gain in speed is tiny and it is actually slowing down because of the pull of the sun, but still not as slow as predicted by celestial mechanics.

To explain why it is going faster than we thought, the team led by the European Space Agency’s Marco Micheli said the likeliest explanation is that ‘Oumuamua is venting material from its surface as it gets heated by the sun, in a behaviour known as outgassing.

This small and steady boost is enough to push it to a cruising speed of 114,000kph; in doing so, this confirms it is a comet, not an asteroid.

“We think this is a tiny, weird comet,” Micheli said. “We can see in the data that its boost is getting smaller the farther away it travels from the sun, which is typical for comets.”

The mystery is by no means over, however. While a typical comet’s outgassing emits dust and gas to form a cloud of material, no such cloud was detected at ‘Oumuamua. This led the team members to believe that the comet might just vent unusually large, coarse dust grains that they can’t see.

Now, the team admits, the recalculated speed will make it even more difficult to find where it actually came from.</s>The traveler, which was given the name `Oumuamua, Hawaiian for "messenger from afar," was first discovered in 2017 by researchers with the Panoramic Survey Telescope Rapid Response Systems (Pan-STARRS), located on Maui. From the moment researchers spotted the oddly shaped object, they weren't exactly sure how to classify it: first it was a comet, then an asteroid, before astronomers deemed it simply an interstellar object.

But now, thanks to the hard work of researchers led by the European Space Agency, `Oumuamua is back to being considered a comet, since it's not solely being guided by the sun's gravity as a planet or asteroid would be, according author Marco Micheli, the lead author on the study.

​"Unexpectedly, we found that `Oumuamua was not slowing down as much as it should have due to just gravitational forces," Micheli told USA Today. "We think this is a tiny, weird comet… we can see in the data that its boost is getting smaller the farther away it travels from the sun, which is typical for comets."

"It's definitely an exciting discovery," he added.

According to Micheli, after analyzing `Oumuamua's trajectory, researchers determined that the 800-meter-long object wasn't solely being pushed on its journey by a gravitational pull of the sun, planets and any other solar system objects.

By slapping the comet label on the mysterious object, researchers are better able to explain `Oumuamua's non-gravitational acceleration, since comets are propelled by outgassing, Space.com reported.

Though `Oumuamua isn't your typical comet, as it lacks a coma — the cloud of gas and dust that envelops around the nucleus of the comet — the mystery remaining behind the object now is its origin.</s>Remember that cigar-shaped interstellar object that whizzed through our Solar System last year? It was pretty weird, right? Well, new research shows ‘Oumuamua is even stranger than we realized.



‘Oumuamua, pronounced “oh-MOO-ah-MOO-ah,” is a comet and not an asteroid, according to research published yesterday in Nature. That’s not a huge revelation, as many scientists had already assumed as much. What’s special about this new paper, however, is that it offers tangible evidence to bolster the case.

Seems like a fine point, but there’s a big difference between asteroids and comets. Both types of objects are leftovers from the formation of the Solar System, but where asteroids are composed of metals and rocky material, comets are made from ice, dust, and rocks. Typically, comets have elongated orbits that take them to the outer reaches of the Solar System, but as they venture inwards, the Sun’s rays cause gas and dust to shear off, producing a glowing cloud of material called a coma and a visible tail.

In the case of ‘Oumuamua, the dark, 2,620-foot-long object was detected on October 19, 2017 using Hawaii’s Haleakala Observatory, and scientists have been tracking it ever since. ‘Oumuamua came to within 20.5 million miles (33 million km) of Earth before heading back toward interstellar space—and it appears to be in a hurry. By combining data collected from the NASA/ESA Hubble Space Telescope, the Canada-France-Hawaii Telescope, ESO’s Very Large Telescope, and the Gemini South Telescope, a team led by Marco Micheli from the European Space Agency noticed something odd about ‘Oumuamua: It’s moving faster than expected.

The extra speed isn’t ludicrous by any means, but it’s still measurable. ‘Oumuamua is still slowing down thanks to the gravitational influence of our Sun, but not at the rate expected by celestial mechanics. The reason, say the authors of the new study, is that ‘Oumuamua is still venting material from its surface. Solar heating is producing an effect called outgassing, and the material ejected from the object’s surface is creating a tiny amount of thrust. It’s like a built-in propulsion system, but one that only becomes active inside the warm, cozy realms of a star system.



To make sure the acceleration effect was not being caused by something else, the researchers considered scenarios like solar wind, a collision with another object, its mass being different than calculated, and other known or suspected astrophysical phenomena. However, “these explanations are all either physically unrealistic or insufficient to explain the observed behavior,” write the authors in the study.

“Also, the unlikely theory that ‘Oumuamua is an interstellar spaceship was rejected: the smooth and continuous change in speed is not typical for thrusters and the object is tumbling on all three axes, speaking against it being an artificial object,” according to Hubble press release. As a related aside, research from late last year came to the same conclusion.



The suspected presence of outgassing was the smoking gun needed to prove that ‘Oumuamua is a comet and not an asteroid.

“We think this is a tiny, weird comet,” said Micheli in a press release. “We can see in the data that its boost is getting smaller the farther away it travels from the Sun, which is typical for comets.”

Now, you’re probably thinking, “What the hell, Dvorsky, a minute ago you said comets produce a visible coma and tail when they get close to the Sun! But ‘Oumuamua doesn’t exhibit any of this!” Fair point, but there is a good reason for this, one demonstrating how freakin’ weird this object really is.

‘Oumuamua, the researchers speculate, is venting unusually large, coarse dust grains. Micheli’s team is theorizing that small dust grains, which that typically litter the surface of comets, eroded during the course of ‘Oumuamua’s epic journey through interstellar space (we have no idea how old this object is, so who knows how long it’s been careening through the Milky Way, which formed 13.5 billion years ago). All that’s left on this comet are the larger dust grains. If ‘Oumuamua is producing a cloud composed of these large particles, it would be too dim for Hubble to see it. But the unexpected deviation in its acceleration suggests these grains are very much there, and that ‘Oumuamua is, or was, in an outgassing phase during its brief time in our Solar System.

Sadly, this new result means it’s going to be exceptionally tough for astronomers to figure out where in the galaxy this object came from. If it had been an asteroid with a boring and predictable speed, scientists should have been able to backtrack its trajectory to its star system of origin. But with its variable speed, that’ll now be a physics task of herculean proportions, and probably something best suited for a future supercomputer.

It came, we saw, it’s quickly leaving. But we’re still learning some amazing things about this messenger from afar.</s>Ever since space scientists first spotted the odd, cigar-shaped object known as 1I/‘Oumuamua in the sky, they have been debating what it is exactly. Suggestions have included an asteroid, a comet and even alien spaceship. Now a study, published in Nature, suggests it may actually be a comet – but an unusual one.

The fascinating findings add to recent discoveries which suggest it may be time to think beyond the division of asteroid versus comet.

The force of gravity is what keeps our feet on the ground, the moon orbiting the Earth and the planets orbiting the sun. It is also the main agent that governs the trajectory of comets as they swing through the solar system. It was his laws of gravity that allowed Newton’s colleague Edmond Halley to predict the return of the comet that is now known as Halley’s comet. The same laws enabled the Rosetta spacecraft to catch up with and fly alongside comet 67P/Churyumov-Gerasimenko.

There are non-gravitational effects that can influence the orbit of a comet, but they are so weak compared to gravity that they are usually negligible. But in the case of object 1I/‘Oumuamua, its orbit is so unusual that non-gravitational effects might play a role in its geometry.

1I/’Oumuamua was first sighted in October 2017. Calculations based on the laws of gravity showed that its trajectory was neither the ellipse of a comet nor the circle of an asteroid. Instead, ‘Oumuamua appeared to have a hyperbolic orbit (see video), meaning it was not gravitationally bound to the sun. This means it is almost certainly an interloper from beyond the solar system. Hence the “1I” part of its name: the first interstellar body.

‘Oumuamua passed within 0.25 astronomical units (AU) of the sun (1 AU is the distance between the sun and the Earth). As a well-behaved comet, it should have developed a coma and tail as ice and dust vaporised from the surface by a process called sublimation. But, despite an organised and concerted observing campaign by the international astronomical community, no trace of a coma or tail was seen.

Indeed, measurements of its composition showed it had more in common with asteroids than with comets: the low reflectiveness of its surface indicated that there was little, if any, ice. It also seemed to have organic material that had been altered by exposure to cosmic radiation – like many asteroids.

Alongside such observations was ‘Oumuamua’s very unusual shape: although the object couldn’t be seen directly, modelling of its light curve (how the amount of reflected light varies as an object rotates) suggested that it was long and thin – cigar-shaped rather than the more usual semi-rounded appearance of asteroids and comets. This led the more fanciful to posit that ‘Oumuamua might be a fabricated, rather than a natural, visitor to the inner solar system, perhaps an alien spacecraft.

So: comet, asteroid or spaceship? We can probably discount the interstellar spaceship – surely it would have at least paused to investigate the mixture of signals emanating from our planet. So comet or asteroid are the options left. The same group that published the discovery report of ‘Oumuamua continued observing the object, acquiring data from instrumentation on practically all the world’s major ground-based telescopes, as well as from the Hubble Space Telescope.

What they discovered was that there was a deviation from the path that ‘Oumuamua should be following if it were influenced by gravitational effects alone. It seemed as if ‘Oumuamua was being slightly pushed away from the sun. The team investigated every non-gravitational effect that they could: was ‘Oumuamua swayed from its path by the pressure of solar radiation? Was the effect of thermal emission as the object rotated significant? Had it had a collision that changed the trajectory?

Other options included it having an unusual distribution of mass or perhaps a centre of gravity displaced from the centre of its body. It could also be highly magnetised, so that it interacted more strongly with the solar wind. Based on their extremely precise and comprehensive series of observations, the team surmised that none of these effects was sufficient to explain ‘Oumuamua’s orbit. Instead, they concluded that significant release of gas by sublimation was the main feasible mechanism – and that would suggest it is a comet.

This is a significant conclusion to reach, not just for ‘Oumuamua, but for cometary and asteroidal science more generally. There has only been a handful of previous observations of non-gravitational effects on cometary orbits, mainly because the measurements required to make the calculations have to be undertaken over a very wide distance. Assuming ‘Oumuamua was outgassing in a similar way to other comets, then the body was losing water and dust at a rate of about 2.4kg per second.

This compares with 20kg per second for 67P/Churyumov-Gerasimenko. It would appear that although ‘Oumuamua seems to have a similar density to solar system comets, it must be much stronger, given the paucity of dust released. Or perhaps the dust was more firmly bound to the surface by the organic compounds present.

So can we really say 1I/‘Oumuamua is a comet, albeit with asteroidal properties? Or could it be an asteroid with cometary properties? Does it matter? I’m not sure that it does. The division between asteroids and comets in the solar system used to be obvious: asteroids had circular orbits, were made of rock and metal and lived in the asteroid belt between Mars and Jupiter. Comets had inclined elliptical orbits, were made of rock and ice and came from further away.

But over the past few years we have discovered families of asteroids scattered all over the solar system, as well as Kuiper Belt Objects. We have also found comets with both short and long orbits. And we have seen unusual features on objects that we thought we knew quite well. The asteroid Ceres, for example, has salts left behind as ice vaporised from the surface by sublimation – a process often seen in comets, but not in asteroids.

Since ‘Oumuamua, along with small bodies in the solar system and instestellar guests, seem to exhibit a spectrum of compositional and dynamic properties, it may be time to think beyond the asteroid versus comet division, and look for a new way to classify these objects. Perhaps we need many more categories. Or maybe just one – the class of Small Body. Clearly ‘Oumuamua is showing us that we have not yet uncovered the variety of objects to be found in our own neighbourhood.
The first known interstellar object to pass through the Solar System, ʻOumuamua, has been identified as a very inactive comet and not an asteroid, as previously thought.