Oumuamua – Solar System’s First Interstellar Asteroid

Oumuamua: In Depth

The first known interstellar asteroid, 1I/2017 U1 ‘Oumuamua, was discovered Oct. 19, 2017 by the University of Hawaii’s Pan-STARRS1 telescope, funded by NASA’s Near-Earth Object Observations (NEOO) Program, which finds and tracks asteroids and comets in Earth’s neighborhood. While originally classified as a comet, observations revealed no signs of cometary activity after it slingshotted past the Sun on Sept. 9, 2017 at a blistering speed of 196,000 miles per hour (87.3 kilometers per second).

The first confirmed object from another star to visit our solar system, this interstellar interloper appears to be a rocky, cigar-shaped object with a somewhat reddish hue. The asteroid, named ‘Oumuamua by its discoverers, is up to one-quarter mile (400 meters) long and highly-elongated perhaps 10 times as long as it is wide. That aspect ratio is greater than that of any asteroid or comet observed in our solar system to date. While its elongated shape is quite surprising, and unlike asteroids seen in our solar system, it may provide new clues into how other solar systems formed.

The observations suggest this unusual object had been wandering through the Milky Way, unattached to any star system, for hundreds of millions of years before its chance encounter with our star system.

“For decades we’ve theorized that such interstellar objects are out there, and now – for the first time – we have direct evidence they exist,” said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate in Washington, in November 2017.

Immediately after its discovery, telescopes around the world, including ESO’s Very Large Telescope in Chile, were called into action to measure the object’s orbit, brightness and color. Urgency for viewing from ground-based telescopes was vital to get the best data.

Combining the images from the FORS instrument on the ESO telescope using four different filters with those of other large telescopes, a team of astronomers led by Karen Meech of the Institute for Astronomy in Hawaii found that ‘Oumuamua varies in brightness by a factor of 10 as it spins on its axis every 7.3 hours. No known asteroid or comet from our solar system varies so widely in brightness, with such a large ratio between length and width. The most elongated objects we have seen to date are no more than three times longer than they are wide.

“This unusually big variation in brightness means that the object is highly elongated: about ten times as long as it is wide, with a complex, convoluted shape,” said Meech. “We also found that it had a reddish color, similar to objects in the outer solar system, and confirmed that it is completely inert, without the faintest hint of dust around it.”

These properties suggest that ‘Oumuamua is dense, composed of rock and possibly metals, has no water or ice, and that its surface was reddened due to the effects of irradiation from cosmic rays over hundreds of millions of years.

A few large ground-based telescopes continued to track the fading asteroid as it receded from our planet. Two of NASA’s space telescopes (Hubble and Spitzer) tracked the object traveling about 85,700 miles per hour (38.3 kilometers per second) relative to the Sun. Its outbound path is about 20 degrees above the plane of planets that orbit the Sun. The object passed Mars’s orbit around Nov. 1 and will pass Jupiter’s orbit in May of 2018. It will travel beyond Saturn’s orbit in January 2019; as it leaves our solar system, ‘Oumuamua will head for the constellation Pegasus.

Preliminary orbital calculations suggest that the object came from the approximate direction of the bright star Vega, in the northern constellation of Lyra. However, it took so long for the interstellar object to make the journey – even at the speed of about 59,000 miles per hour (26.4 kilometers per second) — that Vega was not near that position when the asteroid was there about 300,000 years ago.

Astronomers estimate that an interstellar asteroid similar to ‘Oumuamua passes through the inner solar system about once per year, but they are faint and hard to spot and have been missed until now. It is only recently that survey telescopes, such as Pan-STARRS1, are powerful enough to have a chance to discover them.

“What a fascinating discovery this is!” said Paul Chodas, manager of the Center for Near-Earth Object Studies at NASA’s Jet Propulsion Laboratory, Pasadena, California. “It’s a strange visitor from a faraway star system, shaped like nothing we’ve ever seen in our own solar system neighborhood.”

How Oumuamua Got its Name
The object was officially named interstellar asteroid 1I/2017 U1 by the International Astronomical Union (IAU), which is responsible for granting official names to bodies in the solar system and beyond. In addition to the technical name, the Pan-STARRS team dubbed it ‘Oumuamua (pronounced oh MOO-uh MOO-uh), which is Hawaiian for “a messenger from afar arriving first.”

This artist’s impression shows the first interstellar asteroid: `Oumuamua. This unique object was discovered on 19 October 2017 by the Pan-STARRS 1 telescope in Hawai`i. Subsequent observations from ESO’s Very Large Telescope in Chile and other observatories around the world show that it was travelling through space for millions of years before its chance encounter with our star system. `Oumuamua seems to be a dark red highly-elongated metallic or rocky object, about 400 metres long, and is unlike anything normally found in the Solar System.

Interstellar Space

Interstellar space is the physical space within a galaxy beyond the influence of each star on the plasma


Asteroids are minor planets, especially those of the inner Solar System. The larger ones have also been called planetoids. These terms have historically been applied to any astronomical object orbiting the Sun that did not show the disc of a planet and was not observed to have the characteristics of an active comet. As minor planets in the outer Solar System were discovered and found to have volatile-based surfaces that resemble those of comets, they were often distinguished from asteroids of the asteroid belt. In this article, the term “asteroid” refers to the minor planets of the inner Solar System including those co-orbital with Jupiter.

There are millions of asteroids, many thought to be the shattered remnants of planetesimals, bodies within the young Sun’s solar nebula that never grew large enough to become planets. The large majority of known asteroids orbit in the asteroid belt between the orbits of Mars and Jupiter, or are co-orbital with Jupiter (the Jupiter trojans). However, other orbital families exist with significant populations, including the near-Earth objects. Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, M-type, and S-type. These were named after and are generally identified with carbon-rich, metallic, and silicate (stony) compositions, respectively. The size of asteroids varies greatly, the largest is almost 1,000 km (625 mi) across.

Asteroids are differentiated from comets and meteoroids. In the case of comets, the difference is one of composition: while asteroids are mainly composed of mineral and rock, comets are composed of dust and ice. In addition, asteroids formed closer to the sun, preventing the development of the aforementioned cometary ice. The difference between asteroids and meteoroids is mainly one of size: meteoroids have a diameter of less than one meter, whereas asteroids have a diameter of greater than one meter. Finally, meteoroids can be composed of either cometary or asteroidal materials.

Interstellar Asteroid FAQs

Why is the discovery of asteroid 1I/2017 U1 significant/important?

The discovery of interstellar object 1I/2017 U1 is the first detection of a celestial object in our solar system that originated from another solar system, and the shape of the object itself looks very different than any asteroid or comet we’ve seen in our own solar system.

Was this a surprise?

Not entirely. The discovery of an interstellar object has been anticipated for decades. While it is a historic discovery, the existence of interstellar objects is not a surprise. What is a surprise, however, is the fact that the first detected interstellar object is an asteroid—not a comet as most scientists had expected. Even more surprising is the strange, highly-elongated shape of 1I/2017 U1. None of the known asteroids or comets in our own solar system have such an elongated shape.

Why was this discovery anticipated by scientists?

Theories of how our own solar system was formed tell us that a large percentage of the original planetesimals in the early solar system were ejected into interstellar space through encounters with the giant planet Jupiter. The asteroids and comets that remain in our solar system today are only a small fraction of the original population: the rest of the material was dispersed by young Jupiter, either spewed out into interstellar space or sent crashing into the Sun. Planetary systems that formed around other stars likely evolved in the same way, with each Jupiter-sized planet ejecting its own systems’ asteroids and comets into interstellar space. The space between the stars probably has billions and billions of planetesimals roaming around independently. Scientists understood that inevitably, some of these small bodies would enter our own solar system. This interstellar visit by 1I/2017 U1 reinforces our models of how planetary systems form.

If this discovery was expected, why are scientists still surprised?

Scientists are surprised because they expected an interstellar comet, not an interstellar asteroid. Their models indicate that the material that our own solar system ejected into interstellar space after formation was mostly cometary. Evidence of that comes from the types of objects seen in the Oort Cloud, a vast spherical cloud around our solar system thought to consist of the planetesimals that were nearly ejected from our solar system, but not quite. The vast majority of objects that dive into the inner solar system from the Oort Cloud are comets, and only a very few are asteroids. The very high ratio of comets to asteroids is thought to also apply to objects ejected from our solar system. Scientists assumed the same high ratio of comets to asteroids also hold for material ejected from other solar systems, but the asteroidal nature of 1I/2017 U1 suggests that that may not be the case.

Why are scientists surprised and excited about the shape of 1I/2017 U1?

Scientists have never seen an asteroid as elongated as 1I/2017 U1 in our solar system—not even half this elongated. The most elongated asteroids we see in our own solar system have axis ratios of no more than 3:1. Scientists will have to come up with new theories explaining how such an elongated object as 1I/2017 U1 could form, and how it could have enough strength to hold itself together in one long piece. It is quite possible that entirely different conditions around this object’s parent star gave this object a composition and shape that are not possible in our own solar system.

Are you really sure this object came from outside our solar system?

Yes. The trajectory of this object has been tracked carefully since it was discovered, and its motion follows a hyperbolic path around the Sun. Basically, the high speed at which 1I/2017 U1 is traveling through the solar system cannot be due to acceleration from the Sun’s gravity alone. This object must have approached our solar system already with considerable initial speed. It is simply traveling too fast to have originated in our solar system. The object’s high speed also means that the Sun’s gravity cannot slow it down enough to keep it bound to our solar system. The object will leave, and end up with about the same speed with which it entered; only its direction will have changed.

What do we know about the object’s size and shape?

The object is believed to be at least a quarter-mile (400 meters) long, but it’s the highly-elongated shape that has scientists excited and perplexed. The shape is revealed by high-precision measurements of the object’s brightness. The dramatic cyclical variations in brightness indicate that the object is cigar-shaped, with a length roughly ten times longer than the width. The asteroid spins once every 7.3 hours. Think of a pencil spinning on a table that cycles from being brightest when it its full length is visible, to faintest when its long axis or point is facing toward us. No known asteroid or comet from our solar system varies so widely in brightness, and therefore has such a large ratio between length and width. The most elongated objects we have seen up to now have been no more than three times longer than they are wide. The 10:1 elongation ratio is simply not found for any of the objects within our solar system.

What do we know about the object’s composition? 

Regarding its composition, observations suggest this object is similar to many asteroids found in our solar system – dense, possibly rocky or even metallic. The object’s surface is somewhat reddish due to effects of irradiation from cosmic rays over millions of years.

How do we know this is a natural object? 

From our theories of planetary formation, we expect that as each planetary system forms, it ejects countless small asteroids and comets into interstellar space. We are quite sure that our own solar system did that, and other star systems must have done the same. We think this object is a product of that process in some faraway star system. While its elongated shape is quite surprising, and unlike asteroids seen in our solar system, it may provide us with new clues into how other solar systems formed – some that could be quite different from our own. For the unique insights it might provide, we are excited about studying 1I/2017 U1 further, until it gets too far away for observations.

How fast was this object going when it was nearest the Sun?

The object was traveling fastest when it was nearest the Sun on Sep. 9, 2017. Its peak speed was about 196,000 miles per hour or 87.4 kilometers per second. After that, as it headed away from the Sun, the Sun’s gravity started to slow it down somewhat.

Will it ever return to our solar system?

No, the object is now headed away from the Sun, outbound back into interstellar space, never to return.

Where is this object now (Nov. 20, 2017) and how fast is it going at this time?

The object is currently about 183 million miles (295 million kilometers) from the Sun, traveling outbound above the region between the orbits of Mars and Jupiter at about 89,000 miles per hour, or 40 kilometers per second. It is currently about 124 million miles (200 million kilometers) from Earth.

How fast did this object travel through interstellar space?

This object’s cruising velocity through interstellar space was 59,000 miles per hour (26.3 kilometers per second). That was its speed as it approached our solar system, and the speed it will have after it exits our solar system. At that speed, interstellar object 1I/2017 U1 will cover one light year in about 11,000 years.

It is interesting to note that 1I/2017 U1 is traveling faster than any spacecraft ever launched – including the two Voyagers and New Horizons.

When was 1I/2017 U1 discovered?

The object was discovered on Oct. 19, 2017 by the NASA-funded Pan-STARRS1 telescope which surveys the sky searching for asteroids that could pose an impact hazard to Earth. Rob Weryk, a post-doc researcher at the University of Hawaii’s Institute of Astronomy, was first to identify the object.

Interstellar object 1I/2017 U1 has another name, doesn’t it?

The team from the Pan-STARRS observatory that was the first to detect the interstellar visitor has chosen the name ‘Oumuamua for their discovery. The name is of Hawaiian origin and means “a messenger from afar arriving first.”

When did you realize it was interstellar?

Astronomers on the University of Hawaii team and elsewhere tracked interstellar object 1I/2017 U1 for several days after its discovery before its trajectory was known well enough to confirm that it was, indeed, interstellar in origin. The official confirmation and announcement of this object was made on Oct. 25.

Where did it come from?

1I/2017 U1’s trajectory indicates it came from the general direction of the constellation Lyra. At the speed it was going, it must have taken at least a few hundred thousand years since it was near even the nearest star in that direction, and it may well have been traveling much longer than that. Scientists don’t know the motions of the stars well enough to say where they were located that long ago. We simply don’t know which star system this object came from.

Where is it going?

Interstellar object 1I/2017 U1 is on an outbound trajectory. It will pass above Neptune’s orbit in 2022. As it leaves our solar system it is headed towards the constellation Pegasus.

What was its path through our solar system? 

This object approached our solar system from above the plane of the ecliptic. It passed closest to the Sun on Sept. 9, 2017, well within Mercury’s orbit. Pulled by the Sun’s gravity, the object made a sharp turn under our solar system, and then started its outbound leg, passing under Earth’s orbit on Oct. 14 at a distance of about 15 million miles (24 million kilometers). It passed above Mars’ orbit around Nov. 1, and will pass above Jupiter’s orbit in May of 2018. 1I/2017 U1 will pass above Saturn’s orbit in January of 2019. At present, it is traveling about 85,000 miles per hour (138,000 kilometers per hour) relative to the Sun.  On this outbound leg, the object is once again traveling above the ecliptic plane, but it is moving at a much shallower angle than on its inbound leg.

What is NASA doing now?

A few large ground-based telescopes continue to track this object, even though it is rapidly fading as it recedes from our planet. Two of NASA’s space telescopes (Hubble and Spitzer) are attempting to observe the fading object the week of Nov. 20. Observations from large ground-based telescopes will continue until the object becomes too faint to be detected, sometime next month. NASA’s Center for Near-Earth Object Studies (CNEOS) at the Jet Propulsion Laboratory continues to take all available tracking measurements to refine the trajectory of 1I/2017 U1 until it becomes too faint to observe on its journey out of our solar system.

How long can this interstellar object be observed?

Interstellar object 1I/20167 U1 will be too far and too faint to be detected even in the largest telescopes after mid-December.

If interstellar space is littered with billions and billions of asteroids and comets, why have we never seen one before?

Scientists think that interstellar objects pass through our solar system all the time, but most are too small and too far away from Earth to be detected. 1I/2017 U1 was large enough and by chance passed close enough to Earth to be detected by Pan-STARRS. The discovery may also have been made possible by enhancements to the detection capabilities of the Pan-STARRS near-Earth object search program in recent years.

Will we find more interstellar objects in the future, and if so, how often?

Yes, scientists expect to find more interstellar objects, especially when next-generation asteroid search programs come online. They estimate that an interstellar object similar to 1I/2017 U1 passes inside the orbit of the Earth several times a year, but up until now they have been too faint and hard to detect. Recent upgrades to survey telescopes such as Pan-STARRS increase the chances of finding these objects, and those odds will increase even more when next-generation survey telescopes begin operations.

Last Updated: Nov. 20, 2017
Editor: Tricia Talbert, National Aeronautics and Space Administration
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