Interstellar asteroid is unusual 400+ meter long cigar shape and probably highly metallic

For the first time ever astronomers have studied an asteroid that has entered the Solar System from interstellar space. Observations from ESO’s Very Large Telescope in Chile and other observatories around the world show that this unique object was traveling through space for millions of years before its chance encounter with our star system. It appears to be a dark, reddish, highly-elongated rocky or high-metal-content object.

Although originally classified as a comet, observations from ESO and elsewhere revealed no signs of cometary activity after it passed closest to the Sun in September 2017. The object was reclassified as an interstellar asteroid and named 1I/2017 U1 (`Oumuamua) [1].

“We had to act quickly,” explains team member Olivier Hainaut from ESO in Garching, Germany. “`Oumuamua had already passed its closest point to the Sun and was heading back into interstellar space.”

ESO’s Very Large Telescope was immediately called into action to measure the object’s orbit, brightness and color more accurately than smaller telescopes could achieve. Speed was vital as `Oumuamua was rapidly fading as it headed away from the Sun and past the Earth’s orbit, on its way out of the Solar System. There were more surprises to come.

Combining the images from the FORS instrument on the VLT using four different filters with those of other large telescopes, the team of astronomers led by Karen Meech (Institute for Astronomy, Hawai`i, USA) found that `Oumuamua varies dramatically in brightness by a factor of ten as it spins on its axis every 7.3 hours.

Karen Meech explains the significance: “This unusually large variation in brightness means that the object is highly elongated: about ten times as long as it is wide, with a complex, convoluted shape. We also found that it has a dark red colour, 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, possibly rocky or with high metal content, lacks significant amounts of water or ice, and that its surface is now dark and reddened due to the effects of irradiation from cosmic rays over millions of years. It is estimated to be at least 400 metres long.

Preliminary orbital calculations suggested that the object had come from the approximate direction of the bright star Vega, in the northern constellation of Lyra. However, even travelling at a breakneck speed of about 95 000 kilometres/hour, it took so long for the interstellar object to make the journey to our Solar System that Vega was not near that position when the asteroid was there about 300 000 years ago. `Oumuamua may well have been wandering through the Milky Way, unattached to any star system, for hundreds of millions of years before its chance encounter with the Solar System.

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 so have been missed until now. It is only recently that survey telescopes, such as Pan-STARRS, are powerful enough to have a chance to discover them.

“We are continuing to observe this unique object,” concludes Olivier Hainaut, “and we hope to more accurately pin down where it came from and where it is going next on its tour of the galaxy. And now that we have found the first interstellar rock, we are getting ready for the next ones!”

31 thoughts on “Interstellar asteroid is unusual 400+ meter long cigar shape and probably highly metallic”

  1. Space is vast and stars are very far between. Anyone up to calculating the odds of such an object making such a close flyby of a star? From its point of view it very nearly hit the Sun.

    • Wouldn’t we have to know how many of these things there are?
      It sounds like this is one of the first ones detected. So a sample size of one.

      • We would have to do a detailed survey of say a cubic lightyear of nearby interstellar space. That would tell us how many of these things there are, and hence what the odds are of such an object making a close flyby of a star.
        So that’s isn’t going to happen for while yet.

  2. Ya, my first thought was that Rama came and we missed it. Fortunately, the Ramans always do everything in threes . . . .

  3. Well… if it is tumbling end-over-end, that kind of rules out an interstellar space vehicle, however asteroidal it seems. Or not… could have living pods at both ends. Artificial gravity. I wonder what the period of rotation is. (Follows linkie… returns…)

    7.3 hr = 26,280 s = 0.00024 rad/s … half length 200 m … F = ω² R
    F = 1.14×10⁻⁵ m/s²

    Not much artificial gravity. ⁴⁰/₂ = 20 = r … L = 400… volume = 500,000 m&3
    Assume “ironish” density 9000 kg/m³ = 4.5 billion kg.
    Assuming usual gravitational coefficients, surface gravity of 2.25×10⁻⁵ N/kg at tips.

    So, the spin rate isn’t enough to overcome self-gravity if ironish.

    Actually space needles can be quite long if only 25% ironish (until centripetal force disrupts ’em.) However, not that far different … factor of 2. I guess that’d make it about the longest needle sustainable (if not compact) in end-over-end flipping, assuming 50% or less iron density.


    • They only believe it’s rotating because it’s brightness changes periodically. The strange period could also reflect the pulsing it’s lights instead!

      • the periodic changes in brightness are of course the artificial day/night cycles escaping through the windows of this O’Neil Cilinder that serves as interstellar colony ship.

      • Even if it is rotating, it might not be rotating end over end. It could be rotating around the long axis if the sides are different colours.

    • Just playing with this idea but couldn’t it be a derelict interstellar craft? Because that wouldn’t be inconsistent with tumbling…

    • F = ω² R
      F = 1.14×10⁻⁵ m/s²
      , surface gravity of 2.25×10⁻⁵ N/kg at tips.

      Those numbers are suspiciously close. As the density is a guess they could well be the same number.

      Which would be VERY suspicious.

  4. if any of you read Arthur C. Clarke’s novel “Rendezvous with Rama” , will find this observation amazing, it seems weirdly similar to the beginning of the novel…

  5. Completely fascinated at extra-system objects that have spent millions of years floating about, mostly isolated. This story broke about two days back. You wouldn’t believe the head shaking conspiracy theories that have already crept up. Some of them will make for a great laugh if interested.

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