A cigar from another star

In Rendezvous with Rama (by Arthur C. Clarke), a mysterious object is discovered passing through the Solar System.  The object has a strange shape — it’s a giant cylinder.  It was discovered by the Spaceguard survey, designed to find objects that might impact Earth (so-called near-Earth objects).  Spoiler alert: the cylinder is a spaceship sent by super-intelligent aliens to prospect for other space life.


My all-time favorite sci-fi book (first published in 1973).

Guess what? Rendezvous with Rama just happened in real life! 

A month ago (Oct 19, 2017), the Pan-STARRS survey discovered the first object passing through our Solar System. (Like Spaceguard,  Pan-STARRS was designed to find near-Earth Objects.)

The object is on a hyperbolic trajectory: it is simply passing through the Solar System.  It feels the Sun’s gravity of course, but it is moving too fast to be bound to the Sun. It is just zooming through for a quick visit before it heads back to interstellar space.  Here is a sweet animation of its path:

The object has been officially named 1I/’Oumuamua.

The name comes from Hawaiian ʻou.mua.mua, meaning “scout”, (from ʻou, meaning “reach out for”, and mua, reduplicated for emphasis, meaning “first, in advance of”) and reflects the way this object is like a scout or messenger sent from the distant past to reach out to us. (Source: Wikipedia)

A couple of things about ‘Oumuamua are pretty strange. 

First, it passed very close to the Sun (within about 0.25 Astronomical Units) but it showed no signs of activity.  When comets approach the Sun, small jets release water vapor (and other volatiles), creating a giant coma and tails.  The Rosetta mission got up close to comet 67P/Churyumov–Gerasimenko and saw its jets in action:


Structure in the jets of Comet 67P observed by the OSIRIS instrument on Rosetta on Nov 22, 2014.  Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Why doesn’t ‘Oumuamua have jets (or a coma or tails)?  Maybe it’s rocky with very little ice. But it’s possible that ‘Oumuamua contains ice but only under the surface.  There is a class of objects called Damocloids, that have comet-like orbits but no activity.  The Damocloids are thought to be extinct comets, which passed close to the Sun so many times that they completely lost their water and volatiles. Maybe ‘Oumuamua is similar.

Second, ‘Oumuamua has a weird shape.  It is super-stretched along one axis, which is about 10 times longer compared with the other two axes.  It’s been called a cigar, but it might look like a cucumber. Or a baseball bat.  Or a carrot.  It rotates every 8 hours or so, tumbling along the shortest axis. (Like a cigar/cucumber/baseball bat/carrot thrown up in the air).


Artist’s impression of ‘Oumuamua.  Credit: ESO/M. Kornmesser.

There is some disagreement between different groups on just how stretched-out ‘Oumuamua is; it could be at little as 3:1, more like an interstellar potato than a cigar.  But, the cigar people have the better images!

The sad thing is that we will probably never know what ‘Oumuamua really looks like.  It’s just too far away to get a resolved picture. To do that we would need an image from way up close.  We’ve sent spacecraft to intercept comets and asteroids to see what’s really going on there.  But ‘Oumuamua showed up too suddenly and is moving too fast.  We’ll never catch up (at least not for decades).  Big frowny face.

‘Oumuamua’s origins story probably goes something like this.  It formed as a “planetesimal” (a planetary building block) in a disk orbiting a young star.  Somehow it did not end up being incorporated into a planet.  Instead, the gravity of the growing planets kicked it onto a stretched-out (eccentric) orbit, then kept kicking it until it was ejected into interstellar space.  The most efficient ejection happens when a system with more than one giant planet becomes unstable.  Here is an animation of an instability from my own research:

As you can see, when the giant planets go unstable, the whole outer disk of comet-like objects is completely ejected from the star, left to roam interstellar space.  (Of course, some planets might share their fate, and some might even harbor life…).  Comet-like objects are thought to be much more abundant than rocky ones, and also to be much easier to eject (rather than to be thrown onto the star).  That is why we think it likely that ‘Oumuamua is closer to an extinct comet than to a rocky asteroid (see here for details).

There are a couple of final things that we can learn from ‘Oumuamua.  Of course, let’s keep in mind that this is just one object.  (How much do you learn about a movie from the first frame? How much do you learn about clouds from the first raindrop?)

First, ‘Oumuamua is small!  It’s only about the size of a large stadium.  That’s much smaller than we usually think the building blocks of the planets were.  Is ‘Oumuamua a fragment created in a collision between building blocks?  That might explain its weird shape, but at this point it’s just a guess.

Second, humanity is lagging behind!  If this object was really sent by super-intelligent aliens, then we are just watching it pass by.  Sure, we’re learning some interesting stuff but we don’t have the capabilities to catch up to ‘Oumuamua and make sure it’s not a real spaceship like in Rendezvous with Rama.  I guess that means that we’re not even smart enough to end up in a Galactic zoo…. Sheesh,

The astronomy community is super-jazzed about ‘Oumuamua and I’m sure the story will keep unfolding over the coming months.

UPDATE: Upon writing this article, I was not aware of Project Lyra, a study of how to intercept and analyze ‘Oumuamua.  This exciting concept is nicely-summarized in this article.

Questions?  Comments?  Words of wisdom?

Additional resources:



14 thoughts on “A cigar from another star

  1. In the paper, is equation one the same as the collision cross section used for the growth of planetesimals?

    If so, should the cross section for observation for ‘Oumuamua be the same given that a planetesimal passing within the radius of larger body would collide but a object passing within a given perihelion wouldn’t necessarily be observed, and in the case of ‘Oumuamua is only spotted after it was beyond 1 AU?

    I would understand it being used for an interstellar comet which was brightest when it was near perihelion.

    • You’re right that it’s apples and oranges: in accretion we assume that passing within a given radius implies accretion. Here we are assuming that there is a corresponding radius for detection. How true is that, really? Well, not 100% — there are plenty of factors at play in detecting small bodies: size, albedo, ecliptic latitude, and sky motion come to mind. So, that equation is trying to get a rough estimate but it’s definitely limited…

  2. A little of my own speculation, earlier this year I saw this paper: Planetesimal formation by the streaming instability in a photoevaporating disk https://arxiv.org/abs/1703.07895 and was thinking, if there is planetesimal formation late, after much of the gas is removed and possible a significant fraction of the solids, the objects formed might be smaller than in the usual simulations of streaming instabilities. Maybe the large objects like the observed Kuiper belt objects are formed early, and later small bodies like comets form following photoevaporation.

    • Hmmm — I think this is a question for people doing streaming instability calculations like Jake Simon, Anders Johansen, and their collaborators. I wonder how the size distribution of planetesimals changes as the conditions evolve in the disk… Simon et al (2017) found a near-universal size distribution for the conditions they tested. But of course, what happens in real disks?

  3. Hi Sean
    I’m studying intercepting ‘Oumuamua with Project Lyra. One suggestion for primary science data is its isotopic signature, which could be sampled by striking it with a smaller impactor and sieving the plume with a mass spectrometer. Any other science data nuggets you’d want from an encounter?

    • Hi Adam — intercepting ‘Oumuamua would be amazing! And if you are able to crash into it and see what comes out, that would be even awesomer. What to measure? Isotopes make a lot of sense. D/H would be great if there is some water to detect (Nitrogen 15-to-14 also, for comparison with comets/asteroids and origins of Earth’s water issues). Bulk compositions. That’s what comes to mind first at least…

      • An early analysis of the impact indicates quite a lot of UV emission from the resulting plasma – which’d be too hot for a fly-through (sadly. I liked the mass spectrometry idea!) Should be a strong signal.

  4. Pingback: Allgemeines Live-Blog ab dem 21. November | Skyweek Zwei Punkt Null

  5. Can you explain this aspect of the animation of the Oumuamua trajectory: about 45 seconds in it looks like it’s going backwards.

    Also in regards to reaching it with a planetary space probe, Oumuamua is moving away at 25 km/s. The New Horizons mission using chemical propulsion was able to manage 12 km/s. And the Dawn mission using ion propulsion could manage 10 km/s. So combining these type of propulsion we could get 22 km/s, a little less than the 25 km/s needed for Oumuamua. Tweaking these stages a little should allow us to reach the 25 km/s needed.

    Bob Clark

    • Hi Robert,
      It’s due to the angle and scale at which it is being depicted. The deflection angle was quite large.
      As for intercept speeds, remember that ‘Oumumua has a head start. A probe has to travel faster than its hyperbolic excess to arrive in a finite time.

  6. Thanks. I would like to see an interactive version where you could vary the angle and distance including zoom.

    I was puzzled by the fact it came so close to the Sun to be bent around to curve back its trajectory. This has the effect of increasing the time it spends in the Solar System. But you could argue it came close enough to the Earth to be discovered which makes it likelier to get close to the Sun.

    But it also seemed to get close to Jupiter at about the 24 second point in the video. Jupiter has an 11 year period. It’s not likely it should get close to both Earth and Jupiter. However, because of the angle shown you can’t tell how far it is above the orbital plane during the Jupiter pass. I think it is actually high above the ecliptic during the Jupiter pass, judging from later viewpoints in the video. So It may be further away from Jupiter than it appears. This is another reason why an interactive video would be useful to answer this question.

    It also seems to get close to Mars at about the 52 second point. But again this is hard to tell because of the angle. An interactive video would be helpful again.

    Close passes by Jupiter, the Sun, Earth, and Mars. That can’t be right can it?

    Bob Clark

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