Just how exceptional is 1I/(‘Oumuamua) aka A/2017 U1?

We  now know that 1I/(‘Oumuamua), aka A/2017 U1 is an even more extraordinary object than we believed. It is shaped more or less like a cigar: ten times as long as it is wide. This creates some real physical problems as it is far more elongated than any solar system object, asteroid, comet, or satellite, that we know and is far too elongated to be made up of two bodies in contact, as they would fly apart with its rotation.

By Original ESO/M. Kornmesser, AND here adjusted to 65% length closer to proposed dimension 180 × 30 × 30 m (or 6:1 ratio) – http://www.eso.org/public/images/eso1737a/, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=64265376

The latest JPL solution to the orbit of A/2017 U1 is JPL#14, calculated on November 21st based on 115 observations over 34 days. There are just two additional observations since the last calculation and these have barely changed the calculated orbit. As it becomes more distant and fainter it seems unlikely that there will be many more observations.

We know that it entered the solar system with a velocity of 26.3 km/s, equivalent to 1 light year every 11000 years and will leave the solar system in a direction more or less towards the star 16 Pegasi, in the constellation of Pegasus.

Just how extraordinary is the orbit of ‘Oumuamua?

I can’t remember who was the colleague who asked the question but I have checked the velocity of the five NASA missions on solar system escape trajectories compared to 1I/’Oumuama.

The velocity of ‘Oumuama in the distant future will be (almost) identical to its entry velocity in the solar system: 26.3 km/s.

In contrast, the five NASA probes are all much slower. From fastest to slowest their velocity is:

Voyager 1 – 16.9 km/s

Voyager 2 – 15.0 km/s

New Horizons – 13.6 km/s

Pioneer 10 – 11.9 km/s

Pioneer 11 – 11.2 km/s

Voyager 1 is the fastest ever artificial satellite leaving the solar system, its velocity boosted by close approaches to Jupiter and Saturn, but even so, in an interplanetary race, will be caught and passed quickly by ‘Oumuamua. This will happen around the year 2080, when ‘Oumuamua will be further from the Sun than Voyager 1 although, by then, Voyager 1 will have fallen silent as its nuclear power plant is expected to fail to be able to generate enough power to maintain contact with Earth long before this date.

There are 327 comets that have been calculated reliably to have a hyperbolic, or open orbit that will escape from the solar system. Most of these orbits are just barely hyperbolic with a typical escape velocity from the solar system – that is, their speed in the future, far from the Sun – around 2-3 km/s.

  • The most hyperbolic comet ever observed, is C/1980 E1 (Bowell), which is leaving the solar system with a velocity of 4.1 km/s.
  • Comet Arend-Roland, which was a bright naked-eye object in 1957, is leaving the solar system at 2.2 km/s.

In other words, ‘Oumuamua is leaving the solar system more than 6 times faster than the fastest escaping comet that we have observed and about 10 times faster than an average comet in a hyperbolic orbit.

Truly, both its physical characteristics and its high velocity make it a quite extraordinary object.

8 thoughts on “Just how exceptional is 1I/(‘Oumuamua) aka A/2017 U1?

  1. Can I just add, that Oumuamua is rotating, and not along the axis you would expect given the shape! Oumuamua is rotating like a tumbling pigeon

    This has lead many conspiracies to believe that Oumuamua is actually an alien space craft following the tumbling pigeon space craft design (which theoretically is can be used to create artificial gravity)

    But cool post 🙂

    Liked by 1 person

    1. That is why, if it were a contact binary, for example, it would tear itself apart. The centrifugal force would be greater than the mutual gravity. Its material strength must be great enough to hold it together, which presents some interesting physical issues.

      Incidentally, at 1 light year per 11 000 years of journey, any astronauts on board would have to be:
      a.) Long-lived
      b.) Patient!

      Would you want artificial gravity on a voyage of millions of years, I wonder? However, I still wish that they had called it “Rama” – the name is available and has not been used.

      Liked by 2 people

    1. It is quite fast: twice the speed of approach of Vega. So, I just wonder that, if it has escaped from another solar system, how it has managed to do so with such a high energy orbit? You would have to assume that it escaped from a star that was approaching our Sun at a quite high velocity, or that, somehow, ´Oumuamua was accelerated somehow to an unusually high velocity when escaping, or maybe even a combination of both. However, it looks as if it has been wandering through space for millions of years.


  2. Happy New year!
    A bit of additional information/thoughts about the object:

    – Much of the velocity relative to the sun is actually the proper motion of the sun relative to nearby stars (or what’s called the local standard of rest). ‘Oumuamau came from close to the solar apex and its velocity of 26 km/s is not too far from the solar proper motion (~20 km/s). It is small compared to the orbital velocity of the sun around the galactic centre (~255 km/s). Taking all this, ‘Oumuamua may have left its “home” system at low velocity.
    – Some authors (Fraser et al. on astro.ph) argue that, in addition to tumbling motion, the complicated light curve is due to strong albedo and colour variations. In particular, colour and spectral slope seem to vary strongly.
    – Colour/spectral slope are always in the range typical for the solar system. No spectral features found (low S/N, though). So, apart from the strongly elongated shape, what we know about ‘Oumuamua is not that different from outer solar system objects and some asteroids.
    – A speculation: Could the elongated shape come from deformation by tidal forces, maybe during the encounter with a planet that ejected ‘Oumuamua from its system? I am thinking of Shoemaker-Levy 9 here, only that in the case of ‘Oumuamua the tidal forces did not disrupt the object, maybe due to somewhat higher tensile strength. The higher tensile strength then also helps to survive with that elongated shape later on.


    1. Yes, I did take that factor into account. Even so, there is a big excess of velocity. However, as you suggest, that may also be due to the relative velocity of its host solar system to ours.


  3. One more:
    – One surprising aspect is that the first discovered interstellar object seems to be an asteroid, not a comet.


    1. Maybe it started as a comet and has lost its ices on its travels? However, if it started rocky, how did it get into a high-energy escape orbit if it was ever orbiting a star? This object poses all manner of interesting issues. It could be that it never was part of a planetary system.


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s