Principium editor John Davies offers a personal view of the continuing mystery of the interstellar object Oumuamua. More than one year since its discovery we still do not know what this object is. Here John summarises analysis so far and suggests that a mission to Oumuamua may be our only way of solving the mystery.
The object ʻOumuamua (IAS MPC designation 1I/2017) has intrigued astronomers, science journalists, the popular press and the interstellar studies community since its discovery by Meech et al as announced in Nature Letters on 20 November 2017 .
The key issue is - What is Oumuamua? This article attempts to summarise where we stand on this issue.
I hope to establish two key points -
- The nature of Oumuamua remains unclear but it is certainly unprecedented
- Since it is reachable by currently available launchers a mission should be considered
The article is a piece of science journalism based on my own analysis rather than an academic paper. The conclusions are my own and do not represent an official i4is point of view.
The nature of Oumuamua
Meech (cited above) et al discovered Oumuamua had the following characteristics -
- No evidence of out-gassing even at closest approach to the Sun (0.25 AU, well within the orbit of Mercury) so not a comet.
- Elongated to a degree unknown in any extant asteroid of its size with dimensions of several hundred metres by about one hundred metres.
- Spectrally red.
- Albedo (reflectivity) of 0.04 - typical of comets and the majority of asteroids .
A number of observers confirmed that the cyclical variation in sunlight reflected from Oumuamua suggests that it is tumbling. Assuming that its surface has a uniform albedo this means it could be either extended in one dimension, spindle shaped, or in two dimensions, flattened .
So two more facts established -
- Spindle shaped or flattened.
Naturally such a strange object excited speculation about the possibility of an artificial origin but attempts to detect any non-natural electromagnetic radiation from it failed. In any case, natural scientific caution has produced a consensus that all reasonably possible natural explanations should be sought before artificial origins are considered. For what it's worth the present writer, an engaged amateur in these matters, agrees with this.
The plot thickens
However in June of this year, 2018, "the plot thickens". Oumuamua was discovered to be receding from the sun at an increasing speed. Reporting data from a number of optical telescopes, both terrestrial and orbital, Marco Micheli et al  reported small velocity increases, specifically "all astronometric data can be described once a non-gravitational component representing radial acceleration proportional to ~r−2 or ~r−1 is included in the model".
In other words, the acceleration is inversely proportional to the square of its distance from the Sun or to some lower exponent. It is accelerating, but that acceleration is decreasing as it gets further from the Sun. Micheli et al provisionally conclude that the best explanation of this acceleration is outgassing and that therefore Oumuamua is a very peculiar type of comet.
Here is a quick summary tour of their other possible explanations -
- Solar radiation pressure. The simplest physical phenomenon that could cause a radial acceleration following an r−2 dependency and directed away from the Sun is pressure from solar radiation [but] the magnitude of the observed acceleration implies an unreasonably low bulk density roughly three to four orders of magnitude below the typical density of Solar System asteroids of comparable size.
- Yarkovsky effect  (rejected as too small and in the wrong direction)
- Friction-like effects (rejected since they would slow it down!).
- Impulsive event. ...such as a collision [but] continuous acceleration is a far more likely explanation.
- Binary or fragmented object. ...center of mass of the combined system does in fact follow a purely gravitational trajectory, and the detected non-gravitational signature is an artifact, caused by us tracking only the main component of ‘Oumuamua* [but] no secondary body or fragment is visible ... down to a few magnitudes fainter than ‘Oumuamua, and any object smaller than the corresponding size limit (~ 100 times smaller than ‘Oumuamua) would be insufficient to explain the observed astronometric offsets.
- Photocenter offset. (the effect would be too small)
- Magnetized object. (the effect would be too small)
To support their cometary explanation the writers suggest -
- [this] reconciles ‘Oumuamua’s properties with predictions that only a small fraction of interstellar objects are asteroidal.
- A thermal outgassing model, which treats ‘Oumuamua like a common cometary nucleus, creates a non-gravitational force proportional to ~r−2 in the range of distances covered by our observations.
- Outgassing at this level is not in conflict with the absence of any spectroscopic signs of cometary activity, since the quoted values are well below the spectroscopic limits on production rates.
- The model, however, also predicts 0.2 kg s−1 of dust production, which should have been detectable in the images. While problematic at face value, this discrepancy could be resolved by adjusting the dust grain size distribution, the pore size of the nucleus, and the ice-to-gas ratio.
Examining that first point the term "confirmation bias" sprang to my mind.
Micheli et al go on to say "In-situ observations would be required to determine conclusively the nature, origin, and physical properties of ‘Oumuamua and potentially similar objects yet to be discovered."
In other words, we need to go and have a look! I'll come back to that in the second half of this article.
Interested readers should take the link via spacetelescope.org (see reference ) and read the original paper.
The Loeb/Bialy Conjecture
Most recently Abraham Loeb and Shmuel Bialy  looked in more detail at that "simplest physical phenomenon that could cause a radial acceleration following an r−2 dependency ", as Micheli et al put it. Citing a recent paper by Rafikov  they suggest that "... if outgassing was responsible for the acceleration (as originally proposed by Micheli et al. 2018), then the associated outgassing torques would have driven a rapid evolution in ‘Oumuamua’s spin, incompatible with observations."
Building on Micheli et al, Loeb and Bialy look at the possible nature of Oumuamua if that "simplest physical phenomenon" is the explanation (ie explanation 1 above). They have to account for the Micheli et al objection that "the magnitude of the observed acceleration implies an unreasonably low bulk density roughly three to four orders of magnitude below the typical density of Solar System asteroids of comparable size"
They consider the material and geometry required to achieve that low bulk density while producing the required solar radiation pressure and maintaining integrity for such a thin structure travelling interstellar distances. They cite Belton et al  on the possible flattened shape of the object. They conclude "If radiation pressure is the accelerating force, then ‘Oumuamua represents a new class of thin interstellar material, either produced naturally, through a yet unknown process in the ISM or in proto-planetary disks, or of an artificial origin."
They go on to speculate about the purpose of a possible artificial object, a light sail.
They end by saying "Since it is too late to image ‘Oumuamua with existing telescopes or chase it with chemical propulsion rockets (Seligman & Laughlin 2018, but see Hein et al 2017), its likely origin and mechanical properties could only be deciphered by searching for other objects of its type in the future."
A Mission to Oumuamua
Based on the Project Lyra conclusions I beg to differ from the Loeb/Bialy statement "it is too late to … chase it with chemical propulsion rockets (Seligman & Laughlin 2018, but see Hein et al. 2017) ".
The i4is Project Lyra paper, Hein et al , cited in the Loeb/Bialy paper shows that a Falcon Heavy class launcher could project a probe to reach Oumuamua via a variety of "slingshot" manoeuvres and the Seligman & Laughlin paper , also cited by Loeb/Bialy, supports this view - though they "adopt a complementary approach" considering only a direct mission, no slingshots, launch within a few months of periastron (closest approach to the Sun) and a mission time of a few months rather than the years of the Project Lyra study. They address only future objects and not Oumuamua itself.
This article will not re-examine the feasibility of a mission to Oumuamua since this has been established by Hein et al, supported by Seligman & Laughlin, but will examine the case for a mission based upon what we know now (November 2018).
Abraham Loeb has announced there may be more interstellar objects gravitationally-trapped between Jupiter and Neptune, Identifying Interstellar Objects Trapped in the Solar System through Their Orbital Parameters arXiv:1811.09632 [astro-ph.EP]. They are Centaurs, an established class of asteroids. "Centaur (minor planet)" Wikipedia
And our colleague, Marshall Eubanks suggests that Interstellar Asteroid 1I/’Oumuamua is a member of the Pleiades Dynamical Stream (paper to be published).
Summarising the current state of astronomical opinion -
- The "peculiar comet" explanation is widely supported despite Rafikov's view that this does not accord with observations that its spin has not changed.
- The solar radiation pressure explanation is a minority view and the Loeb/Bialy conjecture that this implies a "solar sail-like" nature is not popular.
To this engaged amateur it seems that the majority view rests on some fairly demanding assumptions -
- that we have a comet whose outgassing material is very unusual, even unprecedented
- that an elongated comet is outgassing so that the resultant thrust vector is through the centre of mass, so that no change in spin is imparted
The solar radiation pressure explanation is unpopular perhaps because -
- it does not fit with the majority view that only a small fraction of interstellar objects are asteroidal.
- that the peculiar shape required by the Loeb/Bialy conjecture implies a new type of astronomical object.
And of course, the eternal search by the popular press for "little green men" and "wacky scientists" - and the consequent wish of scientists to avoid anything which might be interpreted as such.
However, what is almost certain is that we have a very strange object here, that the majority explanation for it seems weak and that we don't know how frequent such objects are since we have only seen one. The Spitzer non-observation of Oumuamua  indicates that, if it is a comet, it is a very strange one, able to produce significant non-Keplerian motion without visible gas and dust emissions. The dismissive scepticism in much of the science media could start to look like damage control and controlling perceptions rather than serious scientific criticism.
Given these circumstances our descendants (and ourselves in later life) may curse us if we do not follow this up. Imagine the situation which seems distinctly likely in, say, ten or twenty years time - that Oumuamua remains a mystery which we now regret not having investigated when the opportunity was there. So my personal conclusion is that this needs further investigation now and that, given that time presses and a mission is feasible if we act soon, that we should build upon the mission planning of the i4is Lyra team and of Seligman & Laughlin and begin a major study of a possible mission to Oumuamua. Kennedy gave NASA eight years to launch to the moon - we may have as little as eighteen months or perhaps three years.
* Editors note - Here and elsewhere the object is named simply Oumuamua except where quoted using other forms such as ‘Oumuamua or 1I/2017 U1, minorplanetcenter.net
 A brief visit from a red and extremely elongated interstellar asteroid, Karen J. Meech et al, Nature Letters, 20 November 2017. https://www.nature.com/articles/nature25020
 The Excited Spin State of 1I/2017 U1 `Oumuamua, Belton et al, https://arxiv.org/pdf/1804.03471.pdf
 Conan Doyle, A Study in Scarlet
 Non-gravitational acceleration in the trajectory of 1I/2017 U1 (‘Oumuamua), Marco Micheli et al, https://www.spacetelescope.org/static/archives/releases/science_papers/heic1813/heic1813a.pdf
 Could Solar Radiation Pressure Explain ‘Oumuamua’s Peculiar Acceleration? Shmuel Bialy and Abraham Loeb, http://iopscience.iop.org/article/10.3847/2041-8213/aaeda8/pdf
 Rafikov, Spin Evolution and Cometary Interpretation of the Interstellar Minor Object 1I/2017 ’OUMUAMUA, https://arxiv.org/pdf/1809.06389.pdf
 Project Lyra: Sending a Spacecraft to 1I/’Oumuamua (former A/2017 U1), the Interstellar Asteroid Andreas M. Hein et al First version [v1] Wed, 8 Nov 2017 20:43:30 UTsion [v3] Fri, 19 Oct 2018, https://arxiv.org/ftp/arxiv/papers/1711/1711.03155.pdf
 Seligman & Laughlin, The Feasibility and Benefits of In Situ Exploration of 'Oumuamua-like Objects http://iopscience.iop.org/article/10.3847/1538-3881/aabd37/meta
 Spitzer Observations of Interstellar Object 1I/'Oumuamua - http://iopscience.iop.org/article/10.3847/1538-3881/aae88f/meta
About the Author
John I Davies is a retired software engineer and mobile telecoms consultant. He was part of the UK space industry including an early design study for the Hubble telescope and the later stages of the European ELDO launcher. His later experience was in mobile data communications, both technical and commercial.