Abstract
‘Oumuamua (1I/2017 U1) is the first known object of interstellar origin to have entered the Solar System on an unbound and hyperbolic trajectory with respect to the Sun1. Various physical observations collected during its visit to the Solar System showed that it has an unusually elongated shape and a tumbling rotation state1,2,3,4 and that the physical properties of its surface resemble those of cometary nuclei5,6, even though it showed no evidence of cometary activity1,5,7. The motion of all celestial bodies is governed mostly by gravity, but the trajectories of comets can also be affected by non-gravitational forces due to cometary outgassing8. Because non-gravitational accelerations are at least three to four orders of magnitude weaker than gravitational acceleration, the detection of any deviation from a purely gravity-driven trajectory requires high-quality astrometry over a long arc. As a result, non-gravitational effects have been measured on only a limited subset of the small-body population9. Here we report the detection, at 30σ significance, of non-gravitational acceleration in the motion of ‘Oumuamua. We analyse imaging data from extensive observations by ground-based and orbiting facilities. This analysis rules out systematic biases and shows that all astrometric data can be described once a non-gravitational component representing a heliocentric radial acceleration proportional to r−2 or r−1 (where r is the heliocentric distance) is included in the model. After ruling out solar-radiation pressure, drag- and friction-like forces, interaction with solar wind for a highly magnetized object, and geometric effects originating from ‘Oumuamua potentially being composed of several spatially separated bodies or having a pronounced offset between its photocentre and centre of mass, we find comet-like outgassing to be a physically viable explanation, provided that ‘Oumuamua has thermal properties similar to comets.
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Acknowledgements
K.J.M., J.T.K. and J.V.K. acknowledge support through NSF awards AST1413736 and AST1617015, in addition to support for HST programmes GO/DD-15405 and -15447 provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy under NASA contract NAS 5-26555. R.J.W. and R.W. acknowledge support through NASA under grant NNX14AM74G issued to support Pan-STARRS1 through the SSO Near Earth Object Observation Program. D.F., P.W.C. and A.E.P. conducted this research at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. We thank S. Sheppard for obtaining the Magellan observations, and E. J. Christensen, W. H. Ryan and M. Mommert for providing astrometric uncertainty information related to the Catalina Sky Survey, Magdalena Ridge Observatory and Discovery Channel Telescope observations of ‘Oumuamua. This work is based on observations obtained at CFHT, which is operated by the National Research Council of Canada, the Institut National des Sciences de l’Univers of the Centre National de la Recherche Scientifique of France and the University of Hawai‘i . It is based in part on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme 2100.C-5008(A) and in part on observations obtained under programme GS-2017B-DD-7 obtained at the Gemini Observatory, which is operated by AURA under cooperative agreement with the NSF on behalf of the Gemini partnership: NSF (United States), NRC (Canada), CONICYT (Chile), MINCYT (Argentina) and MCT (Brazil). This is work is also based on observations made with NASA/ESA HST, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy under NASA contract NAS 5-26555. This work has made use of data from the ESA mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC; https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.
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Contributions
M.M. discovered the non-gravitational acceleration and extracted the high-precision astrometry from most ground-based observations obtained by the team. D.F. performed the different fits and modelling of the non-gravitational acceleration. K.J.M. secured the HST time and designed the observation programme, computed sublimation dust and gas outgassing limits, and provided the assessment of outgassing. M.W.B. led the design of the HST observations and contributed precision astrometry from HST images. O.R.H. obtained the deep stack of images, searched them for dust and companion, and estimated production rates. D.P. performed the thermal sublimation modelling. N.S. conducted thermal model calculations. H.A.W. managed the HST observations and the initial reduction of images. P.W.C. provided support in analysing possible explanations for the observed non-gravitational acceleration. J.T.K. assembled the deep stack of CFHT data to search for dust and outgassing. R.W. identified and searched pre-discovery images of ‘Oumuamua in Pan-STARRS1 data. R.J.W. obtained the observations using CFHT and searched for pre-discovery observations of ‘Oumuamua. H.E. contributed to the HST proposal and to the design of the HST observations. J.V.K. and K.C.C. contributed to the HST proposal. D.K. provided support in analysing possible explanations for the observed non-gravitational acceleration. A.E.P. investigated the magnetic hypothesis.
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Extended data figures and tables
Extended Data Fig. 1 Non-gravitational accelerations of Solar System comets and ‘Oumuamua.
Measured non-gravitational radial accelerations A1 for short-period (red) and long-period (blue) comets from the JPL Small Body Database (https://ssd.jpl.nasa.gov/sbdb.cgi). The solid vertical black line indicates the A1 value for ‘Oumuamua, which falls within the range observed for Solar System comets; the dashed vertical black lines mark the corresponding 1σ uncertainty.
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Micheli, M., Farnocchia, D., Meech, K.J. et al. Non-gravitational acceleration in the trajectory of 1I/2017 U1 (‘Oumuamua). Nature 559, 223–226 (2018). https://doi.org/10.1038/s41586-018-0254-4
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DOI: https://doi.org/10.1038/s41586-018-0254-4
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Bubba Nicholson
Let's see. It's hot. It's accelerating without gravity assist. It's a spaceship.
Darrell Robert Parker
What if the object is the rocky core of an ancient comet, with just enough water left in the interior that as it passes closer to the sun the water transitions to gas and creates the illusion of a artificial object. Has there been any spectral analysis done on this object to determine composition?
AndrewLB Replied to Darrell Robert Parker
Dark red apparently. https://twitter.com/Fitzsim...
Gallerykid
Dammit, Jim, I'm a layman, not an astronomer! Nevertheless, I find this article (and Oumuamua itself) very interesting.
Having just recently learned of Oumuamua's existence, I was largely unaware of its origins and characteristics. How intriguing to know that this object has come to us from interstellar space!
I plan to get myself up to speed on what is known and what is theorized about this body. My biggest questions are about whether we've been able to extrapolate it's course backwards to identify the specific direction from which it came, with an eye toward estimating a likely point of origin; also, whether we plan to visit this object with a probe for pictures, chemical, radiological, magnetic (etc.) analyses of it, to possibly include physical sampling of material to be returned to Earth.
The other thing that interests me about this discovery is, of course, its non-gravitational acceleration. I knew that comets outgas as they approach Sol, but it hadn't really occurred to me that non-cometary bodies might outgas as well; but now having considered it, I can easily see how they could. There's no rule that says non-cometary bodies can't carry materials volatile enough to outgas when heated. And it makes sense that a small body like this might be full of fissures and/or holes.
If Oumuamua was built by gravity (accretion), then porosity might well be expected, since such construction would necessarily result in a very loosely formed structure owing to the extremely weak gravity involved.
However, my own thinking is that such an elongated shape would be very unlikely to result from gravitational construction. Random accretions would likely very strongly favor a more bunched or amorphous configuration.
But I just watched a documentary that speculated Oumuamua might be a fragment derived from the collision of two bodies. Numerous experiments were conducted to determine whether objects resembling Oumuamua's shape might come from such collisions, and it was found that a specific kind of off-center or glancing blow did in fact produce shards similar in shape to Oumuamua.
Given it's shape, I doubt seriously that Oumuamua is an artifact of intelligent design. Although it's not impossible for an interstellar craft to enter our cosmic backyard (consider our own Voyager spacecraft on its way out of the solar system), such an artifact would, IMO, probably not closely resemble a natural object. Like Voyager, an interstellar craft would most likely be purpose-built, and therefore would also be very likely to have a plainly artificial shape. Also, it would be counterproductive for a spacecraft to tumble the way Oumuamua does, IMO.
And I can't help but wonder, given it's speed, how long it's been traveling from its point of origin. Probably many thousands, if not several million years.
I had a brief thought that it may even have been trapped for a time by the Oort cloud or Kuyper belt on its way in, and then got nudged out towards us; but it then occurred to me that an object caught in such a manner would appear to us as having originated there, and not from interstellar space. So I tossed that idea out.
It's hard to overstate the significance, moment, and import of this extremely fortunate discovery! I understand that Oumuamua is now on its way out of the solar system, so we have no opportunity to study it more closely; but having found one interstellar nomad, we may find another; and I hope we find it early enough on its way in towards us that we can send a probe out to it for study, and maybe even sampling. And who knows? Maybe one day we could get an opportunity to nudge one into Earth orbit for long-term study!
AndrewLB Replied to Gallerykid
Well it seems this cigar shaped (a shape we've never seen before), red in color, shinier than an asteroid or comet by a factor of 10, velocity and trajectory changing, coma-less comet, not from our solar system has vanished on its way out of town and not even the spitzer space telescope is able to find it where trajectory calculations said it should be (unless it changed trajectory again). https://arstechnica.com/sci...
We're starting to run out of candidates as to what Oumuamua could be. It was quickly determined that it was not an asteroid due to changes in velocity and trajectory as it rounded the sun. This is caused by out-gassing due to solar radiation turning water ice into gas, creating jets which can cause the observed behavior. Such changes in trajectory and velocity would indicate it being a comet, but such changes would have resulted in the object losing approximately 1/10th of its mass due to out-gassing. Trouble is, there was no out-gassing even though the object passed inside the orbit of Mercury (17% closer) when it passed the Sun.Additionally, the out-gassing would have caused a change in rotational speed. No such change was observed. A recent analysis of this by Roman Rafikov showed that such an elongated object would have been torn apart by the out-gassing required to make such significant changes in speed and trajectory.
So.... not an asteroid... not a comet. And i've read all sorts of theories that it's some sort of new type of comet that has a coma which we can't see.... which to me is even further out than Avi Loeb's speculation that it's Artificial in origin. At least we've observed spacecraft, so we know they exist. We made some ourselves and in fact one of them has even left our solar system and is in interstellar space.
Ive always found it odd how the scientific community can be so dead set against even the consideration that something could have been made by another intelligent civilization considering how a quarter of the stars in our galaxy have earth sized planets in the habitable zone, That's tens of billions of chances that the right conditions could have been met. The fact that we exist and currently have a man-made object traveling in interstellar space is enough for me to consider the idea before some hypothetical new type of comet.
Gallerykid Replied to AndrewLB
There are some observational errors in your reply, I think; but I will generalize my thinking this way: I have yet to see any convincing argument, yours included, that we have anything but a natural object. And while it's true that there are myriad opportunities in the galaxy for intelligent extraterrestrial space-faring life, the likelihood that they could actually travel between their star (not likely to be within 50 LY of Earth) and ours is quite literally, astronomically improbable. For perspective, an episode of "How the Universe Works" had viewers imagine this: if you shrunk Sol to the size of a pollen grain, the galaxy would be the size of the Pacific Ocean, and the nearest star (4.3 LY) would be a mile away.