The interstellar asteroid 'Oumuamua was discovered on October 19, 2017. Once discovered, less than a month's worth of precovery observations were found on various sky surveys. By mid-November, scientists had worked out its orbital eccentricity, and soon after its probable origin and destination.

With such a quick visit and so little data, how did scientists figure out so quickly where it came from and where it was going? Did using space probes for observations impart enough parallax to easily determine that, or did Earth-based observation have enough parallax themselves?


Three accurate observations are sufficient to fix a Keplerian orbit (ie an elliptical or hyperbolic orbit with the sun at the focus)

In practice, observations are not perfectly accurate due to limitations of the equipment and observations over a short time are particularly prone to observational error being magnified. Moreover the orbit will be perturbed by the gravity of the planets, so won't be perfectly Keplerian. For this reason pre-discovery images will help fix the exact orbit more accurately.
As the time difference between first and last observation is critical in the quality of the orbital determination, we talk about the length (in days) of the observation arc as a measure of how well defined the orbit is.

Multiple observations can reduce error by an averaging effect (the Gaussian curve is so named from Gauss's use of it in orbital determination)

However once you have three or more observations of a body you can determine its orbit almost immediately, using a computer to do the calculations for you. It would have been immediately obvious that the object was on a very unusual trajectory.

There is no need to use space probes for the observations. The relative motion of the Earth and the Object provide sufficient parallax to determine distance, position and velocity.

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  • $\begingroup$ In theory, you can determine orbital elements from one observation of position and velocity (arguably, measuring velocity would itself require two observations): naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/cspice/oscltx_c.html -- however, as the linked page itself warns, the results can be far from accurate, and, obviously, averaging out multiple observations is always a good idea. $\endgroup$ – user21 Jun 5 '18 at 17:27
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    $\begingroup$ You need radar to do that. $\endgroup$ – James K Jun 5 '18 at 19:54
  • $\begingroup$ Is there any path that an interstellar asteroid could take through the easily observable solar system that we couldn't extrapolate? Is there a lower limit on the orbital eccentricity we could use to determine an object's trajectory? I take it that an object would have to be significantly more massive than the sun to pass through the solar system without its course being perturbed? $\endgroup$ – jkade Jun 5 '18 at 23:57
  • $\begingroup$ If we can observe it, we can fit an orbit to the observations. No upper or lower limits. There might be exceptional situations in which more than one sensible orbit can fit a small number of observations. Everything gets perturbed, but most orbits are very close to ellipses, or hyperbolae $\endgroup$ – James K Jun 6 '18 at 5:47
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    $\begingroup$ James is right. The three-point method (augmented with extra points when possible) is canonical, and can be done using traditional instruments. Even amateurs can use it. It's sometimes called Gauss' method, and was used for the first time to determine the orbit of Ceres. $\endgroup$ – Florin Andrei Jun 7 '18 at 1:21

The Minor Planet Center (MPC) routinely gives preliminary orbits for newly discovered asteroids with observations spanning only 48 hours. Ordinary elliptical orbits are computed automatically. After further observations are reported, MPC may issue a more precise orbit estimate. The uncertainty never reaches zero, even if we track an asteroid for 100 years or longer.

'Oumuamua's trajectory took longer to determine because it didn't fit the usual elliptical model. Meech et al. 2017 tell in their Methods section how they tried and rejected two different elliptical orbits before settling on a hyperbolic trajectory. Even with observations spanning 6 days, MPC issued their preliminary trajectory with conditions:

Further observations of this object are very much desired. Unless there are serious problems with much of the astrometry listed below, strongly hyperbolic orbits are the only viable solutions. Although it is probably not too sensible to compute meaningful original and future barycentric orbits, given the very short arc of observations, the orbit below has e ~ 1.2 for both values. If further observations confirm the unusual nature of this orbit, this object may be the first clear case of an interstellar comet.

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