Rocketdoc Notes – Week of January 10, 2021

Is Oumuamua our first UFO?


Oumuamua is the first known interstellar object detected passing through the Solar System. It was discovered by Robert Weryk using the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1 telescope at Haleakala Observatory on Maui, Hawaii on 19 October 2017. I know from my previous life that this facility was designed to discover rapidly moving objects in space and obtain definitive photographs. It was spotted 40 days after it passed perihelion (its closest point to the Sun) on 9 September. When it was first observed, it was about 33 million km or 0.22 Astronomical Units (AU) from Earth and already heading away out of the solar system. It never got close enough to Earth to get a photograph other than a blob of light. See plot of Oumuamua’s trajectory in Figure 1 below.


Figure 1- Oumuamua’s Trajectory through the Solar System


Oumuamua appears to have come from roughly the direction of the star Vega in the constellation Lyra. The incoming direction of motion of ʻOumuamua inside the solar system is 6 degrees from the solar apex (the direction of the Sun's movement relative to local stars), which is the most likely direction for approaches from slow travelling objects outside the Solar System. It has a hyperbolic excess velocity, its speed relative to the Sun when in interstellar space, of 26.33 km/s. At that speed it would take over 51,000 years to travel to the nearest star. Hence, it may be an interesting object but it is not a candidate for an interstellar probe.


Oumuamua is a small object estimated to be between 100 and 1,000 meters (300 and 3,000 ft) long, with its width and thickness both estimated to range between 35 and 167 meters (115 and 548 ft). It has a dark red color, similar to objects in the outer solar system. Despite its close approach to the Sun, Oumuamua showed no signs of having a coma (particle tail) but did exhibit non-gravitational acceleration. My explanation is that our usual comets are a mixture of frozen gases and dust particles accumulated over the life of the solar system. Oumuamua has been in interstellar space for millions to billions of years and has not accumulated as much dust as regular comets, so its coma should be largely transparent, and that explains the non-gravitational acceleration observed by astronomers.


The object has a rotation rate of four hours similar to that seen in Solar System asteroids, but many valid models permit it to be more elongated than all but a few other natural bodies. Oumuamua's light curve, assuming little systematic error, presents its motion as tumbling, rather than smoothly rotating, and moving sufficiently fast relative to the Sun that few possible models define a Solar System origin. Extrapolated and without further deceleration, the path of Oumuamua cannot be captured into a solar orbit, so it would eventually leave the Solar System and continue back into interstellar space. Oumuamua's planetary system of origin, and the age of its excursion, are unknown. An artists’ conception of Oumuamua is shown in Figure 2 below.


Figure 2 – Artist Conception of Oumuamua


The bottom line is that Oumuamua appears to be a natural object, albite a strange natural object from very far away, and it is truly unfortunate that we didn’t discover it until it was on its way out of the solar system, and it was too late to mount an exploration probe. The lesson here is that we should be on the lookout for the next interstellar visitor and set aside a general-purpose probe that can be launched on short notice, so we don’t miss the next opportunity.


Thanks for reading.

Dana Andrews

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