In October 2017, astronomers using the Pan-STARRS telescope in Hawaii discovered a small object passing relatively near the Earth (33 million km, 0.22 astronomical units (AU), or about 85 times farther away than the Moon). Initial attempts to fit an orbit to its path, tracked by a series of observations failed. It was realised that the object is on a strongly hyperbolic orbit and is not gravitationally bound to the Sun: dynamically, it is not a part of the solar system—it is an interstellar object, the first to be observed, just passing through. It was first considered to be a comet, but extended observations by large telescopes failed to detect any of the emissions of dust and gas which one would expect from a comet, especially one making its first close approach to a star in many millennia, and perhaps ever. It was then re-classified as an asteroid. Finally, it was given the designation 1I/2017 U1 and the informal name `Oumuamua, which means “scout” or “messenger” in the Hawaiian language.
Further observations deepened the mystery: `Oumuamua was discovered after it had made its closest approach to the Sun on September 9th, 2017 at a distance of 0.25 AU (inside the orbit of Mercury), and as it receded from the Sun, careful tracking of its position indicated it was not following a trajectory as would be expected from Newton’s laws, but rather losing velocity slower than gravitation would account for (or, in other words, it had an outward acceleration added to the deceleration of gravity). This is often the case for comets, whose emission of gas and dust released due to heating by the Sun acts like a rocket to propel the body away from the Sun. But that conflicts with the failure to detect any such emissions from `Oumuamua by telescopes and instruments with more than adequate sensitivity to observe emissions which could account for the acceleration.
If we rule out, for the moment, the possibility of an on-board UFO propulsion system (in which case astronomers would have to rename the object Rama), the only other explanation for this non-gravitational acceleration away from the Sun is radiation pressure from sunlight. Light absorbed or reflected by an object exerts a small but measurable force upon it (the effect is of sufficient magnitude as to affect the orbits of Earth satellites), and this might account for the observed acceleration. However, to produce the observed acceleration, and based upon estimates of `Oumuamua’s size from its brightness and distance, the density of the object would have to have been implausibly low.
Consequently, the most widely accepted explanation for the acceleration was outgassing of carbon dioxide or carbon monoxide, or sublimation of water which, it was argued, could under certain conditions, fall below the detection threshold of the instruments used to observe the object.
In May, 2019, Zdenek Sekanina of the Caltech Jet Propulsion Laboratory published a paper, “Outgassing As Trigger of 1I/`Oumuamua’s Nongravitational Acceleration: Could This Hypothesis Work at All?”, which argues that the observations and comparisons with other comets making their first pass close to the Sun definitively rule out these forms of emissions as explanations for the observed acceleration. This leaves only radiation pressure, which implies that the bulk density of `Oumuamua must be less than 0.001 grams per cubic centimetre, which is less than the density of air (0.0012 g/cm³) at sea level. How could this possibly be? How can an object which reflects enough sunlight to be visible at a distance of 33 million kilometres be lighter than air?
Sekanina’s paper suggests that `Oumuamua is a fluffy dust ball which formed in a disc around a young star and never reached sufficient size to be compacted by self-gravitation. This aggregate could have been the sole survivor of a parent interstellar comet which disintegrated as it passed close to the Sun prior to its discovery.
In October 2018 Shmuel Bialy and Abraham Loeb published “Could Solar Radiation Pressure Explain `Oumuamua’s Peculiar Acceleration?” which explored radiation pressure as the cause of the outward acceleration, reached the same conclusion about the need for the object to have very low density, and observed that a discarded light sail from an alien interstellar spacecraft would have the required density and reflectivity to account for the observations, and could survive the trip from its star system of origin to the solar system without destruction due to collisions with interstellar dust and gas, or disruption due to rotation or tidal forces.
Based on how quickly `Oumuamua was discovered after a survey began which had the ability to find such objects, astronomers estimate that several interstellar objects pass closer to the Sun than Earth’s orbit every year. Perhaps we’ll learn more when we have more than one exemplar of this class of objects to examine.