Is Oumuamua an ET Solar Sail?
Matt Williams, 02/11/2019
On October 19th, 2017, the Panoramic Survey Telescope and Rapid Response System-1 (Pan-STARRS-1) in Hawaii announced the first-ever detection of an interstellar asteroid, named 1I/2017 U1 (aka. ‘Oumuamua).
In the months that followed, multiple follow-up observations were conducted that allowed astronomers to get a better idea of its size and shape, while also revealing that it had the characteristics of both a comet and an asteroid.
Interestingly enough, there has also been some speculation that based on its shape, ‘Oumuamua might actually be an interstellar spacecraft (Breakthrough Listen even monitored it for signs of radio signals!). A new study by a pair of astronomers from the Harvard Smithsonian Center for Astrophysics (CfA) has taken it a step further, suggesting that ‘Oumuamua may actually be a light sail of extra-terrestrial origin.
The study – “Could Solar Radiation Pressure Explain ‘Oumuamua’s Peculiar Acceleration?”, which recently appeared online – was conducted by Shmuel Bialy and Prof. Abraham Loeb. Whereas Bialy is a postdoctoral researcher at the CfA’s Institute for Theory and Computation (ITC), Prof. Loeb is the director of the ITC, the Frank B. Baird Jr. Professor of Science at Harvard University, and the head chair of the Breakthrough Starshot Advisory Committee.
To recap, ‘Oumuamua was first spotted by the Pan-STARRS-1 survey 40 days after it made its closest pass to the Sun (on September 9th, 2017). At this point, it was about 0.25 AU from the Sun (one-quarter the distance between Earth and the Sun), and already on its way out of the Solar System. In that time, astronomers noted that it appeared to have a high density (indicative of a rocky and metallic composition) and that it was spinning rapidly.
While it did not show any signs of outgassing as it passed close to our Sun (which would have indicated that it was a comet), a research team was able to obtain spectra that indicated that ‘Oumuamua was more icy than previously thought. Then, as it began to leave the Solar System, the Hubble Space Telescope snapped some final images of ‘Oumuamua that revealed some unexpected behavior.
After examining the images, another international research team discovered that ‘Oumuamua had increased in velocity, rather than slowing down as expected. The most likely explanation, they claimed, was that ‘Oumuamua was venting material from its surface due to solar heating (aka. outgassing). The release of this material, which is consistent with how a comet behaves, would give ‘Oumuamua the steady push it needed to achieve this boost in velocity.
To this, Bialy and Loeb offer a counter-explanation. If ‘Oumuamua were in fact a comet, why then did it not experience outgassing when it was closest to our Sun? In addition, they cite other research that showed that if outgassing were responsible for the acceleration, it would have also caused a rapid evolution in ‘Oumuamua’s spin (which was not observed).
Basically, Bialy and Loeb consider the possibility that ‘Oumuamua could in fact be a light sail, a form of spacecraft that relies on radiation pressure to generate propulsion – similar to what Breakthrough Starshot is working on. Similar to what is planned for Starshot, this light sail may have been sent from another civilization to study our Solar System and look for signs of life.