On October 25, 2017, the first discovery of an interstellar object (ISO), 1I/’Oumuamua, was announced to the world after it was detected by the Pan-STARRS telescope in Maui, Hawaii. Initially, the object was thought to be a comet from the far reaches of the solar system in a region known as the Oort Cloud, where objects have extreme orbits that take them careening through the inner solar system at speeds exceeding 60 km/s. Astronomers soon realized that 1I/’Oumuamua could not be an ordinary comet; its speed was too high to be bound to our solar system. As 1I/’Oumuamua reached its closest approach to our Sun, it was moving at 87.7 km/s, about 4.2 km/s too fast for it to be bound to our solar system. Additional observations also showed that it was not cometary, but instead appeared more like an asteroid.

Soon after the Minor Planet Center’s official announcement of 1I, a group of astronomers at the University of Washington — Bryce Bolin (also at B612 Asteroid Institute), Lynne Jones, Daniela Huppenkothen, Joachim Moeyens, Mario Jurić, Željko Ivezić and Andrew Connolly — teamed up with researchers Hal Weaver and Carey Lisse at Johns Hopkins University Applied Physics Laboratory and Yan Fernandez from the University of Central Florida, and rapidly obtained observations at the Apache Point 3.5 m telescope in Sunspot, New Mexico. 

The team observed 1I in the New Mexico desert skies, imaging it in three different color filters and obtaining measurements covering a 4-hour lightcurve. Halfway through the observations, the brightness of 1I dramatically increased by 5 times and the object become relatively easy to spot in individual images.  After combining the data from APO with observations by other teams, and applying advanced statistical methods, the team found that 1I had a likely rotation period of ~8.1 hours and an unusually high aspect ratio ~6:1. This aspect ratio revealed that the object is shaped like a fingerling potato. In addition, photometric color measurements implied that 1I has surface similar to primitive C and D type asteroids from the asteroid Main Belt and Jupiter Trojan swarms. These results are in agreement with several independent studies of 1I by groups at the University of Hawaii, University of California, Los Angeles and Queen’s University Belfast. The UW results have been accepted to the Astrophysical Journal Letters (Bolin et al., “APO Time Resolved Color Photometry of Highly-Elongated Interstellar Object 1I/’Oumuamua”), with a preprint available at https://arxiv.org/abs/1711.04927.

“Everyone was quite enthusiastic about this so-far unique opportunity to observe an interstellar asteroid. Nothing like this had been seen before except in the realms of science fiction!” said Bolin, the study’s lead author, B612 Asteroid Institute researcher and a DIRAC Postdoctoral Fellow.  “It was extremely exciting to be part of this effort to learn more about 1I/’Oumuamua before it faded from view. Having a resource like the Apache Point telescope available on short notice made a huge difference in what we could contribute,” added Lynne Jones, an asteroid expert at UW/DIRAC who analyzed ‘Oumuamua’s colors.

Advanced statistical methods played a role in constraining the period and shape with only a short observational baseline. “It was a great proof of concept how different groups within the new DIRAC Institute can work together in new and unexpected ways! I normally work on black holes, but it turns out many of the statistical methods I think about in that context were really useful here.”, said Huppenkothen. “To get to work on asteroids for a change was a new and exciting challenge for me, and exactly the kind of interdisciplinary work we are hoping to foster at DIRAC.“

In addition to publishing the results, the DIRAC team made all of their raw data and analysis notebooks publicly available. “We are strong believers in making it possible for other researchers to verify, re-use, and build upon our work. Transparency, open data, and open science are one of the core tenets of both DIRAC and the Asteroid Institute.” said Professor Mario Juric, Bolin’s postdoctoral advisor at the University of Washington.

We wish to acknowledge the support of the B612 Asteroid Institute, the Charles Simonyi Fund for Arts and Sciences, and the Washington Research Foundation in making this research possible.