The interstellar object ‘Oumuamua was discovered back on Oct. 19, 2017, but the puzzle of its true nature has taken months to unravel and may never be fully solved.
Rewinding to last fall, a team of astronomers led by Karen Meech from the University of Hawaii used Canada-France-Hawaii Telescope (CFHT), United Kingdom Infra-Red Telescope (UKIRT), the W.M. Keck Observatory, Gemini South and European Southern Observatory’s (ESA) Very Large Telescope to observe a unique objected discovered by Pan-STARRS1 on Maui. Meaning “scout from the distant past” in Hawaiian, ‘Oumuamua was originally classified as an interstellar asteroid.
The asteroid classification lead to some consternation on the part of planetary scientists. Current understanding of planetary formation predicts many more interstellar comets than interstellar asteroids. ‘Oumuamua did not show evidence of gas emission or a dusty environment in the original observations. Without these cometary signatures it was deemed an asteroid.
Following the initial discovery observations with Pan-STARRS, a team of astronomers led by Marco Micheli of ESA’s SSA-NEO Coordination Centre and Karen Meech of the University of Hawaii Institute for Astronomy continued to make high precision measurements of the object and its position using many ground-based facilities like CFHT, as well as the Hubble Space Telescope. The final images were taken with Hubble in January, before the object became too faint to observe as it sped away on its outbound orbit.
However, as Meech and her team continued to monitor ‘Oumuamua, observations began to immerge that contradicted the asteroid label.
The first clue: its trajectory. Extensive follow-up observations by CFHT, ESA’s Optical Ground Station telescope in Tenerife, Canary Islands and other telescopes around the world have helped pin it down.
‘Oumuamua was first spotted about a month after its closest approach to the sun, which took it within the orbit of Mercury. Unlike any asteroid or comet observed before, this new object sped past the sun, approaching from “above” the plane of the planets on a highly inclined orbit, moving fast enough — 70,800 miles per hour as of July 1 — to escape the sun’s gravitational pull and eventually depart our solar system.
Contrary to their expectations, the team found that the object was not following the anticipated trajectory if only the gravity of the sun and the planets were determining its path.
“Unexpectedly, we found that ‘Oumuamua was not slowing down as much as it should have due to just gravitational forces,” Micheli said, lead author of the paper reporting the team’s findings, published in the journal, “Nature.”
What could be causing this curious behavior? Rigorous analysis ruled out a range of possible influences, such as radiation pressure or thermal effects from the sun, or interaction with the sun’s solar wind. Other less likely scenarios, such as a collision with another body or ‘Oumuamua being two separate loosely held-together objects, were also discarded.
Comets contain ices that sublimate, or turn directly from a solid to a gas when warmed by the sun. This process drags out dust from the comet’s surface to create a fuzzy “atmosphere” and sometimes a tail. The release of gas pressure at different locations and times can have the effect of pushing the comet slightly off course compared with the expected path if only gravitational forces were at play. The team has not detected any dusty material or chemical signatures that would typically characterize a comet, even in the deepest images taken.
“Oumuamua is small — no more than a half a mile long — and it could have been releasing a small amount of relatively large dust for it to have escaped detection,” Meech said. “To really understand ‘Oumuamua we would need to send a space probe to it. This is actually possible, but it would be very expensive and take a long time to get there so it isn’t practical this time. We just have to be ready for the next one.”
Because of its small size and faintness, current observations of ‘Oumuamua do not provide all the information astronomers need to determine important aspects of the comet’s surface.
“When ‘Oumuamua was discovered, the astronomy community gathered as much data as possible but ultimately the object was just not visible long enough to answer all our questions,” Ken Chambers from PanSTARRS said. “With PanSTARRS monitoring the skies, we hope to discover more ‘Oumuamua-like objects in the future and begin to answer the really interesting questions about this class of objects.”
