The asteroid Oumuamua ("scout from the distant past" in Hawaiian) was discovered on October 19, 2017 by astronomers at thr Pan-STARRS1 survey when it came close to Earth's orbit, within the orbit of Mercury, about a month after its closest approach to the Sun It was called an asteroid - but it may be a comet.

Why the confusion? There are more data about its trajectory. Oumuamua was unlike any asteroid or comet observed before. It sped past the Sun, approaching from "above" the plabe of the planets on a highly inclined orbit, moving fast enough (70,800 miles per hour as of July 1, 2018) to escape the Sun's gravitational pull and eventually depart our Solar System.

So some astronomers wanted to label Oumuamua a comet because current understanding of planet formation predicts more interstellar comets than interstellar asteroids. But there was evidence of gas emission or a dusty environment in the observations, which made it the first interstellar asteroid. The debate was not settled. It never could be, when 2 percent of astronomers can decide Pluto is not even a planet.

Extensive follow-up observations by the Canada-France-Hawai?i Telescope (CFHT), the European Space Agency's (ESA) Optical Ground Station telescope in Tenerife, Canary Islands, and other telescopes around the world provided another surprising twist.



Following the initial discovery observations with Pan-STARRS, astronomers 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.

They 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," said Marco Micheli of ESA's SSA-NEO Coordination Centre and lead author of the paper reporting the team's findings in a statement. 

What could be causing this curious behavior?

They 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 from ESO''s Very Large Telescope, Hubble, and the Gemini South telescope. 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.

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.