‘Oumuamua, the interstellar object racing through the solar system, is now believed to be a comet rather than an asteroid.
First discovered in October 2017, ‘Oumuamua has been observed by both the Hubble Space Telescope (HST) and by ground-based telescopes moving faster through the solar system than scientists had predicted.
While its speed is being slowed by the Sun’s gravitational pull, this is occurring at a rate lower than that expected by calculations of celestial mechanics.
Marco Micheli of the European Space Agency (ESA), leader of a study on ‘Oumuamua’s movements published in the journal Nature, attributes its faster-than-expected movement to the object outgassing or venting material from its surface as a result of being heated by the Sun. He theorizes the small amount of ejected material is giving the object a slight push in its trajectory out of the solar system.
‘Oumuamua was traveling at a speed of 70,836 miles (114,000 km) per hour on June 1.
Outgassing is a feature of comets, not asteroids, and involves the ejection of gas and dust that form a coma or cloud around the objects as a result of solar heating.
In ‘Oumuamua’s case, scientists did not actually see development of a coma or tail but suspect the object is venting large, coarse dust grains, possibly due to its smaller dust grains having eroded during its journey through interstellar space. Hubble cannot detect clouds of large dust grains because they are not very bright.
“We think this is a tiny, weird comet. We can see in the data that its boost is getting smaller the farther away it travels from the Sun, which is typical for comets,” Micheli said.
When ‘Oumuamua was first detected last year, scientists were uncertain as to whether it should be classed as an asteroid or a comet.
The researchers’ initial reason for observing the object with Hubble was an effort to trace its path back to the star system from which it originated. Its unexpected increase in speed makes that a more difficult task, meaning scientists may never know its true origin.