Some small objects in space appear to fire jets of gas that can鈥檛 be seen with telescopes, suggesting they should be classed in a new category between asteroids and comets.
Comets periodically emit bursts of gas and dust as they approach the sun, which melts their icy surfaces. These events give off as much as 10 kilograms of dust per second, which reflects sunlight and makes the bursts visible, often contributing to a tail behind the comet. Asteroids, being rockier, don鈥檛 produce such bursts.
However, at NASA鈥檚 Jet Propulsion Laboratory in California and his colleagues think they have found a handful of objects that look like asteroids at first glance, but are also firing out jets. As they orbit the sun, the objects were seen to periodically undergo jumps of speed that couldn鈥檛 be attributed to other factors. 鈥淭here was a clear acceleration,鈥� says Farnocchia.
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It is thought that the objects, which are mostly no more than a few tens of metres across and orbit near Earth, were firing out comet-like jets but with much lower dust rates, perhaps only about 100 micrograms per second. As such, the jets are invisible to telescopes and only detectable by the acceleration they cause to the objects. The team dub the objects 鈥渄ark comets鈥�.
鈥淭he reason we haven鈥檛 found these before is they鈥檙e very small objects and we needed a lot of observations for these accelerations to become significant,鈥� says Aster Taylor at the University of Chicago, who was part of the study team. 鈥淵ou have to give it months to years to see the acceleration.鈥�
at Queen鈥檚 University Belfast, UK, says the results are 鈥減otentially really interesting鈥�, hinting at a class of inert comets that had gone undetected before. 鈥淲e鈥檝e never really known how low activity can get,鈥� he says. 鈥淭here could be a population of weakly active comets hidden [in the solar system].鈥�
The work was spurred by 鈥极耻尘耻补尘耻补, an object from another solar system seen passing our sun in 2017. Initially, researchers thought 鈥極umuamua was an asteroid, before it exhibited a comet-like burst of acceleration as it passed our sun. These dark comets might be 鈥渟imilar to what happened with 鈥極umuamua鈥�, says Farnocchia.
One of the objects identified by the team, a 30-metre-wide asteroid called 1998 KY26, is set to be visited by a Japanese spacecraft in 2031. That could confirm or refute the team鈥檚 ideas. 鈥淲e might be able to test the predictions,鈥� says Farnocchia. 鈥淢aybe we have to revisit the definition of comets.鈥�
The Planetary Science Journal
