Two massive space objects are heading towards earth, but Nasa does not know whether one of them is a comet or an asteroid.
The space agency has identified the other object as a comet.
Nasa discovered the comet C/2016 in October 2016, and it was expected to fly close to earth this week, as per the website of the agency.
The mystery object, named 2016 WF9, was detected by Nasa's asteroid- and comet-hunting Neowise project on 27 November 2016.
According to scientists, it was relatively large, roughly 500 metres to a kilometre across, and was rather dark. However they had not been able to determine whether it was a comet or an asteroid.
According to James Bauer, deputy principal investigator at Nasa's Jet Propulsion Lab, 2016 WF9 "could have cometary origins.''
"This object illustrates that the boundary between asteroids and comets is a blurry one; perhaps over time this object has lost the majority of the volatiles that linger on or just under its surface," he qouted from a www.stuff.co.nz report.
It was expected to approach earth's orbit in February 2017 at a distance of 51 million km from earth. According to Nasa, the object was "not a threat to Earth for the foreseeable future".
Its body resembles a comet in its reflectivity and orbit, but appeared to lack the characteristic dust and gas cloud that defined a comet.
Meanwhile, the comet, C/2016 U1 NEOWISE, 'has a good chance of becoming visible through a good pair of binoculars, although we can't be sure because a comet's brightness is notoriously unpredictable,' said Paul Chodas, manager of Nasa's Center for Near-Earth Object (NEO) Studies at the Jet Propulsion Laboratory in Pasadena, California.
The comet C/2016 would likely be seen in the southeastern sky shortly before dawn.
The comet is moving further south each day and it would reach its closest point to the sun, inside the orbit of Mercury, on 14 January, before heading back out to the outer reaches of the solar system.