Sun-grazing comet visible in November
The brightest comet in 50 years is likely to be visible from Earth in late November after it makes a "sweltering passage" around the Sun.
Astronomers from the University of Maryland at College Park and the Lowell Observatory have used images obtained from the National Aeronautics and Space Administration's Swift satellite to observe comet C/2012 S1 (ISON) over the past two months and think the comet is making its first trip through the inner solar system.
Nasa said ISON was an object of special interest because of a close encounter it would have with Mars, followed by a "scorching close" approach to the Sun.
Astronomer and Swift team member Michael Kelley said ISON may be observable to Nasa spacecraft working on Mars about October 1 this year, when it would pass about 10.8 million kilometres from the planet.
"Personally, I'm hoping we'll see a dramatic postcard image taken by Nasa's latest Mars explorer, the Curiosity rover," he said.
On November 28, ISON will approach within about 1.2 million km of the Sun's visible surface, which classifies ISON as a Sun-grazing comet.
Nasa said Sun-grazing comets often shed large fragments or even completely disrupted after close encounters with the Sun, but for ISON, neither fate was a forgone conclusion.
At this point, the comet may become bright enough to see just by holding up a hand to block the Sun's glare.
Comet ISON would then move towards Earth and appear in twilight through December.
On December 26, it would come within 64.2 million km of Earth, or about 167 times farther than the Moon.
Lead investigator and astronomer Dennis Bodewits said comet ISON provided a "rare opportunity" to observe its changes in great detail over an extended period.
"[It] has the potential to be among the brightest comets of the last 50 years," he said.
Astronomer Matthew Knight said past comets had failed to live up to expectations once they reached the inner solar system.
"[ISON] looks promising, but that's all we can say for sure now," he said.
"Only observations over the next few months will improve our knowledge of how ISON will perform."
Comet ISON is a clump of frozen gases mixed with dust, which are emitted when the comet ventures near enough to the Sun that the icy material transforms from a solid to gas.
Scientists have estimated that comet ISON's nucleus is 5km across, a typical size for a comet, Nasa says.
The comet was discovered on September 21, 2012, by Russian astronomers Vitali Nevski and Artyom Novichonok using an International Scientific Optical Network telescope located near Kislovodsk.