Get ready for another round of doomsday predictions. A comet en-route to our solar system has dramatically increased in brightness and carries with it a giant, glowing green atmosphere twice the size of Jupiter, according to astronomers. The comet, PANSTARRS (C/2017 S3), is visible with low power telescopes now, but if it does continue to increase in brightness it may be visible to the naked eye in parts of the northern hemisphere in early August. It might also disintegrate due to the gas cloud, and as it's the first time the comet has passed through our solar system, there's no telling what might happen when the comet's visit brings it inside Mercury's orbit and it takes a face-full of unfiltered sunshine.
Writing in Sky and Telescope, astronomer Bob King describes the comet's extreme increase in brightness:
Comet PanSTARRS (C/2017 S3), which had been chillin' at magnitude 12.5, experienced an outburst some time on June 30th, rocketing to magnitude ~9.5 overnight. Located in Camelopardalis northeast of the Double Cluster, it was super obvious in the 15-inch [telescope] at low power on July 2.18 UT, appearing as a dense, tailless cotton ball about 3′ across. It looked suspiciously "gassy," so I applied the Swan Band filter and both the brightness and size of the coma increased, a sure sign of a gas blast from the outburst.
Comet C/2017 S3 may reach magnitude 3 or 4 as it speeds sun-ward in the morning sky en route to an August 16th perihelion. Keep an eye on it...and expect the unexpected. The outburst could continue or even trigger the comet's disintegration.
A gas outburst in this case refers to a crack or other sudden leak in a comet's ice surface, which releases gas previously trapped in pressurized caverns. This gas reflects light and a continuing outburst may either increase the comet's brightness to visible levels, inevitably sparking fearful YouTube videos and a non-trivial amount of references to Nibiru or it may just disintegrate before we get a chance to see it. Easy come, easy go.
Even if it never reaches brightness visible to the naked eye, you'll still be able to see it with binoculars, according to Bob King:
As it scurries east through Auriga and Gemini C/2017 S3 quickly becomes a binocular object at magnitude 7. Even as it brightens rapidly, possibly reaching 6th magnitude on August 7th on its way to perihelion (August 16th), dawn encroaches. We'll probably get our last views when it passes Castor and Pollux on August 4–5 before twilight and daylight finish it off. Thereafter the comet remains near the Sun and rapidly fades from view.
If you're interested in trying to observe the comet, there's more information on how to do so and where in the world the comet will be visible from at Sky and Telescope.