Five years ago, The Hubble Space Telescope spotted a large comet at the farthest distance ever, as it approached the sun from an exit between the orbits of Saturn and Uranus. Now this giant cosmic snowball is coming for its closest passage around the Earth in just a few weeks.
Comet C / 2017 K2 will be at its closest point to us current through the internal solar system on July 14. However, even in its closest part, it will still be farther from us than the average distance between Earth and Mars. This will probably make it difficult to see the comet without at least a small telescope, despite its significant size.
At this point, there is considerable uncertainty about how large the comet’s nucleus is, according to NASA’s solar system ambassador Eddie Irizari and Kelly Keeser Whit at EarthSky, with various observations suggesting a range between 11 and 100 miles (18 and 161 kilometers) wide. This means that C / 2017 is somewhere between quite legally large and among a handful of the largest comets ever discovered, such as Hale-Bopp and Bernardinelli-Bernstein.
The size of the comet’s tail, or coma, is just as massive and obscure. Early observations show that the trail of dust and gases behind the C / 2017 K2 is somewhere between 81,000 and 500,000 miles (130,000 and 800,000 kilometers). So, somewhere between the width of one and six Jupiters – this is a completely epic clue.
To see the comet for yourself, you can search for public online observatories such as the Virtual Telescope Project, which are sure to host viewing parties at some point. You can also take a telescope and start practicing spotting now, using an app like Stellarium, which will also be able to point your lens in the right direction as the comet approaches.
After passing us in July, C / 2017 K2 will continue to the perihelion, which is its closest passage past the sun, before heading back into deep space. Comets tend to behave unpredictably the closer they get to the sun. This may suddenly become more active and brighter, or it may disintegrate and disappear completely.
Whatever happens, this visit will probably be our only chance to meet this comet. Its orbit is so long that it will not return in a few million years.