An old friend from winter returns for an encore in the morning sky. Already visible in binoculars, Comet PanSTARRS (C/2013 X1) may reach naked-eye visibility in June.
2016 hasn't been the best year for naked-eye comets. Not yet, anyway. Skywatchers have had to subsist on whatever morsels fate might send our way. Not that we've lacked in fluffy fuzzballs. There've been plenty to pick from, just few bright ones.
That's why 252P/LINEAR was literally a gift from above. This otherwise faint periodic comet underwent an outburst sometime after perihelion in March and brightened unexpectedly to magnitude +6 in April. A month later, it still remains luminous enough to see in 10×50 binoculars from dark skies.
As it fades, up-and-coming PanSTARRS (C/2013 X1) shows great promise as the next naked-eye comet. PanSTARRS X1 was discovered by the University of Hawai‘i's automated Pan-STARRS 1 (Panoramic Survey Telescope & Rapid Response System) on December 4, 2013. Back then it was a 20th-magnitude wisp. Many of us last encountered X1 in mid-February, as it crossed from Pegasus into Pisces while slowly sinking in the west. Glowing at 9th magnitude and wagging a short tail, it livened up shivery winter evenings.
Since passing perihelion on April 2nd, X1 has been slowly climbing back into the dawn sky. Because it was positioned along the Pisces–Aquarius border in negative declination territory, southern observers were the first to spot it later that month. Over the past week, an earlier rising combined with the comet's westward motion have finally pushed it high enough to clear the horizon at the start of morning twilight for mid-northern latitude observers.
On May 4th, Chris Wyatt in Walcha, New South Wales, Australia used 7×50 binoculars to spot X1 before dawn, describing it as magnitude +7.6 with a well-condensed, tail-less coma 8.7′ across. A Swan Band filter enhanced the view.
This coming week, skywatchers across the U.S. heartland (latitude 40° North) will find X1 about 10° degrees high at the cusp of dawn, low in the southeastern sky in Aquarius. Observers in the northern United States, southern Canada, and central Europe will have to reach a little deeper as the comet only climbs to an altitude of 5–7°. In the land of sweet tea and red-eye gravy, X1 stands 20° or higher before dawn and should be a snap to see with binoculars. The Moon won't be a bother until about May 18th. After that, the next dark-viewing window occurs from about June 1–15.
Plan your cometary rendezvous with care, so you can make the most of the brief window when the comet reaches maximum altitude before the light of dawn compromises the view.
I do this by taking the time of sunrise and subtracting two hours, the length of morning twilight at my latitude. That gives me the time when morning twilight begins — about 3:40 a.m. in mid-May. Since the sky is still quite dark at the start of dawn, I add in a fudge factor of about 20 minutes. That takes me to 4 a.m. Later than that and twilight might make a low-altitude object like X1 to difficult to see. Of course, the brighter the object the bigger your fudge factor can be. Click over to timeanddate.com or the United States Naval Observatory and enter your city to find sunrise, sunset, and twilight length for any day of the year.
Both low altitude and lengthening twilights conspire to make sighting X1 more challenging for all except those living in the southern United States and points south in the coming weeks. Thankfully, even as the comet dips south during most of its apparition, its earlier rising will help to "brake" its declining altitude. Observers in Europe and the northern United States should be able to follow X1 into early June; further south, it will remain visible throughout the summer.
Closest approach to Earth occurs on the first day of summer in the Northern Hemisphere when the comet will presumably peak in brightness around magnitude +6, right at the edge of naked-eye visibility. On June 29th, it begins to climb north again but very slowly.
I hope you're looking forward as much as I am to finding and following this latest visitor from the Oort Cloud. In December 2015, it experienced a major outburst and brightened to magnitude +9. Might the comet have more surprises in store?