See Comet C/2015 ER61 PanSTARRS at its Best

ER61 PanSTARRS
ER61 PanSTARRS
Comet C/2015 ER61 PanSTARRS shortly after outburst on April 8th. Image credit and copyright: John Purvis.

Have you been following the springtime parade of bright comets? Thus far, the Oort cloud has offered up several fine binocular comets, including Comet 2/P Encke, 41/P Tuttle-Giacobini-Kresak, 45/P Honda-Mrkos-Pajdusakova, C/2016 U1 NEOWISE and C/2017 E4 Lovejoy. Now, another comet joins the dawn ranks, as it brightens up ahead of expectations: 2015 ER61 PanSTARRS.

Discovered on March 15th, 2015 by the prolific PanSTARRS-1 NEO survey atop Haleakala in Maui, Hawaii, Comet ER61 PanSTARRS made our who’s-who list of bright comets to watch for in 2017. The odd “ER61” designation stems from the early identification of the object as an asteroid, before it presented observers with a cometary appearance.

ER61 PanSTARRS Skychart
The path of Comet C/2015 ER61 PanSTARRS through the sky from early May through mid-August. Credit: Starry Night Education software.

Late northern hemisphere Spring through Summer sees the comet maintaining a decent elevation above the eastern horizon at dawn, gliding north and parallel to the ecliptic plane through the constellations Pisces, Aries and Taurus from May through mid-August. The comet passed 1.08 AU from the Earth last month on April 4th, and is now racing away from us. The comet’s location near the March equinoctial point on the celestial hemisphere assures an equally good apparition for both the northern and southern hemisphere. As seen from latitude 30 degrees north, the comet sits 30 degrees above the eastern horizon, through the remainder of May. Venus also makes a brilliant beacon to track down Comet ER61 PanSTARRS, as the planet heads towards greatest elongation 46 degrees west of the Sun on June 3rd.

The orbit of Comet ER61 PanSTARRS through the inner solar system. Credit: NASA/JPL.

The comet is also on a 7,591 year long orbit inbound, which takes it out nearly 2,500 AU from the Sun. That’s 190 times the Pluto-Sun distance, and the fourth most distant aphelion of any solar system object known. The 2015-2017 passage of the comet through the inner solar system actually shortened the orbit of Comet ER61 PanSTARRS down to an aphelion of ‘only’ 854 AU due to a 0.9 AU pass near Jupiter last year on March 28th, 2016. A similar orbital shortening by Jove occurred for Comet Hale-Bopp in 1996, which came in on an 4,200 year orbit and departed the inner solar system on a shorter 2,500 year path around the Sun.

The projected light curve for Comet C/2015 ER51 PanSTARRS. The purple line denotes perihelion, and the black dots are actual observations. Adapted from Seiichii Yoshida’s Weekly Information for Bright Comets.

Prospects and Prognostications

Observers reported an outburst from the comet last month in the first week of April, causing it to jump about 2 magnitudes in brightness. Right now, it’s holding steady at +7th magnitude. Unfortunately, the Moon reaches Full phase this week on May 10th, though you’ve still got a slim window to hunt for the comet after Moonset and before sunrise. Once the Moon moves towards a slender crescent phase next late week, we’ll once again have dark predawn skies ideal for comet hunting.

Here are some key dates for Comet C/2015 ER61 PanSTARRS as it glides through the dawn sky:

(Stars highlighted are brighter than +5th magnitude, and passes are less than a degree unless otherwise noted.)

May 10th: Reaches perihelion at 1.04 astronomical units (AU) from the Sun.

May 12th: Passes near the +4.9 magnitude star 19X Piscium.

May 20-23rd: Passes less than 10 degrees from Venus.

May 21st: The waning crescent Moon passes less than 10 degrees to the south.

June 10th: Passes near the +3.6 magnitude star Eta Piscium.

June 11th: Passes near the galaxy M74.

June 16th: Passes into the constellation Aries.

June 19th: The waning crescent Moon passes 9 degrees to the south.

July 13th: Passes near (less than 5′) the +4.6 magnitude star Epsilon Arietis.

July 18th: The waning crescent Moon passes 9 degrees to the south.

July 23rd: Passes near the +4.8 star Zeta Arietis.

The comet versus Venus in the dawn sky – looking eastward on May 15th. Credit: Stellarium.

August 2nd: Crosses into the constellation Taurus.

August 15th: The waning crescent Moon passes 8 degrees to the south.

August 16th: Passes near M45 (The Pleiades)

After mid-August, Comet 2015 ER61 PanSTARRS will drop back down below +10th magnitude, not to return for several millennia to come.

Observing a comet like ER61 PanSTARRS is as simple as knowing where and when to look, then starting to slowly sweep the suspect area with binoculars for a little fuzzball looking like a globular cluster stubbornly refusing to snap into focus. In pre-telescopic times, ER61 PanSTARRS would’ve entered and exited the inner solar system unrecorded.

April ER61 PanSTARRS
Comet C/2015 ER61 PanSTARRS from April 12th. Image credit and copyright: Joseph Brimacombe.

Next up: We’ve got one more predicted comet on tap for 2017, as C/2015 V2 Johnson brightens up to +7th magnitude in mid-June. Keep watching the skies, as the next great comet of the century could always appear unannounced at any time.

Get Ready for Sunday’s Close Flyby of Asteroid 2014 RC

Guess who’s dropping by for a quick visit this weekend? On Sunday, a 60-foot-wide (20-meters) asteroid named 2014 RC will skim just 25,000 miles (40,000 km) from Earth. That’s within spitting distance of all those geosynchronous communication and weather satellites orbiting at 22,300 miles. 

Size-wise, this one’s similar to the Chelyabinsk meteorite that exploded over Russia’s Ural Mountains region in February 2013. But it’s a lot less scary. 2014 RC will cleanly miss Earth this time around, and although it’s expected back in the future, no threatening passes have been identified. Whew!

2014 RC will pass along the outer edge of the geosynchronous satellite belt, home to many weather and communications satellites. The chance of a hit is close to infinitesimal. Click for more information and detailed finder charts. Credit: SatFlare
2014 RC will pass along the outer edge of the geosynchronous satellite belt, home to many weather and communications satellites. The chance of a hit is close to infinitesimal. Click for more information and detailed finder charts. Credit: SatFlare

NEOs or Near Earth Asteroids are defined as space rocks that come within about 28 million miles of Earth’s orbit. Nearly once a month astronomers discover an Earth-crossing asteroid that passes within the moon’s orbit.  In spite of hype and hoopla, none has threatened the planet. As of February 2014, we know of 10,619 near-Earth asteroids. It’s estimated that 93% of all NEOs larger than 1 km have been discovered but 99% of the estimated 1 million NEOs 100 feet (30-meters) still remain at large.

No surprise then that new ones pop up routinely in sky surveys. Take this past Sunday night for example, when the Catalina Sky Survey nabbed 2014 RA, a 20-foot (6-meter) space rock that whistled past Earth that evening at 33,500 miles (54,000 km). It’s now long gone.

Artist view of an asteroid (with companion) passing near Earth. Credit: P. Carril / ESA
Artist view of an asteroid (with companion) passing near Earth. Credit: P. Carril / ESA

2014 RC was picked up on or about September 1-2 by both the Catalina Sky Survey and Pan-STARRS 1 survey telescope atop Mt. Haleakala in Maui. The details are still being worked out as to which group will take final discovery credit. Based on current calculations, 2014 RC will pass closest to Earth around 2:15 p.m. EDT (18:15 UT) on Sunday, September 7th. When nearest, the asteroid is expected to brighten to magnitude +11.5 – too dim for naked eye observing but visible with a good map in 6-inch and larger telescopes.

Seeing it will take careful planning. Unlike a star or planet, this space rock will be faint and barreling across the sky at a high rate of speed. Discovered at magnitude +19, 2014 RC will brighten to magnitude +14 during the early morning hours of September 7th. Even experienced amateurs with beefy telescopes will find it a challenging object in southern Aquarius both because of low altitude and the unwelcome presence of a nearly full moon.


64-frame movie showing Toutatis tumbling through space only 4.3 million miles from Earth on Dec. 12-13. Credit: NASA/Goldstone radar

Closest approach happens in daylight for North and South America , but southern hemisphere observers might spot it with a 6-inch scope as a magnitude +11.5  “star” zipping across the constellations Pictor and Puppis. 2014 RC fades rapidly after its swing by Earth and will quickly become impossible to see in all amateur telescopes, though time exposure photography will keep the interloper in view for a few additional hours.

2014 RC accelerates across the sky from 4 a.m. to 4 p.m EDT in this path created by Gianluca Masi using SkyX Pro software and the latest positions from JPL.
2014 RC accelerates across the sky between 4 a.m. to 4 p.m EDT September 7 in this path created by Gianluca Masi using SkyX Pro software and the latest positions from JPL.

Most of us won’t have the opportunity to run outside and see the asteroid, but Gianluca Masi and his Virtual Telescope Project site will cover it live starting at 6 p.m. EDT (22:00 UT). Lance Benner, who researches radar imaging of near-Earth and main-belt asteroids, hopes to image 2014 RC with 230-foot (70-m) radar dish at the Goldstone complex on September 5-7 and possibly the big 1,000-foot (305-m) radar dish at Arecibo. Both provide images based on radar echoes that show asteroids up close with shapes, craters, ridges and all.