Earth Crosses Comet Johnson Orbit, Tails Twist

Comet C/2015 V2 Johnson shows two tails in this striking image taken on May 26. At center is the comet’s head or coma. The gas tail sticks out to the lower left. The brighter dust tail points to the northwest. Like the gas tail, the dust tail has been narrowing in recent days. Credit: Chris Schur

Comet C/2015 V2 Johnson has been sliding south down the side of Boötes the Herdsman all month and slowly brightening. The “2015” in its name was the year of its discovery. On Nov. 3, 2015, astronomer J.A. Johnson, working with the Catalina Sky Survey, found the then-faint new comet in the constellation Lynx the Lynx not far from the bowl of the Big Dipper.

Comet Johnson (C/2015 V2) crosses from Boötes into Virgo from May through mid-June. Tick marks are shown every three days at 10 p.m. CDT (3:00 UT the following date) with stars to magnitude +7.5. On June 3–4, the comet passes just 5° east of Arcturus. Click for a larger map you can print out and use outside. Map: Bob King; Source: Chris Marriott’s SkyMap

The comet stayed faint until earlier this year when amateur astronomers picked it in more modest-sized telescopes. Johnson’s now visible in little more than a pair of binoculars. I had to bend my neck way back to see it, since it’s high up in the southern sky at nightfall not far from the brilliant, orange star, Arcturus. In 10x50s, I easily spotted a faint, fuzzy patch of light with a somewhat brighter center about ⅔ the size of the full moon from my home.

Comet Johnson shows the skinniest of gas tails (bottom) and a brighter dust tail in this photo taken May 28. North is up, west to the right. Credit: Gianluca Masi, Michael Schwartz

Tonight (May 28), Johnson’s located 10° north-northeast of Arcturus and glows at magnitude +7.5 or 8. As it continues moving south, the comet will brighten a bit more, peaking around magnitude +7-7.5 around mid-June. As you’ve already noticed, the moon has returned to the evening sky. While still just a crescent and not a threat to faint fuzzy things yet, see if you can spot the comet in the next few nights.

Through a 6-inch or larger telescope, Johnson will show a distinctly brighter head and fainter tail pointing toward the northwest. There’s a second tail also, but it’s a faint thread of a thing composed of carbon monoxide gas that fluoresces in UV light from the sun.

This is Comet Johnson on May 1. The two tails (dust top, gas bottom) are angled and the dust tail is broader and slightly curved. Both are now narrowing as Earth gets ready to cross the plane of the comet’s orbit. Credit: Damian Peach

It wasn’t long ago that Johnson’s brighter tail, composed of dust, was broad and fan-shaped and generally pointed opposite the sun, the way comet tails typically do. That’s now changing quickly. On May 31, Earth will pass through the plane of Comet Johnson’s orbit. For a short time, we stare straight across and through dust boiled off the comet’s core earlier this year which lies in the comet’s orbital plane. Since the dust is confined to the comet’s flat orbit, the fanned-out tail plus the earlier debris will morph into a narrower, ray-like tail. Meanwhile, our changing perspective causes the dust tail to appear to point toward the sun.

Comet C/2011 L4 PANSTARRS showed a spectacular anti-tail and a shorter, “normal” dust tail (to left) when Earth passed through the comet’s orbital plane on May 26, 2013. We’re looking straight across the comet’s orbit, so the dust left there earlier by the comet looks like a straight, narrow line. Kind of like looking at Saturn’s rings edge-on. Credit: Joseph Brimacombe

A sunward-pointing dust or gas tail is called an anti-tail. Johnson’s other tail, the skinny gas one, continues to point opposite the sun. But the closer we get to the comet’s orbital plane, the more the two tails will appear to diverge until they’ll be directly opposite each other on the 31st.

When Earth crosses a comet’s orbital plane (shown by the black orbit above), we see dusty debris trailing behind the comet edge-on. It piles up to form a bright, narrow tail that looks like it’s pointing toward the sun, the reason it’s called an anti-tail. The ion or gas tail always points away from the sun. Credit: Io Herodotus / CC BY-SA 3.0

Earth passed through the orbit of Comet C/2011 L4 PANSTARRS (above) in May 2013 — for a brief time, the comet looked just like a straight line drawn with chalk. To better picture an orbital crossing, recall that Saturn’s rings look like a straight line when we see them edge-on, ie. when Earth passes through their orbital plane.

Good luck in finding the comet the next few nights. If you’re using binoculars, look for a faint, fuzzy spot. Telescope users should see the anti-tail.