Comet ISON Goes Green

As NASA and the European Space Agency prepare  their remote photojournalists – Mars Express, Mars Reconnaissance Orbiter and the Curiosity and Opportunity rovers – to capture photos of Comet ISON’s flyby of Mars early next week, amateur astronomers continue to monitor and photograph the comet from backyard observatories across the blue Earth. Several recent color photos show ISON’s bright head or nucleus at the center of  a puffy, green coma. Green’s a good omen – a sign the comet’s getting more active as it enters the realm of the inner solar system and sun’s embrace.

Another  photo of a "greening" Comet ISON taken on Sept. 24 with a 17-inch (43-cm) telescope. Click to enlarge. Credit: Damian Peach
Another great photo of the “greening” of Comet ISON taken on Sept. 24 with a 17-inch (43-cm) telescope. Click to enlarge. Credit: Damian Peach

Sunlight beating down on the comet’s nucleus (core) vaporizes dust-impregnated ice to form a cloud or coma, a temporary atmosphere of water vapor, dust, carbon dioxide, ammonia and other gases. Once liberated , the tenuous haze of comet stuff rapidly expands into a huge spherical cloud centered on the nucleus. Comas are typically hundreds of thousands of miles across but are so rarified you could wave your hand through one and not feel a thing. The Great Comet of 1811 sported one some 864,000 miles (1.4 million km) across, nearly the same diameter as the sun!

Among the materials released by solar heating are cyanogen and diatomic carbon. Both are colorless gases that fluoresce a delicious candy-apple green when excited by energetic ultraviolet light in sunlight.

Sounds like a plan. Newspaper clipping from 1910.
Sounds like a plan. Newspaper clipping from 1910.

Cyanogen smells pleasantly of almonds, but it’s a poisonous gas composed of one atom each of carbon and nitrogen. Diatomic carbon or C2 is equally unpleasant. It’s a strong, corrosive acid found not only in comets but also created as a vapor in high-energy electric arcs. But nature has a way of taking the most unlikely things and fashioning them into something beautiful. If you’re concerned about the effects of cometary gas and dust on people, rest easy. They’re spread too thinly to touch us here on Earth. That didn’t stop swindlers from selling “comet pills” and gas masks to protect the public from poisoning during the 1910 return of Halley’s Comet. Earth passed through the tail for six hours on May 19 that year. Amazingly, those who took the pills survived … as did everyone else.

Comet ISON's location and approximate appearance on October 1 seen from the Curiosity Rover. ISON will pass only 6.7 million miles (10.8 million km) from Mars on Tuesday Oct. 1. Stellarium
Comet ISON’s location and approximate appearance on October 1 seen from the Curiosity Rover. ISON will pass only 6.7 million miles (10.8 million km) from Mars on Tuesday Oct. 1. Stellarium

While Comet ISON is still too faint for visual observers to discern its Caribbean glow, that will change as it beelines for the sun and brightens. If you could somehow wish yourself to Mars in the next few days, I suspect you’d easily see the green coma through a telescope. The comet – a naked eye object at magnitude 2.5-3 – glows low in the northern sky from the Curiosity rover’s vantage point 4.5 degrees south of the Martian equator.

Comet Hale-Bopp shows off its whitish dust tail and fainter, blue ion tail in early 1997. Credit: Bob King
Comet Hale-Bopp shows off its bright dust tail and fainter, blue ion tail in early spring 1997. Credit: Bob King

I’ve noticed that when a comet reaches about 7th magnitude, the green coloration becomes apparent in 8-inch (20 cm) and larger telescopes. Bright naked eye comets often display multiple subtle colors that change chameleon-like over time. Dust tails, formed when sunlight pushes dust particles downwind from the coma, glow pale yellow. Gusty solar winds sweep back molecules from the coma into a second “ion” tail that glows pale blue from jazzed up carbon monoxide ions fluorescing in solar UV.

The highlight of seeing the comet through the telescope was its brilliant, pea-like false nucleus glowing yellow from sunlit dust. The real comet nucleus – the actual comet – lies within the false nucleus and shrouded by dust. Drawing: Bob King
One of the highlights of seeing Comet L4 PANSTARRS through a small telescope was its brilliant, pea-like false nucleus glowing yellow from sunlit dust. The real comet nucleus – the actual comet – lies within the false nucleus and hidden by dust. Drawing: Bob King

During close encounters with the sun, millions of pounds of  dust per day boil off a comet’s nucleus, forming a small, intensely bright, yellow-orange disk in the center of the coma called a false nucleus. Earlier this year, when Comet C/2011 L4 PANSTARRS emerged into the evening sky after perihelion, not only was its yellow tail apparent to binocular users but the brilliant false nucleus glowed a lovely shade of lemon in small telescopes.

With ISON diving much closer to the sun than L4 PANSTARRS, expect a full color palette in the coming weeks. While it may not be easy being green for Sesame Street’s Kermit the Frog, comets do it with aplomb.

NASA’s Mighty Eagle Takes Flight; Finds Its Target

No, it’s not a UFO — it’s NASA’s “Mighty Eagle”, a robotic prototype lander that successfully and autonomously found its target during a 32-second free flight test at Marshall Space Flight Center yesterday, August 16.

You have to admit though, Mighty Eagle does bear a resemblance to classic B-movie sci-fi spacecraft (if, at only 4 feet tall, markedly less threatening to the general populace.)

Fueled by 90% pure hydrogen peroxide, Mighty Eagle is a low-cost “green” spacecraft designed to operate autonomously during future space exploration missions. It uses its onboard camera and computer to determine the safest route to a pre-determined landing spot.

During the August 16 test flight, Mighty Eagle ascended to 30 feet, identified a target painted on the ground 21 feet away, flew to that position and landed safely — all without being controlled directly.

“This is huge. We met our primary objective of this test series — getting the vehicle to seek and find its target autonomously with high precision,” said Mike Hannan, controls engineer at Marshall Space Flight Center. “We’re not directing the vehicle from the control room. Our software is driving the vehicle to think for itself now. From here, we’ll test the robustness of the software to fly higher and descend faster, expecting the lander to continue to seek and find the target.”

In the wake of a dramatically unsuccessful free flight test of the Morpheus craft on August 9, another green lander designed by Johnson Space Center, the recent achievements by the Mighty Eagle team are encouraging.

Here’s a video from a previous test flight on August 8:

Future tests planned through September will have the lander ascend up to 100 feet before landing. Read more here.

The Mighty Eagle prototype lander was developed by the Marshall Center and Johns Hopkins University Applied Physics Laboratory in Laurel, Md., for NASA’s Planetary Sciences Division, Headquarters Science Mission Directorate Image/video: NASA/Marshall Space Flight Center