One of the last spacecraft photos of Comet ISON. It was taken with NASA’s STEREO-A probe on Dec. 6, 2013. This image was compiled using 11 photos stacked atop each other to improve the comet’s visibility. Click to enlarge. Credit: NASA / Toni Scarmato
Comet ISON has left the eyes of spacecraft for the moment and now challenges amateur and professional astronomers from the ground. To date, there has been only one positive observation by an amateur astronomer in Spain and a couple “maybes”. Many have tried to see and photograph the comet’s faint remains, but none have been successful.
The sky facing east 1 1/2 to 1 3/4 hours before sunrise for mid-northern latitude skywatchers. The comet’s position is shown daily and marked every 3 days. Stars plotted to mag. 6. Guide stars are labeled: Oph = Ophiuchus, Her = Hercules, Ser = Serpens and CrB = Corona Borealis. Click for a large version. Created with Chris Marriott’s SkyMap software
Tomorrow morning, ISON climbs to 15 degrees altitude in the morning sky before the onset of twilight. That’s high enough for someone with a fast telephoto lens or fast, wide-field telescope to make a long time exposure without haze and dawn interfering. Photos taken with typical narrower fields of view through telescopes haven’t shown the faintest trace of a nucleus or condensation at the location of ISON’s core. Wide fields might still succeed.
Frame grab from solarsystemcope.com/ison shows Comet ISON (now a debris cloud) later this month when making its closest approach to Earth of 40 million miles. It’s orbit is inclined 62 degrees to the horizontal, taking it high above Earth’s plane.
I’ve been asked whether Earth will get dusted by ISON’s dusty cloud of debris as it passes our planet on the outbound leg of its journey. The answer is almost certainly “no”. There are several reasons why. First, the debris passes far above the Earth’s orbit even at its closest approach on Dec. 26, when what’s left of the comet will be nearly directly above our planet and 40 million miles away. That’s farther than Venus and even farther than Mars during its closest approaches. I’ve read catastrophic talk ISON raining hell fire on Earth. Not gonna happen. Not even meteor fire – at least from the breakup.
Second, while the debris cloud will certainly expand and enlarge, the comet leftovers will continue to travel along the same general path as ISON. They won’t suddenly veer off and make a beeline for Earth. They carry much of the original momentum and direction as the comet that created them.
Comet ISON photographed from the International Space Station on Nov. 23. You can see the twilight glow along the Earth’s limb at bottom and part of the spacecraft in the foreground. Credit: NASA
We also have to consider that as the cloud continues to expand it will rapidly thin. While it’s true a comet’s coma (not the “hard” inner nucleus) can expand to the size of the sun and tails can reach 300 million miles (500 million km) or longer, the amount of material involved spread over those distances is vanishingly small. We’ve passed through at least one comet’s tail (Halley in 1910) with no meteor shower or other ill effects to show for it.
Comets can be powder puffs though, that’s for sure. Even as long ago as January, when ISON was at Jupiter’s distance from the sun, NASA’s Swift spacecraft found it spewing dust at 112,000 lbs. a minute. While our planet’s highly unlikely to get an outbound meteor shower, we may encounter some of ISON’s inbound flotsam and jetsam come mid- January.
Illustration showing Earth encountering Comet ISON dust in mid-January 2014. Credit and copyright: Paul Wiegert
Meteor researcher Paul Wiegert of the University of Western Ontario has been using a computer to model the trajectory of dust ejected by Comet ISON and predicts that starting about January 12 and continuing for several evenings, we stand a chance of a meteor shower from material released well before perihelion.
The dust particles will strike Earth’s atmosphere at around 125,000 mph (56 km/sec), but because they’ll be so tiny, it’s unlikely we’ll actually see anything.
Illustration showing Earth passing through dual debris streams left in the path of Comet ISON in January. Credit: Paul Wiegert / NASA
“Instead of burning up in a flash of light, they will drift gently down to the Earth below,” said Wiegert. In a fascinating twist, Earth will encounter not one but two dust streams from Comet ISON. Dust released by the comet and headed in toward the sun will pepper one side of Earth, while a second stream, blown back from the comet’s former head by sunlight, will pelt the other side.
ISON dust settling into the upper atmosphere may even serve as sites for water vapor to condense and form high-altitude, blue-colored noctilucent clouds.
As we approach the potential shower date, I’ll provide additional information. A possible radiant for the shower is in the Bootes-Draco part of the sky, which in January rises in the northeast not long after midnight. Sure, we may see nothing, but wouldn’t it be cool if ISON made its final appearance as daggers of light right here so close to home?