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SYFY WIRE Bad Astronomy

Comet ISON: Still Alive

By Phil Plait

Comet C/2012 S1 (ISON) is looking good for its rendezvous with the Sun tomorrow!

[UPDATE (Nov. 28 at 17:00 UTC): ISON has dimmed since last night, but is still bright and beautiful in SOHO Images. I've posted an update with new pictures.]

The picture above is a combination of three different views from space-based solar observatories taken at 16:45 UTC (11:45 EST) today. Iâll explain, but the important thing to note is that ISON looks intact, despite earlier worries, and is poised to put on quite  a show.

In the picture, the inner (orangish) circle is the disk of the Sun seen by the Solar Dynamics Observatory, observed in the far ultraviolet. Itâs there to give you a sense of scale; mind you, the Sun is about 1.4 million km (860,000 miles) across.

The red ring is from the NASA/ESA Solar and Heliospheric Observatory, or SOHO, showing the space directly around the Sun. The streamers are from the solar wind, subatomic particles flung out by the Sunâs fierce magnetic field. The blue is also from SOHO, but shows a much larger area around the Sun (the black line to the upper right is an arm that holds a disk of metal used to block out the direct light from the Sun, allowing fainter objects to be seen).

ISON is visible there in the lower right, still marching toward the Sun. You can clearly see two tails; the dusty tail spreading out as pressure from sunlight pushes back on it, and the sharper, narrower ion tail blown back by the solar wind.

The spike in front of the comet is not real, but due to the comet head being overexposed in the image. Digital detectors work by converting light hitting them into electrons. If the object is too bright, the electrons overflow into nearby pixels, constrained by the rows and columns on the pixel array. This effect is called blooming.

The fact that ISON is blooming means itâs getting very bright, probably around magnitude 0.5, about as bright as Mercury as seen from Earth. This is expected, and a good sign! It means gas is pouring off the solid nucleus of the comet and being illuminated by the intense light of the Sun. If it manages to hold itself together, ISON will get even brighter over the next day, and the pictures from SOHO will be spectacular.

Donât forget: Iâll be participating in a NASA Google+ Hangout Thanksgiving day at 18:00 UTC (Iâll be on at 18:30 for an hour), talking about the comet with several scientists, and showing live images from SOHO and other observatories. Stay tuned to the blog and Iâll have the link and an embedded video so you can watch it live. The Planetary Society also has links where you can get more ISON info, and the Comet ISON Observing Campaign is another great source of news.

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