Binocular and telescope owners can watch a fine match-up of the sky’s current brightest comet – Comet C/2009 P1 Garradd – and the rich cluster M92 in Hercules tomorrow morning. The cluster belongs to an ancient group of spherical, star-packed clusters called globulars. Some 330,000 stars are jammed into a ball 109 light years in diameter 26,000 light years away. Despite that rather spectacular distance, it still shines brightly enough at magnitude 6.4 to be easily visible in a typical pair of binoculars from moderately light polluted skies. Look for a small fuzzy spot with a brighter center.
Comet Garradd will be another fuzzy patch only a half a degree to the right or west of M92, so both little glows will be close together in the same field of view in any pair of binoculars. As you’d expect, the comet is much closer to Earth at 142.8 million miles. The separation between them will increase in the coming mornings as Garradd tracks slowly northward through Hercules.
In the diagram above, you can see that the comet has a steeply inclined orbit that takes it well above the plane of the solar system where the planets orbit. That’s why we see it high in the northern sky this month far from the morning planets. Comets that pass relatively close to the sun typically develop two tails – one made of dust carried away by the pressure of sunlight along the comet’s orbit and an ion tail of gases that fluoresce when they’re excited by the ultraviolet energy in sunlight. The dust is released into space as the heat of the sun vaporizes cometary ices.
Since ion tails always point directly away from the sun, while dust tails lag behind in the curve of a comet’s orbit, the two tails point in different directions. Depending upon the sun-Earth-comet viewing geometry, they sometimes overlap or appear separated from one another by varying degrees. Our current viewing angle – looking up from way down below Garradd’s orbit – accentuates the tails’ separation.
The best time to see Comet Garradd and M92 is about 1 1/2 – 2 hours before sunrise, when it’s highest in the eastern sky before morning twilight begins. A small telescope will show the bright coma and a hint of both tails; telescopes of 8 inches or larger will show both tails stretching faintly more than a degree from comet’s head. Seeing a pile of stardust right next to bright, nearby comet should make for a beautiful sight. Try to get out in the next few mornings before moonlight becomes a problem.