Comet 17P/Holmes

On October 24th, a rather dim and obscure comet, HOLMES 17P, suddenly brightened to magnitude 2.5 or so, making it visible naked eye, even with the full moon.

It was spotted in Japan Wednesday evening (our wed. morning). I was viewing it with my 10" at various powers Wednesday night.. Its about 1.6 or so arc minutes in diameter, yellow, and shows a bright inner core with several dimmer shells ...

this sketch by a SF Bay Area amateur astronomer Alexander Avtanski Wednesday captures it quite nicely.

Holmes is in Perseus, almost directly below Mirfak about 4.5 degrees. Through binocs, it looks like a yellowish star, perhaps slightly fuzzy. its a great binocular object.

Here's a finder chart for Sunday, Oct. 28

Thursday evening, it appears via transit timing to have grown to 3 arc minutes. It also appears to have a distinct 'ring' structure to the outer shell, implying its a sphere of gas that was ejected from the core.

If I run the math correctly, 1.6 AU * sin (.7 arc minutes) = 48000 km in a day, thats about 2000 kph the gas cloud is moving away from the core.

Here is an interactive java solar system orbital model from NASA JPL showing Holmes..


17P/Holmes orbit in the solar system...

note it spends all its time between mars and jupiter.

Here is its position in the sky, it makes an interesting retrograde dance around perseus over the next few months...

Friday, was skunked by clouds.

Tonite, Saturday, its totally clear.

I'm measuring Holmes as 40 seconds drift edge to edge on the outer snowball, which I peg as 5-6 arc minutes at that declination. Matching the starfield around it, starry nights says a mag 11 star thats right on the circumference is 2 arc minutes from the center, making it 4 arc minutes diameter.

that means its about doubled in size since Thursday night.

its less obviously yellow now, there's still a distinct 'star' in the middle, the shaped glow seems less obvious, the inner snowball is still about the same size, but the outer shell is even more ring like, and the outer edge seems slightly less distinct than Thursday.