Comet 17P/Holmes is a Jupiter-family comet with a period of 6.9 years. It has a perihelion of just over 2 AU, and its entire orbit lies between Mars and Jupiter. It never comes closer to the Earth than 1.06 AU, and therefore poses no impact risk, and does not produce any meteor showers. It has an estimated nucleus diameter of 3.4 km.

The comet was discovered in 1892 by Edwin Holmes. At the time it was undergoing an outburst of gas that made it visible for several months. Normally, this comet is so dim that it can only be detected photographically. In 2007 it was a stunning binocular object, even from light polluted cities.

On October 24, 2007, it began another outburst. Over just a few hours, it went from a brightness of magnitude 17 to 2.5, a nearly million-fold increase. It appeared as a naked eye object in Perseus, with its brightness rivaling the brightest stars in that constellation. To the eye, it was starlike. But with any magnification it could be seen as an extended, slightly fuzzy object with a distinctly yellow/orange color.

Click on images to enlarge.

The profile of the comet head is shown here. The coma diameter over about four weeks is plotted. The narrower central region is through the pseudonucleus, and the outer coma can be seen falling off in intensity somewhat slower. The odd shape of the most recent profile is caused by the bright star Mirfak being inside the coma. This has been compensated for in calculating the coma width. The profiles are normalized to the same height for clarity.

The changing size of the coma is plotted here. The diameter of the dusty central coma is easily measured, and that is what has been used here. With extreme contrast stretching, the coma is seen to extend about twice as far. That outer coma is mostly gas rather than dust. The growth rate of the coma initially appeared linear, but now seems to be showing a very slight increase with time. The initial dust ejection velocity was about 500 meters per second from the nucleus (similar to what was measured for comets Hyakutake and Hale-Bopp at their most active), but the outer zone of the coma is now increasing in radius at over 700 m/s. This is difficult to explain, but may be related to increased dispersion caused by the solar wind.

As of November 21, the central coma exceeded the angular size of the Moon; the entire coma was much larger than that. The physical diameter of the central coma was about 3 million kilometers, about 2.2 times larger than the Sun. The coma of 17P/Holmes was the largest object in the Solar System. Of course, "object" is used rather loosely here. While the coma appears very large and bright, it is also very tenuous. In the images, dim stars are easily seen right through it.