17P/Holmes is a periodic comet in our solar system, with an orbital period of about 7 years and a nucleus with a diameter of about 3.4 km (until the recent outburst). The comet has been observed and studied by astronomers ever since British amateur astronomer Edwin Holmes discovered it in 1892.

Images from NASA's Hubble Space Telescope (see above image) show how 17P/Holmes mysteriously brightened by nearly a million fold in a 24-hour period beginning in October 23, 2007. The Holmes' outburst is described as the largest known comet outburst to date.

Astronomers do not know why the comet brightened so dramatically. One possible suggestion is that a huge crack opened up inside the comet, exposing chunks of ice beneath the comet’s surface. Sunlight heated the ice and caused pieces of it to break off and form a large cloud of gas and dust around the comet, which reflected light from the Sun, making the comet appear much larger than its actual size. 

Although many comets lose ice when they travel close to the Sun, Holmes appears to have ejected its material when it was moving away from the Sun, adding to the mystery.

This is actually not the first time Holmes had an unexplained outburst. The comet had another outburst when it was discovered by Edwin Holmes in November 1892, and although astronomers kept track of the strange comet until its return in 1906, it was "lost" for almost six decades, The comet was found again only in 1964 with the help of computer predictions by astronomer Brian Marsden.

TFOT recently covered several other astronomical discoveries that were made thanks to the Hubble Space Telescope, including a unique picture of a star with a comet's tail, the brightest supernova ever captured (in the NGC 1260 Galaxy), and a potentially inhabitable Earth-like planet.

More information and images of the Holmes Comet outburst can be found on the Hubble-Site (a recent TFOT site of the week).