Amateur astronomer Juan Antonio Henriquez Santana took a look at Comet Holmes on Oct. 24, 2007, just after midnight, expecting to see the small comet tracing a relatively uninteresting path across the sky. Comet Holmes, a visitor to the inner solar system approximately every six years, should have sped past almost unnoticed by the planet below, visible only to those with reasonably large telescopes.
What Santana saw from his position in the Canary Islands was something quite different – a softly glowing ball. The comet swiftly brightened to become visible to the naked eye as a yellow star in the constellation Perseus. By the next day, it was one of the brightest stars in that constellation.
By mid-November, the comet’s nucleus – just 2.1 miles (3.4 km) in diameter, or the size of New York City’s Central Park, had exuded a coma bigger than the Sun. To put that in perspective, you could fit 1.3 million Earths inside the Sun. The dusty cloud around the comet’s nucleus was, for a time, the largest object in our solar system.
People were able to see the fuzzy, spherical, tail-less comet with the naked eye, even in large cities. In telescopes or binoculars, it looked like a round disk with a bright core.
Comet Holmes in late November 2007. Credit: Wikimedia Commons User Johnpane
Holmes' sudden grab for attention wasn’t unprecedented. The comet was discovered in 1892 by astronomer Edwin Holmes when it temporarily brightened in a similar manner. Astronomers investigating the 2007 outburst speculated that the comet had been hit by a meteoroid, that part of the coma had broken off and disintegrated into tiny particles, or that a storehouse of gas inside the nucleus had just reached the surface, among other theories. Hubble took a look on Oct. 29, Oct. 31, and Nov. 4, 2007, and saw spurs of dust but no large fragments. However, the huge amount of dust near the nucleus would have made such fragments impossible to see unless they were nearly the size of the nucleus itself, astronomers noted.
Hubble's view of the heart of the comet shows the dust distribution near the nucleus.
Whatever the cause of Holmes’ incredible coma, the comet’s solid nucleus remained intact, and Holmes is due to visit our region of the solar system again in 2014, when astronomers will be watching it closely for more clues to its explosive nature.