PLANET FINDER CATCHES A COMET
Last October, astronomers all over the world were astounded when the normally very faint Comet
Holmes erupted in the largest outburst for more than a century. Speaking at the RAS National
Astronomy Meeting in Belfast on Wednesday 2 April, Queen's University Belfast astronomer Dr. Henry
Hsieh will describe how a UK telescope was in the right place at the right time to capture the
first images of this once-in-a-lifetime event.
The SuperWASP-North facility on the island of La Palma was built by UK scientists to discover
planets around other stars. The 8 cameras that make up the system operate robotically,
automatically scanning large areas of the sky each night. By coincidence, at 2339 GMT on the
evening of 23 October 2007, it was pointing towards Comet 17P Holmes.
"By the time SuperWASP spotted the comet, it had already brightened by a factor of 1000," explains
Dr. Hsieh. "But this was still almost 2 hours before anyone else noticed it." (That honour belongs
to amateur astronomer Juan Antonio Henriquez Santana who saw the eruption from Tenerife).
Over the next 2 hours the comet continued brightening, until SuperWASP could no longer accurately
measure it - it was too bright for the cameras!
Comets are bodies orbiting the Sun composed of frozen gases and microscopic solid particles in a
small solid nucleus. When they come close to the Sun, they heat up and some of the icy material
turns to gas, producing characteristic tails. But during this outburst, Comet Holmes released a
large amount of its material all at once.
Two days after the eruption began, sunlight reflecting from the ejected material had made the
comet one million times brighter than it was originally making it easily visible to observers
across the northern hemisphere.
Dr. Hsieh comments "Over the next few weeks, SuperWASP continued to observe Comet Holmes as the
cloud of dust and gas surrounding the 3-km diameter nucleus of the comet steadily expanded.� By
31st October, the cloud was already 900,000 km across or more than twice the distance from the
Earth to the Moon."
"Using our SuperWASP observations, we measured the speed of expansion of the outer edge of this
cloud to be over 1500 km per hour and by 17 November measured the size of the cloud to be more
than 2 million km across - much larger than the Sun."
Two weeks after the outburst, SuperWASP captured an added bonus - the faint and delicate tail of
the comet composed of the gas released from the nucleus. As astronomers watched over the next few
weeks, this tail gradually faded and moved away from the comet.
Although many images were gathered by astronomers around the world, the precise cause of the
outburst is still a mystery. All they know right now is that it happened once before - in 1892 -
and may well happen again.