Comet Hartley 2, officially known as 103P/Hartley, visits the inner solar system about every 6.5 years. The comet was discovered in 1986 and received a visit from a NASA mission, Epoxi, in 2010. It is expected to return in April 2017.
Although the comet is a frequent visitor to the sun, it's still a very active small body. NASA has called Hartley 2 a "weird little comet", and one NASA astronomer characterized Hartley 2 as "a hyperactive little comet, spewing out more water than most other comets its size."
Scientists studying the small comet have raised the possibility that more comets behave the same way, especially if they carry plenty of carbon dioxide or monoxide in their composition.
Hartley 2's visit by Epoxi also revealed some strange scientific discoveries, such as "glittering blocks" on either end of the comet.
Caught by quality control
Hartley 2 is named after its discoverer, Malcolm Hartley, an astronomer working for the Siding Spring Observatory in New South Wales, Australia. He has held several roles at the facility over the years, but in March 1986 he was a quality controller examining the accuracy of images taken by a Schmidt telescope on site.
On March 16 of that year, he saw a strange smudge on a photographic glass plate. "Back then, the observations came in as negatives — stars and other objects in the sky appeared black on a clear background," said Hartley in an interview published by NASA in 2011.
"I noticed a dark haze around a trail. Trails indicate something that is travelling fast through the sky, but asteroids don't have a haze. So I thought it might be a comet."
His find was confirmed by the International Astronomical Union's minor planet center a few days after the discovery. Hartley actually went on to discover at least 10 more comets during his career, until Siding Spring changed the Schmidt telescope he used to perform spectroscopy in 2002.
Hartley's namesake returns to the inner solar system near Earth about once every 6.5 years. Although it's nowhere near as bright as say, Halley's Comet at its finest, Hartley 2's frequent return makes it a valuable object for astronomers seeking to understand how the sun alters these dirty snowballs during repeat trips near the sun.
A last-minute mission switch
Interest in Comet Hartley 2 picked up in 2007, when NASA made a last-minute decision to divert the Deep Impact mission in the comet's direction. The agency originally planned to target Comet Boethin, but that periodic comet vanished from view before Deep Impact could get there.
"We were confident we could find the comet, and we were astonished when it wasn't there," stated the University of Hawaii's Karen Meech, one of the mission's co-investigators, in a 2007 press release. The scientists guessed that the comet broke up into pieces too small to see from Earth.
While Hartley 2 was a promising target, NASA said the drawback to visiting that comet over Boethin was it would take two years longer to bring the spacecraft close by. Because instruments can degrade over time, this initially made Boethin the primary choice.
Deep Impact had already launched in 2005 to first swing by Comet Tempel and eject an impactor into the comet before heading by Earth again in 2007, redirecting its trajectory and launching towards Hartley 2.
The mission's change to Hartley 2 renewed scientific interest in the comet. Other observatories performed more detailed studies of Hartley 2, including the orbiting NASA Spitzer Space Telescope.
Researchers using the observatory examined Hartley 2's nucleus and rate of mass loss that occurs every time it passes near the sun. They estimated that Hartley 2 could last about 700 more years – about 100 more trips around the sun – before it breaks up. [Photos: Close-Up Views of Comet Hartley 2]
Carbon dioxide and 'glittering blocks'
Before reaching Hartley 2, Deep Impact was renamed Epoxi to reflect a search for Earth-size exoplanets around five stars. It took three laps of the sun before Epoxi could get close to Hartley – a trip of 1.6 billion miles (about 18 times the distance between the Earth and the sun.)
Epoxi made its closest approach to Hartley 2 in November 2010, looking to understand more about the comet's interior. It passed as close as 431 miles (694 kilometers) from the comet's surface. Rather than sending a probe into Hartley 2, Epoxi monitored poofs of gas emanating from the comet's surface and surrounding it. It also hunted for water ice on the comet.
Studying the oddball comet up close revealed that outbursts on the comet do not happen uniformly. Jets from the comet, powered by carbon dioxide, were more populous on either side of Hartley 2 than at the middle. The "waist" of the comet instead had jets with water vapor, with only a little carbon dioxide or ice.
The close-up snapshot revealed some other strangeness about Hartley 2. It rotates on two different axes, principally spinning in one direction while also tumbling in another.
Epoxi also spotted some "glittering blocks" on the edges of Hartley 2, of about 165 feet (50 meters) high and 260 feet (80 meters) wide. NASA, although still probing their nature, mentioned that the blocks appear to be about two to three times more reflective than the rest of the surface, on average.
Working in concert with other NASA research teams, Epoxi also revealed a core made up of several types of ices.
"We have evidence of two different kinds of ice in the core, possibly three," stated NASA Goddard Space Flight Center's Michael Mumma. "But we can also see that the comet's overall composition is very consistent. So, something subtle is happening. We're not sure what that is."
— Elizabeth Howell, SPACE.com Contributor