This week, MESSENGER's Mercury Dual Imaging System delivered the 100,000th image of Mercury since the spacecraft entered into orbit around the planet on March 18, 2011.

The instrument - one of seven aboard the spacecraft - has globally mapped the planet in high-resolution monochrome images and in color images through eight of its color filters, uncovering a new view of Mercury and shedding light on the planet's geologic history.

"That our inventory of orbital images of Mercury is now expressed in six figures constitutes an important footnote in the history of solar system exploration," offers MESSENGER Principal Investigator Sean Solomon, of the Carnegie Institution of Washington.

"The MESSENGER mission has at last provided us a view of the innermost planet that is fully global, multispectral, and at a range of illumination conditions. Moreover, we are steadily building a library of targeted high-resolution images that allow us to view features and discern geological processes in unprecedented detail."

Because of Mercury's proximity to the Sun and its slow rotation, designing an imaging system for an orbital mission presented quite a challenge, says MDIS Instrument Engineer Ed Hawkins of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md.

"The sunshade protects the spacecraft from direct solar illumination, but we knew it would constrain a camera's range of pointing," Hawkins says.

"So, we had to come up with a system that would be able to capture the required observations of the planet, maintain the thermal safety requirements and not jeopardize the safety of the spacecraft.

"We finally came up with the idea for a pivoting mechanism that gave the instrument an extra degree of freedom, allowing it to obtain extra observations even when the spacecraft - and the rest of the instruments a euros " were facing away from the planet."

The system has exceeded the team's expectations, he says. "We obtained images of Earth and Venus, but those were primarily to test the instrument. We used fairly simple spacecraft pointing options and exercised basic MDIS exposure control and compression options," he says.

But the instrument's performance during the first flyby of Mercury in January 2008 was the first demonstration of the instrument's full capabilities.

"When we received that first image after the first flyby, it confirmed for us that the imaging system we designed was working, and since then the camera has been operating flawlessly," he says.

Nori Laslo, MESSENGER's Deputy Payload Operations Manager and MDIS Instrument Sequencer, says she can remember when this point "still seemed eons away.

"To have now successfully completed our primary mission, entered our extended mission, and surpassed 100,000 images is spectacular and really speaks to the ability of the MESSENGER team to work as a unit to tackle from all sides whatever challenges are encountered," says Laslo of APL.

"The team is made up of people with many different backgrounds, including engineers, scientists, analysts, sequencers, flight controllers, software developers, information technology specialists, managers, and administrative support, among others.

"Everyone brings different expertise and insight to the table. So the milestone of 100,000 images from orbit is really a group achievement, a product of our combined efforts to make the MESSENGER mission a success."

The 100,000 images from Mercury's orbit constitute an important milestone, says MDIS Instrument Scientist Nancy Chabot, of APL. But there is still much more to come.

"New images are returned from Mercury orbit on nearly a daily basis, and scientists around the world are studying these images to decipher Mercury's history and evolution."