The MESSENGER spacecraft still has a long way to go before it finally settles into orbit around Mercury in March of 2011, but scientists are taking advantage of every milestone in the mission to warm up the spacecraft's science instruments. As MESSENGER began its approach for its August 2 flyby of Earth, its cameras have snapped their first images. The images clearly show a cloudy Earth—and, to scientists' surprise, the Moon as well.
"It's nice to actually have data finally, after all this time," said Louise Prockter, who is the lead scientist for the MESSENGER Dual Imaging System (MDIS) instrument at the Applied Physics Laboratory. “These were our first 'real' images, and they're only going to get better as MESSENGER moves closer to Earth."
Imaging the Moon in the same picture was a pleasant surprise for the imaging team. Prockter explained that images taken from this distance -- 29.6 million kilometers (18.4 million miles) -- were not intended to show a lot of detail on the Earth. Instead, the images "were just trying to support one of the other instruments; we were just trying to look at the earth and find out whether [instrument] boresights overlapped." Consequently, MDIS scientists had given little thought to what else MDIS might pick up in the wide image frame. "When we got the images back, we asked 'what's that speck over there?' and it turned out to be the Moon! It didn't occur to anybody to look for it until then."
Each of MESSENGER's planetary flybys -- Earth on August 2 of this year, followed by two of Venus on October 24, 2006 and June 6, 2007 -- will be valuable for the science team to prepare for the first Mercury flyby on January 15, 2008. But there are challenges to the Earth and Venus flybys, because "MESSENGER was never designed to work at the Earth [distance from the Sun], it was designed to work at Mercury where it's much brighter," Prockter said. The spacecraft has many engineering features designed to dissipate the heat of a Sun that is 11 times brighter what the Earth sees. For example, MESSENGER has a Sun shield that will always be pointed at the Sun to protect the spacecraft from excessive solar heating.
Consequently, Prockter explained, at Earth's distance from the Sun, "We are flying the spacecraft backwards to warm it up. We also have some power issues. We don't have enough power to do everything we'd like to do [at Earth], but we might as well take some images and other data while we're here." Even if the images and data are not yet of Mercury, Prockter is excited. "Now it's under way, and I'm starting to do planning and real sequencing. It's really starting to get interesting!"