The Opportunity, Spirit, and Curiosity rovers may get the headlines for their craters clambering exploits but it's NASA's Mars Reconnaissance Orbiter, quietly observing the planet turn from 178 miles up, that's most critical to our exploration of the Red Planet.
The Mars Reconnaissance Orbiter (MRO) is a $720 million multipurpose spacecraft that acts as an orbital observation post and data relay. It measures 31 x 45 x 21 feet and weighed 4,800 pounds fully fueled on Earth. It began its journey to our neighbor planet in August 2005, when it launched aboard an Atlas V rocket from Cape Canaveral. It reached Mars the following March however did not begin its primary, two-year mission until November 2006 as a solid five months of aerobraking was necessary.
This primary mission was two part: scout the Martian landscape for suitable landing sites for future rover missions and study the planet's climate and atmosphere while keeping an eye out for signs of liquid water near the ice caps. In this endeavor, the MRO was equipped with a wide array of sensors and cameras.
The MRO's primary optics is the high-res HiRISE (High Resolution Imaging Science Experiment) camera, which is used for Martian land-form analysis. This camera utilizes the largest reflecting telescope ever carried on a deep space mission, sees in the 400-1000 nm (blue-green to near-infrared) spectrum, and boasts a 0.3 m resolution from an altitude of 300 km. That's nearly three times better than what you get on Google Earth.
Its secondary optics include the CTX (Context Camera), a greyscale camera with a 6m resolution designed to give, well, context to the super-zoomed images generated by the HiRISE and CRISM (discussed below) as well as create large-scale mosaics—each tile is 19 miles wide—of the surface as generate 3D images over multiple passes. The CTX is credited with capturing the impact craters created by the Curiosity's 55 pound landing ballasts. It also packs the MARCI (Mars Color Imager), another relatively low resolution camera with five-color vision (and two-color ultraviolet). This camera creates a global map of Mars with a 1 - 10 km resolution every day. These maps are essential to cataloging and analyzing Mars' long-term weather systems.
This spacecraft is also loaded with numerous sensors: the CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) and SHARAD (Shallow Subsurface Radar) both of which can detect water, ice, and minerals below the surface; the MCS (Mars Climate Sounder) which monitors the far red end of the spectrum, and the Gravity Field Investigation Package, which like the moon's GRAIL mission, measures the strength of Mar's gravitational field relative to changes in the orbiter's velocity.