Mars Reconnaissance Orbiter becomes the eye and the relay
The sharpest camera ever sent to another planet resolves 30 centimetres per pixel from 300 km up, and the same spacecraft relays much of what the rovers below it send home.
On 10 March 2006 a 2,180-kilogram spacecraft launched the previous August on an Atlas V burned its engines for 1,641 seconds and let Mars capture it into a 400 by 35,000 km polar loop with a 35-hour period. That was the cheap orbit, not the useful one. Getting to the useful one took six more months of aerobraking: roughly 445 passes skimming the top of the atmosphere, letting drag do work the propellant budget could never have paid for, until the orbit had shrunk to 255 by 320 km. Twelve laps a day, periapsis over the south pole, sun-synchronous so that every image is taken at three in the afternoon local time and every shadow falls the same way. Science began in November 2006, once the Sun had moved out from between the two planets.
The point of all that trouble was the camera. HiRISE is a 0.5-metre f/24 telescope — a small observatory — aimed at the ground. From 300 km it resolves 30 centimetres per pixel over a swath more than 6 km wide and up to 60 km long. Nothing sharper has ever been flown to another planet. Around it sit five more instruments: CTX at 6 metres per pixel across 30 km for context, CRISM reading mineral spectra at about 18 metres per pixel to find rocks that only form in water, SHARAD sounding a kilometre down into the crust, MCS slicing the atmosphere every 5 km in altitude, and MARCI, which paints a global weather map every single day.
This changed what landing meant. Viking's planners had to accept ellipses more than a hundred kilometres long and therefore had to pick somewhere boring enough to be survivable anywhere inside them. Pathfinder aimed at the mouth of an outflow channel and hoped. With HiRISE you can count the boulders first. Curiosity went to a target at Gale about 20 by 7 km, chosen for one specific stack of layered rock; Perseverance flew to Jezero carrying an onboard map built from orbital images, matching what its descent camera saw against that map to dodge hazards on the way down.
It is also the switchboard. A 3-metre steerable dish pushes up to 6 megabits per second to Earth, and an Electra UHF radio talks to the surface. Rovers have neither the antenna nor the power to reach Earth directly at any useful rate, so they hand their data to whatever orbiter is overhead. MRO owns the widest pipe at Mars. It photographed Phoenix descending under its parachute in 2008, found Beagle 2 lying half-opened in 2015, imaged Schiaparelli's fresh crater the day after the crash, and its weather camera spotted the 2018 storm that killed Opportunity.
Twenty years takes a toll. CRISM's cryocoolers failed and the spectrometer was formally retired in 2023. HiRISE took its 100,000th image on 7 October 2025 anyway — mesas and dunes in Syrtis Major, about 80 km from Jezero.