Mars Has Twins  |  This Week In Space History

Editors Note: This is part 1 which describes the rovers. Part 2, on Jan 26th – for the tenth anniversary of the second rover landing, will discuss science results.

by dave dooling

The most successful unmanned planetary rovers started their journeys on the Red Planet ten years ago, this week in space. Spirit, the first of two Mars Exploration Rovers, bounced to the surface yesterday Jan. 4, 2004, followed by Opportunity on the other side of the planet on Jan. 25.

They were not the first planetary rovers or even the first on Mars. The Moon had three manned Lunar Rover Vehicles, driven by U.S. astronauts on Apollo 15, 16, and 17 (1971-72), and two unmanned rovers, the Soviet Union’s Lunakhod 1 (1970) and 2 (1973). Lunakhod 2 barely holds the solar system distance record of 42km (26 miles), based on recent satellite images of its tracks. Two Soviet Mars craft carrying tiny rovers reached the surface in 1971 but failed. Sojourner, NASA’s Mars Pathfinder rover, landed in 1997 and lasted 83 sols (Mars days).

Encouraged by Sojourner’s success, the Jet Propulsion Laboratory (JPL) and NASA developed the twin Mars Exploration Rovers. They were much larger, about the size of a go-kart, to amble farther across the Martian surface.

“A big challenge is to be able to get enough traction to get through soil and over rocks but also to be benign enough to get off of the lander without getting entangled in the deflated airbags,” explained JPL engineer Chris Voorhees. The design is “basically like a paddlewheel that is machined onto the outside of the wheel, providing both safety and capability.”

JPL expanded the basic drive system introduced by Sojourner to improve mobility over the planet’s rocky surface. Six wheels in a rocker-bogie arrangement make the rovers more stable than a conventional four-wheel design when going over rocks. (The Soviet Lunakhod pointed in this direction with eight wheels mounted in pairs. The U.S. Lunar Rovers had four wheels each.) On each side, aft and center wheels are on a “bogie” beam linked to a larger “rocker” beam with a center joint mounting the rover body and a strut to the third, forward wheel. All six wheels have electric motors. The fore and aft wheels can steer, allowing the rover to turn a full circle in place, which came in handy when the primary exit for Opportunity was blocked after landing.

According to NASA, the design lets the rover tilt up to 45 degrees although software will make it back away at 30 degrees, and drive over rocks or through holes equal to one wheel diameter (10 inches). All this at a blazing 2 inches per second (0.11 mph) since the operators want to take great care going over uncharted land (even the sharpest images from Mars orbiters cannot show the small rocks that can easily kill an unwary rover).

The drive system, of course, is just a taxi to move science instruments across the surface. Despite popular notions, the rovers were not designed to look for life, but to study the geology of Mars, including conditions that may have supported life in the past. The Viking landers and a series of later missions showed that Mars likely is sterile now.

Steady advances in miniaturized technologies let the rovers carry more cameras and other instruments than past landers. The Panoramic Camera perches atop a mast 1.5m (60 inches) above the surface, and sports two solid-state cameras. The images are only 1,024 pixels square, but the rovers send lots of frames that are stitched into immense panoramas on Earth, or self-portraits giving a bird’s-eye view from above the craft.

Each rover’s robot arm carries multiple tools for studying the surface. One of the most important is the rock abrasion tool (RAT), which can expose unweathered rock for analysis. In operation, the arm presses the RAT against a specimen and grinding wheels scour to expose fresh rock. A bit of humorous speculation led to a Roadrunner-style video showing the RAT sticking and spinning the rover into oblivion.

Lacking a chemical lab, the Mars rovers do most of their analyses using radiation — from heat (infrared) up through gamma rays — reflected or emitted by Mars rocks and soil. A Miniature Thermal Emission Spectrometer measures infrared from rocks and even the sky. A small cobalt-60 source emits gamma rays to measure iron is rocks; small magnets on the arm also help. Curium-244 to emit alpha particles and X-rays to assay the chemistry of rocks based on the radiation they scatter. A microscope imager takes close-up images of samples.

All these, plus solar arrays, electric motors, computers, radios, and other components were packed into two 174kg (384-lb) rover packages for launch by nearly identical Delta II rockets. Spirit was launched on June 10, 2003, from Cape Canaveral, FL, with Opportunity following on June 10. They coasted for about half a year through deep space, silent, packed in a 2.6m (8.5-ft) aeroshell until atmospheric entry.

They arrived in bubble wrap. Specifically, they landed in an air bag system that inflated after the heat of atmospheric entry and ensured that the rovers would not topple over a large rock. After bouncing then rolling to a stop, the bags deflated, exposing the pyramid-shaped landing package. The sides opened, flipping ensuring each rover was right-side up, and activation began.

The rovers landed on opposite sides of Mars, Spirit at Gusev crater and Opportunity in Meridiani Planum. Opportunity unintentionally scored a cosmic “hole in one” by rolling into a crater, 22m (70 ft) wide and 3m (10 ft) deep. Their landing points were named Columbia and Challenger Scientific Stations, respectively, after the space shuttles and crews lost in 2013 and 1986.

After a few days spent unfolding itself and checking all systems, each rover drove off its lander and started exploring the surface. Part 2 of this story, on the 10^th anniversary of the Opportunity landing, will review those results.

The author is education director at the New Mexico Museum of Space History in Alamogordo, NM, and an experienced space reporter. He can be reached at dave.dooling [at] state.nm.us.

Source: http://moonandback.com/2014/01/05/mars-has-twins-this-week-in-space-history/