Legs, Bags, or Wheels?
When choosing landing gear for Mars spacecraft, engineers have to weigh their options-literally.
- By Tony Reichhardt
- Air & Space magazine, August 2007
Say goodbye to airbags, at least for now. The next visitor to Mars, the Phoenix lander (see “Northern Exposure,” August 2007), will touch down on the planet the old-fashioned way—on legs.
NASA’s last three Mars landers, Pathfinder, Spirit, and Opportunity, all parachuted to the ground inside a cluster of tough fabric airbags that absorbed the shock of impact. After cutting loose from the parachute, the airbags bounced, rolled to a stop, and deflated, and out came the rovers.
However cool they might be (if only we had video of Spirit’s 27-foot-high first bounce on Mars), airbags have disadvantages. The bags and their supporting hardware are themselves heavy, which reduces the amount of scientific equipment a lander can carry. And for precision landings, legs generally do a better job. Spirit bounced 28 times before finally settling to a stop 300 yards from its point of first impact.
On the other hand, airbags aren’t bothered by large rocks that might tip a legged lander. On the other other hand, spherical airbags can roll into a deep crater from which a rover would never escape. It’s all about tradeoffs. When choosing a landing system for Mars, engineers consider many factors, down to the location of the target site (lower elevations give the parachute more float time to slow the spacecraft in the thin atmosphere).
The Viking landers of the 1970s touched down on three “crushable” legs—technology borrowed from the moon landers of the previous decade. By the time the smaller Pathfinder returned to Mars in 1996, however, NASA was into saving money, and airbags were selected as a cheap, clever alternative.
The Phoenix spacecraft, originally built for a 2001 Mars mission that was subsequently canceled, uses a lightweight landing system developed for the Mars Polar Lander, which didn’t get the chance to try it: In 1999 a software error caused the spacecraft to crash onto Mars. Phoenix, which weighs about the same as Spirit and Opportunity, would have been light enough to use airbags, but its landing system was designed before Pathfinder proved that bags really work. So legs are back in style, at least temporarily. Next May, after the three-legged Phoenix separates from its parachute at an altitude of 3,300 feet, pulsed rocket engines underneath the spacecraft will fire, slowing Phoenix to 5.5 mph at touchdown. The rockets will fire until the lander makes contact.
Next up is the Mars Science Laboratory, due to launch in 2009. The MSL represents a giant leap for rover-kind. While the golf-cart-size Spirit weighs about 350 pounds, the 2009 rover will be four and a half times as heavy, and as big as a MINI Cooper. Scientists also expect to set it down with pinpoint precision to maximize their chances of finding interesting geology.
That poses quite a challenge for the engineers at NASA’s Jet Propulsion Laboratory in Pasadena, California, who, after tossing out other options, came up with a new kind of Mars landing system based on the Skycrane heavy-lift helicopter, the Sikorsky CH-54B. (You can see an animation of this planned Skycrane maneuver here). Instead of placing rockets on the spacecraft itself, as has been done with all past Mars landers, the engineers put them on a descent stage, which resembles a flying bedstead. The rover, slung underneath the hovering descent stage, is lowered to the ground on cables and touches down on six wheels. Then it drives off. No bags, no legs, no worries.