On December 3, NASA will be launching the Mars Pathfinder, which will be the first solar-powered probe to operate on the surface of Mars, along with the Sojourner, a solar-powered robotic rover.
     The Pathfinder probe has a primary mission lifetime of only 30 days on the surface of Mars. Will Mars dust be a problem for longer solar-powered probes to Mars? NASA Lewis will be flying an experiment on the Pathfinder mission to address this question.
     Views from the Viking landers in 1976 showed that the sky of Mars is a light pink color. The light color comes from sunlight scattered from microscopic dust particles suspended in the upper atmosphere. As dust settles out of the atmosphere after a dust storm, a thin layer of dust may deposit onto the solar arrays. The Viking probes to Mars imaged this deposition, visible in the form of a thin layer of light-colored dust adhering to the surface of rocks after a dust storm. Dust deposition was no problem for the Viking landers, which used a nuclear power source, but could cause difficulties if long-duration missions are to be energized by solar power.
     Calculations by Geoffrey Landis, to be published in an upcoming issue of the scientific journal Acta Astronautica, indicate that dust deposition should be no problem for the short duration Pathfinder probe, but could present difficulties for missions of a year or longer, especially if the missions are to operate during the season when dust storms occur, which is near the southern-hemisphere summer, when Mars is closest to the sun. The experiment on Pathfinder will verify these calculations.
     The experiment, named the "Materials Adherance Experiment -- Solar Cell" (MAE) is mounted on the front left corner of the "Sojourner" rover. It consists of a transparent glass plate covering a small solar cell. Atmospheric dust settling out of the atmosphere will adhere to the glass plate. Once a day the glass plate can be commanded to be rotated away from the solar cell. Comparing the measurement of the solar cell output with and without the dust-covered plate will directly measure the amount of dust on the plate. A quartz crystal monitor (QCM) sensor will simultaneously measure the mass of dust deposited, by measuring the change in vibration frequency of a tiny quartz crystal (similar to the crystal used to keep time in a quartz watch) as dust is deposited on it. The QCM sensor was manufactured by the Optical Instrumentation branch at NASA Lewis. The entire experiment package occupies an area of 12 square centimeters (about two square inches)-- slightly larger than the size of a commemorative postage stamp.
     The transparent cover of the solar cell is rotated by an innovative actuator that makes use of the property of memory metal, or "nitinol," to change shape when heated. A tiny electrical current is used to heat a thin nitinol wire, causing it to contract and rotate the cover. This actuation mechanism has the advantage of requiring only a tiny amount of power, and has no moving parts such as gears or bearings which might jam.
     More information and pictures can be found at the "Solar Energy on Mars" web page http://powerweb.lerc.nasa.gov/pv/SolarMars.html
     The people involved: Dr. Geoffrey Landis, a scientist at the Ohio Aerospace Institute in Cleveland, Ohio, is principal investigator of the MAE Solar Cell experiment. Phillip Jenkins of Essential Research is the engineer who built the MAE solar-cell experiment. Lawrence Oberle of the Optical Instrumentation Branch at NASA Lewis is in charge of building the QCM sensor. Program manager for the NASA Lewis work is Steve Stevenson.