with Simon Mansfield
Mars penetrators subsystem tests successful
JPL - Nov 16, 1997 - Two miniature science probes designed to penetrate the Martian surface and analyze the water vapor content of the planet's subterranean soil in 1999 have successfully completed a crucial subsystem test deep in the New Mexico desert.
This successful check of the batteries and soil collection drill of the mission known as Deep Space 2 (DS2) provides a "green light" for subsequent integrated system tests next spring, said Sarah Gavit, DS2 project manager at NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA. The DS2 mission hardware will be launched in January 1999, mounted on the Mars Surveyor '98 Lander. Both missions will arrive on Mars in December 1999.
DS2 is the second scheduled launch in NASA's New Millennium Program, which is designed to test new advanced technologies prior to their use on science missions in the 21st century. DS2 will validate the ability of small probes loaded with sensitive, miniaturized instruments to analyze the terrain of planets and moons throughout the Solar System.
In the late October test, a 4.4-pound (two-kilogram) prototype probe containing a soil collection drill and a circular group of eight lithium thyonal chloride cells -- forming two batteries -- was shot into the ground at more than 400 mph (644 kilometers per hour). The drill survived a 20,000-G impact, and the batteries, nestled inside a custom-designed casing, survived a 45,000-G impact intact. Both continued to function as designed. One G is the normal force of gravity on Earth.
"The Mars Pathfinder lander experienced about 19 G's when it hit the Martian terrain in July, so you can see that we are working at enormous rates of deceleration," explained Gavit. "One of our biggest challenges has been to find a way for our components to survive such a high deceleration force. The items at highest risk are the batteries, their packaging and the motor drill assembly.
"Although the recent test was one in a long series, it was the first test using flight-like hardware and packaging, so it served as a complete qualification of the battery and drill subsystems," she added.
The probe design features two modules: a circular aftbody, five inches (13 centimeters) in diameter, containing the batteries, that remains atop the surface; and a four-inch-long (10-centimeter) forebody, containing the drill and a soil analysis instrument, that should burrow up to six feet (1.8 meters) into the Martian soil. The two modules are connected via a flex cable that unravels as the forebody dives into the soil after a freefall impact.
Once in the ground, the soil collection drill slowly twists out from the side of the forebody and retracts a tiny soil sample into a chamber within the forebody, where it is analyzed by a water detection instrument. This instrument's key feature is a miniature tunable diode laser, similar in principle to the lasers used in consumer CD players. The soil sample is then heated, creating a vapor that passes through the path of the laser beam if water is indeed present. This resulting change in the intensity of the laser light indicates the amount of water, if any, to be found in the Martian soil sample.
The aftbody features batteries developed just for DS2. These batteries can operate down to minus 112 degrees F (minus 80 degrees Celsius), making them the only batteries of this type with the dual capability of being able to survive the strong impact and work in low temperatures. The aftbody also includes a micro- telecommunications system that, together with miniaturized electronics in the forebody, will relay the probe's findings to the orbiting Mars Global Surveyor spacecraft for transmission to Earth via NASA's Deep Space Network.
The Oct. 29 test took place at the New Mexico Institute of Mining Technology's Energetic Materials Research and Test Center in Socorro, NM. It was the 53rd test of DS2 hardware since the spring of 1996, beginning with early tests of preliminary battery and drill designs, among many other components.
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