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A prototype of NASA's Phoenix Mars Scout stretches its 6.6-foot-long arm to scoop soil at Death Valley National Park in California. On Mars, the probe will dig a trench up to 20 inches deep to analyze the history of surface ice. (NASA)

The Not-So-Big Dig

With the equivalent power of an electric can opener, engineers try to do more than scratch the Martian surface.

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(Continued from page 1)

The requirements for a Mars drill are daunting. The machine must collect cuttings and cores, analyze the samples, and transmit the findings to Earth. It must weigh less than 90 pounds and run on an energy budget of less than 100 watts, drilling a roughly two-inch diameter hole to produce a one-inch core.

Earthbound miners, when boring a two-inch diameter hole for blasting basalt, commonly use 2,400-pound compressors that drive 55-pound drills. Even that is a relatively small outfit compared to the drills used for water or oil wells. No one has ever drilled autonomously on Earth; the rigs require constant human input, and they rely on brute force and extreme horsepower to penetrate the mantle. By contrast, the drills under development for Mars will have motors that operate with the equivalent power of an electric can opener.

Last February, Dohm and other scientists joined two teams of engineers from Swales Aerospace of Pasadena, California, and Maryland’s Raytheon/UTD, at the Idaho National Laboratory (INL) in Idaho Falls to test two candidate drills for Mars. The rigs’ designs were at opposite ends of current development in low-power, low-mass drills to retrieve core samples and cuttings for analysis.

Swales’ entry, which has a target depth of 65 feet, used a custom-made drill string: interconnecting pieces of pipe that make up the ever-lengthening shaft of a drill. In a drill string, the shaft is assembled in sections, which are added as the borehole deepens. To collect a core sample, the entire string is withdrawn from the hole and disassembled.
Raytheon/UTD’s rig, which had drilled through limestone to a depth of 4.5 feet in lab tests, used a tethered corer design. The core and cuttings are winched up from the bottom of the hole inside of the drill and collected at the surface. The advantage of the tethered system is that it reduces the weight of the drill because there is no drill string. That allows for greater depths to be bored with less energy.
Drilling conditions in Idaho Falls that February day were in some ways tougher than on Mars. Adjacent to an INL parking lot, the teams set up custom-made white canvas tents that flapped like sails under blue skies and wispy clouds. The tents protected the teams from both stiff winds and prying eyes. But an unusual weather pattern pushed daytime temperatures to the mid-50s, causing snowmelt to flood the ground and pour into the boreholes. The regolith kept collapsing, and anyone who stepped off the hastily laid plywood footpaths would find himself ankle deep in Idaho mud. “Quagmire” came to mind, a term that shouldn’t normally apply to Idaho in midwinter.
Both drills had uniquely designed bits to bore into ice or very hard rock, but not the soupy two-foot-thick layer of gravel, sand, and soil that formed atop the basalt bedrock.

The Swales team has been developing its Modular Planetary Drill System since 2005. “You want to have a cool acronym— we don’t,” said Argie Rumann, a senior systems engineer, in a notable attempt at humor during an intense week of field tests.

Swales brought a third-generation rig that measured 11 feet tall, four feet wide and weighed 425 pounds with its ground support equipment platform. To work on Mars, a shallow drill would need to shrink in all those dimensions by a factor of 10. The drill and retrieval system, as well as the instruments to analyze the Martian samples, would need to fit in a space of about 35 cubic feet—the size of a kitchen stove—and weigh less than 88 pounds.

Rumann, along with team leader Jose Guerrero and colleague Dominic Wu, piled dry ice and sandbags around their borehole to keep the meltwater at bay. Guerrero designed the patented bit, a donut-shaped cutting head that collects a core while the flutes spiraling up the outside deliver the cuttings into a separate collection chamber.

With the drill inching down at 50 rpm, they finally got into the basalt at a depth of 17 to 24 inches. The tent pulsated with vibrations and the drill string howled like a couple of coyotes. As drillers will tell you, there is an art to listening to the down-hole sounds of the drill string that they claim is vital to anticipating problems. Ignoring the warning sounds could result in an irretrievably stuck drill. But Mars is hundreds of millions of miles away, with a radio lag time of more than 10 minutes, so Earth monitors could not react to real-time changes in drill vibrations and sounds. Software must be programmed into the drill to stop it when sensors detect problems with torque, temperature, or other conditions.

“Developing the software to evaluate what the drill is doing and to react [to it] will take a team of programmers a year,” Guerrero said. Much easier is miniaturizing the drill and its scientific instruments. That, he says, “is just a matter of money. The more we spend, the easier it is to get lower weight.”
The Raytheon/UTD tethered core drill turned out to have its own problems. Jennifer Farrand and Matt Tucker, a couple of young, serious engineers, brought the drill from Maryland after 18 months of development. Their drill has an assembly that anchors itself to the sides of the hole and exerts force on the bit from down-hole instead of from the surface.

But on that unseasonable February day, the saturated Idaho regolith collapsed and captured their bit. The ground then refroze during the night. That forced Farrand and Tucker to rent an electric impact drill and bore by hand a series of holes around their stuck core until they could dig it out. Farrand took it in stride: “These conditions may arise on Mars if drill bit friction thaws ice beneath the surface. The refreezing could capture the bit permanently.” She smiled, “We’re always learning.”
The Raytheon/UTD drill is similar to a NASA/Baker Hughes Mars drill rig that Jeffrey George and his team at the Johnson Space Center tested in the Canadian arctic in May 2006. George’s team spent two weeks on Ellesmere Island testing their seven-foot-tall by 1.75-inch-diameter drill in what the scientists considered ideal weather.

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