The Not-So-Big Dig
With the equivalent power of an electric can opener, engineers try to do more than scratch the Martian surface.
- By Tom Harpole
- Air & Space magazine, November 2006
(Page 2 of 4)
The problem was a key flaw in the drill’s design: Its threads were not carrying the cuttings to the surface. Instead, the cuttings were getting clogged in the hole, binding the drill stem. (Nevertheless, later X-ray analysis of the core showed 58 separate layers of regolith along with various pebbles and an increasing density down to the bottom of the core.) NASA fixed the problem on later flights. On Apollo 16, Charlie Duke drilled to the full eight feet in about one minute. On Apollo 17, Gene Cernan did it in just under three.
Robotic spacecraft also have used drills. The Soviets put drills on their Luna soil-sample return probe to the moon (capable of penetrating about 13 inches) and Venera spacecraft to Venus (just over an inch) in the 1960s and 1970s. The European Rosetta mission, which launched in 2004, is designed to land on a comet in 2014, drill down about eight inches and analyze the contents.
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.