Stevenson said his proposal should be attractive to the scientific community because it is of the same scale, price-wise, as planetary exploration. To date, NASA has flown unmanned missions past all the planets except Pluto, has made a few highly successful soft landings on Mars, has probed the clouds of Jupiter, is getting ready to probe the atmosphere of Titan, and has sent four spacecraft into interstellar space. Sending something into the Earth, Stevenson believes, will have comparable payoffs in the quest for knowledge.
"When we fly to other worlds, we are often surprised by what we find, and I think the same will be the case if we go down."
A Million Tons of Molten Iron
According to Stevenson, the crack that will have to be blasted into the Earth's surface to launch the probe will need to be several hundred meters in depth, and about a foot (30 centimeters) wide, to accommodate a volume of about 100,000 to several million tons of molten iron.
The instant the crack opens, the entire volume of iron will be dropped in, completely filling the open space, he said. Through the sheer force of its weight, the iron will create a continuing crack that will open all the way to the planet's core 3,000 kilometers (1,800 miles) below. Anything on a smaller scale may not work; anything larger will be even more expensive, so Stevenson thinks a crack of those dimensions is about right.
"Once you set that condition up, the crack is self-perpetuating," Stevenson said. "It's fundamentally different from drilling, where it gets harder and harderand eventually futilethe farther you go down."
The iron will continue to fall due to gravity because it is about twice the density of the surrounding material. Riding along in the mass of liquid iron will be one or more probes made of a material robust enough to withstand the heat and pressure. The probe will perhaps be the size of a grapefruit but definitely small enough to ride easily inside the 12-inch (30-centimeter) crack without getting wedged, Stevenson said.
Inside the probe will be instrumentation for data collection, which will be relayed through low-intensity mechanical waves of some sort. Because radio waves cannot propagate through Earth, this is the only way to get the data transferred, Stevenson said.
Based on the rate the molten iron would fall due to gravity, the ball would move downward into Earth at roughly human running pace (about 10 miles/16 kilometers per hour), Stevenson said.
"Each of the principles involved is based on sound knowledge of crack propagation, fluid dynamics, mechanical-wave propagation, and 'stress states,'" Stevenson said. "If these things didn't already work in nature, we would have no volcanoes and poorly performing bathroom plumbing, but little to fear from a pebble shattering our windshields."
The biggest question should not be the cost, but whether we should pursue the goal of exploring Earth's interior, he said. "That said, I'd suggest we do it if we can keep the cost under [U.S.] $10 billion."
This proposal is modest compared with the space program, Stevenson said, and may seem unrealistic only because so little effort has been devoted to it. "The time has come for action."
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