Beyond Planet Earth: An Elevator to the Moon

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A lunar elevator arrives at a station on the Moon.  © AMNH/M. Garlick.

A lunar elevator arrives at a station on the Moon.

 © AMNH/M. Garlick.

Below, astrophysicist Michael Shara, who curated the forthcoming exhibition Beyond Planet Earth: The Future of Space Exploration, explains how a lunar elevator would work—and why it might inspire a new sport.

We humans are barely toddlers when it comes to space exploration. Our first baby steps off our home planet 50 years ago took us to low Earth orbit. By 1973, 12 intrepid men had walked on the moon’s surface. Since then we have sent robots to every planet in our solar system. The Hubble Space Telescope has shown us that the ordinary matter we are made of comprises only 4 percent of the mass of the universe. The Kepler orbiting telescope has proved that billions of worlds orbit the stars of our Milky Way galaxy. What will we accomplish in space in the coming centuries, as our steps become surer and bolder?

The new exhibition Beyond Planet Earth: The Future of Space Exploration takes you on the adventures awaiting humanity in the next few hundred years. Suborbital tourism, deflecting asteroids, establishing lunar and Martian scientific bases, terraforming Mars, and searching for life in Europa’s oceans will all happen in the coming century. While we can’t predict what the spaceships carrying us and our robots will look like, we do know where we’re going, the challenges of getting there, and the opportunities available when we arrive at destinations as alien as anything out of “Star Trek.”

The cover of the Fall issue of Rotunda, the Museum Member magazine, shows what a lunar base at the South Pole of the Moon might look like. The South Pole is a likely place for a lunar base for two reasons. First, there’s probably a lot of frozen water there, from comets that crashed there and remained frozen in nearly perpetual darkness. Just as important, the Sun is almost continuously visible from the tops of the rims of South Pole craters, so that large arrays of solar panels could continuously supply power to a lunar base. A huge infraredoptical- ultraviolet telescope, larger than a football field and with a rotating liquid mirror, would capture images of celestial objects with a resolution unmatched even by Hubble or Webb.

The Moon’s South Pole would also be a logical base for a lunar elevator, shown at left with its cable stretching back to Earth. This isn’t fantasy. A real lunar elevator for moving people and cargo such as helium-3, a rare isotope found in lunar soil that is thought to be a clean candidate for nuclear fuel, to and from the Moon could be built with current technologies and materials. (Visitors will see a model of a lunar elevator in the exhibition.)

The principle of a lunar elevator is elegant and simple. Any object—let’s say a space station—placed along a line joining the centers of the Moon and the Earth, and more than one-ninth the distance from the Moon to the Earth, will fall toward Earth.

That’s because Earth is 81 times as massive as the Moon, so its gravitational pull exceeds that of the Moon as soon as you travel more than 26,500 miles toward Earth from the Moon. If you attach a cable from the lunar surface to the space station, the station is tethered: it “wants” to fall toward Earth because of Earth’s dominant gravity, but it can’t because it’s held in place by the cable. Voilà: you’ve just built lunar-Jack’s beanstalk pointing up to Earth from the lunar equator. Now imagine extending the cable 238,000 miles, to just above the Earth’s atmosphere. Attach gripping, rotating wheels to the mechanical arm of a solar-powered gondola connected to the cable, and you have a rocket-free way of transporting anything and anybody between the Earth and the Moon’s surface.

Well, almost. You do have to “jump” to about 100 miles above the Earth’s surface to catch the gondola as it moves by at 1,000 miles per hour due to the Earth’s speed of rotation. But rocket airplanes suitable for this purpose are already being built by commercial companies like Virgin Galactic. One of the great advantages of this scheme is that you never need to speed up to, or slow down from, Earth-orbital speed of 17,500 miles per hour.

Thus the dangerous heating and mechanical stresses generated when reentering Earth’s atmosphere would be hundreds of times less on a rocket-plane-lunar elevator trip to the Moon than a trip involving rockets. A one-way trip would take about a week and could be as comfortable as an Alaskan or Caribbean cruise, though somewhat more expensive. Tourism could help support the operation of a lunar elevator.

The lunar elevator also offers the opportunity for the most extreme sport I can think of: space jumping. If you stepped off the end of a cable stretching down from the tethered space station to about 60 miles above the Earth—in a space suit, of course—you would begin to fall faster and faster. Reaching about 2,500 miles per hour when you began to encounter the outer atmosphere, you would use a combination of carefully timed drogue parachutes, a parasail, and a main parachute to slow down enough to avoid being torn apart by wind resistance. If you were really good, and lucky, you might land safely within a 100-yard bull’s-eye—just 15 minutes after you left space.