The last time NASA sent people to the moon, they landed somewhere near the moon's equator. It was simpler to get home from there and safer for those early missions.
But as NASA plans to return astronauts to the moon in the coming decades, it is the moon's north and south poles that scientists and engineers are aiming for - drawn by the prospect of perpetual sunlight, water ice, intriguing geology and a gentler environment.
Scientists at the Johns Hopkins University's Applied Physics Lab will help the agency prepare for those missions. The lab, near Laurel, won a contract to lead a four-year, $6.9 million study for NASA.
A team of 30 scientists from several institutions, led by APL planetary scientist Ben Bussey, will use data from lunar missions to provide basic information for such a trip. Specifically, the team will:
* Scout for safe landing sites, the most likely places to find water ice, and the most promising places to probe the moon's early history.
* Try to map the unique lighting at the moon's poles, looking for places where the sun's energy is constant.
* Develop plans for astronomy and other science that can be tackled only from the unique vantage point of the moon's poles.
"The primary reason for going back to the poles ... is the lighting," Bussey said in an interview. Unlike the Earth's polar regions, the moon's poles offer more relatively benign conditions of sunlight and temperature.
Apollo astronauts spent only a few days on the surface, and all landed near the moon's equator. But astronauts on extended stays near the equator would face 14 Earth days of sunlight, followed by 14 days of darkness. Temperatures would blaze to 250 degrees Fahrenheit, then plummet to 250-below.
At the poles, however, the sun may shine nearly all the time, a low, steady light that circles the sky, near the horizon, once every 28 days. Temperatures would stay closer to minus-50 degrees F - arctic conditions, for sure, but far easier for lunar base engineers to design for.
"Nothing is easy, but the fact you've got less [temperature] change makes it an easier engineering problem," Bussey said.
Base planners are drawn to the poles, too, by the prospect of finding places - probably on high mountains or crater rims - that enjoy perpetual sunshine, which could provide a reliable and steady source of electric power and outdoor illumination for a lunar base. But scientists still haven't identified those places.
One goal of the APL-led project is to produce a year-round "illumination map" of the polar regions, using imagery from past, current and future missions to establish precisely where explorers might find the most sunlight.
Or darkness. Scientists believe the floors of some deep polar craters may be perpetually shaded by the surrounding rims and mountains. Temperatures there - minus-364 degrees - might preserve buried deposits of water ice left eons ago by crashing icy comets or asteroids.
If so (and if it can be extracted), the water might help sustain a lunar base, saving NASA the expense of hauling water from Earth. With solar energy, water also might be broken down into hydrogen and oxygen, providing rocket fuel for the flight home.
Data gathered by NASA's Clementine orbiter in 1994 and the Lunar Prospector mission in 1998 and 1999, have yielded tantalizing hints with remote sensing but "nothing definitive" about the presence of water at the moon's poles, Bussey said.
The APL team will look at that data again and consider new evidence from NASA's Lunar Reconnaissance Orbiter, due to launch in April. But definitive proof, he said, must wait for robot landers. "You have to go down to the surface," he said.
Others on the team will use polar images to begin measuring slopes and rock distributions, looking for safe landing sites and promising base locations.
And with the promise of a permanent, manned base at one of the moon's poles, APL-led investigators also will explore what kinds of scientists could make best use of the vantage point.
Planetary geologists, for example, might want to drill into layers of rock and ice for clues to the early history of the solar system. Polar exploration could provide insight into early comet and asteroid impacts on the moon and on Earth, where 4.5 billion years of weathering and continental drift have erased the evidence.
The south pole's Aitken Basin, the moon's biggest, deepest impact crater, will be a key target. That asteroid strike "probably stripped off the entire lunar crust," Bussey said, "so we'll have part of the inner mantle on or close to the surface. That will tell us a lot about the history of the moon."
Another team member, Mark Postman, of the Space Telescope Science Institute in Baltimore, will explore opportunities that a polar base would offer astronomers. He is especially eager for NASA to place a solar-powered telescope there and aim it at Earth.
"Our objective is to monitor how the Earth's bio-signature varies over time," he said. That's the evidence of life - oxygen, methane, water and other signals - that's imprinted into sunlight as it's reflected off our planet. The information should help astrobiologists recognize signs of life as they analyze the light reflected off planets circling other stars.
"For this application," Postman said, "the moon is really the ideal place to be."