When a NASA spacecraft returns to lunar orbit Tuesday morning for the first time in a decade, the space agency will be building on 40 years of largely unheralded, but fundamentally important discoveries wrung out of shoestring-budget missions.

Scientists in the post-Apollo era have been quietly revealing the secrets of our satellite and laying the groundwork for a new wave of moon missions. Their efforts have turned up tantalizing evidence that water ice exists on the moon, which would make the long-held dream of sending manned missions to other planets just a little bit easier.

The NASA mission is only the third American trip to the moon since those famed Apollo missions of the late ’60s and early ’70s.

Early Tuesday morning, the Lunar Crater Observation and Sensing Satellite will be live-streaming a flyby as it swings into orbit around the moon, where it will await instructions to plunge into a dark crater on the moon’s surface. That will throw up a plume of debris that LCROSS’ counterpart, the Lunar Reconnaissance Orbiter, will fly through, allowing it to determine whether water ice exists below the surface of the moon.

If water ice is indeed confirmed, it would be a major victory for post-Apollo lunar science, which has suffered from a lack of public interest since the end of manned missions.

“In general, the exploration of the moon has been driven by the manned program,” said Steven Dick, NASA chief historian. “Before the Apollo program, you had the Ranger, the Surveyor and Lunar Orbiter missions, the unmanned missions. Those were all driven so that people could land there.”

Now, as NASA builds the capability to go back to the moon, unmanned missions like LCROSS are again making their way to our satellite. As they generate a new wave of interest, the importance of the missions between Apollo and the new wave could increase.

Immediately in the wake of the Apollo 11 lunar landing, the other shoe dropped on the program’s funding. With the United States mired in Vietnam and burdened by a list of other expenditures, Dick said that the will to put money into lunar exploration fell away. Americans set their sights on domestic issues close to home as well as on farther-flung targets in space.

“The general consensus was ‘We beat the Russians, and that’s what we set out to do,’” he said.

The U.S. spent an estimated $25 billion in 1969 money on the Apollo program. That would be at least $115 billion in 2008 money and as much as $362 billion, depending on the method of calculation. (NASA’s 2009 budget came in at $18.1 billion.)

The last three Apollo missions were canceled and for 22 years, no spacecraft visited the moon. Why bother? It was lifeless. It seemed impossible that water could survive. We could do a lot of lunar observations from Earth. Really, it became one of the least interesting objects in the solar system, despite its proximity.

Then, in 1994, a Department of Defense funded projected nicknamed Clementine (pictured) launched for the moon. A low-budget affair planned as a technology demonstration, “it did not have science as a primary objective,” a National Research Council report noted. Nonetheless, the mission raised an intriguing possibility: ice on the moon.

Bouncing radio waves to the bottom of permanently shadowed craters at the moon’s south pole, astronomers back on Earth detected reflections that didn’t look as if they’d hit silicate rock. The waves that the Deep Space Network antenna on Earth picked up looked as if they’d hit water ice.

This wasn’t entirely shocking, but it was exciting. It had been suggested as far back as 1961 that water ice might exist on the moon in areas that are never exposed to sunlight. Water left over from the moon’s infancy or deposited there by meteorites or comets could have just stuck around in the craters at temperatures that never rise above minus-280 degrees Fahrenheit. Now, there was some experimental evidence to suggest that there was water ice on the moon.

Though more recent observations called the original finding into question, the little mission sparked renewed interest in the moon. NASA worked up its first lunar mission in a quarter century, the Lunar Prospector, to investigate the possibility of ice, among other lunar mysteries.

The Prospector’s neutron flux data returned evidence of large amounts of hydrogen, “probably” in the form “abundant water ice.” To confirm the observation, at the end of the Prospector’s useful life, the team attempted to crash the craft into a crater and send up a plume of debris with some water ice in it.

Unfortunately, no water ice was detected in the plume, but there were a variety of reasons that could have happened. The high-risk experiment didn’t rule out the presence of ice on the moon.

And there the science sat until President Bush started to ramp up manned solar system exploration through the Constellation Program. As with the Apollo program, robotic missions are preceding the possible return of humans to the moon in the Constellation program.

“The idea now is to go back with the unmanned missions as reconnaissance to go back to the moon with manned missions,” Dick said.

Humans would need water to explore the solar system, so the possible ice on the moon suddenly became a whole lot more interesting.

Come Oct. 9, when LCROSS plows into the moon more than 40 years after the Apollo program began, we could finally know for sure whether there’s ice hiding in a deep, dark crater.

“This mission is the culmination of a dedicated team that had a great idea,” said Daniel Andrews, LCROSS project manager at Ames Research Center in a release. “And now we’ll engage people around the world in looking at the moon and thinking about our next steps there.”