Last week marked the 40th anniversary of the Apollo 17 mission Ð the sixth and last manned lunar landing mission.
The Apollo 17 crew included mission commander Eugene Cernan, lunar module pilot Harrison "Jack" Schmitt and command module pilot Ronald Evans.
Apollo 17 lifted off on Dec. 7, 1972 the only nighttime launch in the Apollo program and after a three-day voyage (which the onboard astronauts took the famous and iconic "blue marble" photograph of Earth) arrived in lunar orbit. Cernan and Schmitt then departed the command module America, and in the lunar module Challenger, descended toward their planned landing site, the Taurus-Littrow valley in the lunar highlands, arriving there on Dec. 11.
Cernan and Schmitt a geologist by profession and the first and only trained scientist to visit the moon Ð spent three days at Taurus-Littrow, performing three seven-hour moonwalks and conducting a variety of scientific investigations during the course of these.
Forty years ago this Friday, on Dec. 14, 1972, Schmitt, and then Cernan, ascended the ladder of Challenger from the lunar surface.
They then lifted off from the moon and met with Evans and America. Afterward, the three astronauts and America departed lunar orbit and headed toward Earth and splashed down into the Pacific Ocean on Dec. 19.
No one has visited the moon since the Apollo 17 crew departed it four decades ago.
Although there were originally three additional Apollo missions scheduled to fly after Apollo 17, these were cancelled due to budget cuts.
Meanwhile, even unmanned lunar exploration soon ground to a halt. At the same time, the former Soviet Union continued to launch a few more Luna missions during the subsequent years, including the Luna 21 mission, which deployed a rover in early 1973 and the Luna 24 mission, which successfully returned lunar samples to Earth in 1976, with the completion of this latter mission even that program ceased.
It wasn't until the 1990s that probes of any kind visited the moon; the earliest ones were the Japanese Hiten probe in 1991 and the American Clementine probe a Department of Defense mission in 1994. They were followed by the low-budget American Lunar Prospector mission in 1998.
Although these were not especially sophisticated missions, Clementine data indicated the possibility that ice might exist in permanently-shadowed craters near the moon's poles and data from Lunar Prospector somewhat strongly supported this.
The past few years have seen a resurgence in interest in unmanned lunar missions, both here in the U.S. and in several other countries.
In addition to the American probes, lunar-orbiting spacecraft have been launched by Japan, China, India and the European Space Agency.
The two primary American missions have been the Lunar Reconnaissance Orbiter (LRO) mission, which has been photographing the moon's surface in unprecedented detail, including various images of the Apollo landing site and the Lunar Crater Observation and Sensing Satellite (LCROSS) spacecraft.
The two probes were co-launched in 2009, and in October of that year, LCROSS impacted a crater near the moon's south pole, with water being definitely detected in the resulting debris plume.
A recent American effort was the Gravity Recovery And Interior Laboratory (GRAIL) mission, which consisted of two spacecraft since named Ebb and Flow that have been orbiting the moon closely in tandem with each since the beginning of this year in an effort to perform high-resolution mapping of the moon's gravity field.
According to recent results from the GRAIL mission, the moon's crust nowhere thicker than 27 miles is a crushed and pulverized "rubble pile" due to the violent and enormous impacts the moon has undergone since its original formation.
Ebb and Flow are being targeted to impact the moon's surface just before 3:30 p.m. MST Monday, Dec. 17.
So when will humans ever visit the moon again? This question cannot be easily answered.
The American Constellation program, originally proposed and initiated in 2004, had established a timetable of a flight to the moon to take place by 2020, however in reality, Constellation was never adequately funded and it was cancelled in 2010.
While potential lunar missions are presently being discussed, there are no formal American plans for a return to the moon anytime in the near- to mid-foreseeable future.
Meanwhile, the Chinese space program, which has successfully launched several manned Earth-orbital missions over the past nine years as well as two unmanned lunar orbiting missions within the past five years, has announced plans to launch an unmanned lunar rover mission late next year and to send astronauts to the moon in the 2025-2030 timeframe.
Perhaps the next lunar visitors will come from private efforts.
There is already the Google Lunar X-Prize, a $30-million prize to be awarded to the first private-developed effort to land and deploy a lunar rover, with a deadline date of the end of 2015.
The firm Space Adventures, which has facilitated several private citizen visits to the International Space Station (ISS) over the past decade, is currently marketing an around-the-moon trip, and meanwhile just last week came the announcement of a new company, Golden Spike, that is envisioning taking private visitors on lunar orbiting, and even lunar landing, journeys.
The prices for such trips are not cheap Ð the cost of a Golden Spike lunar landing mission is being estimated at $1.5 billion Ð so the next human to set foot on the moon might be one of our planet's wealthiest.
But in the precedent being set by the Space Adventures ISS flights, the costs could conceivably become more accessible to more "average" citizens within a generation or two.
The first human to walk upon the moon, Neil Armstrong, passed away earlier this year.
One can perhaps hope that the next person to do so is already alive, and may take that "one small step" sometime within the not-to-distant future.
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