The drawing outlines
essential steps in a Lunar Orbiter mission following launch and transit to
the Moon's vicinity.
In step 1 the spacecraft fired its velocity control rocket to make a
In step 2 the rocket fired again to deboost the spacecraft into its
initial orbit of the Moon.
Here its orbit was adjusted, and the first pictures were made (3) before
the Orbiter changed orbital parameters (4)
to assume an elliptical orbit that brought it closer to the lunar surface
for further photographic coverage (5).
early years of the space race, the United States engaged in a series of
key programs designed to fly past, orbit, impact, and soft-land on the
Moon. The implementation of projects such as Ranger, Lunar Orbiter, and
Surveyor was critical to the eventual success of the Apollo program,
NASA's major program to land astronauts on the surface of the Moon before
the end of the 1960s.
United States took an early interest in the Moon. Less than a year after
the launch of the Soviet Sputnik satellite that set off the “space race,”
the U.S. Air Force Ballistic Missile Division assembled a series of small
probes designed to enter orbit around the Moon. These spacecraft, called
Able (and later, Pioneer), carried rudimentary infrared TV scanners to
take pictures of the lunar surface for about two weeks while in orbit
around the Moon. Unfortunately, none of the three Air Force Able probes
ever made it to the Moon. After a failure in August 1958, the second,
launched in October 1958, reached about one-quarter of the way to the Moon
and returned valuable data on the Van Allen radiation belts that encircle
the Earth. The third Able, launched in November 1958, failed to reach
beyond 1,500 kilometres of the Earth.
Ranger 7 took this image, the
first picture of the Moon by a U.S. spacecraft,
on 31 July 1964 at 13:09 UT (9:09 AM EDT) about 17 minutes before
impacting the lunar surface
Force attempts were followed by two Army launches in 1958 and 1959,
carrying probes designed for more simple flybys of the Moon. They were
also to test an experimental imaging system. The first, Pioneer 3, reached
about one-fourth of the way to the Moon. The second, Pioneer 4, had the
distinction of being the first U.S. spacecraft to reach “escape velocity,”
i.e., speed sufficient to leave Earth orbit. Due to an engine malfunction,
however, the probe passed by the Moon at a range of about 59,545
kilometres, much farther than originally planned. Pioneer 4 eventually
became the first U.S. probe to go into solar orbit.
final group of early lunar probes was built by Space Technology
Laboratories for NASA, and all were designed to reach lunar orbit. They
were launched by the Atlas Able booster. All three (of a planned four),
launched between November 1959 and December 1960, failed to reach the
Surveyor 7 mosaic of the rim area
of Tycho from the highland region north of the crater on the Moon.
Surveyor 7 landed 10 January 1969 and took about 21,000 photos over a
some of which were used to make up this mosaic.
these failures behind it, NASA embarked on several new projects designed
specifically to support the Apollo program. The first of these was Ranger,
managed by NASA's Jet Propulsion Laboratory (JPL). Originally, the Ranger
spacecraft, launched by the Atlas Agena B, were designed to land simple
instrumented capsules made of balsa wood on the lunar surface. The first
two experimental spacecraft failed in their test missions. Ranger 3, the
first fully operational spacecraft, missed the Moon and flew into solar
orbit. Ranger 4 lost power during the three-day-long outbound flight.
Because of an accurate initial trajectory, however, it hit the surface of
the Moon on April 26, 1962, becoming the first U.S. spacecraft to reach
the Moon, three years after the Soviets achieved a similar success with
Luna 2. Ranger 5, launched in October 1962, also missed the Moon.
these failures, NASA introduced a new Ranger design that had more modest
missions goals. Instead of landing a probe on the lunar surface, these
“Block III” Rangers were designed to take high-resolution photos prior to
impact using a set of six cameras. Ranger 6, launched in January 1964,
successfully impacted on the Moon, but the camera system did not function.
Success finally came with Ranger 7, launched in July 1964, which took more
than 4,000 impressive photos of the Moon as it careened to the northern
rim of the Sea of Clouds. The photos were, in many cases, one thousand
times better than photos of the Moon taken from the Earth. Ranger 8 was
another resounding success. It took thousands of photos prior to impact on
the Sea of Tranquillity in February 1965, an area that was the target for
the first Apollo landing in 1969. Ranger 9 continued the spate of
successes, taking nearly 6,000 spectacular photos before crashing into the
Alphonsus crater in the lunar highlands in March 1965.
NASA program, also managed by JPL was the much more ambitious Surveyor
project. NASA originally conceived the Surveyor program in 1963 as a
lander/orbiter combination project, but later scaled it down to only
soft-landing. Each lander comprised a three-legged triangular aluminium
structure with a large solid propellant retro-rocket engine at the base.
The lander was equipped with an advanced imaging system. After three tests
of the Atlas Centaur booster in 1965-1966, NASA launched Surveyor 1 in May
1966. The mission was a resounding success. The spacecraft landed
successfully in the Ocean of Storms on June 2, 1966 and took more than
11,000 photos of the surface over a month-long period. Although Surveyor 2
failed, Surveyor 3 successfully landed on the Moon in April 1967. In
addition to an imaging system, the lander also included a remote scooper
arm to determine the density of lunar soil. Experiments showed that the
lunar soil had the consistency of wet sand. More than two years later, in
November 1969, Apollo 12 astronauts Charles Conrad, Jr. and Alan Bean
landed their Intrepid Lunar Module about 180 meters from Surveyor 3 and
recovered some its parts to evaluate the environmental effects of a long
period on the Moon's surface. Surveyor 4 was a failure, but Surveyors 5,
6, and 7 successfully landed on the Moon in 1967 and 1968, returning vast
amounts of photographs and data on the Moon that were critical to
designing experiments for the Apollo missions. In total, the five
successful Surveyors returned more than 87,000 photos of the Moon and
showed that it was feasible to soft-land a large probe on the Earth's only
conducted the Surveyor program in parallel with the Lunar Orbiter project,
which was designed to accurately map the lunar surface (down to one-meter
resolution). The program's primary goal was to allow mission planners to
select the safest, but most scientifically interesting landing sites for
the Apollo missions. The project was managed by NASA's Langley Research
Centre. Each Lunar Orbiter weighed about 380 kilograms and was launched by
the Atlas Agena D rocket. Each carried an imaging system that could
develop exposed film, scan the photos, and broadcast them back to Earth.
Five Lunar Orbiters were launched between August 1966 and August 1967. All
were highly successful, and in total, mapped roughly 99 percent of the
lunar surface. Lunar Orbiter 1 became the first spacecraft to take a
picture of the Earth from the Moon. While the first three orbiters mapped
potential Apollo landing sites, the last two conducted more general
scientific surveys and also mapped the lunar gravitational field.
two of the three final piloted Apollo missions to the Moon in 1971 and
1972, the crews of Apollo 16 and 17 each released a single small satellite
known as the Particles and Fields Satellite to conduct scientific
experiments. Each of these probes weighed about 80 pounds (36 kilograms)
and operated for a few months, successfully returning data on magnetic
fields and other interplanetary phenomena.
the Pioneer, Ranger, Surveyor, and Lunar Orbiter spacecraft, NASA also
attempted to send three Explorer spacecraft to the Moon. Two of them,
launched in 1967-1968, were International Monitoring Platform (IMP)
vehicles. After an initial failure, the second IMP, Explorer 35,
successfully studied the solar wind, the interplanetary magnetic field,
the lunar gravitational field, and other interplanetary phenomena from
lunar orbit. The third vehicle, Explorer 49, reached lunar orbit in June
1973 and studied low-frequency radio emissions in the solar system. It was
the last U.S. mission to the Moon for 21 years. With the end of the Apollo
program in 1972, NASA redirected its deep space program to other targets.
United States finally resumed robotic exploration of the Moon in 1994 with
the launch of Clementine, jointly managed by the Ballistic Missile Defence
Organization (BMDO) and NASA. Clementine, carrying 25 technology and
scientific instruments on board, successfully entered orbit around the
Moon in February 1994. During its first two months in lunar orbit, it took
about 1.6 million digital images of the Moon. In December 1996,
Clementine's scientific team caused a sensation when they announced that
the spacecraft's instruments had detected ice at the bottom of a lunar
crater near the Moon's south pole.
recent NASA mission to the Moon was that of Lunar Prospector, a part of
NASA's Discovery program of low-cost missions. Lunar Prospector
successfully entered orbit around the Moon in January 1998. During its
year-long primary mission, the probe discovered an estimated six billion
tons of water ice trapped in the shadows of the Moon's polar areas, thus
bolstering the value of Clementine's earlier findings. The mission was
managed by NASA's Ames Research Centre.