Exploring the planets with spacecraft began with the first trip to the Moon, two years after the former Soviet Union set the space program into full motion with the launch of Sputnik I, in October 1957. In September 1959, Lunik (Luna) 2 impacted onto the lunar equator. The next month Luna 3 completed the first flyby of a planetary body, returning a series of pictures of the lunar limb and a few views of the Moon's farside, neverbefore seen by humans.
And Zond III returned images like this:
All told, the Soviet lunar program sent 20 Luna spacecraft and 5 Zond craft to the Moon; most but not all were successful. Several reached the lunar surface. Lunokhod 1 was the first unmanned roving vehicle to operate on another planetary body, landing on November 17, 1970.
The first U.S. missions to the Moon were literally a crash program! Between 1961 and 1965, NASA launched three Ranger spacecraft (7, 8, 9) like bullets to land (impact) onto the surface, destroying themselves in the process. Each Ranger had a television camera mounted in its nose that looked forward at the surface as it approached. They instantly telemetered progressively higher resolution images, until the moment of collision. The first image taken from Ranger 7 on July 31, 1964 was sent back when the spacecraft was 17 minutes from the lunar surface impact point. The large craters Alphonsus, Ptolemaeus, and Arzechel was evident.
The last images had resolutions of just a few meters. Here is a typical example, captured by the Ranger 7 camera. Each trapezoid frames the next scene in the succession:
One of the hallmarks of pre-Apollo lunar exploration was the Lunar Orbiter program, in which five "photolab" spacecraft orbited the Moon between August 1966 and August 1967. The former Soviet Union was actually the first to orbit the Moon, in April of 1966, but its image returns were minimal. The first three Lunar Orbiters used orbits inclined to the equator, and the last two were in near-polar orbits. Here is a cutaway sketch of one of the Orbiters:
Each Orbiter had a payload consisting of two cameras holding 70 mm black and white film. One camera, with a wide angle lens, produced photos whose resolutions ranged from 8 m to 150 m (26 ft to 492 ft) depending on varying orbital altitudes. The second High Resolution camera improved these values by a factor of eight but covered much smaller areas. The photos, developed onboard as negatives, consisted of continuous strips, electronically scanned by a "flying spot" passed into a photomultiplier tube that generated an analog signal transmitted to Earth. On Earth, the signal drove an imaging device which produced 35 mm positive strips that photointerpreters joined into mosaics.
The 2,000, or so, photos from these Lunar Orbiter missions covered almost the entire lunar surface, leading to detailed geological maps that have been interpreted mainly through superposition and cross-cutting relationships (see next page). Here are four typical scenes:
In the top left is an oblique view of the Apennine Mountains, with an uplifted crust that is similar to highlands in terrain characteristics, lapped by mare lavas from younger basalts that filled Oceanus Procellarum. The right photo above shows lava plains with sinuous rilles (flow trenches) in Oceanus Procellarum and part of crater Prinz. Each strip in the mosaic is 4 km (2.5 mi) wide.
The bottom left photo shows mare ridges (squeeze-ups) and another rille. The photo at the bottom right seemingly is a bland view of a flat mare surface, except that a close look discloses an upper flow unit that is marked by a distinct front that creates a lobe marking the advance. These structures clearly signify basin-filling by multiple extrusions.
A primary goal for the Orbiters was high resolution photos of candidate landing sites. The scene below (5 km by 8 km, 3 miles by 5 miles) documents an elliptical site in Oceanus Procellarum, near the Apollo 12 touchdown point, considered acceptable despite a seemingly high crater density.
Constructed from photo strips acquired by Lunar Orbiter IV, at an average altitude of 2,700 km (1,678 miles), the next classic image shows the great lunar "bullseye," known as the Orientale Basin, a multiringed (perhaps four; the outermost with a diameter of 900 km [559 miles]) impact structure invaded by lavas (Mare Orientale). This feature lies just beyond the visible western limb of the Moon; the western Oceanus Procellarum is at the upper right.
The Orbiters confirmed that impact cratering is a (perhaps "the") dominant process operating at the lunar surface. This next image is actually a photograph made by an astronaut from a spacecraft orbiting the Moon. This is a classic young smaller crater, with an intact rim and well-preserved ejecta beyond it.
Copernicus is one of the most studied craters on the Moon. Here it is in a Lunar Orbiter II panorama. Below that is the famous close-up view of the Copernicus interior - this image was named "Picture of the Year":
But, signs of volcanism were ubiquitous: photointerpreters indentified some volcanic calderas, lunar rilles were either flow channels or collapsed lava tubes, domes were evident, and the floors of many large impact craters contained clear indications of volcanism. Best known among confirmed volcanic features on the Moon is the Marius Hills seen in these two Orbiter views:
However, such constructive volcanic features are rare on the Moon. Stratocones have not been found. A few domes, such as the individuals found in the Marius Hills or the domes atop a single larger dome at the Rumker Hills (shown below), are characteristic of upwellings of basaltic lavas.
Some of the rilles on the Moon are very much like volcanic channels on Earth. The Schroeter Valley is a good example:
Both volcanic and tectonic features are shown in this next image which shows part of Mare Nubium. A large wrinkle ridge (lava pushup) is subparallel to a long escarpment, Rupes Recta, that is the surface expression of a fault scarp 115 km long, and up to 300 meters high:
Many large lunar craters contain structures that are almost certainly volcanic features. This Orbiter V view of the interior of Tycho (see previous page; one Full moon image shows a dark interior, the similar image below it has a bright interior, resulting from the method of photo processing) shows, on a larger scale, many features similar to those on terrestrial caldera floors:
This Orbiter V close-up view of the interior of Tycho shows details of volcanic structures such as tumuli and squeeze-ups much like those on terrestrial caldera floors. such as tumuli. The lavas that enter the crater form in the lunar subcrust after the impact unloads rock from above causing hot rock below to melt and migrate to the new crater floor (much like blood flows to a surface wound):
Lunar Orbiter I sent back a view of Earth from above the Moon's surface. This is reputed to be the very first such image of our planet from "way out":
In 2008, the data used to produce the above image of Earth were digitized and reprocessed to generate this image, showing more detail:
The Orbiter program paved the way for the eventual Apollo landings. But it was necessary - and wise - to land unmanned probes on the surface to learn first hand its properties. Hence, the Surveyors.