Exploration of the inner solar system
author Paul Boșcu, June 2017

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The first obvious place to send a space probe was the Moon. It’s only 384,400 km away, so both NASA and the former Soviet Union launched unmanned lunar missions in the 1960s. Luna 3 was first to send photographs of the back side of the moon, which we cannot see with telescopes from Earth. Probes from the United States named Ranger 6, Ranger 7, and Ranger 8 took a total of 17,267 pictures before they each smashed on the Moon. These pictures confirmed that there were thousands of craters too small to be seen clearly from Earth.

The Luna program was launched in the highly competitive political climate of the 1960s. Luna 9 became the first spacecraft to soft-land on the Moon, proving that the surface is firm. Though other spacecraft failed just before and after it, Luna 10 became the first spacecraft to enter lunar orbit. Luna 11 and 12 added more orbital maps. On December 24, 1966, Luna 13 landed on the Moon and provided TV panoramas. Luna 14 was the last of this series and entered orbit on April 10, 1968.

A total of nine Rangers were launched between 1961 and 1965. All but the last three failed, though Ranger 4 did become the first artificial object to impact the Moon’s far side. Ranger 7 relayed the first close-range lunar photos. Ranger 8 provided more coverage, and Ranger 9 sent images shown live in the first television spectacular about the Moon.

The first Surveyor soft-landed on the Moon in June 1966. Also that summer, the first of five Lunar Orbiters was launched to take photos and test ground tracking systems. Surveyor 2 crashed, but Surveyor 3 landed in April 1967. The Apollo 12 crew landed nearby in and retrieved a piece of it. Surveyor 4 crashed, and 5 landed in the Sea of Tranquility. Surveyor 6 landed and took off from the surface, the first American spacecraft to do that. The last Surveyor reached the Moon on January 10, 1968. It landed in the crater Tycho.

The Soviet Union may’ve been unlucky when it came to exploring Mars but it experienced dramatic success with missions to Earth’s other neighbor: Venus. The Venera program ran from 1961 through 1984 and boasted 10 landing probes that gathered data on Venus’s surface, as well as 13 flyby or orbital probes that sent data home from the atmosphere surrounding Venus. The landers, in particular, were technological marvels that were able to withstand the extreme conditions at Venus’s surface: high pressure, temperatures hot enough to melt lead, and a corrosive atmosphere.

Probes to the Moon or to Mars were capable of surviving and transmitting data for weeks or months (or, in some cases, years), but the surface conditions on Venus were much less hospitable. Venus’ atmosphere is so thick that the atmospheric pressure at the planet’s surface is almost 100 times that at the Earth’s surface. Venus’s thick clouds trap the Sun’s heat, resulting in a surface temperature of about 260 degrees Celsius. Because of this the space probes that managed to reach Venus’s surface lasted for only around an hour, so the amount of data they were able to return home was limited.

Ironically, although surface conditions on Venus are much harsher than on Mars, landing on the former is actually easier than landing on the latter. The thick atmosphere means that a series of parachutes can slow a spacecraft down from interplanetary speeds to a safe landing speed without having to rely on complex retrorockets to be fired before landing (like the ones needed on Mars due to the planet’s thin atmosphere).

Venus is a tremendous challenge for spacecraft. Many of the 19 Soviet missions failed, but Venera 4 was the first spaceship to enter the atmosphere of another planet; Venera 7 was the first to land on another planet; and Venera 9 transmitted the first photographs from the surface of another planet.

Models of Venus evolved from steamy jungles to dry deserts to global oceans of carbonated water. In 1967, Carl Sagan suggested that “float bladder macro-organisms” might live in the atmosphere. Radar studies of the surface in 1970 detected dark, circular regions that were interpreted as giant impact basins. All of these models would be found lacking before the end of the 1970s.

Venera 7 accomplished what no space mission had up to that point: It landed a capsule on another planet. Previous Venera missions had included atmospheric probes, which sent back data as they traveled through Venus’s atmosphere, but these spacecraft didn’t survive the trip to the surface. Launched in 1970, the Venera 7 spacecraft made it to Venus and successfully jettisoned its landing capsule, which parachuted down to the planet’s surface. Using a raised antenna, the capsule sent home radio signals with data regarding Venus’s surface temperature for 23 minutes before communications ceased.

The transmitter actually got stuck and sent back the surface temperature over and over again. However, the Venera 7 mission still counts as the first time that data was sent back from the surface of another planet.

Soviet Venera 8 landed on Venus on July 22, 1972. With a new refrigeration system, it survived the intense heat for 50 minutes.

Scientists were surprised that Venera 8 measured light levels that were similar to a cloudy day on Earth. They had not included a camera because they assumed that almost no light would penetrate the thick clouds.

The four Venera spacecraft covered in this section all succeeded in capturing images of Venus’s surface. These photos were valuable because they were mankind’s first views of the surface of a mysterious world whose surface can’t be seen from space (unless you have radar eyes!). Though science-fiction-style steamy jungles had mostly been ruled out due to Venus’s hot surface temperatures, scientists were surprised to see a world that was more Earthlike than they’d originally imagined.

Although its main goal was to take measurements of the Venusian atmosphere with a variety of instruments, Venera 9, launched in 1975, wound up taking the first pictures returned from the surface of another planet. The spacecraft had two cameras, one on each of its sides (although one didn’t work because a lens cap failed to pop off correctly). The functioning camera used a periscope to peek through Venera 9’s hull so it could capture images without exposing itself to the extreme temperature and pressure conditions on the Venusian surface.

Later in 1975, Venera 10 was similarly successful in taking images of Venus’s surface, which revealed a smooth surface similar to a terrestrial lava flow. Complete lens cap failures on Venera 11 and 12 caused a slight delay in the flow of photos from Venus until 1981 when Venera 13 and 14 successfully returned color images of the planet’s surface.

Venera 13 and 14 used a more-sophisticated camera design that allowed them to send higher-resolution images back to Earth for a combined total of approximately three hours. The color images were made by taking versions of the same image using different color filters on the camera.

Unlike their predecessors, both cameras worked on Venera 13 and 14, allowing a complete view of the terrain surrounding the landers. Turns out the surfaces at both of these landing sites are covered with smooth, platelike rocks that are likely lava flows. The sky is yellowish, and the surface appears brownish-yellow.

Venera 15 and 16, launched just five days apart in 1983, achieved what previous Venera missions couldn’t: They mapped Venus’s surface. Although the images sent back from previous Venera missions were both novel and useful, Venus’s thick atmosphere prevented traditional cameras from seeing through to the surface. Thus, a global study of the planet’s surface was impossible until radar instrumentation was added to the Venera spacecraft.

Global surface maps are essential for understanding the geologic history of planets and how features at different parts of a planet’s surface relate to each other, when and how they were formed, and so on. Such maps can also be used to determine approximately how old the surface is, and how recently it was likely to have had active volcanoes or other activity.

Twinning of spacecraft, or launching a pair of identical spacecraft within a very close time frame, was typical in the Soviet space program (as well as the American space program), because the combined effort of the two spacecraft was able to provide much more coverage for image mapping than either would’ve been able to do alone. In the case of Venera 15 and 16, the two spacecraft were designed identically, and the surface-imaging instrumentation took up enough space that these missions didn’t have entry probes.

Each spacecraft contained dish antennae for transmitting a signal through the thick Venusian clouds and then picking up that signal as it bounced off the surface. They also used solar arrays to power the equipment.

The twin Vega spacecraft took advantage of the United States’ decision to cancel its planned probe to Comet Halley, which passed near the Earth in 1986. The Soviet Union stepped in to build a comet probe and soon realized that planetary alignments would allow a spacecraft to fly past Venus on its way to the famous comet. The Soviet Union built two more copies of its now well-tested Venera spacecraft, with a few variations. The most innovative of these tweaks was the addition of a balloon, or aerostat, that was released during the spacecraft’s descent into the Venusian atmosphere.

The 3.4-meter diameter balloon carried an attached gondola of instruments on a long tether. The two aerostats successfully measured Venus’s atmospheric composition and tracked its wind currents, sending back data for 46 hours until their batteries ran out. During that time, Vega 1 and 2 floated more than one-third of the distance around the planet at an altitude of about 54 kilometers above the surface. After releasing the landers and aerostats, the two Vega spacecraft used Venus’s gravity to travel on to their rendezvous with Comet Halley in 1986. By mid-1986, they’d sent back more than a thousand pictures of the comet’s nucleus.

Early planetary exploration didn’t belong solely to the Soviet Union. American planetary research started with the Mariner Program, the United States’ first long-running interplanetary spacecraft program, which was intended to start exploring the strange new worlds of Mercury, Mars, and Venus. This program consisted of ten space missions, seven of which were successes. Two additional missions (Mariner 11 and 12) were planned but ended up rolling into the start of the later Voyager Program.

The Mariner spacecraft, which wound up becoming the basis for almost all of NASA’s planetary exploration for decades to come, were based on either octagonal or hexagonal core units to which the instruments (cameras, antennae, and the like) were attached. Like most other space missions, solar panels provided the main source of power for both the spacecraft and its instrumentation. Atlas rockets, either Centaur or Agena, were used to launch the Mariner spacecraft into space.

Even though its focus was on putting a human on the Moon before the Soviets could, NASA also set its sights on exploring the rest of the solar system in the 1960s. The Moon was close enough to make human space travel there feasible, but the initial forays to other planets had to be done by robotic spacecraft. Thus Mariner 2 became the first spacecraft to fly past another planet (Venus). Later, Mariner 5 returned to Venus.

The leadoff mission of the Mariner series, Mariner 1, would’ve done a flyby of Venus, but it was destroyed shortly after launch due to a problem with the launch rocket. Mariner 2, which was originally intended as a backup to the Mariner 1 mission, was a stunning success and became the first spacecraft in the world to fly past another planet. On December 14, 1962, Mariner 2 came as close as 34,833 kilometers to the Venusian surface.

Another achievement of the Mariner 2 flyby was the variety of important data it collected, such as cloud temperatures, Venusian surface temperatures (the planet measured about 400 degrees Celsius on the surface), and other data concerning Venus’s atmosphere. Additionally, the sensors and instruments aboard Mariner 2 discovered that Venus has no major magnetic field or radiation belt surrounding it.

Mariner 5 was originally intended as a backup Mars mission, but after the success of the Mars-bound Mariner 4, it was retrofitted and sent to Venus. It succeeded in taking measurements of Venus’s atmosphere, refining the measurements made by Mariner 2 and providing further clues about the planet’s hot surface temperature and thick atmosphere.

The final spacecraft in the Mariner Program, Mariner 10 (1973) was the first spacecraft ever to take advantage of gravity-assisted trajectory, a technique that uses the gravitational attraction of one planet to propel a spacecraft toward its next goal in an effort to conserve both fuel and power. In this case, Mariner 10 was slated to fly past Venus and then continue on to Mercury, where it could make multiple flybys.

Mariner 10 used its ultraviolet cameras to take detailed images of the structure of the Venusian clouds, but the real prize of the mission was Mercury. Three Mercury flybys, in 1974 and 1975, revealed an ancient, battered surface covered with impact craters, but with signs of old volcanic activity and some cracks and faults. Mariner 10 was only able to map one hemisphere of Mercury; the other hemisphere remained unseen for more than 30 years until the MESSENGER mission.

Little was known about the surface of Venus throughout most of the 20th century because previous exploration of the planet in the 1970s had been foiled by thick clouds that prevented astronomers from seeing through to the surface. The United States’ Magellan mission, launched in 1989 by the Space Shuttle Atlantis, used a sophisticated radar system that was specially designed to be able to see through the clouds of Venus. Magellan was in orbit over the poles of Venus, which allowed it to map more than 98 percent of the planet’s surface.

Magellan’s radar images have been combined with altimetry data to produce three-dimensional stereo views of Vensus’s surface, made by combining slightly different views of the surface taken on different orbits.

After Magellan accomplished its mapping goals, the spacecraft’s systems began to fail, and it was given one final task: to test a new technique called aerobraking (using a planet’s atmosphere to change the orbit of a spacecraft). Spacecraft controllers flew the spacecraft deep into Venus’s atmosphere until it burned up. The technique destroyed Magellan but helped engineers figure out how to use aerobraking to change the orbits of other spacecraft. Consequently, this technique has since been used successfully by Mars Global Surveyor and Mars Odyssey.

The Magellan images revealed a Venusian surface that’s surprisingly Earth-like in many ways but lacking common geologic features found on the Earth. Most of Venus’s surface features are volcanic in nature, including huge lava plains, large shield volcanoes, and smaller lava domes. Lava channels that can be up to 6,000 kilometers long suggest that the lava was very fluid and likely erupted at a high rate. However, because Venus is a very hot planet, with a surface temperature as high as 454 degrees Celsius, water isn’t stable at its surface, meaning the familiar geologic features carved by water on Earth are missing on Venus.

Magellan found no evidence of plate tectonics on Venus, meaning that volcanic activity on Venus operates very differently from Earth.

One surprise found on Venus was that the surface appeared to be uniformly younger than expected, as shown by the small number of impact craters, with an average surface age of about 500 million years. In geologic time, this is quite young. Scientists are still debating whether a single cataclysmic resurfacing event, such as a global volcanic eruption, could’ve occurred 500 million years ago, or whether the apparent surface age comes from ongoing geologic activity that slowly covers in parts of the planet’s surface.

After the heyday of American lunar exploration during the Project Apollo years, little further study of the Moon was done until the Clementine mission of the 1990s. This mission produced global digital maps of the Moon and was followed by the Lunar Prospector mission, which studied the Moon’s composition from orbit.

The Clementine mission, a joint effort of NASA and the Ballistic Missile Defense Organization, took American space exploration back to the Moon when it launched on January 25, 1994. Although the primary purpose of Clementine was to test how spacecraft and sensors behave when in space for long periods of time, NASA was also able to sneak in some science from a variety of different instruments.

Clementine actively observed the Moon using ultraviolet and infrared imaging, laser altimetry, and other measurements. With the combined results of this various data, scientists pieced together a multispectral picture of the Moon’s surface, including a more in-depth study of lunar surface material and a modern digital global map of the Moon’s surface. These maps help scientists better understand the geologic history of the Moon and have been used as a basis for future lunar exploration.

The Lunar Prospector mission was charged with mapping the composition of the lunar surface. The small, cylindrical spacecraft launched on January 7, 1998, and entered lunar orbit four days later. During its voyage into lunar orbit, the spacecraft deployed three 2.5 meter arms that contained instruments and equipment for measuring the Moon’s gravity, atmosphere, temperature, and other quantities necessary for creating a surface compositional map of the Moon.

Lunar Prospector sent home data concerning the Moon’s atmosphere and crust, as well as the potential for ice on the satellite’s surface. A major goal of the mission, which fit into the Prospector name, was to explore the Moon from the perspective of resources that might be available to help support either future human exploration or commercial enterprises. If water were found on the Moon, for instance, it would be a very important resource for future human bases.

The crash of Lunar Prospector in 1999 resulted in the first burial of human remains on the Moon. A lipstick-sized canister of ashes of geologist Eugene Shoemaker was strapped inside the spacecraft. As the first head of the United States Geological Survey’s Center of Astrogeology, Eugene Shoemaker is considered the father of astrogeology, the geologic study of celestial bodies such as the Moon, asteroids, and Mars. He headed the Apollo lunar geology efforts and trained the astronauts.

Separately or with his wife Carolyn Spellman, Shoemaker discovered 32 comets and 1,125 asteroids, including Shoemaker–Levy 9 that impacted Jupiter in 1992. He was well known for his conviction that comets and asteroids pose a threat to life on Earth. He supported the theory of Luis and Walter Alvarez that an impact was responsible for the Cretaceous–Tertiary extinction of the dinosaurs 65 millions ago.

Eugene Shoemaker died in a violent car collision while studying impact craters in Australia in July 1997 at age 69.

In 1995 the Solar and Heliospheric Observatory (SOHO) was launched, a collaborative project between NASA and the European Space Agency (ESA). SOHO’s primary mission was to study and discover more about the Sun from close range. Specifically, its mission goals were to understand the core structure of the Sun, study the solar corona, and demystify the details behind solar winds. The observatory consists of two separate modules, a Service Module and a Payload Module.

The Service Module powers the spacecraft, controls its temperature, and sends data back to Earth. The Payload Module houses all the spacecraft’s instruments. Instrumentation on SOHO came from both European and American developers. Twelve main instruments are designed to work together to relay info back to Earth, and scientists worldwide use the data gleaned from these instruments. The major instruments include ultraviolet imagers and solar wind and energetic particle detectors.

SOHO orbits between the Sun and the Earth, around a stable point known at the First Lagrangian Point, or L1. At L1, the gravity from the Sun and the Earth balances out, resulting in a particularly stable orbital configuration. SOHO orbits the L1 point so it can always communicate with Earth.

Originally intended to operate for about two years, SOHO is still operating, after more than 20 years, and has returned valuable data regarding solar winds, solar structures, coronal waves, and new phenomena such as solar tornadoes. SOHO has also spotted hundreds of comets and played a large role in helping meteorologists give more-accurate weather predictions.

NEAR (Near-Earth Asteroid Rendezvous) made history as the first spacecraft to orbit an asteroid. Launched in February 1996, NEAR was destined to rendezvous with the near-Earth asteroid 433 Eros to help provide more information about the composition, magnetic field, mass, and other attributes of the asteroid. Even though NEAR was never built as a lander, in February 2001, at the end of its mission, the probe was commanded to go into a lower and lower orbit until it eventually touched 433 Eros’s surface.

NEAR sent back high-resolution images and other measurements, and it was a successful first launch of NASA’s Discovery Program of low-cost spacecraft. The asteroid’s extremely low gravity allowed the probe to survive the landing, creating a first in the grand scheme of space probe research: Something that wasn’t a lander landed.

NEAR orbited 433 Eros for nearly a year before landing and returned dozens of high-resolution photographs of the asteroid. Before it landed, the spacecraft was renamed NEAR Shoemaker in honor of the geologist Eugene M. Shoemaker.

The Stardust mission was the first sample return of particles from a comet. The spacecraft launched in February 1999 and reached Comet Wild 2 in January 2004. As the spacecraft performed a close flyby of the comet, it collected samples from the comet’s coma (essentially the ice and dust particles that make up the comet’s “tail”). The comet particles were captured in a very low-density collector called an aerogel. The capsule containing the sample returned to Earth in 2006, landing with a parachute in the Utah desert. The main Stardust spacecraft continued past Earth and flyed to Comet Tempel 1 in 2011. Stardust ceased operations in March 2011.

The Stardust mission is historically significant because it marks the first time scientists had an opportunity to study an actual sample that came from primitive comet material. Because comets are thought to be primordial remnants from when the solar system was young, researchers hope to use this information to study the origin and evolution of the early solar system. In 2014 scientists announced the identification of possible interstellar dust from the Stardust capsule.

As the spaceship approached Earth, it released the sample-return capsule. On January 15, 2006, the capsule entered Earth’s atmosphere. Parachutes were deployed, and the capsule landed safely in the Utah desert.

Deep Impact wasn’t just the name of a NASA mission — it was what scientists wanted their spacecraft to leave on Comet Tempel 1 so they could study the comet’s insides. The plan was to send part of the spacecraft on a collision course with the comet and use the impact to eject material from inside the comet into outer space, where it could be studied. Impact occurred on July 4, 2005. Afterwards Deep Impact was placed on an extended mission to study other comets until contact was lost in 2013.

The spacecraft used during the Deep Impact mission was designed in two Sections: a “Smart Impactor” was for crashing into the comet. It was armed with a camera so it could take pictures of the comet’s nucleus right up to the point when the impactor itself was destroyed. A flyby spacecraft was for photographing the crash and continuing on with a flyby. The flyby spacecraft was solar-powered and shielded from flying comet debris and boasted different types of cameras and other measuring devices.

The impact made a large crater and ejected a large plume of material, as NASA scientists had hoped it would. However, images and compositional measurements taken by the flyby spacecraft showed more dust, and less ice, than scientists had anticipated. Due to the composition and chemistry, scientists were able to better pinpoint where the comet may have originated — the far reaches of the Kuiper Belt.

Genesis was a revolutionary spacecraft designed to collect charged particles emitted from the Sun and return them safely to Earth. It would assume a tight orbit around L1 for 2.5 years, collect samples, and then head back to Earth. On September 8, 2004, Genesis began its descent to a Utah landing site. However, its parachutes did not open, and the spacecraft plummeted at high speed into the ground. The capsule containing the samples split open during the crash, exposing the sample medium to the outside atmosphere. However, approximately 4 milligrams of samples were saved.

After reentering Earth’s atmosphere, Genesis would deploy its parachutes for a slow descent toward the surface. A specially equipped helicopter was to snag the spaceship midair and carry it to land. Genesis was the first spacecraft to return extraterrestrial materials to Earth since the Apollo missions.

In 2005, the European Space Agency (ESA) brought exploration of Venus back into the spotlight with Venus Express, the agency’s first foray into studying Venus. Venus Express was prepared and launched in November 2005, reaching Venus about five months later. Venus Express has made a number of interesting discoveries, including observations of frequent lightning, a vortex in the atmosphere over the South Pole of the planet, and evidence that liquid water could’ve existed on Venus long ago.

Venus Express has completed its 500-day mission and was an extended mission until late 2014. Contact with the spacecraft was lost on 18 January 2015, when ESA scientists last detected the spacecraft's X-band carrier signal.

The MESSENGER spacecraft, whose name is short for MErcury Surface, Space ENvironment, GEochemistry and Ranging, launched on August 3, 2004. The probe took a long and looping trip through the inner solar system, relying on flybys of Earth, Venus and Mercury to slow down enough to be captured by Mercury's gravity. MESSENGER finally arrived at Mercury on March 17, 2011, becoming the first probe ever to orbit the heat-blasted world, and just the second spacecraft ever to study it up close.

MESSENGER's original mission at Mercury was supposed to last just one year, but NASA extended operations twice so the probe could continue its observations, which team members say have revolutionized our understanding of the planet.

MESSENGER mapped the planet in unprecedented detail, discovered that Mercury hosts a strangely offset magnetic field and confirmed that permanently shadowed craters near Mercury's poles harbor deposits of water ice. During the course of its four years at Mercury, MESSENGER captured more than 250,000 images and used its seven scientific instruments to gather extensive data sets that will keep scientists busy for years to come.