Space Exploration, Unmanned

Manned space exploration receives most of the glory and the attention, but unpiloted spacecraft have been used as tools longer and for a greater variety of reasons. Some even believe that humankind should cease piloted space exploration in favor of the much less expensive unpiloted probes when we consider the accomplishments of these robotic craft.

The beginnings of unpiloted space exploration were made by the Soviets. The first artificial earth satellite, Sputnik, or ‘‘Fellow Traveler’’ carried only a radio transmitter and batteries and no other instruments despite its 80-kilogram-plus orbiting weight. Its batteries lasted about two weeks after the 4 October 1957 launch. The Soviets had been working on an earth-orbiting laboratory, Object D, as their first satellite, but Sergey Korolev, the most influential chief designer in their program, was anxious that the U.S. would launch one first.

He thus convinced his colleagues to prepare a couple of ‘‘simple satellites’’ for orbit, which eventually became Sputniks-I and -II. Object D was thus pushed down to third in the launch list. When the U.S. orbited its Explorer-I satellite on 31 January 1958, it carried a radiation detector and some other small instruments. The radiation detector picked up evidence of a radiation belt around the earth. Object D, when it became Sputnik-3, confirmed this. The era of robotic exploration had begun.

Several dozen Explorers and hundreds of Soviet Cosmos satellites have explored near-earth space, but bodies such as the moon, Mars, and Venus have attracted the most attention from the public. The moon came under early and frequent exploration. After three American failures in 1958, the Soviet Luna-1 flew past in 1959, discovering the solar wind, and the American Pioneer-4 was launched.

The Soviets followed up that year with a hard lander, Luna-2, the first person-built object to land on another celestial object, and Luna-3, which imaged the lunar far side. With Ranger-4, the Americans hit the moon. By then, President John F. Kennedy had pledged America to land a person on the moon and return them to earth by 1970, so the unpiloted programs took on the air of being preparatory to the main event.

The Rangers, which flew until 1965, impacted the moon taking pictures all the way down. The Soviets had by then turned their interest to soft landers, suffering several failures that made them into lunar impactors. The Soviets successfully launched a Lander and orbiter to the moon early in 1966. The American Surveyor series began landings soon after. In the summer of 1966 the first American Orbiter arrived at the moon.

During the next two years, America, convinced that it was in a race to make the first piloted lunar landing, sent seven Surveyors, and five Orbiters to the moon, all but a couple of Surveyors successfully. The Soviets continued to fly orbiters, then graduated to lunar fly-arounds with returns to earth on two Zond missions in the second half of 1968, seemingly in further rehearsal of piloted mission. However, the Americans orbited the moon with a piloted mission near Christmas and landed on 20 July 1969. Any race, if there was one, was over.

The Soviets then turned to sample return missions, in which soil samples from the lunar surface would be returned to earth. The first successful one of these was Luna-16, which landed on 20 September 1970. This was followed over the next six years with several more missions, two with Lunokhod rovers. The similarly between the Soviet science accomplishments on their unpiloted missions and the accomplishments of American piloted missions lent support for those who opposed piloted space exploration as too expensive and dangerous for its worth.

After a 14-year hiatus, the Japanese Muses-A orbiters visited the moon the first month of 1990. This was the first non-U.S. or non-Soviet probe to reach the moon. The Japanese are expected to launch penetration missions early in the 21st century. Meanwhile, the U.S. mounted two missions in the 1990s. The first was the Clementine, intended for technology demonstration and launched by the Ballistic Missile Defense Agency. The second was Lunar Prospector, one of the NASA’s ‘‘faster, cheaper, better’’ missions searching for water and mineral deposits useful for eventual lunar bases occupied by humans. (‘‘Faster, cheaper, better’’ is an effort to avoid the excessive costs of space missions.)

Venus was the target of 17 Soviet spacecraft in the 22 years between 1961 and 1983 and five American overall. The early probes revealed Venus to be as inhospitable as it was beautiful. Mariner-2 discovered a 425°C surface temperature and a thick atmosphere. The Soviets decided to land on the planet, regardless. Their Venera-3 spacecraft was intended to explore the atmosphere, but communications failed just after atmospheric entry and it impacted, although this was quite good navigation for 1966.

Then followed a series of spacecraft that returned atmospheric data before being crushed by the thick carbon dioxide atmosphere. First Venera- 4 on 18 October 1967 returned data on the composition of the atmosphere and the surface temperature, now measured at 500°C. Venera-5 lasted until it reached 26 kilometers of the surface and its sister, Venera-6, went within 11 kilometers. Venera-7 made the first landing on another planet on 15 December 1970 and lasted 23 minutes.

The Soviets then made probes that landed and lasted longer and longer. They carried increasingly complex sensor suites, eventually including imaging equipment. Venera-8 measured wind speeds while descending and lasted 50 minutes after landing. Venera-9 and -10 were orbiters and landers. Venera-9 worked on the surface for 53 minutes, including sending the first picture from another world on 25 November 1975. Venera-10’s lander achieved 65 minutes of life on the surface.

The Americans, after a couple of Mariner flypast missions in the 1960s, sent a pair of Pioneers in 1978. The first, Pioneer-12, was an orbiter with a radar-mapping device that lasted until 1992. The second, Pioneer-13, released a cluster of four atmospheric probes. The Soviets also sent two probes during the 1978 launch window (the period when the planets are best aligned), reaching 95 and 110 minutes of surface life.

The Soviets ended their exploration of Venus with four straight successful probes, Venera-13, - 14, -15, and -16. The first pair were fly-pasts with landers, the last two orbiters and landers, all had color imaging equipment, and the orbiter returned a map of Venus’s northern hemisphere. Mapping was the chief goal for the 1989-1994 mission by the Magellan American spacecraft that used its synthetic aperture radar on 99 percent of the planet’s surface. It was the final probe to reach Venus.

In contrast to the relative success exploring the inhospitable Venus, probing relatively benign Mars proved problematic. Only three of sixteen Soviet probes of Mars have seen full success, while eight of fourteen American probes were satisfactory; less than a third of the combined missions were thus successful. After five Soviet failures and the failure of its partner, Mariner-III, Mariner-IV flew past the Red Planet and returned pictures of craters on Mars. After eleven years and eight failed missions, the Soviet Mars-3 soft-landed on 2 December 1971.

Even still, it relayed only 20 seconds of data to its orbiter, which lasted nine months. After a few more fly-pasts, the US Mariner-IX went into Martian orbit at about the same time as Mars-3. A planet-wide dust storm was in progress, and the temperature variations from it inspired the concept of a nuclear winter that scientists theorized would follow any widespread atomic exchange. The Soviet Union succeeded with another orbiter, Mars-5, after the failure of Mars-4, to survey possible Mars-6 and -7 landing sites. However, both probes failed.

The U.S. then had a pair of highly successful orbiters and landers, Viking-1 and -2, which arrived during the 1976 launch window and relayed data until 1980. After over a decade of hiatus, the Soviets tried again with Phobos-1 and -2 in 1988, both including a Phobos lander. Both failed, as did the next American mission, Mars Observer. A replacement spacecraft, Mars Global Surveyor, was launched in November 1996, and entered Martian orbit the next year.

The Russians attempted the ambitious Mars-96 mission that same launch window, an orbiter, two landers, and two soil penetrators. The booster failed, leaving the spacecraft to crash into the ocean with 270 grams of plutonium intended to generate electrical power.

The American Mars Pathfinder, one of NASA’s ‘‘faster, cheaper, better’’ missions, landed on Mars and released a rover, Sojourner. This mission returned a torrent of data on the Martian surface for 83 days after its arrival 4 July 1997. America then suffered a series of embarrassing failures at Mars. The first was Mars Climate Orbiter, which burned up on approaching the planet due to one of its sub-teams calculating in metric units and the other in imperial units. This was followed by the loss of the Mars Polar Lander and its Deep Space-2 Penetrators. All of these Soviet and U.S. failures prompted a panel at a NASA conference, entitled ‘‘Why is Mars So Hard?’’

Japan sent its Nozomi probe on its way to Mars in 1998, but at the time of writing, it had not arrived. Whether successful or not, it is, however, the first interplanetary probe launched by a country other than Russia or the U.S.

Looking at the deepest end of space exploration is NASA’s Great Observatories program, which includes infrared and x-ray observatories, but the most famous is the optical Hubble space telescope. It was placed into orbit by the Space Shuttle on 25 April 1990, and serviced several times, each visit yielding a better spacecraft. It has mapped the entire sky and seen matter at the origins of the universe. It places the exploration of the Moon, Venus, and Mars essentially in our back yard.