Probes to the planets

Although the planets of our solar system have been extensively studied from earth, most of our present knowledge about them has come from the discoveries made by interplanetary probes. Since the first successful “fly-by” of Venus, by Mariner 2 in 1962 (only about five years after the launching of the first artificial satellite), great progress has been made in the unmanned exploration of the planets. Probes have soft-landed on Venus and Mars, for example, and the Pioneer and Voyager missions have sent back much new information about the outer planets.

Venus fly-bys

Following one American and three Soviet failures to fly spacecraft close to Venus, the American probe Mariner 2 was successful in 1962, when it passed within 21,600 miles (34,760 kilometers) of the top of the Venusian atmosphere. During its 35-minute period of collecting data, Mariner2 relayed much new information back to earth. The spacecraft carried various instruments, including radiometers to measure temperatures deep within the dense clouds that surround Venus. The results were surprising; the radiometers indicated that temperatures near the surface are greater than 800° F. (425° C), hotter than the melting point of lead. Mariner2 also carried a magnetometer to measure the strength of the magnetic field near the planet and in interplanetary space, as well as a solar plasma detector to measure the concentration and intensity of the solar wind.
Two other fly-by missions (both American) have since taken place: Mariner 5 (closest approach, 2,480 miles (3,991 kilometers) on October 19,1967) and Mariner 10 (closest approach, 3,000 miles (4,828 kilometers) on February 5,1974). The latter craft was similar to the successful Mariners 6, 7, and 9 that were sent to Mars between 1969 and 1971. Mariner 10 carried two cameras sensitive to visible and ultraviolet light and returned the first close-up pictures of Venus.
After bypassing Venus, Mariner 10 continued for a rendezvous with Mercury in March 1974. The craft flew past the planet—sending back detailed photographs of the surface-then went around the sun, returned for a second rendezvous with Mercury, and relayed more high-quality pictures of the planet.

Venus landers

The dense clouds around Venus limit the usefulness of the remote sensing techniques used by fly-by spacecraft. As a result, the Soviet Union and the United States planned missions to land probes on the planet’s surface.
Most of the early craft were Soviet. Venera 3 (launched on November 16,1965) reached Venus and impacted on the surface, thereby becoming the first man-made craft to reach another world (apart from the moon). Unfortunately, however, it did not return any data. Venera 4 successfully entered the Venusian atmosphere on October 18,1967 (one day before the Mariners fly-by of the planet). At a height of 28,000 miles (45,000 kilometers) above the surface, the parent craft released a spherical probe which, slowed by a parachute, descended through Venus’s atmosphere. The probe transmitted data for more than 1 j hours, but it is believed that the transmissions ceased before it reached the surface, at an altitude of about 15 miles (25 kilometers). Slowing the probe’s rate of descent by using a parachute was successful and was employed on later Venera missions.
The Venera 5 and 6 probes also ceased transmitting before they reached the surface, because they could not withstand the tremendous temperatures and pressures deep in Venus’s dense atmosphere. As a result of the experience gained from these early missions, Venera 7 (launched on August 17,1970) was more robustly constructed. It successfully soft-landed on Venus and sent back the first transmissions from the planet’s surface. It found that the surface temperature is about 885° F.

(475° C) and that the atmospheric pressure at ground level is approximately 13,000 pounds per square inch (900,000 kilograms per square meter)—about 90 times that at sea level on earth. The Venera 7 probe was built to withstand temperatures up to 980° F. (530° C) and continued transmitting for 20 minutes on the planet’s surface. Venera 8 was the first craft to return information about the light levels at the surface, as well as data on the soil’s density and composition.
The first photographs of the Venusian surface were sent back by the Venera 9 and Venera 10 craft, which landed in October 1975. The pictures were of surprisingly high quality considering the extremely severe conditions on the planet’s surface. Venera 13 and Venera 14 transmitted the first color pictures of the surface in 1982. They used a special photographic technique in which three black-and-white photographs, each taken through either a red, blue, or green filter, were combined to produce color images. These landers also performed the first detailed analyses of Venusian soil samples.
In 1983, two additional Soviet spacecraft mapped most of the northern hemisphere of Venus. Venera 15 finished its mapping in July 1984; Venera 16, in April 1984. The two probes provided clear images of features as small as 0.9 mile (1.5 kilometers) across.
Five years after the Mariner 10 mission, the United States sent two other probes to Venus. Pioneer Venus 1 was launched on May 20, 1978 and was designed to orbit the planet and collect information about its atmosphere. Pioneer Venus2, launched about 11 weeks later, carried four probes for making a detailed analysis of the atmosphere as they descended to the surface. Both missions were successful; among the findings were a larger than expected proportion of argon in the atmosphere and the fact that cloud-top temperatures on the night side of Venus are higher than those on the day side. Pioneer Venus 1 also made the first detailed map of the planet’s hidden surface by using radar.
The U.S. space probe Magellan was launched on May 4,1989 and began orbiting Venus on August 10,1990. Radar images received from the probe show details of features as small as 330 feet (100 meters) across.

The two Viking Mars missions were almost identical. The craft (which were of the same basic design) followed similar flight paths and used the same landing technique. As each probe approached Mars, its rockets fired to enter into orbit around the planet. After selection of a landing site, the landing capsule separated from the parent craft (which continued to orbit Mars) and began to descend, maneuvering into the correct attitude for atmospheric entry at an altitude of about 790,000 feet (240,000 meters). Its parachute opened at about 20,000 feet (6,000 meters) and the heat shield was jettisoned. After drifting downward, the lander then jettisoned the parachute and fired its retrorockets to decelerate sufficiently for a soft landing.

Mars orbiters

The first close-up pictures of Mars were returned from television cameras on board the American Mariner 4 spacecraft, which passed within 6,118 miles (9,846 kilometers) of the planet in 1965. Other successful Mars missions followed in 1969 with Mariners 6 and 7, which sent back more than 200 pictures of the Martian terrain.
The Mariner 9 craft (launched on May 30, 1971) became the first object from earth to become an artificial satellite of another planet. Carrying a narrow-angle camera capable of resolving features as small as 325 feet (100 meters) across, Mariner9 remained operational for 349 days, during which it relayed more than 7,000 high-quality images of Mars and its two moons, Phobos and Deimos. Mariner 9 also carried an infrared radiometer to measure the temperature of the surface of Mars. The results indicated a range of more than 300° F. (150° C) between the equator and the poles.

Mars landers

The Viking 1 spacecraft was launched on August 22,1975 and, after a 300-day-journey, went into orbit around Mars. It then began sending back pictures of the Martian surface in order to locate a suitable site for the lander part of the spacecraft. After 16 days, a good site was found, and on July 20,1976, Viking Lander 1 separated from the Viking orbiter and successfully soft-landed on the surface of Mars (in Chryse Planitia). While the lander analyzed the soil and transmitted close-up pictures of the surface, the parent craft remained in orbit, acting as the communication relay between the lander and earth. Viking 2 was launched shortly after Viking 1, and its lander touched-down on September 3,1976, in Utopia Planitia, about 4,600 miles (7,400 kilometers) from the Viking 1 site.
Both of the landers were comprehensively equipped with instruments for analyzing the atmosphere and soil. The tasks they performed included a survey of the chemical composition of the soil, the detection of seismic tremors, and, perhaps most important, the search for life on the surface. Soil for the lifedetection experiment was scooped up by a mechanized arm on the side of the lander and divided into three portions, each of which was placed in one of three biology experiments. The fully automatic equipment then tried to incubate the soil samples with sunlight, water, and nutrients, simultaneously testing for signs of life. (The experiments were designed on the assumption that any forms of life would ingest and excrete certain basic chemicals). The results were inconclusive.
Each of the Viking landers had two cameras positioned at a height equivalent to the average human eye level. Remotely controlled from earth, the cameras were capable of performing a complete 360° scan of the surrounding landscape. They could also be operated as a pair to provide stereoscopic views. The cameras used a small mirror that scanned a narrowvertical strip of the landscape, reflecting the image to a photocell that converted the light into an electrical signal that was transmitted back to earth. The camera was then rotated by a small amount and another vertical line scanned, the entire process being repeated until a complete view had been obtained. At the control center on earth, the signals were processed to produce photographs.

Probes to the outer planets

The exploration of the outer solar system has been dominated by the achievements of American spacecraft. The first probe to the outer planets, Pioneer 10, was launched on March 2,1972, the start of a 21-month journey to Jupiter. During its flight, the craft passed through the asteroid belt, a potentially hazardous zone because collision with even an extremely small asteroid might disable the probe. Fortunately, Pioneer ZCHand all later long-distance probes) passed through unscathed. Pioneer-Saturn was launched about a year later, on April 6,1973, destined for both Jupiter and Saturn. The probes were identical, each carrying scientific instruments to study both interplanetary space and their destination planets. In addition, each probe had a large dish antenna—8 feet (2.4 meters) in diameter— for communicating with earth. The cameras on board the spacecraft worked in basically the same way as those on the Viking landers, scanning narrow vertical strips and converting the image into digital form for sending back to earth.
After the successes of the Pioneer missions, the United States sent two other probes to the outer solar system— Voyager 1 (launched on August 20,1977) and Voyager2 (launched on September 5, in the same year). The two craft were identical, and their scientific instruments were more complex than those of any previous unmanned probes. One aspect of their design was given special attention: the radiationshielding of the electronic components. Pioneer had found that the radiation field around Jupiter was much stronger than expected and so the two Voyagers had thicker shielding of their sensitive microelectronic components. Moreover, the Voyagers’ camera systems were different from those on the Pioneer crafts. On the Voyagers, the scanning platform was designed so that it could be rotated to any angle in space. Each probe carried two video cameras, one with a wide-angle lens, the other with a narrow-angle lens; the cameras were remotely controlled from earth. Each camera had a rotating filter wheel in front of its lens, and color pictures were obtained by combining three black-and-white images, each taken through a blue, green, or orange filter—a similar system to that later used on the Soviet Ve/7-era 13 probe. The images were then converted into digital form for transmission back to earth. Another pair of instruments recorded ultraviolet and infrared radiation, which can provide important clues to atmospheric composition or the surface chemistry of solid objects. Other detectors searched the interplanetary space for magnetic fields, charged particles, radio discharges, and cosmic rays. Communication with earth was through a bowl-shaped antenna 12 feet (3.6 meters) across.
Voyager 1 came closest to Jupiter on March 5,1979. In the images, scientists found the faint trace of a ring never before seen.
On July 9,1979, Voyager 2 arrived at Jupiter and reobserved the planet’s ring, lo’s volcanoes, and other revelations made during the previous fly-by. Europa was examined closely during the second encounter.
When Voyager 1 reached Saturn, it encountered the planet and its large moon Titan. Titan’s atmosphere is so thick with haze that the spacecraft could not record any features. Voyager 1 came closest to Saturn on November 12,1980. Voyager2 did not reach the planet until nearly a year later, on August 25, 1981. The spacecraft’s observations of Saturn and its satellites were spectacular.

By directing Voyager2 on an extremely precise trajectory past Jupiter and Saturn, NASA engineers preserved an opportunity to send the probe past both Uranus and Neptune. On January 24,1986, Voyager 2 functioned perfectly as it passed within 51,000 miles (81,600 kilometers) of the cloud tops of Uranus. The space probe discovered two more rings and ten more moons around the planet. And on August 24,1989, the spacecraft soared about 3,000 miles (4,800 kilometers) over Neptune and passed within about 24,000 miles (38,400 kilometers) of its large moon Triton. The images Voyager 2 took revealed more about Neptune than had been learned since its discovery in 1846.
After this last planetary encounter, the Voyagers headed for interstellar space, which they should reach around 2015. In the meantime, they will continue communicating information about the stars they pass.

The trajectories of four missions to the outer planets are shown left, with the dates of the probes’ closest approaches to each planet. With the exception of Pioneer 10(which encountered only Jupiter), the probes’ flight paths were carefully calculated so that the gravitational attraction of each planet was used to swing the craft into the correct trajectory for the next planetary encounter.