Important Missions for Solar System Exploration An overview from early ’50’s to today… Part III

Mariner 8: Mariner-71H (also called Mariner-H) was the first of a pair of American spacecraft intended to explore the physical and dynamic characteristics of Mars from Martian orbit. The overall goals of the series were to search for an environment that could support life; to collect data on the origin and evolution of the planet; to gather information on planetary physics, geology, planetology, and cosmology; and to provide data that could aid future spacecraft such as the Viking Landers.

Mariner 9: Mariner 9 was the first spacecraft to orbit another planet. It carried an instrument payload similar to Mariners 6 and 7, but, because of the need for a larger propulsion system to control the spacecraft in Mars orbit, it weighed more than Mariners 6 and 7 combined. When Mariner 9 arrived at Mars, the atmosphere was so dusty that the surface was obscured. This unexpected situation made a strong case for the desirability of studying a planet from orbit rather than merely flying past. Mariner 9's computer was thus programmed from Earth to delay imaging of the surface for a couple of months until the dust settled. After 349 days in orbit, Mariner 9 had transmitted 7,329 images, covering over 80% of Mars' surface. The images revealed river beds, craters, massive extinct volcanoes, canyons including Valles Marineris, a massive system of canyons over 4,000 kilometers [about 2,500 miles] long. The vast chasm is named in honor of the spacecraft. Mariner 9 also found evidence of wind and water erosion and deposition, weather fronts, fogs, and more. Mars' tiny moons, Phobos and Deimos, were also photographed. The findings from the Mariner 9 missions laid the groundwork for the Viking program.

Pioneer 10: Pioneer 10 was the first spacecraft to travel through the asteroid belt and reach the outer solar system, flying past Jupiter at a distance of about 130,354 km (81,000 miles) from the cloudtops. During its Jupiter encounter, Pioneer 10 imaged the planet and its moons, and took measurements of Jupiter's magnetosphere, radiation belts, magnetic field, atmosphere, and interior. These measurements of the intense radiation environment near Jupiter were crucial in designing the Voyager and Galileo spacecraft. Also, as the first spacecraft to use a planetary gravity assist to change its velocity, Pioneer 10 then headed out of the solar system in the direction opposite to the Sun's motion through the Milky Way galaxy. Except for Voyager 1, which is travelling in the opposite direction, Pioneer 10 is farther from Earth than any other human artifact. Even so, it will take another 2 million years for Pioneer 10 to reach the first star on its trajectory. Pioneer 10 carries a plaque intended to communicate something about its home planet should the spacecraft ever meet up with another intelligent species. Routine tracking of Pioneer 10 ended in 1997, but the spacecraft continued to send out signals until January 2003.

Pioneer 11: Pioneer 11 was the second spacecraft (following Pioneer 10) to visit the outer solar system and the first spacecraft to visit Saturn. Borrowing momentum from Jupiter during its flyby, Pioneer 11 was hurled above the ecliptic (the plane in which most of the planets orbit the Sun) on a trajectory that brough it about 22,000 km (13,000 miles) from Saturn. It took close-up pictures and discovered an additional ring.

Mariner 10: Mariner 10 was the first spacecraft to use a gravity assist trajectory, accelerating as it entered the gravitational influence of Venus, then being flung by the planet's gravity onto a slightly different course to reach Mercury. It was also the first spacecraft to encounter two planets at close range. Placed into orbit around the Sun going in the opposite direction from Earth's orbit, Mariner 10's orbit crossed Mercury's the first time at a distance of 703 kilometers (437 miles) from the planet, the second time at 48,069 kilometers (29,870 miles), and a third time at 327 kilometers (203 miles). In total, Mariner 10 imaged about half the planet. Mercury was revealed to have a very cratered, Moon-like surface and a faint, mostly helium atmosphere.

Helios 1: Helios 1 was a joint German- American deep space mission to study the main solar processes and solar-terrestrial relationships. Specifically, the spacecraft's instruments were designed to investigate phenomena such as solar wind, magnetic and electric fields, cosmic rays, and cosmic dust in regions between Earth's orbit and approximately 0.3 AU from the Sun.

Viking 1: The Viking Mars mission was performed by two spacecraft, Viking 1 and Viking 2, launched within a couple of weeks of each other. Each spacecraft consisted of an orbiter and a lander, which traveled attached together for nearly a year to reach Mars orbit. The orbiters then began taking pictures of the Martian surface, from which a landing site was selected. The landers then separated from the orbiters and soft landed. The orbiters continued imaging and, between Viking 1 and Viking 2, imaged the entire planet at what was then high resolution. The orbiters also conducted atmospheric water vapor measurements and infrared thermal mapping. The Viking 1 orbiter flew within 90 kilometers of Phobos to take images of this larger, inner moon of Mars. The Viking landers took full 360-degree pictures, collected and analyzed samples of the Martian soil, and monitored the temperature, wind direction, and wind speed. The Viking missions revealed further details of volcanoes, lava plains, huge canyons, and the effects of wind and water. Analysis of the soils at the landing sites showed them to be rich in iron, but devoid of any signs of life.

Viking 2: The Viking Mars mission was performed by two spacecraft, Viking 1 and Viking 2, launched within a couple of weeks of each other. Each spacecraft consisted of an orbiter and a lander, which traveled attached together for nearly a year to reach Mars orbit. The orbiters then began taking pictures of the Martian surface, from which a landing site was selected. The landers then separated from the orbiters and soft landed. The orbiters continued imaging and, between Viking 1 and Viking 2, imaged the entire planet at what was then high resolution. The orbiters also conducted atmospheric water vapor measurements and infrared thermal mapping. The Viking 2 orbiter flew within 22 kilometers of Deimos and took high-resolution pictures of this smaller of the two moons of Mars. Unlike the Viking 1 orbiter, the orbit of Viking 2 was inclined well above the equator so that it could better study the polar regions. The Viking landers took full 360- degree pictures, collected and analyzed samples of the Martian soil, and monitored the temperature, wind direction, and wind speed. The Viking missions revealed further details of volcanoes, lava plains, huge canyons, and the effects of wind and water. Analysis of the soils at the landing sites showed them to be rich in iron, but devoid of any signs of life.

Helios 2: Helios 2 was the second spacecraft launched to investigate solar processes as part of a cooperative project between the Federal Republic of Germany and the United States in which the former provided the spacecraft and the latter the launch vehicle.

Voyager 1: After historic visits of to Jupiter and Saturn, Voyager 1 is now on course to be the first human-made object to leave our solar system. In space for more than 25 years, it has already traveled farther from our Sun than any other spacecraft. It is not clear when Voyager 1 will reach the heliopause boundary, where the influence of our Sun ends. The boundary is thought to exist somewhere from 8 to 22.5 billion kilometers (5 to 14 billion miles) from the Sun. When Voyager 1 does cross over, scientists will be able to measure the interstellar environment without the influence of the Sun for the first time.

Voyager 2: As of September 2003, Voyager 2 was about 10,657,000,000 km (6,622,000,000 miles) from our Sun. Even at speeds of more than 56,000 kph (35,000 mph), it will take nearly 20,000 years for the Voyagers to reach the middle of the comet swarm and possibly twice this long for them to pass the outer boundaries of cometary space. By this time, they will have traveled a distance of two light- years, equivalent to half of the distance to Proxima Centauri, the nearest star.

Pioneer Venus: The Pioneer Venus mission had two components: an orbiter and a multiprobe. These spacecraft were launched separately, the orbiter arriving at Venus just a few days ahead of the multiprobe. The orbiter carried 17 experiments, including instruments to see through the clouds and map the surface, as well as measure cloud distribution, atmospheric composition, the magnetic field, the solar wind, properties of the ionosphere of Venus, the gravity field, and gamma ray bursts. The multiprobe spacecraft was actually four separate atmospheric probes hitching a ride on a spacecraft bus. The probes separated from the bus 12.9 million kilometers (8 million miles) before arrival at Venus and entered the atmosphere in different regions of the planet at the same time. The probes made measurements of the structure and composition of the atmosphere all the way to the surface. Meanwhile, the probe bus made measurements in the upper atmosphere, and the Orbiter simultaneously made measurements for comparison.

ISEE-3/ICE: International Sun-Earth Explorer 3's was part of a trio of spacecraft sent into space to study interplanetary space. It was the first spacecraft orbit at a libration point and also the first to detect the solar wind approaching Earth. It was later renamed International Cometary Explorer and sent to study comet Giacbini-Zinner and comet Halley. When last contacted, the spacecraft was on a trajectory that will bring it close to Earth in August It could possibly be recaptured, returned to Earth and ultimately be displayed at the Smithsonian Institution.