Space Probes and What Was the First Space Probe to Visit the Outer Planets

Space Probes and What Was the First Space Probe to Visit the Outer Planets
Page content

What is a Space Probe

Space probes are unmanned vehicles, usually a robotic spacecraft, carrying instrumentation for scientific research and space exploration. They use a complicated system of navigation and propulsion enabling them to reach the farthest parts of the Solar System.

The outer planets, namely Jupiter, Saturn, Uranus and Neptune, had been of particular interest to scientists since the space program began, due to the scant amount of information we had on them. Many probes were launched to the Moon and the inner planets by the former Soviet Union as well as the United States. But, it was not until the 1970s that exploration of the outer planets truly began.

Before we examine what was the first space probe to visit the outer planets and what the future of space probes and space exploration is, we will take look at the major systems of a space probe, the instrumentation they carry, methods of propulsion and certain maneuvers sometimes deployed, such as a gravitational slingshot.

Major Subsystems of a Space Probe

A space probe has several major subsystems. We will look at each one briefly:

  • Power Supply Subsystem: The power supply provides electrical current to the spacecraft. For probes designed to charter the inner Solar System, solar-cell arrays are used, converting the light from the Sun to electricity. For probes designed for deep space exploration nuclear power is used.

  • Attitude Control Subsystem: A spacecraft’s attitude control system involves the use of solar panels which are directed towards the Sun. Sensors are pointed at targets for scientific analysis, whereas the antennas are oriented towards the Earth.

  • Propulsion Subsystem: The propulsion system is configured according to mission goals and enables the spacecraft to maneuver during flight.

  • Engineering Instrumentation System: This system is responsible for monitoring the overall ‘health’ of the spacecraft. Information to Earth is relayed via the computer and communications systems.

  • Environmental Control Subsystem: Within a spacecraft, the internal temperature needs to be maintained so as to ensure the proper functioning of equipment. This system is designed to maintain the environment within the spacecraft.

  • Computer Subsystem: The computer system is responsible for controlling all other systems. Duties include checking the spacecraft internally, maintenance and processing and storing scientific data.

  • Communications Subsystem: This system transmits data back to Earth. Commands are also sent via this system and tracking of the probe is also conducted through this system.

  • Structural Platform Subsystem: The mechanical portion of the spacecraft, this system supports and protects all the other systems.

  • Guidance Control Subsystem: Mainly responsible for overall guidance and proper direction of the charted course and to detect and correct any deviations by giving the appropriate commands.

We have looked at the major systems of a space probe. We will now look at the instrumentation they carry.

Instrumentation on a Space Probe


A spacecraft carries various kinds of instruments for scientific research and gathering data. We will look at some of those instruments briefly:

  • Magnetometer: This instrument is used to measure the magnetic field of a planet.

  • Plasma Analyzer: Used to detect particles originating from the solar wind.

  • Charged Particle Instrument: Used to detect cosmic rays within the Solar System.

  • Cosmic Ray Telescope: Gathers information on the energy ranges of cosmic ray particles and their composition.

  • Geiger Tube Telescope: This instrument is designed to determine the intensities and energy spectra of subatomic particles, such as electrons and protons, through a planet’s radiation belt.

  • Ultraviolet Photometer: This instrument uses ultraviolet light to determine the amount of hydrogen and helium within a planet as well as in space.

  • Radiometer: Used to provide information of the temperature of clouds on a planet and the heat generated from within.

  • Imaging Science System: Uses a narrow angle and a wide angle camera system to provide imagery of the planets.

  • Radio Science System: Determines the physical properties of planets, such as atmospheres, ionospheres, masses and densities, and the amount of particles within a ring system around a planet as well as their sizes.

  • Interferometer Spectrometer: Determines the total energy balance of a planet and its atmospheric composition.

  • Planetary Radio Receiver: Uses a sweep-frequency radio receiver to ascertain the radio-emission signals coming from a planet.

  • Visible and Infrared Mapping Spectrometer: A remote sensing instrument, it works by capturing images of the planet by using visible and infrared light to learn about the surfaces of moons, rings and a planet’s atmosphere.

  • Radar: Used to produce maps of the surface of the moon and possibly even the planet.

There are other instruments that are also used on the spacecraft. However, the ones mentioned here are the most significant ones used. We will now look at the method of power generation used in deep space probes in some detail.

Power Sources

Cassini RTG

Deep space probes, such as the ones that visited the outer planets, are too far away from the Sun to utilize the energy of the Sun. Instead they rely on nuclear power. A deep space probe uses a nuclear power generator called a radioisotope thermoelectric generator (RTG) to provide electricity for the functioning of the spacecraft. Journeys to the outer planets are several years long, sometimes even a decade or more, and currently this form of electrical energy generation is the most efficient and useful.

Thermoelectric generators produce electricity from heat. An RTG is not a fission powered generator, but instead it is dependent on the energy released from the radioactive decay of isotopes of elements, such as uranium and plutonium. The mechanism works by transferring energy from the decay of radioactive atoms to heat which in turn produces a temperature difference between two different semiconductor materials. The energy from the decay raises the temperature of one of the materials and when they are connected to form a circuit, one has a higher temperature, which produces an electric voltage. The voltage then generates an electric current within the circuit which is then transferred to the spacecraft for its functioning. All scientific instruments as well as subsystems depend upon this method to provide electricity to the spacecraft and as a means of propulsion.

Gravitational Slingshot

Sometimes a space probe uses a technique known as gravitational slingshot or gravitational assist maneuver to alter the speed and path of a spacecraft. A gravitational slingshot works by using a celestial body, such as a planet or a moon, which pulls on the spacecraft due to its gravity and motion (orbital angular momentum), thus speeding it up or slowing it down in its path. It is commonly used for longer missions in order to save fuel, but may take longer for the craft to reach its final destination.

Deep Space Probe Exploration of the Outer Planets

The outer planets, also known as the Jovian planets, consist of Jupiter, Saturn, Uranus and Neptune. The first probe to explore Jupiter was the Pioneer 10 spacecraft. The Pioneer 11 spacecraft sent back information on Jupiter and Saturn. But it was the Voyager 2 spacecraft launched by the National Aeronautics and Space Administration (NASA) in 1977 that explored all the outer planets in great detail for the first time. We will look at the outer planets and the data Voyager 2 gathered on them:


1. Jupiter: Jupiter is the largest planet in the Solar System. The gas giant has a mass two and half times that of all the planets combined. The Jupiter system is composed of 63 known moons. Jupiter is primarily composed of hydrogen and helium. Trace amounts in the atmosphere of water vapor, methane, ammonia, hydrogensulfide, neon, oxygen and sulfur also exist.

The internal structure of Jupiter is thought to be of a dense core composed of a variety of elements, surrounded by a layer of liquid metallic hydrogen with an outer layer of molecular hydrogen. As one moves towards the core, the temperature and pressure steadily increase. The atmosphere on Jupiter is extremely thick. Clouds comprised of ammonia crystals and ammonium hydrosulfide are arranged into bands. The bands occur at different altitudes. Circulation patterns similar to Earth cause a great amount of turbulence and massive storms. The Great Red Spot is one such anti-cyclonic storm on Jupiter that is larger than Earth.

The Voyager 2 spacecraft began its survey of the planet in 1979. Many important discoveries were revealed for the first time by the spacecraft. These included the satellites of Jupiter, radiation belts and its planetary rings. The discovery of volcanic activity on Io, one of Jupiter’s moons, and the discovery of the Great Red Spot also came as a big surprise.

Voyager 2 On Closest Approach to Saturn

2. Saturn: Saturn is the second largest planet in the Solar System. Saturn is also a gas giant similar to Jupiter. There are 62 known moons in the Saturnine system. The interior of Saturn is comprised of a core of variety of elements, surrounded by metallic hydrogen, followed by a layer of liquid hydrogen and liquid helium and an outer gaseous layer. The atmosphere of Saturn is comprised mostly of hydrogen and some helium. Trace amounts of ammonia and methane have also been detected.

Saturn’s cloud patterns are very similar to that of Jupiter. Saturn is probably best known for its rings which are comprised mostly of water ice and some amorphous carbon. Voyager 2 came in contact with Saturn in 1980. The probe’s cameras detected complex structures in the rings of Saturn; particles in the rings ranging from dust grains to the size of automobiles. Instruments aboard Voyager 2 also studied the atmosphere of Saturn and determined the planet’s surface temperature at the deepest levels to be -203 degrees Celsius.

Exploration of the Outer Planets (Continued)


3. Uranus: The composition of Uranus is much like Neptune’s and quite different from Jupiter and Saturn. Even though Uranus is considered to be a gas giant, primarily composed of hydrogen and helium, much of the atmosphere of Uranus contains ices such as water, ammonia and hydrocarbons,. It is the fourth largest planet in the Solar System.

There are 27 known moons of Uranus. Uranus is known to have the coldest know atmosphere anywhere in the solar system. There is no solid surface on Uranus, similar to the other gas giants. One issue that has perplexed scientists is the low internal heat produced from within Uranus, which is much lower than the other gas giants. Uranus also has ring system which is quite complex. The upper atmosphere of Uranus experiences very strong winds, similar to other gas giants. The axis of rotation of Uranus is tilted sideways. This means that poles of the planet are near the equator. Voyager 2 approached Uranus in 1986. The probe discovered 10 previously unknown moons of Uranus, the ring system around the planet, its axial tilt and studied the atmosphere of Uranus in great detail.

Neptune as Photographed by Voyager 2

4. Neptune: Neptune is similar to Uranus in composition. It is the first planet to be discovered by mathematical prediction rather than by observation. Neptune’s atmosphere is primarily composed of hydrogen and helium with traces of hydrocarbons, water and ammonia. The interior of Neptune is similar to that of Uranus. It is composed of ices and rock.

There are 12 known moons of Neptune. Neptune has a very active atmosphere with visible weather patterns. The most notable thing about Neptune’s atmosphere are the winds on Neptune. The winds on Neptune are the strongest anywhere in the solar system which speeds reaching in excess of 2,100 km/h. Neptune also has a very cold atmosphere, similar to that of Uranus.

There is also a ring system around Neptune, but it is not a highly complex system. Voyager 2 encountered Neptune in 1989 and made a close fly-by of one of Neptune’s moon Triton. Vast amounts of data were gathered by the probe on both Neptune and Triton. Voyager 2 also discovered the Great Dark Spot on Neptune, similar to Jupiter’s Great Red Spot. The spot has since disappeared from the planet’s surface.

What the Future Holds

Juno Space Probe

The future of deep space probe exploration looks promising. Many countries are now beginning to send probes to the Moon and are entering a new age in the development of space exploration. NASA is partnering with many space agencies around the world for future projects. One such mission is called Juno, which will conduct an in-depth examination and study of the planet Jupiter. It is set to launch in August of 2011.

Another mission, part of NASA’s discovery program, is called GRAIL. Set to launch in September of 2011, GRAIL will fly twin spacecrafts in orbit around the Moon to study the gravity field of the satellite in great detail.

Another mission, set to launch in 2012, is the Mars Science Laboratory. This will be a long-duration laboratory on the planet mars for sampling the planet and studying its contents.

Other missions are also being planned to the outer planets. NASA’s new Millenium program launched a program in 2009 called Space Technology 8. This program introduced four new subsystem-level technologies to newly developed space probes, thus enhancing them and making a leap forward. New propulsion systems, such as ion drives, are also on the drawing boards as an alternative to nuclear power.



2. Deep Space Probes:To the Outer Solar System and Beyond, Gregory Matloff, Springer, 2005


4. Gas Giants, Brian J. Knapp, Atlantic Europe Publishing, 1992


Image Credits: Wikimedia Commons/NASA/Hannes Grobe, Alfred Wegner Institute for Polar and Marine Research/NASA/Bricktop/NASA/JPL/USGS/Don Davis/NASA/NASA/Aushulz/NASA/Bricktop