Robotic Spacecraft:

Robotic spacecraft are specially designed and constructed systems that can function in specific hostile environments. Their complexity and capabilities vary greatly and their purposes are diverse. There are, roughly, eight broad classes of robotic spacecraft according to the missions the spacecraft are intended to perform:

  1. Flyby spacecraft
  2. Orbiter spacecraft
  3. Atmospheric spacecraft
  4. Lander spacecraft
  5. Rover spacecraft
  6. Penetrator spacecraft
  7. Observatory spacecraft
  8. Communications spacecraft

Flyby Spacecraft:

Flyby spacecraft conducted the initial reconnaissance phase of solar system exploration. They follow a continuous solar orbit or escape trajectory, never to be captured into a planetary orbit. They must have the capability of using their instruments to observe targets they pass. Ideally, they can pan to compensate for the target's apparent motion in optical instruments' field of view. They must downlink data to Earth, storing data onboard during the periods when their antennas are off Earthpoint. They must be able to survive long periods of interplanetary cruise. Flyby spacecraft may be designed to be stabilized in 3 axes using thrusters or reaction wheels or to spin continuously for stabilization.

Examples of the flyby spacecraft category is Voyager 2, which conducted encounters in the Jupiter, Saturn, Uranus, and Neptune systems, Stardust, Mariner, Pioneer.

Orbiter Spacecraft:

A spacecraft designed to travel to a distant planet and enter into orbit about it must carry with it a substantial propulsive capability to decelerate it at the right moment to achieve orbit insertion. It has to be designed to live with the fact that solar occultations will occur, wherein the planet shadows the spacecraft, cutting off any solar panels' production of electrical power and subjecting the vehicle to extreme thermal variation. Earth occultations will also occur, cutting off uplink and downlink communications with Earth. Orbiter spacecraft are carrying out the second phase of solar system exploration, following up the initial reconnaissance with in-depth study of each of the planets.

Examples include Magellan (above), Galileo, Mars Global Surveyor, and Cassini.

Atmospheric Spacecraft:

Atmospheric spacecraft are designed for a relatively short mission to collect data about the atmosphere of a planet or satellite. One typically has a limited complement of spacecraft subsystems. For example, an atmospheric spacecraft may have no need for propulsion subsystems or attitude and articulation control system subsystems at all. It does require an electric power supply, which may simply be batteries, and telecommunications equipment for tracking and data relay. Its scientific instruments may take direct measurements of an atmosphere's composition, temperature, pressure, density, cloud content and lightning.

Typically, atmospheric spacecraft are carried to their destination by another spacecraft. Galileo carried its atmospheric probe on an impact trajectory with Jupiter in 1995 and increased its spin rate to stabilize the probe's attitude for atmospheric entry. After probe release Galileo maneuvered to change from an impact trajectory to a Jupiter Orbit Insertion trajectory. An aeroshell protected the probe from the thousands of degrees of heat created by atmospheric friction during atmospheric entry, then parachutes deployed after the aeroshell was jettisoned. The probe completed its mission on battery power, and the orbiter relayed the data to Earth. The Pioneer 13 Venus Multiprobe Mission deployed four atmospheric probes that returned data directly to Earth during descent into the Venusian atmosphere in 1978.

Balloon packages are atmospheric probes designed for suspension from a buoyant gas bag to float and travel with the wind. The Soviet Vega 1 and Vega 2 missions to Comet Halley in 1986 deployed atmospheric balloons in Venus' atmosphere en route to the comet. DSN tracked the instrumented balloons to investigate winds in the Venusian atmosphere. (The Vega missions also deployed Venus landers.) While not currently funded, informal plans for other kinds of atmospheric spacecraft include battery powered instrumented airplanes and balloons for investigations in Mars' atmosphere.

Examples of the atmospheric spacecraft category is Huygens, which is being carried to Saturn's moon Titan by the Cassini spacecraft, Galileo Atmospheric Probe, Mars Balloon, Titan "Aerover" Blimp, Vega 1 Venus Balloon.

Lander Spacecraft:

Lander spacecraft are designed to reach the surface of a planet and survive long enough to telemeter data back to Earth. Examples have been the highly successful Soviet Venera landers which survived the harsh conditions on Venus while carrying out chemical composition analyses of the rocks and relaying color images, JPL's Viking landers at Mars, and the Surveyor series of landers at Earth's moon, which carried out similar experiments. The Mars Pathfinder project, which landed on Mars on July 4, 1997, and deployed a rover, was intended to be the first in a series of landers on the surface of Mars at widely distributed locations to study the planet's atmosphere, interior, and soil. A system of actively-cooled, long-lived Venus landers designed for seismology investigations, is being envisioned for a possible future mission.

Although example of a semi-lander mission is the B612 rescue mission, a proto-type of a ion engine craft that `lands' on an asteroid, then applies thrust to push it into another orbit (real slow).

Examples of the lander spacecraft category is Mars Pathfinder, Venera 13 Surveyor.

Penetrator Spacecraft:

Surface penetrators have been designed for entering the surface of a body, such as a comet, surviving an impact of hundreds of Gs, measuring, and telemetering the properties of the penetrated surface. As of November 2000, no Penetrator spacecraft have been successfully operated. Penetrator data would typically be telemetered to an orbiter craft for re-transmission to Earth. The Comet Rendezvous / Asteroid Flyby (CRAF) mission included a cometary penetrator, but the mission was cancelled in 1992 due to budget constraints.

Examples of a penetrator spacecraft is the twin Deep Space 2 penetrators which piggybacked to Mars aboard the Mars Polar Lander and were to slam into Martian soil December 3, 1999. They were never heard from. Others include Deep Impact (mission to a comet), Ice Pick (mission to Europa), and Lunar-A (mission to Earth's Moon).

Rover Spacecraft:

Electrically-powered rover spacecraft are being designed and tested by JPL as part of Mars exploration effort. The Mars Pathfinder project included a small, highly successful mobile system referred to as a micro-rover by the name of Sojourner. Mars rovers are also being developed by Russia with a measure of support from The Planetary Society. Rover craft need to be be semi-autonomous. They are steerable from Earth. Their purposes range from taking images and soil analyses to collecting samples for return to Earth.

Examples of a rover spacecraft is, of course, the famous Sojourner Rover, shown here in an image from the surface of Mars.

Observatory Spacecraft:

An observatory spacecraft does not travel to a destination to explore it. Instead, it occupies an Earth orbit or a solar orbit from where it can observe distant targets free of the obscuring and blurring effects of Earth's atmosphere.

NASA's Great Observatories program studies the universe at wavelengths from visible light to gamma-rays. The program includes four Observatory Spacecraft: the familiar Hubble Space Telescope (HST), the Chandra X-Ray Observatory (CXO -- previously known as AXAF), the Compton Gamma Ray Observatory (GRO), and the Space Infrared Telescope Facility (SIRTF).

The HST is still operating. GRO has completed its mission and was de-orbited in June 2000. CXO was launched in July 1999 and continues to operate. SIRTF is set to launch in 2003. In the coming decades many new kinds of observatory spacecraft will be deployed to take advantage of the tremendous gains available from operating in space.

Examples observatory spacecraft are HST, Chandra (X-ray Observatory), Compton (Gamma-ray Observatory), IRAS (Infrared Astronomical Satellite).

Communications Spacecraft:

Communications spacecraft are abundant in Earth orbit, but they are largely incidental to JPL's missions. The Deep Space Network's Ground Communications Facility does make use of Earth-orbiting communications spacecraft to transfer data among its sites in Spain, Australia, California, and JPL.

In the future, communications spacecraft may be deployed at Mars, Venus, or other planets to communicate with orbiters, rovers, penetrators, and atmospheric spacecraft operating in their vicinity. Their purpose would be to augment the Deep Space Network's capabilities to communicate with the resident spacecraft.


In the 1960s, mission designers recognized that a unique opportunity was going to present itself more than a decade later. Starting in the late 1970s, the giant gaseous outer planets -- Jupiter, Saturn, Uranus and Neptune -- would line up in such a way that single spacecraft might hop from one to the next, using the gravity of each one to keep speeding it on its way. Taking advantage of this alignment -- which occurs only once every 175 years -- NASA approved the Voyager Project, designed to send twin spacecraft to the outer solar system.

Voyager 2 was launched first from Cape Canaveral, Florida, on August 20, 1977; Voyager 1 was launched on a faster, shorter trajectory on September 5, 1977. Both spacecraft were delivered to space aboard Titan-Centaur expendable rockets. Voyager 1 made its closest approach to Jupiter on March 5, 1979, and Voyager 2 followed with its closest approach occurring on July 9, 1979. The first spacecraft flew within 206,700 kilometers (128,400 miles) of the planet's cloud tops, and Voyager 2 came within 570,000 kilometers (350,000 miles).

The Voyager 1 and 2 Saturn flybys occurred nine months apart, with the closest approaches falling on November 12 and August 25, 1981. Voyager 1 flew within 64,200 kilometers (40,000 miles) of the cloud tops, while Voyager 2 came within 41,000 kilometers (26,000 miles).

Voyager 1's flight path at Saturn bent it up and away from the ecliptic, the plane in which most planets orbit the Sun. Voyager 2, meanwhile, continued on for two more planetary encounters. Voyager 2 flew by Uranus on January 24, 1986, coming within 81,500 kilometers (50,600 miles) of the planet's cloud tops. Voyager 2 made a final flyby of Neptune on August 25, 1989, passing within 5,000 kilometers (3,000 miles). At the time, the planet was the most distant member of the solar system from the Sun. (Pluto once again became most distant in 1999.)

Following their planet flybys, both Voyagers are heading out of the solar system. Flight controllers believe both spacecraft will continue to operate and send back valuable data until at least the year 2020. On February 17, 1998, Voyager 1 passed the Pioneer 10 spacecraft to become the most distant human-made object in space.

The total cost of the Voyager mission from May 1972 through the Neptune encounter (including launch vehicles, nuclear-power-source RTGs, and DSN tracking support) is 865 million dollars. At first, this may sound very expensive, but the fantastic returns are a bargain when we place the costs in the proper perspective. It is important to realize that on a per-capita basis, this is only 20 cents per U.S. resident per year, or roughly half the cost of one candy bar each year since project inception.

Each Voyager spacecraft comprises 65,000 individual parts. Many of these parts have a large number of "equivalent" smaller parts such as transistors. One computer memory alone contains over one million equivalent electronic parts, with each spacecraft containing some five million equivalent parts. Since a color TV set contains about 2500 equivalent parts, each Voyager has the equivalent electronic circuit complexity of some 2000 color TV sets. The science instruments consisted of dual cameras, infrared spectrometer and radiometer, ultraviolet spectrometer, photopolarimeter, plasma detector, low-energy charged particle detector, cosmic ray detector, magnetometer, planetary radio astronomy, plasma wave detector.

Like the HAL computer aboard the ship Discovery from the famous science fiction story 2001: A Space Odyssey, each Voyager is equipped with computer programming for autonomous fault protection. The Voyager system is one of the most sophisticated ever designed for a deep-space probe. There are seven top-level fault protection routines, each capable of covering a multitude of possible failures. The spacecraft can place itself in a safe state in a matter of only seconds or minutes, an ability that is critical for its survival when round-trip communication times for Earth stretch to several hours as the spacecraft journeys to the remote outer solar system.

Both Voyagers were specifically designed and protected to withstand the large radiation dosage during the Jupiter swing-by. This was accomplished by selecting radiation-hardened parts and by shielding very sensitive parts. An unprotected human passenger riding aboard Voyager 1 during its Jupiter encounter would have received a radiation dose equal to one thousand times the lethal level.

The Voyager magnetometers are mounted on a frail, spindly, fiberglass boom that was unfurled from a two-foot-long can shortly after the spacecraft left Earth. After the boom telescoped and rotated out of the can to an extension of nearly 13 meters (43 feet), the orientations of the magnetometer sensors were controlled to an accuracy better than two degrees.

Each Voyager used the enormous gravity field of Jupiter to be hurled on to Saturn, experiencing a Sun-relative speed increase of roughly 35,700 mph. As total energy within the solar system must be conserved, Jupiter was initially slowed in its solar orbit---but by only one foot per trillion years. Additional gravity-assist swing-bys of Saturn and Uranus were necessary for Voyager 2 to complete its Grand Tour flight to Neptune, reducing the trip time by nearly twenty years when compared to the unassisted Earth-to-Neptune route.


The Galileo mission consists of two spacecraft: an orbiter and an atmospheric probe. The orbiter will be the sixth spacecraft to explore the Jovian magnetosphere, but the first to be placed into orbit around the giant planet. Scientific objectives addressed by the orbiter are to: (1) investigate the circulation and dynamics of the Jovian atmosphere; (2) investigate the upper Jovian atmosphere and ionosphere; (3) characterize the morphology, geology, and physical state of the Galilean satellites; (4) investigate the composition and distribution of surface minerals on the Galilean satellites; (5) determine the gravitational and magnetic fields and dynamic properties of the Galilean satellites; (6) study the atmospheres, ionospheres, and extended gas clouds of the Galilean satellites; (7) study the interaction of the Jovian magnetosphere with the Galilean satellites; and, (8) characterize the vector magnetic field and the energy spectra, composition, and angular distribution of energetic particles and plasma to a distance of 150 Rj.

The structure of the orbiter is divided into two sections. The main body of the spacecraft, comprised of the electronics bays, propellant system, RTG and science booms, and high-gain antenna, rotates at rates of 3.25 or 10.5 rpm. The despun section, aft of the main body, uses an electric motor to drive it counter to the rotation of the main section. This dual spin attitude control system accommodates instruments which require stable, accurate pointing (the imaging instruments) and those which benefit from repetitive, broad-angular coverage (the various particles and fields instruments). The length of the spacecraft is 9 m and, with the high-gain antenna (HGA) deployed, is 4.6 m in diameter.

Power is provided to the spacecraft through the use of two radioisotope thermal generators (RTGs), each of which is located at the end of a short boom. The magnetometer sensors and plasma wave antenna are located on yet another boom, 10.9 m in length.

Although it was intended that communications with the Deep Space Network (DSN) would be primarily through the HGA (which would remain pointing toward the Earth at all times), thermal constraints forced the use of the two low-gain antennas prior to the first Earth flyby. HGA deployment was planned thereafter, but at least three of the HGA "ribs" were unable to be moved much beyond their launch configurations, thereby jeopardizing the total science return of the mission. Several attempts have been made to deploy the antenna through a variety of techniques.

Jupiter's atmosphere displays a rich variety of activity that is not well understood. Equatorial cloud belts are associated with atmospheric jet streams that alternate between east and west directions, at different latitudes.

Between the jet streams there are numerous circulating ovals of clouds, some of which have very long lifetimes. The largest of these, the Great Red Spot, has been in existence for over 300 years, since the first recorded observations of the planet.

Galileo was also in position to observe one of the most spectacular events of the century was the impact of Comet Shoemaker-Levy 9 on Jupiter in July 11, 1994.


The Cassini Orbiter's mission consists of delivering a probe (called Huygens, provided by ESA) to Titan, and then remaining in orbit around Saturn for detailed studies of the planet and its rings and satellites. The principal objectives are to: (1) determine the three-dimensional structure and dynamical behavior of the rings; (2) determine the composition of the satellite surfaces and the geological history of each object; (3) determine the nature and origin of the dark material on Iapetus' leading hemisphere; (4) measure the three-dimensional structure and dynamical behavior of the magnetosphere; (5) study the dynamical behavior of Saturn's atmosphere at cloud level; (6) study the time variability of Titan's clouds and hazes; and, (7) characterize Titan's surface on a regional scale. The spacecraft was originally planned to be the second three-axis stabilized, RTG-powered Mariner Mark II, a class of spacecraft developed for missions beyond the orbit of Mars. However, various budget cuts and rescopings of the project have forced a more special design, postponing indefinitely any implementation of the Mariner Mark II series.

Cassini is currently planned to take a similar tour of the solar system as did Galileo, referred to as a VVEJGA (Venus-Venus-Earth-Jupiter Gravity Assist) trajectory. Several opportunities exist for Cassini to make observations of asteroids, although exact encounters remain to be determined after the spacecraft has been launched as it depends on the launch date. Shortly after entering orbit around Saturn, Huygens will separate from the Cassini orbiter and begin its entry into the atmosphere of Titan. Cassini is then expected to make at least 30 loose elliptical orbits of the planet, each optimized for a different set of observations.

Cassini's instrumentation consists of: a radar mapper, a CCD imaging system, a visible/infrared mapping spectrometer, a composite infrared spectrometer, a cosmic dust analyzer, a radio and plasma wave experiment, a plasma spectrometer, an ultraviolet imaging spectrograph, a magnetospheric imaging instrument, a magnetometer, an ion/neutral mass spectrometer.

Telemetry from the communications antenna as well as other special transmitters (an S-band transmitter and a dual frequency Ka-band system) will also be used to make observations of the atmospheres of Titan and Saturn and to measure the gravity fields of the planet and its satellites.

The Huygens probe, supplied by the European Space Agency (ESA), will scrutinize the clouds, atmosphere, and surface of Saturn's moon Titan. It is designed to enter and brake in Titan's atmosphere and parachute a fully instrumented robotic laboratory down to the surface. The Huygens probe system consists of the probe itself, which will descend to Titan, and the probe support equipment (PSE), which will remain attached to the orbiting spacecraft. The PSE includes the electronics necessary to track the probe, to recover the data gathered during its descent, and to process and deliver the data to the orbiter, from which it will be transmitted or "downlinked" to the ground.

After a seven-year journey bolted to the side of the Cassini Orbiter, Huygens was set free on Dec. 25, 2004. The Probe coasted for 21 days en route to Titan. Huygens made a parachute-assisted descent through Titan's atmosphere, collecting data as the parachutes slowed the probe from super sonic speeds. Five batteries onboard the probe were originally sized for a Huygens mission duration of 153 minutes, corresponding to a maximum descent time of 2.5 hours plus a half hour or more on Titan's surface. In fact, they lasted much longer than that. These batteries were capable of generating a total of 1800 Watt-hours of electrical power.

During its descent, Huygens' camera returned more than 750 images, while the Probe's other five instruments sampled Titan's atmosphere to help determine its composition and structure. Huygens collected 2 hours, 27 minutes, 13 seconds of descent data, and 1 hour, 12 minutes, 9 seconds of surface data, which turned out to be far more surface data than was ever expected.

Titan is an excellent place to search for new forms of life because the temperature and pressures are near the triple point of methane (CH4) -> methane rain, snow, ice, clouds, rivers, etc. Earth's environment is near the triple point of water - water serves as the solvent for organic chemicals to interact = life. On Titan, methane can serve this same purpose.


  • What are the advantages and disadvantages between flyby, orbiter, atmospheric, lander, rover and penetrator probes?
  • Describe the Galileo mission and main science results.
  • Describe the Cassini mission and main science results.