With the landing of the Mars Science Laboratory (MSL) on Mars, on 6 August 2012, it is appropriate to have a look at previous U.S. missions to Mars. Mars is relatively easy to study as it is close to the Earth and has no clouds which obscure its surface. Its generally red appearance led it to being associated with war and when its two tiny moons were discovered they were named Phobos (fear) and Deimos (terror). Mars is also of interest as it is possibly the only other planet in the solar system that may, once, have harboured forms of life.

Mariner Series
The Mariner series of interplanetary spacecraft consisted of a number of essentially dissimilar spacecraft which explored the inner planets, Mercury, Venus and Mars.

Mariner-3 was the first U.S. Mars probe and was launched on November 5, 1964. However, the spacecraft disappeared into deep space. It was to perform a flyby of Mars during which it was to take photos of the planet, undertake occultation studies and carry out other interplanetary experiments.

After this initial failure, Mariner-4, which had been launched on November 28, 1964, flew past Mars at a distance of 9789km on July 14, 1965, and transmitted 21 photos of the Martian surface back to Earth. Unexpectedly, these photos showed many craters, a feature which was later proven to be an exception.

There was no evidence of volcanic activity or water erosion. No magnetic field or a radiation belt was found while a very thin atmosphere, consisting of 95 percent carbon-dioxide, was detected. Both Mariner-3 and -4 carried a camera with transmission device, a solar plasma probe, an ionization chamber, a radiation detector, four Geiger-Mueller counters, a helium vector magnetometer, a cosmic ray telescope and two cosmic dust detectors.

The success of the Mariner-4 mission was followed by Mariner-6 and -7, launched on February 24, 1969, and March 1969 respectively. They passed Mars on the 27th and July 31st respectively and August 5th, 1969, over the southern hemisphere of the planet and the Equator at distances of 3400 and 3500km. They returned, first 74 and then 126 photos of the planet. Instruments on board detected carbon-dioxide, carbon-monoxide, and atomic hydrogen, oxygen and carbon in the upper atmosphere. The surface temperature was measured as -50 to 13oC in the daytime and -103 to -52oC at night.

Both spacecraft carried two TV cameras, an infra-red radiometer for thermal mapping, an ultraviolet spectrometer to identify chemical constituents of the upper atmosphere, an infra-red spectrometer to measure the lower atmosphere and the surface composition and a celestial mechanics’ experiment to determine the mass of Mars and the distance between Earth and Mars.

After the launch failure of Mariner-H on May 8, 1971, the Mariner-9 spacecraft was launched on May 30, 1971, and was placed into an orbit around Mars of 1397 x 17,916km with an inclination of 64.3 degrees on November 14, 1971. The mission provided a wealth of data on the planet, although initially images offered little detail of the surface due to a prevailing dust storm. It was not until one year later that the spacecraft revealed the true surface of the planet: valleys of enormous size, 4000m deep, 5000km long and 150km wide, and the highest volcano known in the solar system, Nix Olympica, 25km high and 500km wide at its baseline. The photos also showed intriguing wind erosion patterns which kept scientists busy with analysis for years.

Valles Marineris
It was apparent that the earlier Mariners happened to have observed very untypical regions of Mars. Now the hemispheres were seen to be quite different, although the dividing line was not at the equator but at a 50 degree angle to it.

The southern hemisphere was found to be heavily cratered, ancient and inactive and resembled the Moon. The northern hemisphere, on the other hand, appeared geologically active with lava fields, fractures, etc. The spacecraft also made observations of the two moons of Mars and the images obtained revealed them to be irregularly shaped.

The instrumentation of Mariner-9 consisted of an infra-red interferometer spectrometer for studies of the planet’s surface and composition, its atmospheric constituents, temperature, etc., an infra-red radiometer to measure the surface temperature, an ultraviolet spectrometer to study the atmospheric composition, structure and temperature, as well as two television cameras.

A total of 7329 pictures were transmitted until the instruments on the spacecraft were closed down on October 27, 1972. Mariner-9 is expected to burn up in the Martian atmosphere in 2025.

Viking Series
The Viking program comprised of two spacecraft, each one consisting of an orbiter, to be placed in a Martian orbit, and a lander to land on the surface of Mars. The latter provided the most spectacular images of the planets surface following their landings on July 20th and September 3rd, 1976.

Each lander carried two camera systems, a meteorology boom, a surface sampling instrument, a seismometer, a biology instrument, a gas chromatograph/mass spectrometer and an X-ray fluorescence spectrometer.

The three life experiments were based on the assumption that life was carbon based and was probably in the form of microbes or bacteria. In one experiment, a sample of soil was incubated in simulated Martian sunlight for up to five days in an atmosphere of carbon-dioxide and carbon-monoxide labeled with a radioactive tracer. The unused tracer was then removed and the sample heated to 625 degrees C to vaporise any organic material, releasing any tracer that had been taken up. As a result, fixation of some tracer occurred, but the amount was such that it was probably due to chemistry rather than biology.

In another experiment, a sample of soil was fed liquid nutrients rich in vitamins and amino acids labeled with a radioactive tracer, then incubated at 10 degrees C for up to 11 days. Measurements were then made of any gases resulting from the consumption of the nutrients. An early release of carbon-dioxide was detected, probably due to biology rather than chemistry.

In the third experiment, a sample was put into a liquid nutrient of organic compounds and inorganic salts. It was then incubated for up to 12 days in an atmosphere of helium, krypton and carbon-dioxide. The atmosphere was sampled at intervals for hydrogen, nitrogen, oxygen, methane and carbon-monoxide. It was found that carbon-monoxide and oxygen were expelled, which was probably due to chemistry. The overall conclusion of the life science experiments was that the results were ambiguous and could be explained either by chemical reactions or by primitive life forms.

Data collected by the landers also showed that the winds were generally less than 20km/h but that, during storms on the southern hemisphere, they could exceed 180km/h. Little seismic activity was found. The surface is red in color and has a wide variety of rocks. Analysis of the surface samples resulted in a 21 percent silicon content, 13 percent iron as well as aluminum, magnesium, calcium, sulphur and other elements. It was estimated that 42 percent of the oxygen was bound up in compounds, such as iron oxides, which accounts for the red color.

The two cameras provided images over 350 degrees from the spacecraft to the horizon. Images were in black/white, color and in three infra-red bands. Stereo images were also produced. Each image required 20 minutes to build up—moving objects were recorded as a line, and there were no lines found on any of the images.

Viking-1 Landing Site
The orbiters carried two narrow angle television cameras for high resolution imaging, an atmospheric water detector to map the atmosphere of Mars and to detect any water, an infra-red thermal mapper, and radio equipment to be used in occultation experiments to provide data on the planet’s size, gravity, mass, density and other physical characteristics. The results obtained by the orbiters showed dense carbon-dioxide clouds over the poles during the summer. In the winter, these condensed as ice causing a drop in the atmospheric pressure. It was found that the polar caps have water ice as well as the seasonal carbon-dioxide ice. The ice cap extends as far as 50 to 60 degrees latitude in winter.

Imagery revealed large surface channels scoured by flooding and many smaller channels apparently caused by past water flows. Huge canyons indicated substantial surface movement.

The entire surface of Mars was mapped with resolutions ranging from 200 to 8m. The study of the atmosphere revealed that the constituents were the same at high altitude as at the surface, indicating efficient mixing by the winds. No ozone was detected so that the solar ultraviolet radiation reached the surface, breaking water down into HO and O.

Both Phobos and Deimos were examined by the orbiters. The moons are probably former asteroids that came near Mars and were captured by the planet. In particular, Phobos was mapped using high resolution and was found to be quite dark and may consist of carbonaceous chondrite with an outer layer of rock.

Mons Olympus
Viking-1 attained an orbit of 1500 x 50,600km with an inclination of 37.8 degrees on June 19, 1976. The lander separated on July 20, 1976, and landed at 22 degrees 18’ N, 48 degrees 0’ W.

Viking-2’s orbit was attained on August 7, 1976, and was 1502 x 35,728km with an inclination of 55.6 degrees. Its lander separated on September 3, 1978, and landed at 47 degrees 14’ N, 135 degrees 18’ E.

Although the design life of the Vikings was three months, the Viking-1 orbiter was not shut down until August of 1980, after an operational life of 49 months. The Viking-2 orbiter remained operational until July of 1978 (a period of 22 months), while its lander functioned until April of 1980 (45 months). The Viking-1 lander was kept operational by public subscription until it ceased to function in November of 1982, after 76 months of operation.

The Mars Observer was a 2487kg spacecraft that was launched on September 25, 1992, and was to be placed in a Martian orbit of 380,000 x 550km, later to be modified to 375 x 350km with an inclination of 92.8 degrees. The spacecraft was to undertake a mapping mission of the planet, which would have lasted for the duration of a Martian year, i.e., 687 days. The spacecraft was nicknamed the USS Thomas O. Paine.

Contact with the spacecraft was lost prior to the Martian orbit insertion, which was planned for August 24, 1993. It is not known if orbit insertion was achieved, or if the spacecraft flew past Mars into a solar orbit.

Mars Global Surveyor
Launched on November 7, 1996, the Mars Global Surveyor, with a mass of 1060kg, reached Mars on September 11, 1997, and the intention was that, by January of 1998, it would have been in a Martian polar orbit of 350 x 410km, with an inclination of 93 degrees. However, during the aerobraking phase of the spacecraft’s flight around Mars, one of the large solar panels suffered too much stress. The aerobraking phase was amended and the desired orbit was finally achieved in February 1999.

Due to the delay in the aerobraking phase, the scientific program was delayed by one year. In July of 1998, the alignment of the Sun was incorrect for mapping and it was necessary to wait until March 9, 1999, before mapping could start. Budget restrictions, at that point in time, prevented the extension of the observation program beyond the one year time period. However, following excellent results, the mission was extended to February of 2001. The program was subsequently extended to April 2002.

After the observation program, the satellite continued to be used as a data relay station for subsequent lander craft until, on November 5, 2006, contact with the spacecraft was lost.

Mars Pathfinder
Launched on December 4, 1996, the Mars Pathfinder was a 890kg Mars exploration spacecraft consisting of a cruise vehicle, an entry vehicle and a lander. The lander included the Mars Pathfinder Microrover, named Sojourner, a 11.5kg vehicle which was 63cm long, 48cm wide and 31cm high.

The 890kg spacecraft made a successful landing on Mars on July 4, 1997. The landing site was at Ares Valles, at approximately 19 degrees 20’ N, 33 degrees 55’ W, an area that was initially targeted for the landing of Viking-1, whose schedule 21 years ago called for the event to occur on July 4, 1976.

Sojourner + Yogi
After the landing, the Mars Pathfinder was renamed the Carl Sagan Memorial Station. The next day, the Sojourner was released. Although intended to operate only for one week, Sojourner continued to operate until September 27, 1997, when contact was lost. The mission was formally terminated on November 4, 1997.

Mars Climate Orbiter
The Mars Climate Orbiter was launched on 11, December 1998, and was to be placed in an orbit around Mars to observe climatic changes in the martian atmosphere.


Mars Climate Observer
The Mars Climate Orbiter reached Mars on September 23, 1999, and was to achieve an operational orbit of 373 x 437km and an inclination of 92.9 degrees by November 23, 1999. However, a navigation error brought the spacecraft in at a distance of 60km and it is believed to have burned up and been destroyed by the Martian atmosphere.

Mars Polar Lander
Launched on January 3, 1999, the Mars Polar Lander was to touch down in the south polar region of Mars between the 75 and 80 degrees South latitudes on December 3, 1999. The 560kg probe would have landed suspended from a parachute, to conduct a primary mission, which was to last three months.

The spacecraft reached Mars on December 3, 1999, and the lander separated as scheduled, however, contact was lost. Using images provided by the Mars Global Surveyor, the Mars Polar Lander was eventually located on the surface of Mars.

2001 Mars Odyssey
Originally referred to as the Mars Surveyor 2001 Orbiter, the 2001 Mars Odyssey was launched on April 7, 2001, and placed in an orbit around Mars on October 23, 2001. Using aerobraking, the 379kg spacecraft maneuvered into a Martian orbit of 419 x 450km with an inclination of 93.1 degrees, which was reached on January 17, 2002.

During its primary mission, which lasted a full Mars year (23 months), it provided scientists with the most detailed and complete global maps of Mars to that date, with daytime and nighttime infra-red images at a resolution of 100m revealing details of frozen water deposits and surface textures and minerals. It also provided a communications relay for the Spirit and Opportunity Mars rovers.

In August of 2004, NASA approved an extension of the mission until September of 2006 to provide an additional Mars’ year of information. In addition, the spacecraft assisted in March 2006 with the Mars Reconnaissance Orbiter mission by monitoring atmospheric conditions at the arrival of that spacecraft. Odyssey also analyzed potential landing sites for the Phoenix mission.

Mars Explorer Rover
On June 10 and July 8, 2003, the two Mars Explorer Rover missions were launched. The two spacecraft were similar to that used for the Mars Pathfinder mission and a similar landing technique was adopted, using a parachute and airbags to cushion the impact. The spacecraft had a total mass of 365kg.

The rovers had a much greater mobility than the 1997 Mars Pathfinder rover and were able to travel about 100 m across the surface each day. Also, they carried equipment that allowed them to search for evidence of water that may have been present in Mars’ past. While the two rovers were identical, they landed in different regions of Mars.

Each rover, with a mass of 174kg, carried five instruments, all designed to analyze rocks and soils.

The MER-A, also named Spirit, landed on Mars on January 4, 2004, at Gusey Crater at 14.6 degrees S, 184.7 degrees W, a site which was later named as the Columbia Memorial Station. Spirit’s primary mission started on January 15, 2004, and was completed in May of 2004. The mission was subsequently extended to September of 2006.

In 2006, one of the rover’s six wheels stopped working but, using the remaining five wheels, the vehicle carried on. In March 2009, while traversing a low plateau called Home Plate, the rover became stuck in the sand. Efforts to get the rover moving again failed and, in November 2009, another wheel failed. Using the remaining wheels, Spirit was moved to angle in such a way that it would get the maximum amount of solar energy during the Martian winter in order to continue as a stationary science platform.

NASA decided in May of 2011 to cease attempts to establish contact with vehicle. Last contact was on March 22, 2010, and it is believed that the robot’s internal components and electrical connections had all been damaged by the cold temperatures of the Martian winter. It had traveled a total of 7.7km.

MER-B carried the Opportunity rover and landed at Meridiani Planum at 2.1 degrees S, 6.0 degrees W on January 25, 2004. On January 31, 2004, the rover craft left the landing platform. Its primary mission was completed in May of 2004. The mission was subsequently extended to September of 2006 when the rover reached Victoria Crater, about 10km from the landing site. As at April 2012, the rover was still operational, having traveled a total of 34.3km.

Mars Reconnaissance Orbiter
The Mars Reconnaissance Orbiter (MRO) was launched on August 12, 2005. It was a 1900kg spacecraft built by Lockheed Martin. It was placed in an orbit around Mars to search for evidence that water had persisted on the surface of Mars for a long period of time. It also zoomed in for extreme close-up photography of the Martian surface, analyzed minerals, looked for subsurface water, traced how much dust and water are distributed in the atmosphere, and monitored daily global weather.

Other experiments scanned underground layers for water and ice, identified small patches of surface minerals to determine their composition and origins, tracked changes in atmospheric water and dust and checked global weather every day.

As the spacecraft approached Mars in March of 2006, it passed under the southern hemisphere at an altitude of about 300km. It then performed a maneuver on March 10, 2006, to slow the orbiter down to achieve its capture orbit of 300 x 45,000km.

The orbiter’s primary mission ended in November of 2009. Following that, the spacecraft continued to be used as a communications relay.

Phoenix
Launched on August 4, 2007, Phoenix was a Mars lander built for NASA by Lockheed Martin Space Systems. The objectives of the mission included the study of the history of water in the Martian polar regions and to search for evidence of a habitable zone as well as to assess the biological potential of the ice-soil boundary.

The spacecraft made use of a lander structure, subsystem components and protective aeroshell originally built for the Mars Surveyor 2001 Lander spacecraft that had been cancelled. Before re-entry, flight path data was sent to the on-board computer, which controlled descent and landing and guided the spacecraft to its landing site. A cruise assembly, carrying flight control systems and solar arrays necessary for the trans-Martian flight, was jettisoned five minutes before entry into the Martian atmosphere, at an altitude of about 125km. The spacecraft then slowed itself down by means of friction—a heat shield protected it from the high temperatures.

When the lander’s speed was about Mach 1.7, a parachute deployed, followed with the jettisoned heatshield. The landing radar was activated and the spacecraft’s legs extended. At an altitude of about 1km, the lander separated from the parachute and then used its thruster to decelerate further.

When Phoenix was at an altitude of 12m or traveling at 2.4 m/s, the spacecraft began traveling at a constant velocity. The landing engines were turned off when sensors located on the footpads of the lander detected touchdown. The landing took place on May 25, 2008, at 68.2 degrees N, 125.7 degrees E.

After the landing, the robotic arm of the lander collected Martian arctic soil and dropped it into the TEGA to vaporize any water that would be in the sample. Baking at lower temperatures did, however, not indicate any water. The temperature was later increased to search for minerals that decompose at different temperatures. As the oven could only be used once, there were no further experiments of this nature.

Phoenix successfully accomplished its primary three-month mission and two bonus months of operations before contact was lost in November of 2008. Images taken by the Mars Reconnaissance Orbiter indicated that carbon dioxide ice deposits had settled on the lander’s two circular solar panels, which caused the panels to snap off or bend. The mission was formally closed in May 2010.

About the author
Jos Heyman is the Managing Director of Tiros Space Information, a Western Australian consultancy specializing in the dissemination of information on the scientific exploration and commercial application of space for use by educational as well as commercial organisations. An accountant by profession, Jos is the editor of the TSI News Bulletin and is also a regular contributor to the British Interplanetary Society’s Spaceflight journal.