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Interplanetary Missions of 1997

The year 1997 saw more space activities than any other within the last decade. Human spaceflight included eight shuttle launches with missions involving on-orbit science, preparations for the International Space Station and maintenance of Mir.

Originally published in the Canadian Space Gazette, Winter 1998

Aboard Russia's Mir space station, the international cadre of astronauts and cosmonauts continues to study the weightless environment and gains the experience needed before construction of ISS begins. Our robotic probes explored locations all through the solar system. NASA's Discovery Program, now in full gear, had missions that passed an asteroid and landed on Mars. Other spacecraft such as Galileo and Ulysses continued to send back pictures and information of the worlds they examine to Earth. Mainstream media paid an unusually high level of attention to these events.

On June 27, the Near Earth Asteroid Rendezvous spacecraft performed a 1200 km flyby of the main belt asteroid Mathilde. NEAR photographed its surface at a top resolution of 200 m per pixel and spectrally mapped the illuminated areas of the asteroid. NEAR was the first Discovery mission launched. The 805 kg spacecraft rode a Delta II rocket into space from Cape Canaveral, Florida on February 17, 1996. NEAR is now approaching Earth for a flyby on January 23, 1998. At the closest part of its pass, the spacecraft will fly 555 km above Iran. It will use our planet's gravity to pull it off its old path and into the orbital plane of its primary objective, Eros. In January, 1999, it will rendezvous with asteroid Eros, settle into orbit, map the surface in detail, study the geology and determine the distribution of minerals on the near Earth asteroid. The only other asteroids visited by spacecraft are Ida and Gaspra.1 Galileo photographed them when it passed through the main asteroid belt, which is roughly between the orbits of Mars and Jupiter.

The Galileo probe, launched from the space shuttle Atlantis in 1989, continued touring Jupiter and its moons through 1997. Armed with a suite of scientific instruments, Galileo studied Jupiter's atmosphere, the surfaces of its satellites, their compositions and the entire system's fields and particles. As it circled the largest planet in our solar system, Galileo also focused in on the Great Red Spot. It resolved unprecedented detail in the centuries old hurricane which is large enough to swallow Earth. From numerous flybys, scientists in the six nations that are part of the Galileo Project have come to better understand the internal structure of Jupiter's large satellites, Callisto, Europa, Ganymede and Io. It has been determined that all of the moons have metallic cores surrounded by rock, except Callisto. The surfaces of Europa and Ganymede have a deep layer of frozen, or possibly liquid, water. The spacecraft discovered further eruptions and flows on the volcanic moon, Io, where sulfur is abundant enough to fill lakes. The icy moon Europa grabbed the most attention because of its familiar surface. High resolution photographs revealed a surface that resembles the polar seas of Earth with icebergs and cracked pack ice. The dynamism of this frozen crust suggests a liquid ocean hidden beneath. If this is so, thermal oceanic vents, like those at the bottoms of our deep sea trenches, would provide a perfect haven for life. Galileo's primary mission drew to a close on December 7, 1997 and was succeeded by the Galileo Europa Mission (GEM). On December 16, this mission's first flyby occurred over Europa at an altitude of just 200 km. The two year GEM will have seven more flybys of Europa, four with Callisto and possibly one or two with Io, depending on Galileo passing safely through the intense radiation belts close to Jupiter.

The Mars Pathfinder was the second Discovery Program craft to leave for space. Following its December 4, 1996 launch, the spacecraft cruised through deep space for seven months until its fiery arrival in the upper martian atmosphere, 125 km above the surface. As it plummeted towards the ground, the thin martian atmosphere slowed the vehicle from 7.5 km per second to just 400 m per second. At approximately 9 km altitude, pathfinder unfurled a large parachute and then jettisoned its spent heat shield. Immediately after, the lander unraveled on a Kevlar restraint from beneath the back cone, inflated a cocoon of airbags, fired its retrorockets for two seconds and then met the rocky surface at approximately 20 m per second. After rolling to a halt, Pathfinder deflated the airbags and opened its solid petals, on one of which the rover Sojourner was mounted. The Imager for Mars Pathfinder panned the surrounding terrain and proved that the lander came down within the target zone, an ancient flood plain called Ares Vallis. It had been over fifteen years since we last saw a sunrise from the surface of Mars. Subsequent observation by the camera on the lander and the one on the rover Sojourner revealed the legacy of the massive floods. The large rocks around Pathfinder were arranged as by the force of fast flowing water. Close up probing by Sojourner revealed smooth pebbles and conglomerate rocks. Analysis of data from Sojourner's Alpha Photon X-Ray Spectrometer (APXS) showed that many of the rocks are basaltic and some possibly sedimentary. The Mars Pathfinder mission also studied the planet's atmosphere. On some days the spacecraft imaged wispy high altitude clouds. From the windsock experiment, we learned that gentle breezes from the west were common at the landing area. Examination of the craft's solar panels showed that a layer of rust colored dust had settled out of the atmosphere on them.

Although none occurred during the time the lander was alive, past violent wind storms were evident in the wakes of dust around rocks and in the pits from airborne debris seen on the rocks themselves. Mars Pathfinder far outlived its nominal lifetime. When the last transmission from the spacecraft was received, the lander had lasted three times the thirty days it was designed for and Sojourner had lived for twelve times the seven days it was designed for. The venerable lander succumbed to the cold of the coming martian winter. However, it is possible that the rover is still following its preprogrammed routine, circling its mother ship, avoiding large rocks and vainly attempting to regain radio contact with the lander. This highly successful science and technology demonstration mission came to a close in November, 1997.

On September 11, 1997, the Mars Global Surveyor (MGS) fired its main rocket for 25 minutes which slowed it down just enough to be caught by the Red Planet's gravity. This concluded the spacecraft's 750 million km deep space cruise and dropped it in to a highly elliptical orbit around Mars. The $148 million probe was launched from Cape Canaveral aboard a Delta II rocket back on November 7, 1996. Although it was launched before Mars Pathfinder, it took longer to reach their common destination because of its low energy trajectory. Upon arrival, Mars Global Surveyor began lowering its orbit by aerobraking against the Martian atmosphere. Aerobraking is a technique where a spacecraft dips into the upper fringes of a planet's atmosphere to slow it down instead of transporting more rocket fuel with it to do the job. Aerobraking was first tried at the end of the Magellan radar mapping mission to Venus. As MGS continued its aerobraking, the one solar panel that had failed to fully deploy at the beginning of the cruise proved to be troublesome. Mission control detected dangerous levels of stress on the solar panel's hinge and decided to modify the aerobraking as to pass more gently through the upper atmosphere. This change postponed the start of planetary mapping until March, 1999. Despite the delay, MGS has already begun studying the martian atmosphere and surface. It has also made an important discovery by detecting weak magnetic features within the planet's crust instead of a planet wide magnetic field. The hiatus has also presented some unexpected benefits. From its periapsis, or the low point of its martian orbit, MGS can photograph surface features from just over 100 km altitude, much lower than the final mapping orbit. It has already returned detailed images of massive land slides, ancient river beds and canyons.

At 4:43 am on October 15, 1997, a massive Titan IVB rocket rumbled off its sea side pad at Cape Canaveral and rose into the predawn sky. Aboard was Cassini, a joint venture of NASA, the European Space Agency (ESA) and the Italian Space Agency (ASI). This spacecraft is a behemoth compared to newer space probes in regard to its mass, duration of mission and cost. Its launch signaled the end of an age of big budget probes such as the Vikings, Galileo and the late Mars Observer. Disregarding the Titan launcher and Centaur booster, the spacecraft had a full mass of 5600 kg. Cassini is 6.8 m high and 4 m wide, it cost $191.5 million in 1995 and the total mission expense will be around $1.9 billion,2 since development began in 1989. To achieve the necessary velocity to reach Saturn in the outer solar system, Cassini will harness the orbital velocity of several planets through a technique called gravity assist. It is currently heading closer to the Sun on its way to Venus. On April 21, 1998, it will accelerate past that cloud covered planet and then arc around for another pass of Venus on June 20, 1999 to pull it out towards Earth. After skimming past its home planet on August 15, 1999, it will swing out through the asteroid belt and unfortunately miss the millennial celebrations on Jupiter during its pass of that planet on December 30, 2000. Finally, after the 3.2 billion km cruise, Cassini will arrive at Saturn on July 1, 2004. On November 6, 2004, Cassini will release the Huygens probe supplied by the Italian Space Agency (ASI). This craft will parachute through the atmosphere of Saturn's largest moon, Titan and land on hard ground or possibly an methane ocean. During its four year tour of the Saturnian system, the Cassini orbiter will study the giant planet's atmosphere and rings and investigate the variety of moons circling it. It will also radar map the hidden surface of cloud covered Titan.

Far below the orbital plane of the planets, Ulysses is looping around for its second pass of the Sun's south pole. The Ulysses spacecraft, another joint mission between NASA and ESA, was launched by the space shuttle Discovery on October 6, 1990. It headed out to Jupiter for a gravity assist on February 8, 1992 and was flung below the orbital plane of the planets and into a polar orbit about the Sun. In this orbit, it passed the Sun's south pole in 1994 and the north pole in 1995. Throughout the mission, Ulysses studies the heliosphere, the vast region where the Sun's atmosphere is present and its solar wind outflows. The intensity of the heliosphere changes with respect to the Sun's latitude. Ulysses will pass over the solar south pole in September, 2000.

Also in the distant reaches of the Solar System, but not forgotten, are Pioneer 10, 11 and the Voyager twins. Pioneer 10 was launched on March 2, 1972, was the first spacecraft to venture through the asteroid belt and the first to visit Jupiter. It is now the farthest spacecraft from Earth. Two experiments were still running aboard Pioneer 10, even after a quarter of a century in deep space, when scientists at NASA's Ames Research decided to stop listening for the craft's signal in April, 1997. Pioneer 11 left Earth on April 5, 1973, traveled past Jupiter and then visited Saturn.3 Pioneer 11's signal is now on the edge of silence. Voyager 1 and 2 were launched aboard a Titan III E on September 5 and August 20, 1977, respectively. Although Voyager 1 launched later, its high energy trajectory took it to Jupiter before its twin. Voyager 1 flew by Jupiter and Saturn. Voyager 2 conducted a remarkable grand tour of Jupiter, Saturn, Uranus and Neptune. Together they returned a phenomenal amount of information regarding the outer solar system. Today, the two spacecraft are part of the Voyager Interstellar Mission which is searching for the boundary between the heliosphere and interstellar space called the heliopause.

This was a year of transition. It marked the beginning of an age of frequent small spacecraft missions and the dusk of large powerful designs like Cassini. Although this year had discovery on many fronts, 1997 was just a prelude to what is in store. An unprecedented amount of exploration is ahead of us.

John P. Kavanagh is a freelance space exploration writer
currently studying cartography at the University at Buffalo.

  1. Geissler, Paul E. "Galileo Explores Its Home Planet" Planetary Report May/June 1995: 8-13
  2. Miner, Ellis D. "Voyager: The Discoveries Continue" Planetary Report July/August 1995: 4-9
  3. Bonnet, R. M. "Taking the Next Step" Planetary Report January/February 1995: 8-11
  4. Curtis, Anthony R. Space Almanac. Houston: Gulf, 1990. P527, 530, 662
  5. NASA, Jet Propulsion Laboratory. Cassini: Voyage to Saturn 23 January 1998
  6. NASA, Jet Propulsion Laboratory. Galileo Home Page 23 January 1998
  7. NASA, Jet Propulsion Laboratory. Mars Pathfinder Home 23 January 1998