n this ERA of tight space budgets and infrequent deep-space missions, solar system researchers have pushed to squeeze more science from existing interplanetary spacecraft. By drastically altering their orbital paths using gravity-assisted maneuvers combined with relatively brief rocket firings, veteran probes can be sent on productive extended missions.
This technique was successfully employed by the International Sun-Earth Explorer 3 (ISSE 3) spacecraft to accomplish two important space firsts: the exploration of the geomagnetic tail from to 1 million kilometers behind the Earth and an encounter with a comet.
The ISEE 3 program consisted of 3 satellites ISEE 1 and ISEE 3 were the principal US contribution to the International Magnetospheric Study, and ISEE 2 which was built and managed by ESA. ISEE 1 and ISEE 2 were launched on October 22, 1977 into almost coincident orbits.
The ISEE 3 was launched in August 1978; no thought was given to redirecting it to a comet at some future date. The spacecrafts primary mission, monitoring the solar wind upstream from the Earth, was challenging enough. To do this on a continuous basis, ISEE 3 had to be placed in orbit around the Sun-Earth L1 liberation point, located about 1 million km sunward from the Earth. Such an orbit is unstable, but with gentle rocket firings the spacecraft was kept from wandering too far away.
ISEE 3 remained in its planned orbit for about four years, gathering data on solar-wind plasma, magnetic and electric fields, charged particles, and cosmic rays. However, in 1982 an irresistible proposal for a radically different mission objective was put forward: by executing an intricate series of propulsive and lunar gravity assist maneuvers, ISEE 3 could be targeted for an encounter with Comet Giacobini-Zinner.
ISEE 3 began its complex lunar swingbys in March 1983. After four gravity assists, a fifth one on December 22, 1983, skirted ISEE 3 within 120 km of the lunar surface. The last close passage with the Moon catapulted the probe out of the earth-Moon system into a heliocentric trajectory that would eventually intercept Giacobini-Zinner. Once on its way. NASA announced a new name for the spacecraft International Cometary Explorer (ICE).
The spacecraft Giotto was named after artist Giotto di Bondone, who had used the appearance of Comet Halley in 1301 as the model for the Star of Bethlehem in his 1304 painting titled “Adoration of the Magi”. The purpose of the Giotto mission was to study Comet Halley during the comet’s perihelion passage in 1986. Giotto also encountered Comet Grigg-Skjellerup during an extended mission in 1992. The major objectives of the mission were:
Obtain the first close-up images of a nucleus of a comet
Determine the elemental and isotopic composition of volatile components in the cometary coma, particularly parent molecules
Characterize the physical and chemical processes that occur in the cometary atmosphere and ionosphere
Determine the elemental and isotopic composition of dust particles
Measure the total gas-production rate and dust flux and size/mass distribution and derive the dust-to-gas ratio
Investigate the macroscopic systems of plasma flows resulting from the cometary-solar wind interaction
The European Space Agency (ESA) launched Giotto on an Ariane-1 rocket on July 2 1985. The spacecraft design was based on the GEOS research satellites, and a 600kg shield had to be added to ensure it would survive a close comet encounter. Giotto carried 10 instruments, which included a multicolor camera, mass spectrometers, dust impact detector, plasma instruments, energetic particle analyzer, and a magnetometer. The power source was a 5,032 cell solar array that provided 190 watts of power during the first comet encounter, and four silver-cadmium batteries was carried along as well. On March 13, 1986, Giotto approached Comet Halley for a targeted 500 km flyby. The spacecraft crossed the bow shock of the solar wind, and into the comet’s coma. At this point the camera was switched to tracking mode to follow the brightest object (the comet nucleus) in its field of view and was returning images back to Earth. The images revealed the comet nucleus to be a dark peanut-shaped body with two bright jets spewing material out. The dust impacts were lower than expected until a few minutes prior to closest approach when the impact rate rose sharply, as the spacecraft apparently crossed into the path of one of the jets. Just 14 seconds prior to closest approach, the spacecraft was struck by a large dust particle which knocked the spacecraft off Earth point, and it took approximately 30 minutes for the spacecraft to recover and point its antenna back to Earth and reestablish communications.
Passing by the comet at a velocity of 68 km/second, the spacecraft had suffered some damage. Some of the instruments no longer worked or were partially functional. The star mapper’s baffle was peppered with holes.
Subsequent analysis of Giotto’s data showed a new wealth of data collected. Water accounted for about 80% by volume of all of the material being thrown out by the comet. Seven jets were identified which threw out 3 tons/sec of material. The largest grain detected was 40mg, though the large particle that hit the spacecraft was estimated to be from 0.1 to 1 gram. The data from the plasma and ion mass spectrometer instruments indicated that the surface of Comet Halley is covered in a layer of organic material. The actual closest approach was measured at 596 km. On April 2, 1986, Giotto was placed into hibernation. In April 1990, Giotto was reactivated. Three of the instruments proved fully operational, four partially damaged but usable, and the remainders, including the camera, were unusable. In 1985, Giotto scientists asked the ICE Flight Dynamics Team to help identify any extended-mission potions for the spacecraft. The group proposed using an Earth swingby on July 1, 1990. On July 2, 1990, Giotto made a close encounter with Earth and was retargeted to a flyby of Comet Grigg-Skjellerup on July 10, 1992. Aimed directly at the comet, Giotto missed the comet by 200 km in the closest ever flyby of a comet. Flying by at 14 km/second and with a dust production rate about 1/200 of Halley’s, the Comet Grigg-Skjellerup encounter was expected to have very little dust damage to the spacecraft.
Another encounter was with the Hartley 2. However, Hartley 2 has only been observed during a single apparition, in 1991.
After the 1992 comet encounter, Giotto was placed into an Earth-return orbit using almost all-remaining fuel. The satellite was put back into hibernation on July 23, 1992 and the Giotto mission officially terminated. In its present orbit, Giotto will pass silently within 300,000 km of the Earth on July 1, 1999.
Sakigake (‘Pioneer’) is a test spacecraft similar to Suisei (Planet-A). It flew by Comet P/Halley on its sunward side at a distance of about 7 million kilometers on March 11, 1986. It carried three instruments to measure plasma wave spectra, solar wind ions, and interplanetary magnetic fields, all of which worked normally. The spacecraft was spin-stabilized at two different rates (5 and 0.2 rpm). It was equipped with hydrazine thrusters for attitude and velocity control, star and sun sensors for attitude determination, and a mechanically despun offset parabolic dish for long-range communication. Sakigake made an Earth swingby on January 8, 1992. The closest approach was at 23h08m47s (JST, = UTC+9h) with a geocentric distance of 88,997 km. This was the first planet-swingby for a Japanese spacecraft. During the approach, Sakigake observed the geotail. A geotail passage occurred at 290 Re on June 14, 1993, before ISTP’s multi-spacecraft investigation of that region. The second Earth swingby was on June 14, 1993 at 40 Re, and the third on October 28, 1994 at 86 Re. Almost no hydrazine remains so no further maneuvers were accomplished. Telemetry contact was lost on 15 November 1995 at a distance of 106 million km. Future mission planning had included a 23.6 km/s, 10,000 km flyby of Comet P/Honda-Mrkos-Pajdusakova on Feb 3, 1996 (approaching the nucleus along the tail) some 0.17 AU from the Sun, and a 14 million km passage of Comet P/Giacobini-Zinner on Nov 29, 1998.
Suisei (the Japanese name meaning ‘Comet’) was launched on August 18, 1985 into heliocentric orbit to fly by Comet P/Halley. It is identical to Sakigake apart from its payload: a CCD UV imaging system and a solar wind instrument. The main objective of the mission was to take UV images of the hydrogen corona for about 30 days before and after Comet Halley’s descending crossing of the ecliptic plane. Solar wind parameters were measured for a much longer time period. The spacecraft is spin-stabilized at two different rates (5 and 0.2 rpm). Hydrazine thrusters are used for attitude and velocity control; star and sun sensors are for attitude control; and a mechanically despun offset parabolic dish is used for long-range communication. Suisei began UV observations in Nov. 1985, generating up to 6 images/day. The spacecraft encountered Comet P/Halley at 151,000 km on sunward side during March 8, 1986, suffering only 2 dust impacts. Fifteen burns of Suisei’s 3 N motors over 5-10th of April 1987 yielded a 65 m/s velocity increase for a 60,000 km Earth gravity assist swingby on August 20, 1992, although the craft was then lost behind the Sun for the summer. The hydrazine was depleted on 22 February 1991. Preliminary tracking indicated a 900,000 km flyby had been achieved. ISAS had decided during 1987 to guide Suisei to a Nov. 24, 1998 encounter with P/Giacobini-Zinner, but due to depletion of the hydrazine, this, as well as plans to fly within several million kilometers of Comet P/Tempel-Tuttle on Feb. 28, 1998 has been cancelled.
In the decade following Sputnik I, the United States and the USSR between them launched about 50 space probes to explore the moon. The first probes were intended either to pass very close to the moon (flyby) or to crash into it (hard landing). Later probes made soft landings with instruments intact and achieved stable orbits around the moon. Each of these four objectives required increasingly greater rocket power and more precise maneuvering; successive launches in the Soviet Luna series were the first to accomplish each objective. Luna 2 made a hard lunar landing in September 1959, and Luna 3 took pictures of the moon’s far side as the probe flew by in November 1959. Luna 9 soft-landed in February 1966, and Luna 10 orbited the moon in April 1966; both sent back many television pictures to earth. In addition to the 24 lunar probes in the Luna program, the Soviets also launched five circumlunar probes in its Zond program.
Early American successes generally lagged behind Soviet accomplishments by several months but provided more detailed scientific information. The U.S. program did not bear fruit until 1964, when Rangers 7, 8, and 9 transmitted thousands of pictures, many taken at altitudes of less than 1 mi (1.6 km) just before impact and showing craters only a few feet in diameter. Two years later, the Surveyor series began a program of soft landings on the moon. Surveyor 1 touched down in June 1966; in addition to television cameras, it carried instruments to measure soil strength and composition. The Surveyor program established that the moon’s surface was solid enough to support a spacecraft carrying astronauts.
In August 1966, the United States successfully launched the first Lunar Orbiter, which took pictures of both sides of the moon as well as the first pictures of the earth from the moon’s vicinity. The Orbiter’s primary mission was to locate suitable landing sites for the Apollo Lunar Module, but in the process it also discovered the lunar mascons, regions of large concentration of mass on the moon’s surface. Between May 1966, and November 1968, the United States launched seven Surveyors and five Lunar Orbiters. Clementine, launched in 1994, engaged in a systematic mapping of the lunar surface. In 1998, Lunar Prospector orbited the moon in a low polar orbit investigating possible polar ice deposits, but a controlled crash near the South Pole detected no water.
While the bulk of space exploration initially was directed at the earth-moon system, the focus gradually shifted to other members of the solar system. The U.S. Mariner program studied Venus and Mars, the two planets closest to the earth; the Soviet Venera series also studied Venus. From 1962 to 1971, these probes confirmed the high surface temperature and thick atmosphere of Venus, discovered signs of recent volcanism and possible water erosion on Mars, and investigated Mercury. Between 1971 and 1973 the Soviet Union launched six successful probes as part of its Mars program. Exploration of Mars continued with the U.S. Viking landings on the Martian surface. Two Viking spacecraft arrived on Mars in 1976. Their mechanical arms scooped up soil samples for automated tests that searched for photosynthesis, respiration, and metabolism by any microorganisms that might be present; one test suggested at least the possibility of organic activity. The Soviet Phobos 1 and 2 missions were unsuccessful in 1988. The U.S. Magellan spacecraft succeeded in orbiting Venus in 1990, returning a complete radar map of the planet’s hidden surface. The Japanese probes Sakigake and Suisei and the European Space Agency’s probe Giotto both rendezvoused with Halley Comet in 1986, and Giotto also came within 125 mi (200 km) of the nucleus of the comet Grigg-Skjellerup in 1992. The U.S. probe Ulysses returned data about the poles of the sun in 1994, and the ESA Solar and Heliospheric Observatory (SOHO) was put into orbit in 1995. Launched in 1996 to study asteroids and comets, the Near Earth Asteroid Rendezvous (NEAR) probe made flybys of the asteroids Mathilde (1997) and Eros (1998) and is scheduled to orbit Eros in 2000. The Mars Pathfinder and Mars Global Surveyor, both of which reached Mars in 1997, were highly successful, the former in analyzing the Martian surface and the latter in mapping it. The Mars Climate Orbiter and Mars Polar Lander, however, were lost upon their arrival at Mars in 1999, as was Deep Space 2, twin probes that were to penetrate the Martian surface near the South Pole. These losses set back NASA’s Mars exploration programwhich includes six probes originally scheduled to be launched between 2001 and 2005by at least two years. Japan’s Nozomi orbiter, launched in 1998 and originally scheduled to reach Mars in 1999, will arrive four years later because of a need to conserve fuel.
Space probes have also been aimed at the outer planets, with spectacular results. One such probe, Pioneer 10, passed through the asteroid belt in 1973, then became the first object made by human beings to escape the solar system. In 1974, Pioneer 11 photographed Jupiter’s equatorial latitudes and its moons, and in 1979 it made the first direct observations of Saturn. Voyagers 1 and 2, which were launched in 1977, took advantage of a rare alignment of Jupiter, Saturn, Uranus, and Neptune to explore all four planets. Passing as close as 3,000 mi (4,800 km) to each planet’s surface, the Voyagers discovered new rings, explored complex magnetic fields, and returned detailed photographs of the outer planets and their unique moons. Launched in 1989, the Galileo spacecraft followed a circuitous route that enabled it to return data about Venus (1990), the moon (1992), and the asteroids 951 Gaspra (1991) and 243 Ida (1993) before it orbited Jupiter (199599); it also returned data about the Jupiter’s atmosphere and its largest moons (Io, Ganymede, Europa, and Callisto). The joint U.S.-ESA probe Cassini, launched in 1997, will explore Saturn, its rings, and some of its moons upon arriving in 2004.