A number of early spacecraft carried particles and fields instrumentation. Explorer 1 carried Van Allen's geiger counter that provided the first evidence for the radiation belts. Explorer 10 was the first to provide data across the Earth's magnetopause when this battery-powered spacecraft made one pass into the near tail region out in the magnetosheath and then back to Earth. A fuller investigation of the location and shape of the magnetopause awaited the launch later in 1961 of Explorer 12 that made its first crossings of the magnetopause near noon and made its way around to the dawn magnetopause before ceasing operations. Beginning with the Explorer 18 mission in 1963, a series of Explorer spacecraft were then deemed Interplanetary Monitoring Platforms. IMP 1, 2 and 3 explored the near tail. IMP 5 had a higher inclination and provided some of the first observations of the polar cusp. IMP 7 and 8 were launched in 1972 and 1973 into near circular orbits with a radius of about 35 RE and an inclination of about 30o as shown in Figure 3.1. They were intended to orbit the Earth out of phase by 180o so that there would always be one spacecraft in the solar wind. However, the magnetometer on IMP 7 failed shortly after launch and IMP 8 alone remained to monitor the solar wind. In addition to the magnetometer the IMP 7 and 8 spacecraft carried a DC electric field instrument, a plasma wave instrument, solar wind plasma analyzers from both the Massachusetts Institute of Technology and the Los Alamos National Laboratory, a University of Iowa low energy proton and electron energy analyzer and a variety of energetic particle devices. This mission and its payload have been described by [King et al., 1982]. IMP 8 is most remarkable for its value to innumerable correlative studies and for its longevity. IMP-8 made measurements for close to 30 years. Because it is about as close to Earth as a useful solar wind monitor can get and still provide significant coverage of the solar wind, its data have often been used in studies even when more recent data from missions such as ISEE-3 and ACE at the L1 libration point have been simultaneously available.
At low altitudes several early missions are worthy of mention. The Navy satellite 1963-38C was the first spacecraft to detect field-aligned currents through their magnetic signatures. A. J. Dessler has told the story that when Zmuda and Armstrong sent their original paper to the Journal of Geophysical Research, he told them that their interpretation of the signatures as waves was wrong and that disturbances were field-aligned currents instead. As a result they revised the paper. However, field-aligned currents are carried by Alfven waves and today we would say that the best interpretations of many of the structures seen with 1963-38C were in terms of waves.
Under the Explorer banner the Atmospheric Explorer satellites were a low altitude series of satellites, some of which dipped low into the atmosphere temporarily to get data in regions where drag would deorbit them if they lingered. This mission is important because it established rules for the sharing of data and a common data system by which these data could be shared. While these rules may seem quite restrictive by today's standards and the data system quite primitive, the Atmospheric Explorer project was still ahead of its time.
The first of the Orbiting Geophysical Observatories was launched in 1964 into a highly elliptical orbit. It was followed by the launch in 1965 of the second OGO (or POGO as it was sometimes called) into a low altitude polar orbit. These spacecraft were complex 3-axis stabilized spacecraft with momentum wheels and thrusters, long booms for sensors and scanning platforms for instruments. There were deployment problems and interactions between the various control systems so the first two OGOs soon were spin stabilized. The third OGO was sent into a highly elliptic orbit like OGO-1 and performed as designed for the order of a month and then was spin stabilized. OGO-4 was a POGO that functioned well. OGO-5 was another eccentric orbiter that functioned as planned. OGO-6 was a POGO at low altitude that again functioned properly. The high altitude OGO spacecraft added immeasurably to our understanding of the solar wind interaction with the magnetosphere. OGO-1 provided the first high resolution measurements of both the magnetopause and bow shock. OGO-3 discovered ELF hiss and chorus that are responsible for the pitch angle diffusion of the radiation belt electrons. OGO-5 provided some of the first measurements of the polar cusp as did IMP-5 and the ISIS-1 satellite in the ionosphere, all of whose initial results were published in 1971. OGO-5 also provided a fairly complete study of the plasmasphere and it provided evidence for the erosion of the magnetopause by the solar wind. This evidence and complementary evidence in the near tail led to the postulate of the near-Earth neutral point model of substorms.
In the late 1970s an international program of solar terrestrial studies was initiated under the banner of the International Magnetospheric Study (IMS) [Russell and Southwood, 1983]. Although not as comprehensive as the later International Terrestrial Program (ISTP) of the 1990s, the IMS program contained both a ground-based and space-based component and as, in the later ISTP program, the satellites were multinational in origin. The first of these spacecraft was GEOS-1 that was intended to be a geo-stationary satellite carrying a magnetometer, a static electric field instrument using the drift of an electron beam in the Earth's field to measure this field, magnetic and electric wave instruments, a static electric field instrument with wire booms, thermal plasma composition and energetic particles [Knott, 1982]. GEOS 1 did not reach its intended orbit but was operated as an eccentric equatorial spacecraft with an orbital period of 12 hours. The GEOS 2 spacecraft was launched in 1978 into the desired synchronous orbit. The GEOS 1 and 2 orbits are shown in Figure 3.2. The effectiveness of both spacecraft was diminished by the early failure of the magnetometers. The accessibility of this data by the greater solar terrestrial community was quite limited.
The next pair of spacecraft to be launched as part of the IMS were the ISEE-1 and 2 mother-daughter spacecraft that were launched into the same 23 RE apogee orbit on a single Delta 2 launch vehicle in the fall of 1977 [Ogilvie, 1982; see also the special issue in IEEE Trans. Geoscience Electronics GE-16, 1978]. These two spacecraft followed each other around in the same orbit at a variable separation shown in Figure 3.3 so that the motion of magnetospheric boundaries could be measured and temporal profiles turned into spatial profiles. ISEE 1 and 2 made 10 sweeps through the magnetosphere from 1977 to 1987 measuring the outer magnetosphere, tail, magnetopause, magnetosheath and bow shock as shown in Figure 3.4. It carried a magnetometer, plasma analyzers from Los Alamos, Iowa and Goddard, energetic particles, steady and oscillating electric field instrument, oscillating magnetic fields, plasma composition and an experiment on ISEE-1 that sounded the plasma with radio waves and propagated them to ISEE-2 to find the integrated electron density by the delay introduced in the propagation.
Their companion spacecraft, ISEE-3, was launched in August 1978 into a potato chip-like orbit around the L1 or forward Lagrangian point, the place where the pull of the Earth's and sun's gravitational fields balance. As shown in Figure 3.5 this orbit had a dimension of 130 RE in the east-west direction centered on the solar direction and moved back and forth along the direction to the sun a total of 90 RE. There was also some small orbital motion in the north-south direction. The payload consisted of a magnetometer, a solar wind analyzer, an electron analyzer, plasma composition measurements, energetic particles, electric and magnetic waves, radio waves and cosmic rays. Some of these instruments are shown in Figure 3.6 on a sketch of the ISEE-spacecraft. ISEE-3 ably performed its function of monitoring the solar wind until it was commanded out of its orbit to perform a study of the distant tail in 1983/84 for which it was poorly instrumented. In 1985, it was taken further afield to fly by the comet Giacobini-Zinner. It is now on the other side of the sun and has provided little data since 1985.
The next pair of coordinated satellites were Dynamics Explorers 1 and 2 in 1982. Originally conceived as the Electrodynamics Explorers, two spacecraft in the same middle magnetospheric orbit, they were descoped and separated and turned into a magnetospheric mission with a coplanar ionospheric comparison. This mission was the first to carry a moderately high altitude auroral imager to observe the aurora from high altitudes. Dark holes in the images from this camera led the principal investigator to announce (mistakenly) the existence of a large flux of small comets. The controversy over this pronouncement has lasted for most of two decades. The precession of the polar orbit is shown in Figure 3.7. The Dynamics Explorers also included the innovative centralized data system. Since the computers could not handle the load, this system was premature and investigators, who could, took their data out of the system and processed them at their home institutions.
The next Explorer solar terrestrial mission to be launched was the Active Magnetospheric Particle Tracer Experiment whose purpose was to explore the entry of particles into the magnetosphere from the solar wind using the release of large amounts of barium and lithium as shown in Figure 3.8. The mission launched in August 1984 consisted of three spacecraft, the Charge Composition Explorer (CCE) in an 8.8 RE equatorial orbit, the Ion Release Module (IRM) (provided by Germany) and a small United Kingdom subsatellite (UKS) that acted as a daughter spacecraft to the 18.7 RE apogee IRM. The IRM and the UKS satellites co-orbited with variable separation until January 1985 when the UKS satellite failed. The AMPTE spacecraft made many contributions to magnetospheric physics but none of the ions created by the barium or lithium releases were ever detected by the detectors on CCE even when the releases were made inside the magnetosphere in the near geomagnetic tail. The mission and its payload have been described in the special issue of the IEEE Trans. on Geoscience and Remote Sensing GE-23(3), May 1985.
The last of the solar terrestrial missions we discuss in this section is the Chemical Release and Radiation Effects Satellite (CRRES) that was launched and died in 1991. This spacecraft released large amounts of ions in the terrestrial magnetosphere at low and moderately high altitudes. It also made many useful measurements of the plasmas and field in the equatorial middle magnetosphere.
Recent Magnetospheric Missions
A particularly important mission for solar terrestrial physics has been the International Solar Terrestrial Program. The trajectory of the Geotail spacecraft contributed by Japan and launched in 1992 is shown in Figure 3.9. Presently the orbit has evolved again into an about 30 x 8 RE orbit. The early trajectory of the Wind spacecraft is shown in Figure 3.10. The Wind spacecraft has also used lunar swingbys to keep its line of approaches aligned with the Earth sunline but unlike Geotail, Wind tries to stay in the solar wind. The Polar spacecraft orbits mainly within the magnetosphere. Gravitational torques have cause precession of the orbit plane as shown in Figure 3.11. A third partner in this effort was Russia and a number of former socialist republics that launched the Tail and Auroral Interball spacecraft whose trajectory are shown in Figure 3.12. In 1997 ACE was launched and placed at the L1 Lagrangian point. ACE took over much of the solar wind monitoring function from the Wind spacecraft. In 2000 the four-spacecraft Cluster mission was launched into a high inclination orbit about 4x20 RE and the IMAGE mission began imaging the plasmas of the magnetosphere. A particular crucial mission for the magnetosphere is the planned Magnetosphere Multiscale mission that will explore the coupling between small and large scale phenomena.
With the exception of Pluto and Mercury, a vigorous program of exploration has been carried out in the solar system. This is not to say that we have more than scratched the surface of what there is to know. Rather, we have learned much about the planets from Venus to Neptune in the relatively short span of only four decades. The lacuna in the exploration of Pluto and Mercury is particularly unfortunate since these two planets are the end members of the solar system, bookends that bracket the radial evolution of the properties of the planets. We need to understand both why Pluto and Mercury are as they are and why planetary formation ceased exterior and interior to these planets. Efforts are now under way to address the exploration of both objects. The MESSENGER Discovery mission was launched in mid 2004 to Mercury with ESA and JAXA planning to send a dual orbiting mission to Mercury within the decade. NASA is readying the Pluto flyby New Horizons as part of the New Frontiers Program.
Rather than simply listing a chronology of the planetary program, we will proceed from the inside out, starting with the brief flybys of Mercury by Mariner 10.
Only one mission has been launched to Mercury, Mariner 10 that flew by three times from March 1974 to March 1975. The middle encounter flew across the dayside of the planet. The two March encounters flew by the nightside, very close to it. The Mariner spacecraft were all three axis stabilized, particularly suited for imaging. About half of the surface of Mercury was imaged. Very little atmosphere was detected but there was a significant magnetic field, strong enough to deflect the solar wind, and most probably due to a dynamo inside the planet. The MESSENGER Discovery mission is presently on its way to rendezvous with Mercury and enter orbit.
Mariner 2 flew by Venus in 1962 taking images but not seeing any disturbance in the solar wind flow. Venus, while similar to the Earth in size, clearly had a much weaker magnetic field. The surface could not be seen through the clouds but the clouds themselves had some surprises since they rotated around the planet in 4 days despite the fact that Venus itself rotates only once every 243 days. Mariner 5 repeated the Mariner 2 flyby but came much closer, close enough to detect a perturbation in the solar wind flow but not close enough to detect any planetary magnetic field.
The Venera series of missions carried atmospheric probes, landers and orbiters to Venus. The Venera 2, 4, 6 and 8 missions were atmospheric probes that returned some plasma and magnetic field data on the interaction. The Venera 9 and 10 missions orbited Venus and mapped out the region of solar wind interaction but did not unambiguously resolve the question of a planetary magnetic field. Venera 11, 12, 13 and 14 dropped landers onto the surface of Venus and provided atmospheric, rock composition and lightning data as well as pictures of the surface. Venera 15 orbited Venus and mapped the surface with radar. The landers all descended with parachutes. The orbiters were three axis stabilized for some observations but slowly rotated about the solar direction at other times. The final mission in the series was the VEGA mission that flew two spacecraft by Venus and went on to comet Halley. As they flew by they each dropped off a balloon that drifted in about 24 hours from the nightside to the dayside. These balloons showed a very dynamic atmosphere but no evidence for lightning in their photometer readings at the local times (morning) where they obtained data.
The Pioneer Venus spacecraft reached the planet in 1978. Four atmospheric probes and an orbiter measured first the vertical structure of the atmosphere and then the atmosphere and ionosphere of Venus for a period of almost 14 years, more than a complete solar cycle. The probes descended by parachutes that were discarded at lowest altitudes. The orbiter was spin stabilized. This mission proved that there was no significant magnetic field at Venus and gave most strong evidence for planetary lighting confirming the contemporaneous data from the Venera landers.
Finally, The Magellan spacecraft arrived at Venus and mapped the surface almost completely with greater resolution than the Venera 15 mission. This spacecraft was 3 axis stabilized and stored its data for later transmission to Earth. Magellan, like the Pioneer Venus orbiter, gave valuable experience with atmospheric drag toward the end of its lifetime and burned up in the Venus atmosphere. Presently, a new ESA mission called Venus Express is being readied for launch with arrival and insertion into orbit scheduled for June 2006.
Perhaps surprisingly considering the present strong Mars program, Mars until recently has not been explored as extensively as Venus. Mariner 4 flew by Mars in 1964 and detected a planetary bow shock. Mariner 7 and 9 (with 6 and 8 failing) first flew by and then orbited Mars allowing a complete map of its surface to be created. The Soviet program provided the first orbiters of Mars, Mars 2 and 3 in 1972 and Mars 5 in 1974. These missions allowed the bow shock to be mapped and the loss of planetary ions to the solar wind to be discovered. In 1975 the first successful landing on Mars took place, the Viking 1 and 2 landings. This was followed by a hiatus in Mars exploration until the Soviet Phobos mission was put into Mars orbit in 1989. This mission demonstrated how weak the Martian magnetic field was. The next successful mission to Mars was the Mars Global Surveyor that provided high resolution images, global laser altimetry and measurements of strong localized remanent magnetic fields. After several mishaps to Mars-bound spacecraft (Mars'96, Mars Polar Lander, Deep Space 2, Mars Climate Observer and Nozomi), the Mars Pathfinder, the 2001 Mars Odyssey Spacecraft, Mars Express and the 2004 landings of the Mars Excursion Rovers got Mars exploration going again. However, the solar wind and aeronomy of Mars are still very much under explored.
Jupiter, Saturn, Uranus and Neptune
The largest planet in the solar system has now been visited by six spacecraft. Pioneers 10 and 11 in 1973 and 1979, Voyagers 1 and 2 in 1979; Ulysses in 1992; Galileo (in orbit) from 1995 to September 2003 and Cassini in 2000. Galileo mapped the entire equatorial magnetosphere of Jupiter and provided data repeatedly from each of the Galilean moons. Figure 3.13. shows the trajectories of Voyager 1 and 2.
Pioneer 11 reached Saturn in 1979 with Voyagers 1 and 2 arriving in 1980 and 1981. Cassini entered Saturn orbit in July 2004. Voyager 2 arrived at Uranus in 1986 and Neptune in 1989. Both Voyager 1 and 2 continue to operate at this writing.
Future Planetary Missions
The most robust exploration of any planet is that of Mars. The Mars Reconnaisance Orbiter is being readied for launch.The Mars Surface Laboratory has been selected for flight. The Phoenix Scout mission is being build to land on Mars.
The small body program is also strong. The NEAR mission successfully mapped Eros. Lunar Prospector mapped the moon from Polar orbit. The Stardust mission has sampled Tempel I and is heading back to Earth. Rosetta has been launched to explore the comet 67P Churyumov-Gerasimenko. The Deep Impact mission is heading for its encounter with comet Tempel 1 and the Dawn Discovery mission is being readied for launch to Vesta and Ceres.