Galileo's atmospheric entry probe, based on the design of the large probe of the Pioneer Venus multiprobe, was finally released on 13 July 1995 when the spacecraft was still 80 million kilometers from Jupiter. The probe hit the atmosphere at 6.5° N / 4.4° W at 22:04:44 UT on 7 December 1995, traveling at a relative velocity of 48 kilometers/hour, and returned valuable data for 58 minutes as it plunged into the Jovian cauldron. The entry probe endured a maximum deceleration of 228 g's about a minute after entry when temperatures scaled up to 16,000°C. The probe's transmitter failed 61.4 minutes after entry when the spacecraft was 180 kilometers below its entry ceiling, evidently due to the enormous pressure (22.7 atmospheres). Data, originally transmitted to its parent and then later transmitted back to Earth, indicated an intense radiation belt 50,000 kilometers above Jupiter's clouds, few organic compounds, and winds as high as 640 meters/second. The entry probe also found less lightning, less water vapor, and half the helium than had been expected in the upper atmosphere.

The Galileo orbiter, meanwhile, fired its engine at 00:27 UT on 8 December, becoming Jupiter's first human-made satellite. Its orbital period was 198 days. Soon after, Galileo began its planned 11 tours over 22 months exploring the planet and its moons, including flybys of Ganymede (for the first time on 27 June 1996) and Europa (on 6 November 1997). Having fulfilled its original goals, NASA implemented a two-year extension to 31 December 1999 with the Galileo Europa Mission (GEM) during which the spacecraft conducted numerous flybys of Jupiter's moons, each encounter yielding a wealth of scientific data. These included flybys of Europa nine times (eight between December 1997 and February 1999 and once in January 2000), Callisto four times (between May 1999 and September 1999), and Io three times (in October 1999, November 1999, and February 2000). On the last flyby of Io, Galileo flew only 198 kilometers from the surface of Io sending back the highest resolution photos yet of the volcanically active moon.

On 8 March 2000, NASA announced plans to extend Galileo's mission again, with a new phase beginning October 2000 called the Galileo Millennium Mission. The idea was to coordinate investigations of Jupiter and its environs—particular the interaction of the solar wind with the planet's magnetosphere—with the Cassini spacecraft (on its way to Saturn) that was expected to fly past Jupiter in December 2000. This second extension was largely possible due to the extreme accuracy of navigation during the prior phases of the mission that had saved a significant amount of maneuvering propellant. The RTGs on board the spacecraft were still delivering about 450 W in 2000, although power capacity was declining at a rate of about 7 W per year. Under the Millennium Mission, Galileo flew by Ganymede, the largest moon in the solar system, on 20 May at a range of 809 kilometers and made its final flyby (also of Ganymede) on 28 December 2000 at a range of 2,337 kilometers. In January 2001, both Galileo and Cassini together encountered the magnetosphere bow shock within a half hour of each other.

As the mission entered its final phase, mission scientists arranged for three final flybys of Io, primarily to obtain more data on the moon's magnetic field and heat generation that drives its volcanism. A final flyby of Callisto (in May 2001) resulted in some very high-resolution photographs and led to the Io encounters in August and October 2001 and in January 2002, the last one at a staggering 101.5 kilometers range, the closest it had gotten to any moon in its entire mission. This close encounter increased Galileo's orbit such that it could be easily commanded in the future to terminate its mission in Jupiter's atmosphere. A flyby of Amalthea in November 2002 provided key information on the moon's density and preceded its closest flyby of Jupiter itself. Beginning March 2003, Galileo was contacted once a week only to verify its status. Many years of operation in the Jovian system had exposed the spacecraft to intense radiation, taking a toll on many systems and instruments. Because Galileo had not been sterilized, to prevent contamination, it was decided to have the vehicle burn up in the Jovian atmosphere instead of risking impact on a moon such as Europa.

Having completed its 35th orbit around Jupiter and after accompanying the planet for three-quarters of a circuit around the Sun, Galileo flew into the atmosphere at a velocity of 48.2 kilometers/second, just south of equator, on 21 September 2003 at 18:57 UT. In its nearly eight-year mission around Jupiter, Galileo had returned an unprecedented amount of data on the planet and its environs. For example, Galileo discovered far less lightning activity (about 10% of that found in an equal area on Earth) than anticipated, helium abundance in Jupiter very nearly the same as in the Sun (24% compared to 25%), extensive resurfacing of Io's surface due to continuing volcanic activity since the Voyagers flew by in 1979, and evidence for liquid water ocean under Europa's icy surface (with indications of similar liquid saltwater layers under the surfaces of Ganymede and Callisto). Jupiter's ring system was found to be made of dust from impacts on the four small inner moons. Galileo also discovered materials linked to organic compounds (clay-like minerals known as phyllosilicates) on the icy crust of Europa, perhaps produced by collisions with an asteroid or a comet. The spacecraft also identified the first internal magnetic field of a moon (Ganymede) that produces a "minimagnetosphere" within Jupiter's larger magnetosphere. By March 2000, the spacecraft had returned about 14,000 images back to Earth.

Hiten and Hagoromo

Nation: Japan (3) Objective(s): lunar flyby and lunar orbit Spacecraft: MUSES-A and Hagoromo subsatellite Spacecraft Mass: 185 kg (MUSES-A), 12 kg (Hagoromo) Mission Design and Management: ISAS Launch Vehicle: Mu-3S-II (no. 5) Launch Date and Time: 24 January 1990 / 11:46:00 UT Launch Site: Kagoshima / Launch Complex M1

Scientific Instruments:

Hiten:

cosmic dust detector (MDC)

Results: This two-module Japanese spacecraft was designed to fly past the Moon and release an orbiter. It was the first Japanese lunar mission and also the first robotic lunar probe since the flight of the Soviet Luna 24 in 1976. MUSES-A (for Mu-launched Space Engineering Satellite), named Hiten ("musical angel") after launch, was put into a highly elliptical orbit around Earth that intersected with the Moon's orbit. Due to a problem with the orbital injection burn, the probe's orbital apogee was 290,000 kilometers, much less than the hoped for 476,000 kilometers, but after a number of subsequent maneuvers, Hiten reached its originally planned nominal orbit. At 19:37 UT on 18 March, during its first flyby of the Moon, Hiten released into lunar orbit, a small 12-kilogram "grandchild" satellite named Hagoromo. Hagoromo did not carry any science instruments and was designed to only transmit telemetry and diagnostic data back to Earth, but it made Japan the first nation besides the Soviet Union and the United States to put a spacecraft into lunar orbit.

Hagoromo's initial orbital parameters were 20,000 × 7,400 kilometers. Although the maneuver successfully demonstrated the use of the swingby technique to enter lunar orbit, communications with Hagoromo had already been lost shortly before release on 21 February when its S-band transmitter failed. Hiten, meanwhile, passed by the Moon at 20:04:09 UT on 18 March at a distance of 16,472.4 kilometers and continued on its trajectory, simulating the orbital path of the proposed Geotail spacecraft. By 4 March 1991, Hiten had carried out seven more lunar flybys and began a phase of aerobraking into Earth's atmosphere—a feat it carried out for the first time by any spacecraft on 19 March (at 00:43 UT) at an altitude of 125.5 kilometers, which lowered Hiten's relative velocity by 1.712 meters/second and its orbital apogee by 8,665 kilometers. A second aerobraking over Earth occurred at 11:36 UT on 30 March at 120 kilometers altitude, reducing velocity by 2.8 kilometers/second and apogee by another 14,000 kilometers.

With its primary mission now over, Hiten began an unexpected extended mission to experiment with a novel method to enter lunar orbit. After a ninth and tenth flyby of the Moon, the latter on 2 October 1991, Hiten was put into a looping orbit that passed the Earth–Moon L4 and L5 Lagrange Points in October 1991 and January 1991, respectively, where it activated its MDC cosmic dust detector, jointly built with Germany. Circling back to the Moon for its eleventh flyby of the Moon at 13:33 UT on 15 February 1992 at a range of 422 kilometers, Hiten used a portion of its last remaining propellant to fire its engine (for 10 minutes) and insert itself into lunar orbit. This profile to enter lunar orbit that required very little delta-V on the part of the spacecraft was developed by JPL mathematician Edward Belbruno (1951– ). Initial orbital parameters around the Moon were 422 × 49,400 kilometers. A final amount of propellant was then used two months later, on 10 April 1993 to deorbit Hiten, which crashed onto the lunar surface at 18:03:25.7 UT at 55.3° E / 34.0° S.

Ulysses

Nation: ESA and USA (1) Objective(s): heliocentric orbit Spacecraft: Ulysses Spacecraft Mass: 371 kg Mission Design and Management: ESA / NASA / JPL Launch Vehicle: STS-41 Discovery Launch Date and Time: 6 October 1990 / 11:47:16 UT Launch Site: Kennedy Space Center / Launch Complex 39B

Scientific Instruments:

solar wind plasma experiment (BAM)
solar wind ion composition experiment (GLG)
magnetic fields experiment (HED)
energetic particle composition/neutral gas experiment (KEP)
low energy charged particle composition/anisotropy experiment (LAN)
cosmic rays and solar particles experiment (SIM)
radio/plasma waves experiment (STO)
solar x-rays and cosmic gamma-ray bursts experiment (HUS)
cosmic dust experiment (GRU)

Results: The Ulysses mission was an outgrowth of the abandoned International Solar Polar Mission (ISPM) that originally involved two spacecraft—one American and one European—flying over opposite solar poles to investigate the Sun in three dimensions. Eventually, NASA cancelled its spacecraft, significantly eroding the confidence of international partners in the reliability of NASA as a partner, and the mission merged into a single spacecraft, provided by ESA. The scientific payload was shared by ESA and NASA, with the latter providing the RTG power source (similar to one used on Galileo), a Space Shuttle launch, and tracking from its Deep Space Network. Ground control operations were shared by both the Americans and Europeans. The vehicle was designed to fly a unique trajectory that would use a gravity assist from Jupiter to take it below the ecliptic plane and pass the solar south pole and then above the ecliptic to fly over the north pole.

Eventually, 13 years after ESA's science council had approved the mission, and considerably delayed by the Challenger disaster, on 6 October 1990, about 7.5 hours after launch, Ulysses was sent on its way into heliocentric orbit via an Inertial Upper Stage/PAM-S motor combination. Escape velocity was 15.4 kilometers/second, higher than had been achieved by either of the Voyagers or Pioneers, and the fastest velocity ever achieved by a human-made object at the time. After a mid-course correction on 8 July 1991, Ulysses passed within 378,400 kilometers of Jupiter at 12:02 UT on 8 February 1992, becoming the fifth spacecraft to reach Jupiter. After a 17-day period passing through and studying the Jovian system, the spacecraft headed downwards and back to the Sun. From about mid-1993 on, Ulysses was constantly in the region of space dominated by the Sun's southern pole, as indicated by the constant negative polarity measured by the magnetometer. South polar observations extended from 26 June to 6 November 1994, when the vehicle was above 70° solar latitude. It reached a maximum of 80.2° in September 1994. Its instruments found that the solar wind blows faster at the south pole than at the equatorial regions. Flying up above the solar equator on 5 March 1995, Ulysses passed over the north polar regions between 19 June and 30 September 1995 (maximum latitude of 80.2°). Closest approach to the Sun was on 12 March 1995 at a range of 200 million kilometers.

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