Beyond_Earth-_A_Chronicle_of_Deep_Space_Exploration_1958-2016.pdf

Viking 1

Nation: USA (56)

Objective(s): Mars landing and orbit

Spacecraft: Viking-B

Spacecraft Mass: 3,527 kg

Mission Design and Management: NASA / LaRC (overall) / JPL (Orbiter)

Launch Vehicle: Titan IIIE-Centaur (TC-4 / Titan no. E-4 / Centaur D-1T)

Launch Date and Time: 20 August 1975 / 21:22:00 UT

Launch Site: Cape Canaveral / Launch Complex 41

Scientific Instruments:

Orbiter:

• 1. imaging system (2 vidicon cameras) (VIS)
• 2. infrared spectrometer for water vapor mapping (MAWD)
• 3. infrared radiometer for thermal mapping (IRTM)

Lander:

• 1. imaging system (2 facsimile cameras)
• 2. gas chromatograph mass spectrometer (GCMS)
• 3. seismometer
• 4. x-ray fluorescence spectrometer
• 5. biological laboratory
• 6. weather instrument package (temperature, pressure, wind velocity)
• 7. remote sampler arm

Aeroshell:

• 1. retarding potential analyzer
• 2. upper-atmosphere mass spectrometer
• 3. pressure, temperature, and density sensors

Carl Sagan, a member of the Viking science team, posing with a life-size model of the Viking Lander in Death Valley, California in the mid-1970s. At the time, Sagan was professor of astronomy at Cornell University. Credit: NASA/JPL

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Results: Viking 1 was the first of a pair of complex deep space probes that were designed to reach Mars and collect evidence on the possibility (or lack of) for life on Mars. Each spacecraft was composed of two primary elements, an orbiter (2,339 kilograms) and a lander (978 kilograms). The Orbiter design heavily borrowed from the Mariner buses, while the Lander looked superficially like a much larger version of the Surveyor lunar lander. Prior to launch, the batteries of the first spacecraft were discharged, prompting NASA to replace the original first spacecraft with the second, which was launched as Viking 1. After three mid-course corrections (on 27 August 1975, and 10 and 15 June 1976), the spacecraft entered orbit around Mars on 19 June 1976. Initial orbital parameters were 1,500 × 50,300 kilometers. The following day, the orbiter moved into an operational orbit at 1,500 × 32,800 kilometers. The same day, when the Orbiter began transmitting back photos of the primary landing site in the Chryse region, scientists discovered that the area was rougher than expected. Using the new photos, scientists targeted the lander to a different site on the western slopes of Chryse Planitia ("Golden Plain"). The Lander separated from the Orbiter at 08:32 UT on 20 July 1976, and after a complicated atmospheric entry sequence during which the probe took air samples, Viking Lander 1 set down safely at 22.483° N / 47.94° W at 11:53:06 UT on 20 July 1976 (about 28 kilometers from its planned target). Once down, the spacecraft began taking high-quality photographs (in three colors) of its surroundings. Besides high-resolution images, the lander also took a 300° panorama of its surroundings that showed not only parts of the spacecraft itself but also the gently rolling plains of the environs. Instruments recorded temperatures ranging

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4 November 1985 when signals took approximately 2.5 hours to reach Earth. Light conditions were 400 times less than terrestrial conditions. Closest approach to Uranus took place at 17:59 UT on 24 January 1986 at a range of 81,500 kilometers. During its flyby, Voyager 2 discovered 10 new moons (given such names as Puck, Portia, Juliet, Cressida, Rosalind, Belinda, Desdemona, Cordelia, Ophelia, and Bianca—obvious allusions to Shakespeare), two new rings in addition to the "older" nine rings, and a magnetic field tilted at 55° off-axis and off-center. The spacecraft found wind speeds in Uranus' atmosphere as high as 724 kilometers/hour and found evidence of a boiling ocean of water some 800 kilometers below the top cloud surface. Its rings were found to be extremely variable in thickness and opacity. Voyager 2 also returned spectacular photos of Miranda, Oberon, Ariel, Umbriel, and Titania, five of Uranus' larger moons. In flying by Miranda at a range of only 28,260 kilometers, the spacecraft came closest to any object so far in its nearly decade-long travels. Images of the moon showed a strange object whose surface was a mishmash of peculiar features that seemed to have no rhyme or reason. Uranus itself appeared generally featureless in the photographs taken. The spectacular news of the Uranus encounter was interrupted the same week by the tragic Challenger accident that killed seven astronauts during their Space Shuttle launch on 28 January. Following the Uranus encounter, the spacecraft performed a single mid-course correction on 14 February 1986—the largest ever made by Voyager 2—to set it on a precise course to Neptune.

Voyager 2's encounter with Neptune capped a 7 billion-kilometer journey when on 25 August 1989 at 03:56 UT, it flew 4,800 kilometers over the cloud tops of the giant planet, the closest of its four flybys. It was the first human-made object to fly by the planet. Its 10 instruments were still in working order at the time. During the encounter, the spacecraft discovered six new moons (Proteus, Larissa, Despina, Galatea, Thalassa, and Naiad) and four new rings. The planet itself was found to be more active than previously believed, with 1,100 kilometer winds. Hydrogen was found to be the most common atmospheric element, although the abundant methane gave the planet its blue appearance. Images revealed details of the three major features in the planetary clouds—the Lesser Dark Spot, the Great Dark Spot, and Scooter. Voyager photographed two-thirds of Neptune's largest moon Triton, revealing the coldest known planetary body in the solar system and a nitrogen ice "volcano" on its surface. Spectacular images of its southern hemisphere showed a strange, pitted "cantaloupe"-type terrain. The flyby of Neptune concluded Voyager 2's planetary encounters, which spanned an amazing 12 years in deep space, virtually accomplishing the originally planned Grand Tour of the solar system, at least in terms of targets reached if not in science accomplished. Once past the Neptune system, Voyager 2 followed a course below the ecliptic plane and out of the solar system. Approximately 56 million kilometers past the encounter, Voyager 2's instruments were put in low power mode to conserve energy. After the Neptune encounter, NASA formally renamed the entire project the Voyager Interstellar Mission (VIM).

Of the four spacecraft sent out to beyond the environs of the solar system in the 1970s, three of them—Voyagers 1 and 2 and Pioneer 11—were all heading in the direction of the solar apex, i.e., the apparent direction of the Sun's travel in the Milky Way galaxy, and thus would be expected to reach the heliopause earlier than Pioneer 10 which was headed in the direction of the heliospheric tail. In November 1998, 21 years after launch, non-essential instruments were permanently turned off, leaving seven instruments still operating. Through the turn of the century, JPL continued to receive ultraviolet and particle/fields data. For example, on 12 January 2001, an immense shock wave that had blasted out of the outer heliosphere on 14 July 2000, finally reached Voyager 2. During the six-month journey, the shock wave had ploughed through the solar wind, sweeping up and accelerating charged particles. The spacecraft provided important information on high-energy shock-energized ions. On 30 August 2007, Voyager 2 passed the termination shock and then entered the heliosheath. By November 5, 2017, the spacecraft was 116.167 AU (17.378 billion kilometers) from Earth, moving at a velocity of 15.4 kilometers/second relative to the Sun, heading in the direction of the constellation Telescopium. At this velocity, it would take about 19,390 years to traverse a single light-year. Data from the remaining five operating instruments—the cosmic ray telescope, the low-energy charged particles experiment, the magnetometer, the plasma waves experiment, and the plasma spectrometer—could be received as late as 2025.

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This false color image of Jupiter's Great Red Spot taken by Voyager 1 was assembled from three black-and-white negatives. Credit: NASA/JPL

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Voyager 1

Nation: USA (57) Objective(s): Jupiter flyby, Saturn flyby Spacecraft: Voyager-1 Spacecraft Mass: 721.9 kg Mission Design and Management: NASA / JPL Launch Vehicle: Titan IIIE-Centaur (TC-6 / Titan no. 23E-6 / Centaur D-1T) Launch Date and Time: 5 September 1977 / 12:56:01 UT Launch Site: Cape Canaveral / Launch Complex 41

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Image showing the general trajectories of the four NASA probes, Pioneers 10 and 11 and Voyagers 1 and 2, sent out of the solar system. As of February 2017, Voyager 1 was at a distance of 20.6 billion kilometers from the Sun while Voyager 2 was at 17 billion kilometers. Both Voyagers are headed towards the outer boundary of the solar system in search of the heliopause, the region where the Sun's influence wanes and the beginning of interstellar space can be sensed. Credit: NASA/JPL

Scientific Instruments:

1. imaging science system (ISS)
2. ultraviolet spectrometer (UVS)
3. infrared interferometer spectrometer (IRIS)
4. planetary radio astronomy experiment (PRA)
5. photopolarimeter (PPS)
6. triaxial fluxgate magnetometer (MAG)
7. plasma spectrometer (PLS)
8. low-energy charged particles experiment (LECP)
9. plasma waves experiment (PWS)
10. cosmic ray telescope (CRS)
11. radio science system (RSS)

Results: Voyager 1 was launched after Voyager 2, but because of a faster route, it exited the asteroid belt earlier than its twin, having overtaken Voyager 2 on 15 December 1977. It began its Jovian imaging mission in April 1978 at a range of 265 million kilometers from the planet; images sent back by January the following year indicated that Jupiter's atmosphere was more turbulent than during the Pioneer flybys in 1973–1974. Beginning on 30 January, Voyager 1 took a picture every 96 seconds for a span of 100 hours to generate a color timelapse movie to depict 10 rotations of Jupiter. On 10 February 1979, the spacecraft crossed into the Jovian moon system and in early March, it had already discovered a thin (less than 30 kilometers thick) ring circling Jupiter. Voyager 1's closest encounter with Jupiter was at 12:05 UT on 5 March 1979 at a range of 280,000 kilometers, following which it encountered several of Jupiter's Moons, including Amalthea (at 420,200-kilometer range), Io (21,000 kilometers), Europa (733,760 kilometers), Ganymede (114,710 kilometers), and Callisto (126,400 kilometers), in that order, returning spectacular photos of their terrain, opening up a completely new world for planetary scientists.

The most interesting find was on Io, where images showed a bizarre yellow, orange, and brown world with at least eight active volcanoes spewing material into space, making it one of the most (if not the most) geologically active planetary body in the solar system. The presence of active volcanoes suggested that the sulfur and oxygen in Jovian space may be a result of the volcanic plumes from Io which are rich in sulfur dioxide. The spacecraft also discovered two new moons, Thebe and Metis. Following the Jupiter encounter, Voyager 1 completed an initial course correction on 9 April 1979 in preparation for its meeting with Saturn. A second correction on 10 October 1979 ensured that the spacecraft would not hit Saturn's moon Titan. Its flyby of the Saturn system in November 1979 was as spectacular as its previous encounter. Voyager 1 found five new moons, a ring system consisting of thousands of bands, wedge-shaped transient clouds of tiny particles in the B ring that scientists called "spokes," a new ring (the "G-ring"), and "shepherding" satellites on either side of the F-ring—satellites that keep the rings well-defined.

During its flyby, the spacecraft photographed Saturn's moons Titan, Mimas, Enceladus, Tethys, Dione, and Rhea. Based on incoming data, all the moons appeared to be composed largely of water ice. Perhaps the most interesting target was Titan, which Voyager 1 passed at 05:41 UT on 12 November at a range of 4,000 kilometers. Images showed a thick atmosphere that completely hid the surface. The spacecraft found that the moon's atmosphere was composed of 90% nitrogen. Pressure and temperature at the surface was 1.6 atmospheres and –180°C, respectively. Atmospheric data suggested that Titan might be the first body in the solar system (apart from Earth) where liquid might exist on the surface. In addition, the presence of nitrogen, methane, and more complex hydrocarbons indicated that prebiotic chemical reactions might be possible on Titan. Voyager 1's closest approach to Saturn was at 23:46 UT on 12 November 1980 at a range of 126,000 kilometers.