Beyond_Earth-_A_Chronicle_of_Deep_Space_Exploration_1958-2016.pdf

226

Chang’e 2

Nation: China (2)

Objective(s): lunar orbit, Sun–Earth L2, asteroid flyby

Spacecraft: Chang’e erhao

Spacecraft Mass: 2,480 kg

Mission Design and Management: CNSA

Launch Vehicle: Chang Zheng 3C

Launch Date and Time: 1 October 2010 / 10:59:57 UT

Launch Site: Xichang / LC2

Scientific Instruments:

• 1. CCD camera (TDI)
• 2. descent camera
• 3. directional antenna surveillance camera
• 4. solar wing surveillance camera
• 5. engine surveillance camera
• 6. gamma-ray spectrometer
• 7. x-ray spectrometer
• 8. laser altimeter

Results: The original mission of Chang'e 2 (or CE-2) was as backup to Chang'e 1 (CE-1), to basically repeat that mission with an improved suite of instruments. After Chang'e 1's highly successful mission, additional tasks were attached to CE-2, such that this mission essentially became a pathfinder mission to Chang'e 3 (CE-3), a landing mission. Unlike CE-1, CE-2 was launched on a more ambitious direct translunar trajectory (at 212 × 356,996 kilometers at 28.5° inclination), which required the more powerful Chang Zheng 3C launch vehicle. A midcourse correction on 2 October 2010 was so accurate that further adjustments were unnecessary on the way to the Moon. The spacecraft successfully entered lunar orbit after 4 days and 16 hours of flight (as opposed to 12 days for CE-1) at an orbit of 120 × 80,000 kilometers. Three adjustments followed on 7, 8, and 9 October that resulted in CE-2 being in its operational circular orbit at 100 kilometers. All of its instruments, activated during the coast to the Moon, continued operations in lunar orbit without problems. On 26 October at 13:27 UT, CE-2 fired its four 1 kgf thrusters for over 18 minutes to bring down perilune to 15 kilometers so that the spacecraft could photograph the planned landing site of CE-3 in Sinus Iridum. It returned to its nominal orbit two days later after obtaining a large number of high resolution images of the surface, some down to 1.2 to 1.5 meters resolution. In February 2012, Chinese authorities released a full map of the Moon at 7 meters resolution, claimed at the time as the highest resolution map of the Moon. Of all recent probes to the Moon, only the photographs from Lunar Reconnaissance Orbiter (LRO) had higher resolution. CE-2's main lunar orbital mission concluded on 1 April 2012 but because the spacecraft still had a relatively large amount of maneuvering propellant still left, mission planners decided in early 2013 to formulate an extended mission, one that would culminate with a flight to the Sun–Earth L2 Lagrange Point. By 23 May 2013, CE-2 completed a second survey of Sinus Iridium from a low altitude and filled in some surface details where earlier data was of relatively low detail. On 8 June 2011, CE-2 raised its apolune to 3,583 kilometers. The next day, CE-2's main engine (50 kgf thrust) fired for 18 minutes, boosting the spacecraft out of lunar orbit. Finally, on 25 August 2011 at 15:24 UT, the 1 kgf thrusters fired to place CE-2 in a 180-day period Lissajous orbit around L2, about 1.5 million kilometers from Earth. China thus became only the third country or entity (after the United States and ESA) to send a spacecraft to a Lagrange Point. There, Chang'e 2 studied charged particles near Earth's magnetic tail and observed possible x-ray and gamma-ray bursts from the Sun. The spacecraft departed L2 on 15 April 2012 (although this was not announced by the Chinese until 14 June) and headed for a flyby encounter with the asteroid 4179 Toutatis, about 7 million kilometers from Earth. On 13 December 2012 at 08:30:09 UT, Chang'e 2 flew by Toutatis at a distance of just 1.9 kilometers (much better than the 30 kilometers hoped), making China the fourth nation or entity after the U.S., ESA, and Japan to perform an asteroid flyby. The encounter occurred at a relative velocity of 10.73 kilometers/second, giving very little time (about a minute) for useful imaging but some excellent pictures were returned. After the Toutatis encounter, CE-2 remains in heliocentric orbit, with Chinese controllers maintaining contact in 2014 when the probe was as far as 100 million kilometers from Earth. On 23 October 2016, chief scientist for China's Lunar Exploration Project, Ouyang Ziyuan, announced that Chang'e 2 had "fulfilled its mission," that it remained in heliocentric orbit (apparently as "the smallest man-made asteroid in the solar system," which was not true as some of NASA's early Pioneers were smaller), and that the spacecraft would be returning "somewhere closer to the earth around 2029." There was no word on whether the Chinese still maintained any contact with the spacecraft.

227

Juno

Nation: USA (95)

Objective(s): Jupiter orbit

Spacecraft: Juno

Spacecraft Mass: 3,625 kg

Mission Design and Management: NASA / JPL

Launch Vehicle: Atlas V 551 (AV-029)

Launch Date and Time: 5 August 2011 / 16:25:00 UT

Launch Site: Cape Canaveral Air Force Station / SLC-41

Scientific Instruments:

• 1. gravity science system (GS)
• 2. microwave radiometer (MWR)
• 3. vector magnetometer (MAG)
• 4. JADE and JEDI plasma and energetic particle detectors
• 5. Waves radio/plasma wave sensor
• 6. ultraviolet imager/spectrometer (UVS)
• 7. infrared imager/spectrometer (JIRAM)
• 8. JunoCam

Results: Juno, NASA's second New Frontiers mission (after New Horizons), was designed to study Jupiter from polar orbit around the gas giant. Its specific science goals include studying the planet's composition, gravity field, magnetic field, and polar magnetosphere, as well investigating the nature of the planet's core, the amount of water in its atmosphere, mass distribution, and the winds in its clouds. Juno is the first mission to Jupiter not to use radioisotope thermoelectric generators (RTGs) for power and relies on three giant solar arrays symmetrically arranged around the spacecraft that provide 450 watts of power in orbit around Jupiter.

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At launch, the optimal mission was planned to last about 14 Earth months in Jovian orbit. Juno entered parking orbit around Earth (at 194 × 226 kilometers at 28.8° inclination) and then a hyperbolic escape orbit less than 45 minutes after launch. As it headed outwards toward the asteroid belt, on 1 February 2012, Juno carried out its first mid-course correction, a firing lasting 25 minutes. Further course corrections were implemented on 30 August and 14 September 2012. The spacecraft passed the halfway point to Jupiter at 12:25 UT on 12 August 2013. Juno returned to the vicinity of Earth, using a gravity assist maneuver to pick up more speed, on 8 October 2013. Several of its instruments were activated as preliminary tests to see if they were in working condition. Twice after the flyby, Juno entered "safe mode" and shut down all inessential systems but ground controllers were quickly able to return the spacecraft to normal operating mode. On 13 January 2016, Juno broke the record for the furthest distance from the sun—793 million kilometers—where a spacecraft has been powered by solar power. The record had previously been held by ESA's Rosetta space probe and set in October 2012. After a mid-course correction on 3 February, on 27 May, Juno crossed from the primary gravitational influence of the Sun to that of Jupiter. A month later, on 30 June, Juno entered Jupiter's magnetosphere. Finally, after an almost five-year trip, Juno fired its Leros-1b engine for 35 minutes and 1 second (from 03:18 to 03:53 UT Earth receive time) and entered an elliptical and polar orbit—known as a "capture orbit"—around Jupiter of 8.1 million × 4,200 kilometers with an orbital period of 53.5 days. Five of its scientific instruments were powered up on 6 July with the rest activated by the end of the month. According to the original plan, Juno's dedicated science mission was designed to begin only after two complete capture orbits, nearly two months after orbital insertion, and after a minor orbital correction on 13 July.

This stunning view of Jupiter was acquired by NASA's Juno spacecraft on 19 May 2017 using its JunoCam from an altitude of 12,857 kilometers. Although the small bright clouds that dot the entire south tropical zone appear tiny, they are actually towers approximately 50 kilometers wide and 50 kilometers high that cast shadows below. Credit: NASA/SWRI/MSSS/Gerald Eichstadt/Sean Doran

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In its first capture orbit, Juno made its first pass through perijove (lowest point) on 27 August when it approached the gas giant at a range of only 4,200 kilometers at 13:44 UT. This would be the closest the spacecraft would get to the planet during its primary mission, and it was the first time that its entire suite of scientific instruments was activated to study the planet as it flew by at a velocity of 208,000 kilometers/hour. Just prior to the perijove flyby on 19 October, at about 06:47 UT, Juno suddenly entered "safe mode" when a software performance monitor caused a reboot of the spacecraft's main computer. The switch to safe mode necessitated a postponement of exit from capture orbit to a new orbit with a period of 14 days; the burn was postponed to allow engineers to verify the performance of two helium check valves in the spacecraft's fuel pressurization system (associated with its main engine). On 24 October, the spacecraft exited safe mode and carried out a 31-minute burn using its smaller thrusters to slightly adjust its orbit. Juno successfully carried out its third flyby of Jupiter on 11 December at 17:04 UT, at a range of 4,150 kilometers, this time focusing on investigations on Jupiter's interior structure via its gravity field. (Only the JIRAM instrument was inactive during the flyby). During the flyby, Juno's JunoCam (a visible-light camera) captured spectacular images of one of Jupiter's so-called eight "string of pearls"—large counterclockwise rotating storms in the planet's southern hemisphere. For the fourth close flyby of Jupiter, at 12:57 UT on 2 February, NASA opened up an online vote for the public to choose in selecting what pictures should be taken. Juno's perijove for this encounter was 4,300 kilometers, while it was traveling at 57.8 kilometers/second. About two weeks later on 17 February 2017, NASA announced that Juno would remain in its 53-day capture orbit for the remainder of its primary mission and still be able to accomplish its science goals. Mission planners took this decision to avoid firing Juno's main engine due to concerns about the valves on the engine that did not operate as expected the previous October. There was concern that firing the engine would put the vehicle in a "less-than-desirable" orbit. In its capture orbit, scientists believe that Juno can still complete its primary science mission in 12 orbits, performed through 2018. If the spacecraft is operational at that time, there will remain the option of an extended mission. The orbiter performed its fifth close pass of Jupiter on 18 May 2017, swooping down to about 3,500 kilometers above the planet's clouds. All science instruments operated as planned. An even more spectacular flyby, its sixth, occurred on 10 July, when Juno flew over the Great Red Spot at an altitude of 9,000 kilometers, just 11 minutes and 33 seconds after reaching perijove (at 3,500 kilometers). The spacecraft remained in fine operation in the same orbit through its seventh (on 1 September) and eighth (on 24 October) pass to perijove. As part of an educational program with the LEGO group, Juno carries three aluminum figurines into outer space, each about 3.8 cm large, that of Galileo Galilei, the Roman god Jupiter, and his wife Juno. There is also a plaque, provided by the Italian Space Agency, dedicated to Galileo and including a facsimile of handwritten text by him. In June 2016, musicians Trent Reznor (of Nine Inch Nails) and Atticus Ross shared a nearly 9-minute piece of music to celebrate the Juno mission. The track originally scored NASA's short film about Juno entitled "Visions of Harmony."