Beyond_Earth-_A_Chronicle_of_Deep_Space_Exploration_1958-2016.pdf

215

Dawn

Nation: USA (91)

Objective(s): Vesta and Ceres orbit

Spacecraft: Dawn

Spacecraft Mass: 1,217.7 kg

Mission Design and Management: NASA / JPL

Launch Vehicle: Delta 7925H-9.5 (no. D327)

Launch Date and Time: 27 September 2007 / 11:34:00 UT

Launch Site: Cape Canaveral Air Force Station / SLC-17B

Scientific Instruments:

• 1. framing camera (FC)
• 2. visible and infrared mapping spectrometer (VIR)
• 3. gamma ray and neutron detector (GRaND)

Results: Dawn, the ninth mission in NASA's Discovery Program, was launched on a nearly-decade long mission to study two very different objects which both accreted early in the history of the solar system, the asteroid Vesta (arrival in 2011) and the dwarf planet Ceres (arrival in 2015). The investigation of these objects—the two largest in the asteroid belt—was driven by the three principal scientific motivations: to investigate the conditions at the origin of the solar system; to find the nature of the building blocks from which the inner planets were formed; and to contrast the very different evolutionary paths of Ceres and Vesta. Besides its complement of scientific equipment, Dawn also carried three xenon ion thrusters (derived from the technology used on the Deep Space 1 spacecraft), each with a thrust of 91 mN and a specific impulse of 3,100 seconds. Some of the scientific equipment was provided by German and Italian institutions. After launch, Dawn was accelerated to escape velocity of 11.50 kilometers/second by the PAM-D solid propellant third stage which fired at 12:29 UT. The spacecraft passed lunar orbit at around 14:30 UT on 28 September and entered solar orbit at roughly 1.00 × 1.62 AU. Long-term cruise with the ion thrusters began on 17 December 2007, and completed on 31 October 2008, nearly 11 months later. Subsequently, a single trajectory correction on 20 November 2008 orchestrated a gravity assist flyby past Mars at a range of 542 kilometers on 17 February 2009. Over two years later, Dawn began to approach its first target, Vesta, returning progressively higher resolution images of the protoplanet. At around 05:00 UT on 16 July 2011, Dawn gently slipped into orbit around Vesta at an altitude of about 16,000 kilometers, thus becoming the first spacecraft to orbit any object in the main asteroid belt.

This image from Dawn shows Kupalo Crater on Ceres. The crater, one of the youngest on the minor planet, measures 26 kilometers across. The image was taken on 21 December 2015 from Dawn's low-altitude mapping orbit from a distance of approximately 385 kilometers. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

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Using its xenon-ion engine, it moved into a closer "survey" orbit at an altitude of about 2,700 kilometers by 2 August, staying at that orbit until the end of the month. Later, on 27 September, it moved into a closer orbit at an altitude of 680 kilometers (with an orbital period of 12.3 hours) staying there until 2 November, during which period it fully mapped Vesta six times, including in color and in stereo. By 8 December, the spacecraft was in a 4.3-hour orbit at an average altitude of just 210 kilometers. Original plans were to carry out a 70-day mission at that low orbit but this was extended to 1 May, during which the spacecraft took 13,000 photos covering most of Vesta as well as more than 2.6 million visible and IR spectra.

This mosaic image shows the mysterious mountain Ahuna Mons on Ceres. The images were taken by NASA's Dawn spacecraft from a low-altitude mapping orbit, about 385 kilometers above the surface, in December 2015. Credit: NASA/ JPL-Caltech/UCLA/MPLS/DLR/IDA/PSI

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From 23 June to 25 July, Dawn was back up to 680 kilometers conducting more mapping, including of areas that had not been visible before. Data from the extended period of study in 2012 allowed scientists to estimate the size of its dense iron-nickel core (about 220 kilometers across), and conclusively identify Vesta as one of the few remaining remnants of large planetoids that formed the rocky planets of the solar system. In December 2012, investigators announced that Dawn had detected sinuous gullies on the surface of Vesta that might have been caused by liquid water—similar gullies on Earth are carved by liquid water. The data collected by Dawn suggested that Vesta is more closely related to terrestrial planets such as Earth (and its Moon) than to typical asteroids. Dawn's findings also showed that Vesta is the source of more meteorites on Earth than Mars or the Moon. Pictures also identified immense basins on the surface of Vesta such as the 400-kilometer diameter Veneneia and the 500-kilometer diameter Rheasilvia basins, created by impacts two and one billion years ago, respectively. At 07:26 UT on 5 September 2012, Dawn escaped the gravitational grip of Vesta and headed towards its second destination, the dwarf plant Ceres. During its stay at Vesta, the two bodies travelled around the Sun for 685 million kilometers. On the way to Ceres, Dawn stopped normal ion thrusting and spuriously entered "safe mode" on 11 September 2014. Fortunately, ground controllers were able to determine the source of the problem—a coincidental combination of high-energy particles that disabled the ion propulsion system and a previously unknown bug in the spacecraft software—and resumed normal ion firing by 15 September. Approach operations to Ceres began in January 2015 although during the approach phase, Dawn took fewer photographs of its target (than with Vesta) due to problems with two of its four reaction wheels, the first of which failed on 17 June 2010. Needing at least three wheels to be operational, the mission team devised a plan to allow the spacecraft to operate using only two (in case another failed) in combination with hydrazine reaction control thrusters. A second wheel indeed failed, on 8 August 2012, just as Dawn was spiraling away from Vesta and beginning its trip to Ceres. The mission team implemented plans to conserve the much needed hydrazine (now more valuable than ever). All of these strategies allowed Dawn to approach Ceres slightly compromised but largely operational. At 00:39 UT on 7 March 2015, Dawn finally entered initial (polar) orbit around Ceres, thus becoming the first mission to study a dwarf plant, ahead of the New Horizons encounter with Pluto four months later. Dawn also became the first spacecraft to orbit two different celestial bodies (other than the Sun, of course). In planning for the mission, scientists had envisioned four different circular mapping orbits—called RC3, Survey, HAMO, and LAMO—around Ceres, from one as high as 13,600 kilometers (RC3) to as low as 385 kilometers (LAMO). Dawn remained in its first mapping orbit, RC3, from 23 April to 9 May 2015 carrying out photography, taking spectra at infrared and visible wavelengths, and searching for lofted dust as evidence of water vapor. On 9 May the spacecraft began using its ion engine to spiral down to reach its second mapping orbit by 3 June at about 4,400 kilometers, with its science mission beginning two days later. The orbital period at this point was 3.1 days. During this phase, lasting eight orbits, Dawn carried out extensive scientific observations over the sunlit side of Ceres. This phase, completed by 30 June, was punctuated by a minor alarm on 27 June due to anomalies in two of the scientific instruments, and a problem on 30 June when the spacecraft went into safe mode due to a problem with its orientation (later traced to a mechanical gimbal system that swivels one of its three ion engines). On 13 August 2015, it arrived in its third mapping orbit and stopped using its ion thrusters. Orbital altitude was 1,470 kilometers with a period of 19 hours. Its science mission in this orbit began four days later. Over the next six months, the spacecraft mapped Ceres six times. Mission Director at JPL Marc Rayman noted that this would be "some of the most intensive observations of its entire mission." At 23:30 UT on 23 October, Dawn turned one of its ion engines (no. 2) to move to its next orbit, a passage that took about seven weeks. On 7 December, the engine was turned off as the spacecraft reached its final mapping orbit at about 385 kilometers altitude. After some further orbital tweaking on 11–13 December, Dawn's orbit was synchronized with Ceres' rotation around its axis. Finally, on 18 December, the spacecraft began its next science phase in its new orbit, which it maintained for nearly nine months until 2 September 2016. During this period, the spacecraft obtained extensive data with its combined gamma-ray and neutron spectrometer as well as its infrared mapping spectrometer. Circling the dwarf planet every 5.4 hours, the standard high-resolution mapping programs were also in effect. By 3 May, Dawn's time in orbit around Ceres exceeded its time in orbit around Vesta in 2011–2012. A couple of months later, on 30 June 2016, JPL announced that Dawn had concluded its fully completed prime mission. By this point, it had taken 69,000 images and completed 2,450 orbits around both Vesta and Ceres. In addition, the ion engines had fired for 48,500 hours. NASA approved an extended mission at the time, opting not to have Dawn travel to a large asteroid known as Adeona but to continue to explore Ceres. On 2 September 2016, Dawn began a new five-week journey to a higher orbit after a highly successful stay at lower altitudes. The spacecraft reached a new orbit at 1,480 kilometers with an orbital period of 18.9 hours by 6 October, having begun an extended science mission 10 days later. On 4 November, it began climbing higher again and reached its sixth science orbit (known as "extended mission orbit 3" or XMO3) on 5 December at about 7,520 × 9,350 kilometers altitude. Here it measured the cosmic ray noise to calibrate the data on Ceres' nuclear radiation that was collected when it was at 385 kilometers. Dawn was basically in good condition in early 2017 despite a temporary switch to "safe mode" on 17 January. On 23 April 2017, mission controllers discovered that two of the remaining reaction wheels on board the spacecraft had stopped working, thus jeopardizing attitude control. By using hydrazine, controllers were able to return Dawn to standard flight configuration. A few days later, on 29 April, Dawn successfully observed Ceres at opposition, i.e., from a position between Ceres and the Sun, allowing the spacecraft to view the bright Occator Crater from a new perspective. Soon after celebrating the tenth anniversary of its launch, NASA announced on 19 October 2017 that it had authorized a second extension to the mission. During the extension, Dawn will descend to lower than before—possibly 200 kilometers—to continue studies of the dwarf planet, focused on measuring the number and energy of gamma rays and neutrons. One of the most important discoveries made by Dawn was the existence of widespread ice just below the surface of Ceres, announced in December 2016. Dawn carries a memory chip with the names of more than 360,000 people who submitted their names as part of an outreach effort in 2005 and 2006.

216

Chang’e 1

Nation: China (1)

Objective(s): lunar orbit

Spacecraft: Chang’e yihao

Spacecraft Mass: 2,350 kg

Mission Design and Management: China National Space Administration

Launch Vehicle: Chang Zheng 3A (no. Y14)

Launch Date and Time: 24 October 2007 / 10:05:04 UT

Launch Site: Xichang / LC 3

Scientific Instruments:

• 1. stereoscopic CCD camera
• 2. Sagnac-based interferometer spectrometer imager
• 3. laser altimeter
• 4. microwave radiometer
• 5. gamma and x-ray spectrometer
• 6. space environment monitor system (a high-energy particle detector and 2 solar wind detectors)