Beyond_Earth-_A_Chronicle_of_Deep_Space_Exploration_1958-2016.pdf

211

SMART-1

Nation: European Space Agency (3)

Objective(s): lunar orbit

Spacecraft: SMART-1

Spacecraft Mass: 367 kg

Mission Design and Management: ESA

Launch Vehicle: Ariane 5G (no. V162) (L516)

Launch Date and Time: 27 September 2003 / 23:14:46 UT

Launch Site: Centre spatial Guyanais / ELA-3

Scientific Instruments:

• 1. advanced Moon micro-imager experiment (AMIE)
• 2. demonstration of a compact x-ray spectrometer (D-CIXS)
• 3. x-ray solar monitor
• 4. SMART-1 infrared spectrometer (XSM)
• 5. electric propulsion diagnostic package (EPDP)
• 6. spacecraft potential, electron and dust experiment (SPEDE)
• 7. Ka band TT&C experiment (KATE)

Results: The Small Missions for Advanced Research in Technology (SMART)-1 spacecraft was a technology demonstrator designed to test solar-electric propulsion and other deep space technologies on the way to the Moon. A second part of the mission would focus on studying polar mountain peaks that are in perpetual sunlight as well as the dark parts of the lunar parts that might contain ice. The ESA spacecraft, the first European spacecraft to enter orbit around the Moon, had a French-built Hall effect thruster (known as PPS®1350) derived from a Russian ion propulsion system originally designed by OKB Fakel, a Russian company that specializes in attitude control thrusters using ion and plasma sources. The thruster used xenon propellant to generate 88 mN of thrust (about the weight of a postcard) and a specific impulse of 1,650 seconds. The engine was powered by solar arrays which generated the 1,350 watts needed to power the ion engines. Initially launched into a geostationary transfer orbit of 7,035 × 42,223 kilometers by the Ariane 5 hypergolic EPS upper stage (with a 2,600 kgf thrust Aestus engine), SMART-1 used its electric propulsion system to slowly spin out into higher and higher elliptical orbits in what was a highly efficient mission profile. Two days every week, mission controllers at the European Space Operations Centre (ESOC) in Darmstadt, Germany, repeated burns of the ion engine, gradually expanding the spacecraft's spiral orbit. By the time it was 200,000 kilometers out, the Moon's gravity began to exert a significant influence on SMART-1. Nine days after its last perigee (on 2 November 2004), the spacecraft passed through the L1 Lagrange Point into a region dominated by the Moon's gravitational field. At 17:47 UT on 15 November, the vehicle then passed through its first perilune, having moved into polar orbit around the Moon. Initial orbital parameters were 6,704 × 53,208 kilometers, with an orbital period of 129 hours. During the following weeks, SMART-1's ion engine fired to gradually reduce orbital parameters to allow closer views of the surface; it reached its operational orbit (with an orbital period of about 5 hours) by 27 February 2005. While in orbit, SMART-1's instruments studied the Moon's topography and surface texture as well as mapping the surface distribution of minerals such as pyroxenes, olivines, and feldspars, thus improving the data returned by Clementine. The mission was designed to end in August 2005 but was extended a year to August 2006 with plans for an impact. On 17 September 2005, the ion engine was fired for the last time, having exhausted all its propellant, leaving the vehicle in a natural orbit determined only by lunar gravity (and the gravitational influences of Earth and the Sun) and the occasional use of its attitude control thrusters. By that time, the ion engine had fired for 4,958.3 hours, a record length of operation in space for such an engine. The mission of SMART-1 finally ended at 05:42:22 UT on 3 September 2006 when the spacecraft was deliberately crashed onto the nearside of the Moon in Lacus Excellentiae at 46.2° W / 34.3° S. Its impact (at 2 kilometers/second) created a dust cloud visible with Earth-based telescopes.

212

Rosetta and Philae

Nation: ESA (4)

Objective(s): comet orbit and landing

Spacecraft: Rosetta Orbiter / Rosetta Lander

Spacecraft Mass: 3,000 kg (includes 100 kg lander)

Mission Design and Management: ESA

Launch Vehicle: Ariane 5G+ (V158) (no. 518G)

Launch Date and Time: 2 March 2004 / 07:17:44 UT

Launch Site: CSG / ELA-3

Scientific Instruments:

Rosetta Orbiter:

• 1. ultraviolet imaging spectrometer (ALICE)
• 2. comet nucleus sounding experiment by radiowave transmission (CONSERT)
• 3. cometary secondary ion mass analyzer (COSIMA)
• 4. grain impact analyzer and dust accumulator (GIADA)
• 5. micro-imaging dust analysis system (MIDAS)
• 6. microwave instrument for the Rosetta orbiter (MIRO)
• 7. optical, spectroscopic and infrared remote imaging system (OSIRIS)
• 8. Rosetta orbiter spectrometer for ion and neutral analysis (ROSINA)
• 9. Rosetta plasma consortium (RPC)
• 10. radio science investigation (RSI)
• 11. visible and infrared thermal imaging spectrometer (VIRTIS)

Philae:

• 1. alpha proton x-ray spectrometer (APXS)
• 2. cometary sampling and composition instrument (COSAC)
• 3. Ptolemy evolved gas analyzer
• 4. comet nucleus infrared and visible analyzer (CIVA)
• 5. Rosetta lander imaging system (ROLIS)
• 6. comet nucleus sounding experiment by radiowave transmission (CONSERT)
• 7. multi-purpose sensors for surface and sub-surface science (MUPUS)
• 8. Rosetta lander magnetometer and plasma monitor (ROMAP)
• 9. surface electric sounding and acoustic monitoring experiments (SESAME)
• 10. sample and distribution device (SD2)

Results: Rosetta was a European deep space probe launched on an originally projected 11.5-year mission to rendezvous, orbit, land, and study the 67P/Churyumov-Gerasimenko comet. Part of ESA's Horizon 2000 cornerstone missions, which includes SOHO (launched 1995), XMM-Newton (1999), Cluster II (2000), and INTEGRAL (2002), Rosetta consists of two parts—an orbiter (Rosetta) and a lander (Philae)—each equipped with a variety of scientific instruments. Originally, the mission was targeting comet 46P/Wirtanen but when the launch was delayed due to problems with the Ariane 5, the mission was redirected to Churyumov-Gerasimenko. Rosetta was launched into an escape trajectory with a 17-minute burn of Ariane's EPS second stage, putting the spacecraft on a trajectory that culminated in a 0.885 × 1.094 AU heliocentric orbit inclined at 0.4° to the ecliptic.

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Its voyage to its target comet was punctuated by a series of gravity-assist maneuvers, the first of which occurred at 22:09 UT on 4 March 2005 when Rosetta flew by Earth (over the Pacific, west of Mexico) at a distance of 1,954.7 kilometers. A most risky flyby of Mars followed on 25 February 2007, when Rosetta came a mere 250 kilometers close to the Red Planet, experiencing a short and critical period out of contact with Earth and in Mars' shadow. Both these flybys produced spectacular photographs of Earth and Mars, respectively. The assist sent the spacecraft towards Earth for a second time, arriving and flying past our planet at a range of 5,295 kilometers on 13 November 2007. Before the final Earth flyby (on 12 November 2009), Rosetta performed a close flyby (just 800 kilometers) of asteroid 2867 Steins in the main asteroid belt at 18:58 UT on 5 September 2008, collecting a large amount of information. A second asteroid flyby, that of 21 Lutetia at 16:10 UT on 10 July 2010 at a range of 3,162 kilometers, produced spectacular images (using the OSIRIS instrument) of a battered minor planet riddled with craters. Resolution was as high as 60 meters in a body whose longest side is around 130 kilometers. Soon after, in June 2011, Rosetta was placed under "hibernation" as it made its way beyond the orbit of Jupiter—where there was no solar energy to power the vehicle—and back again close to the Sun. On 20 January 2014, its internal clock "awoke" the spacecraft and sent a signal back to Earth that all was well. Now only 9 million kilometers from its primary target, Rosetta began its final race to comet 67P/C-G. On 6 August 2014, at a distance of 405 million kilometers from Earth (about halfway between the orbits of Mars and Jupiter), Rosetta finally rendezvoused with the comet as it completed the last of 10 maneuvers (that began in May 2014) to adjust velocity and direction. During close operations near the comet, on 15 September, scientists identified a landing site for the spacecraft, "Site J" (later named "Agilkia"), located near the smaller of the comet's two "lobes." By this time (10 September 2014), the spacecraft was in a roughly 29-kilometer orbit around 67P/C-G, becoming the first spacecraft to orbit a cometary nucleus. Just prior to the planned landing, on 12 November, controllers identified a problem in Philae's active descent system thruster which provides thrust to avoid a rebound, but it was decided to move on with the landing and rely only on the harpoons instead of the thruster to keep the spacecraft moored. At 08:35 UT on 12 November, the two spacecraft separated, initiating Philae's 7-hour descent to the comet at a relative velocity of just 1 meter/second. A signal confirming the touchdown arrived at Earth at 16:03 UT (about 28 minutes, 20 seconds after the actual event). It later transpired the Philae had actually landed three times on the comet (at 15:34:04, 17:25:26, and 17:31:17 UT comet time) as the two harpoons did not fire as intended after each touchdown. Later analysis showed that all of the three methods to secure the lander had faced some problems: the ice screws, which were designed for soft materials, did not penetrate the hard surface of the Agilkia region; the thruster failed to fire due to a problem with a seal; and the harpoons also did not fire due to an electrical problem. As a result, Philae bounced on the surface several times before settling down about one kilometer away from its intended landing site in an area known as Abydos. All of its instruments were subsequently activated for data collection. For a short period, ESA controllers did not know the disposition of the lander as it went into hibernation, but on 14 November, contact was reestablished with Philae, following which all of its collected data was transferred to the mothership. Due to exhaustion of the primary battery, last contact with Philae was at 00:36 UT on 15 November, thus coming to about 64 hours of independent operation (and 57 hours on the surface). During its mission, Philae completed 80% of its planned "first science sequence," returning spectacular images of its surroundings, showing a cometary surface covered by dust and debris in size measuring anywhere from a millimeter to a meter. Philae also found complex molecules that could be the key building blocks of life, monitored the daily rise and fall of temperature, and assessed the surface properties and internal structure of the comet. ESA controllers hoped that the lander could be revived in August 2015 when sunlight fell on the lander and its solar panels, but assumed that Philae's mission was essentially over by November 2014. As hoped, the Philae lander was awoken after seven months of hibernation. At 20:28 UT on 13 June 2015, controllers at ESA's European Space Operations Center in Darmstadt received signals (about 663 kbits of data over 85 seconds) from the lander, suggesting at least initially that Philae was "doing very well" and "ready for operations," according to DLR Philae Project Manager Stephan Ulamec. A second smaller burst was received at 21:26 UT on 14 June followed by six more bursts by 9 July 2015, after which time Rosetta was no longer in range to receive data from Philae. A year after landing, in November 2015, mission teams still remained hopeful that there would be renewed contact with the lander, especially as the Rosetta orbiter began to approach the lander again. But in February 2016, ESA announced that it was unlikely that Rosetta would ever pick up any more signals from Philae again, partly due to failures in a transmitter and a receiver on board. On 5 September 2016, ESA announced that they had conclusively identified the landing site of Philae in images taken by Rosetta's OSIRIS narrow-angle camera when the orbiter approached to just 2.7 kilometers of the surface. Rosetta, meanwhile, had continued its primary mission orbiting Comet 67P/Churyumov-Gerasimenko as the comet itself arced closer to the Sun. In November 2014, the orbiter adjusted its orbit several times to position it about 30 kilometers above the comet, interrupted by a brief "dip" down to 20 kilometers for about 10 days in early December. On 4 February 2015, Rosetta began moving into a new path for an encounter, timed for 12:41 UT on 14 February, at a range of just six kilometers. The flyby took the spacecraft over the most active regions of the comet, allowing scientists to seek zones where gas and dust accelerates from the surface. In June 2015, ESA extended Rosetta's mission to at least September 2016 (an extension of nine months from its original "nominal" mission). During this extension, Rosetta was party to Comet 67P/C-G's closest approach to the Sun, a distance of 186 million kilometers, on 13 August 2015. At the perihelion, gases and dust particles around the comet reached peak intensity, clearly visible in the many spectacular images sent back by the orbiter. Finally, at 20:50 UT on 30 September 2016, Rosetta carried out a final maneuver sending it on a collision course with the comet from a height of 19 kilometers. During the descent, Rosetta studied the comet's gas, dust, and plasma environment very close to the surface and took numerous high-resolution images. The decision to end the mission was predicated on the fact that the comet was heading out beyond the orbit of Jupiter again, and thus, there would be little power to operate the spacecraft. Confirmation of final impact arrived at Darmstadt at 11:19:37 UT on 30 September 2016, thus ending one of ESA's most successful planetary missions. Besides collecting a vast amount of data on the properties of the comet, including its interior, surface and surrounding gas, dust, and plasma, Rosetta's key findings include the discovery of water vapor in comet 67P/G-C (vapor that is significantly different from that found on Earth), the detection of both molecular nitrogen and molecular oxygen for the first time at a comet, the existence of exposed water ice on the comet's surface, and the discovery of amino acid glycine (commonly found in proteins) and phosphorus (a component of DNA and cell membranes) in the comet.