- (A translation of this article.)
Akatsuki (24th science satellite; project name "Planet-C" or "VCO" [Venus Climate Orbiter]) is a Venus probe of the Japan Aerospace eXploration Agency (JAXA) and the Institute of Space and Astronautical Science (ISAS). Equipped with cameras of various observational wavelengths, it will observe the Venusian atmosphere in three dimensions. On 2010 May 21, it was launched from the Tanegashima Space Center. It was intended to enter orbit around Venus on 2010 December 7, but the orbital insertion failed, leaving the craft on a heliocentric trajectory near Venus's. On 2015 December 7, another attempt at insertion into an orbit about Venus was made[1], and on December 9 its success was confirmed.[2][3][4]
Mission Summary edit
[Akatsuki's] main purpose is the clarification of the mechanisms behind what are, in meteorology to date, inexplicable phenomena in Venus's atmosphere—the planetary-scale high-speed winds called "super-rotation", for instance. One might say that Akatsuki is a Venusian weather satellite. It is hoped that the fruits of this mission can be tied together in a comprehensive understanding of planetary meteorological phenomena. Moreover, it will make investigation of the physical properties of the Venusian surface, its volcanic activity, etc. by means of infrared light, as well as, in the duration between departure from Earth and arrival at Venus, observation of the distribution of interplanetary dust (Zodiacal light).
Once arrived at Venus, it will enter an elliptic orbit spanning elevations of 300—80,000 km over an approximate orbital period of 30 hours. After decreasing [something about apsides] approximately 20 hours, it will be almost synchronized with Venus's super-rotation and make continuous observation of the behavior of the Venusian atmosphere over approximately two years.
Design edit
The basic design, following Hayabusa, has the satellite's main unit at about 500 kg. The probe maintains its orientation via reaction-wheel-based three-axes control. The reaction wheels (MW, for "momentum wheel"), compared to Hayabusa's three, are increased by one to four. The orbit maneuvering engine (OME) is capable of 500 N of thrust; for it's nozzle and combustion chamber, the OME uses monolithic ceramics made from high-quality silicon nitride (Si
3N
4).[5]
The craft is equipped with two each of high-gain antennas (HGAs, radial-line-powered slot planar arrays, 32 kbps), middle-gain antennas (MGAs, horn antennas, 512 bits/s), and low-gain antennas (LGAs, super-wide-angle lenses, 8 bits/s) (though the HGAs are a dedicated transmitter and receiver, whereas the MGAs and LGAs are all transceivers that can do both simultaneously); it is mainly the HGA that is used to communicate with Usuda Deep Space Center's 64 m parabolic antenna. For situations where communication is necessary but Usuda is not visible, use of NASA's Deep Space Network is also possible.
As for observational instruments, the probe is equipped with a 1μm camera (IR1) to capture surface-originated infrared rays, cloud-scattered sunlight, etc., a 2μm camera (IR2) to capture infrared emissions from the atmosphere below the clouds and investigate low-altitude clouds, the distribution of rare gasses, etc., a mid-range infrared camera (LIR) to capture infrared emissions from clouds and investigate their structure, and an ultraviolet imager (UVI) to capture cloud-scattered sunlight and investigate the distribution of, e.g., gaseous sulfur dioxide. Also equipped, for the sake of understanding whether or not lightning discharges develop, is a lightning and airglow camera (LAC). Further, an ultra-stable oscillator (USO) is equipped, which besides its use as telecommunications equipment, supports radio occultation measurements to investigate the atmosphere's layered structure by exploiting the fact that when probe-to-earth radio waves graze the Venusian atmosphere, the radio signal's frequency and intensity will feel the atmosphere's influence.
The solar paddle mounting axis and the fuselage's z axis (the axis joining [i.e., lying perpendicular to] the surfaces on which the OME nozzle and HGAs are mounted) being orthogonal, the paddles can always turn to face the sun. Sunlight must not strike on the surfaces where the axles are mounted for the probe to use the radiation. One of these surfaces has the MGAs; the other has the observational instruments. The remaining two surfaces house the LGAs.[7] During the interplanetary cruise, the paddle axles are basically intended to be perpendicular to the plane of the ecliptic; when attitude adjustments are occasioned, they are to point the observational sensors away from the sun.
Development edit
At the time the project was proposed, it was planned that, four months from February of 2007, the probe would reach a temporary orbit around earth via an M-V rocket, in six months enter a heliocentric orbit, a year later, in June of 2008, go on an Earth swing-by to aim for Venus, and then arrive at Venus in September of 2009.[8] However, with the development of a new type of solid-fuel rocket and the discontinued use of the M-V rocket, the launch was amended to employ the 17th flight of an H-ⅡA rocket. The joint between the probe's main unit and the rocket could not be made to work at the diameter of the M-V rocket, so a fresh adapter was manufactured for use with the H-ⅡA. Further, based on the H-ⅡA rocket's launch capabilities for this planet, the mission was underweight, and to effectively use the surplus payload capacity, five tiny auxiliary satellites (IKAROS, UNITEC-1, Waseda-SAT2, KSAT, and Negai☆″) rode along at launch. IKAROS was stowed inside the aforementioned adapter, while UNITEC-1 and, in a casing called J-POD (the JAXA Picosatellite deployer), Waseda-SAT2, KSAT, and Negai☆″ were attached to its outside. Akatsuki's development cost was 14.6 trillion yen.
In October of 2010, the official name "Akatsuki" was announced.[9]
Operations edit
Operation immediately after Launch edit
For the 17th flight of an H-ⅡA (H-ⅡA F17), loaded with Akatsuki and five small auxiliary satellites including IKAROS, the launch was planned to fall between 2010 May 18 and 2010 June 3; the first planned launch time was May 18 at 06:44:14 (JST) but in the vicinity of the launch pad the stipulated overabundance of clouds containing frozen moisture was observed for five postponements of launch. Three days later, at 06:58:22 on the 21st, the postponed launch occurred at the instant planned.
Upon H-ⅡA F17's entry into Earth orbit, the second-stage engine momentarily shutting down, three of the spacecraft (Negai☆″, Waseda-SAT2, and KSAT) separated; afterwards, it transitioned to a heliocentric orbit via second-stage reignition, and approximately 27 minutes and 29 seconds after launch Akatsuki [11] and the remaining two craft (IKAROS and UNITEC-1) separated. Because of the launch rocket's surplus capability, the mission dispensed with the terran and lunar swing-bys, entered a Hohmann-transfer-like trajectory with its second stage, and proceded directly to Venus.
On June 28, Akatsuki, near aphelion (approximately 1.07 AU), ignited the OME thruster for 13 seconds, the world's first successful in-orbit demonstration of a ceramic thruster.[12] The OME thruster was scheduled to make two course corrections, but, because of the launch-time trajectory control's higher-than-expected accuracy, one was cancelled.
On October 28, four cameras, the LAC excepted, photographed part of Sagittarius. Based on prior predictions of the transmitted images, all of the cameras were confirmed to be in good health. Afterwards, other photographs were made, for example of Taurus, of the Earth with the Moon, etc.
Orbital Insertion Maneuver edit
At 07:50 AM on December 6, the probe oriented itself for the Venus orbital insertion maneuver (VOI-1), pointing the OME to achieve the desired motion.[13] The plan resembled the following table:
| Planned Event | Time (JST) | Time Relative to Closest Approach to Venus (December 7, 09:00 JST) |
|---|---|---|
| Trajectory-control engine (OME) ignition | December 7, 08:49:00 | 11 minutes before |
| Earth occultation begins; loss of communication with ground station | December 7, 08:50:43 | roughly 9 minutes before |
| OME burn finishes | December 7, 09:01:00 | 1 minute after |
| Earth occultation ends; communications resume | December 7, 09:12:03 | roughly 12 minutes after |
| Sunset begins | December 7, 09:36:37 | roughly 37 minutes after |
| Sunset ends | December 7, 10:40:44 | roughly 1 hour, 41 minutes after |
| z-axis oriented to face Earth | December 7, 10:59:00 | roughly 2 hours after |
| MGA-to-HGA handoff | December 7, 12:09:00 | roughly 3 hours after |
| Venusian orbit determination; scheduling of course corrections follows | December 7, 21:**:** | roughly 12 hours after |
Had the OME thruster fired for 12 minutes as planned, Akatsuki would have entered an elliptic trajectory around Venus for about four days. The intention was that, after those four days, at its closest approach to Venus, it would have made a trajectory change to lower its orbital period to about two days, then, after those two days, another burn to enter a survey orbit with a period of about 30 hours. Or if the OME had fired for at least 9 minutes and 20 seconds, there would have been the potential to enter a 50-day highly elliptic orbit about Venus.
On December 7 at 08:52:36, doppler data from the probe confirmed that Akatsuki was decelerating due to the retrofire begun at 08:49 the same morning.[14] The log of transmissions to Akatsuki lasted about three and a half minutes. But, at 08:50 the same day, Akatsuki was hidden behind Venus when viewed from Earth, and, as planned, communication with Earth was cut off. Communications were expected to resume at 09:12 that morning, but radio from the probe was instead received at 10:28, not from an MGA as expected, but an LGA.[15] That afternoon, with the solar paddles and one of the LGAs pointed to the sun, the other LGA pointed backwards, it was established that the craft had entered a slowly rotating "safe interrupt mode". To guarantee electrical power in case of serious trouble for the fuselage, this mode puts top priority on maintaining orientation. Until the day after, December 8, was spent returning to three-axis-control mode and acquiring telemetry. According to the telemetry, Akatsuki had, about two and half minutes after the OME thruster ignition, experienced a strange transverse force that substantially disturbed its attitude and, immediately afterwards, entered safe interrupt mode, which shut down the thruster. Becase the deceleration had been inadequate, it could not enter Venusian orbit.[16]
On December 27, it was announced that the cause of the trouble was the blockage of a non-return valve installed (to prevent backflow) in a pipe leading from a high-pressure helium tank to a fuel tank.[17][18][19] For normal combustion, adjustments are made so that the fuel, versus the oxidizer, is mixed in a higher proportion[note 1]; here overheating occurred based on the hypothesis that the pressure supplied by the fuel tank lessened, the flow of fuel to the thruster stagnated, and the oxidizer-fuel mixing ratio became inefficient, all because the non-return valve was blocked. Consequently producing abnormal combustion, enduring rotational momentum beyond the expectations for the fuselage, and at the same time reaching temperatures well above design, it is likely that the craft damaged[note 2] part of its thruster.
In June of 2011, JAXA announced the results of a ground test: the origin of the non-return valve blockage was quite probably ammonium nitrate crystals produced by the reaction of the fuel (hydrazine) with the oxidizer (dinitrogen tetroxide). It was conjectured that, because the ingredients for the resin in the oxidizer-side valve seal had a disposition to transmit dinitrogen tetroxide, a slow diffusion of oxidizer towards the fuel non-return valve reacted with the fuel to produce crystals that clogged it. Additionally, due to the abnormal combustion, it was confirmed that damage to the ceramic thruster would recur, probably magnified when the thruster was ignited a second time.[20]
Orbital Correction Management edit
Akatsuki, having in consequence executed a Venusian powered swing-by, had transfered to an orbit with a period of 203 days (slightly inside Venus, which has an orbital period of 224.7 days), a perihelion of 90 million kilometers, and an aphelion of 110 million kilometers.[21] As Venus would make about ten laps and fall behind one in the time that Akatsuki would make roughly 11 circuits around the sun, the two would again draw near in December of 2016 and January of 2017.[22][23] On 2010 December 8, JAXA reported to ISAS that they would look for the possibility of retrying an insertion to Venusian orbit at one of those times.[24]
On December 9, Akatsuki booted three of its cameras (LIR, UVI, and IR1) and photographed Venus from a distance of about 600,000 kilometers.[25]
In January 2011, it made the news that JAXA was considered plans for Akatsuki such as "slightly correcting the orbit, and observing a Venus-crosser on its next encounter with Venus"[26] or "shortening the time until a re-encounter by one year by decelerating (amending the orbit to one with a longer period) and waiting to overtake Venus from behind"[27]. But at that time, there was no official announcement from JAXA concerning these ideas. If the craft remained on the original trajectory for 2011, it would rejoin Venus over the interval from 2016 to 2017, but because it would do so at a distance much farther than in the case of VOI-1, some course correction would be needed anyway.
In March of 2011, for the purposes of both science data acquisition and checking the soundness of the survey instruments, four cameras (LIR, UV1, IR1, and IR2) were used to multi-image Venus from a distance of at least 10,000 kilometers, and the cameras' health was confirmed. Then dangers such as the possibility of approaching the rated maximum temperature[note 3] or deterioration of the thermal insulation were hypothesized for April 17, which would have been the first time passing perihelion, when the temperature of things like the satellite exterior and surface-mounted equipment would climb. Because of this, techniques like reorientation, etc. were employed on approach to perihelion to minimize the influence on instruments due to heat input.[28]
On 2011 June 30, the space agency JAXA reported, among other things, on the non-return valve's vulnerability to blockage and a plan for a Venus rendezvous in November 2015: the second (November of 2011) or third time (June 2012) passing perihelion the probe would effect with the OME or RCS thrusters a change in the mission's orbit, shortening it and lowering the aphelion.[29]
At 11:50 (JST) on 2011 September 7, to determine whether or not the OME was usable, and also with the goal of investigating of the extent of attitude disruption when using the OME, the first OME test fire (with a firing time of two seconds) was carried out. On September 9, JAXA announced the first test's results: OME thrust was estimated at 1/9 [of the specified operating force]. Based on this result, on September 14, the second OME test (for the same [?] five seconds) was executed with the intent of validated the OME firing situation. JAXA released the second test's results on September 15, which agreed with the first's. Thus, the first plan for Venus orbital insertion [i.e., using the OME] was deemed infeasible. However, as the RCS could be used in leiu of the OME, a plan was proposed to make the same orbital insertion time, November of 2015.[30]
On 2011 September 30, JAXA reported abandonment of further use of the OME in favor of flying by RCS and that the oxidizer would be dumped.[31]
On 2011 October 26, JAXA sent a report to ISAS on Akatsuki's present condition and maneuvers to encounter Venus. It described the dumping of oxidizer used by the OME—for six minutes on 2011 October 6 and for nine minutes on on the 12th and 13th each—as having gone exactly as planned. Further, the plan to use RCS was also detailed: a burn for a change of 90 m/s on 2011 November 1, another of 90 m/s on the 10th, and one of 70 m/s (to be adjusted to include any corrections needed to the first two changes) on the 21st. On November 1, an interview was given on the first maneuver, according to which the RCS firing time was 9 minutes and 48 seconds; details of the plan for the next would be settled by later telemetry data analysis. On 2011 November 10, a publication from JAXA describing the second burn came out reporting that it lasted 544 seconds (9 minutes and 4 seconds) starting from 13:37. The same report refined the first burn time to 587.5 seconds (9 minutes and 47.5 seconds). On 2011 November 21, came an announcement from JAXA relating the third maneuver: 342 seconds (5 minutes and 42 seconds) begun at 13:57.
On 2011 January 1, at an ISAS inquiry meeting, a report was given on the study investigating Akatsuki's Venusian orbital insertion failure and the current counterplans.[32] The same day, JAXA reported to ISAS the completion of the adjustments scheduled prior in the plan for going forward given above. The results of those adjustments were that it was feasible by RCS use for the probe to fly a trajectory that would bring it Venus in 2015, including a reattempt at Venus orbital insertion, even in case of [something about pressure increases]. Considering the probe's life span, [people] were scrutinizing the possibility of Venus orbital insertion after 2015, besides keeping up every sort of adjustment and test to further that potential.[33]
On 2015 August 5, JAXA announced that it had fired the RCS engines three more times, on the 17th, 24th, and 30th [of the previous month], for a total of seven and a half minutes, making course corrections that moved the second insertion from November to December[34][35]; according to observations made until August 2, the corrections had achieved their purpose.[36]
On 2015 August 31, JAXA disclosed that Akatsuki had passed perihelion, its nearest point to the sun.[37] JAXA had confirmed the fuselage integrity over a period of about one month.[37]
Venusian Orbital Insertion edit
JAXA confirmed that, from 08:51 on 2015 December 7, the RCS engines had been fired as planned for about 20 minutes for Akatsuki's Venus orbit insertion, as well as the achievement of the necessary burn time[38]; on December 9 the Venusian orbital insertion was verified.[2][3][4]
Public Relations edit
In October of 2009, simultaneous with the announcement of the official name, a marketing campaign appealling to the public for names and messages to send to Venus ("Register! your message to Akatsuki's Venus"[39]) began. If one sent a message from a mobile device, personal computer, or similar to a site, it replied with an image having "Venus Probe 'Akatsuki' Commemorative Transport Pass" written on it (in the image, one's message typeset along the bottom of an illustration of Akatsuki arriving at Venus) as an attachment; this kind of campaign was borrowed from the Japan spaceprobe Nozomi, but for Akatsuki could incorporate handwritten text and custom illustrations for celebrities' and organizations' messages and thus stressed quality over quantity.[40][41] 260,214 people both within and outside of Japan sent entries before the 2010 January deadline, and eventually 148,204 people entered through organizations like schools and planentaria.[42] Among others, a campaign by volunteer fans wanting to send an illustration of Vocaloid Hatsune Miku to space collected the messages of 13,849 people.[40][42] The messages were finely printed on about 90 12 cm ✕ 8 cm ✕ 0.2mm aluminum plates that also act as a balance weight to adjust the fuselage's center of mass; these were installed in the three passes of heat treatment given to the spacecraft.[40][42][43]
In the role of Akatsuki's mission mascots were "Akatsuki-kun" and "Kinsei-chan", drawn on the official website, in spin-off merchandise, etc.[44][45] Akatsuki-kun appears officially on Twitter to explain misson progress, on-board instruments, and so on, and converse with his older brothers Hayabusa-san (until 2010 June 13) and IKAROS-kimi or with his friend Mitibiki-san.[46]
Footnotes edit
- ^ While the most efficient fuels mix with oxidizer at a 1:1 ratio (perfect combustion), for Akatsuki the ratio is 1:0.8. There must be less combustion than is ideal in order to not exceed the ceramic thruster's heat-resistance.
- ^ The ceramic thruster can tolerate 1,500℃—a temperature far higher than a traditional metalic thruster could—but the temperature of perfect combustion reaches 2,000℃.
- ^ In orbit around Venus (the original heat budget), there would have been [heat input] of 2,649 W/m². (The highest considered in planning was 2,800 W/m².) With the failure of the Venus insertion, the probe entered heliocentric orbit, a trajectory on which heat input was believed to reach 3,655 W/m², roughly 1,000 W/m² higher than the value for circling Venus.