Akatsuki, IKAROS and Shin-en

JAXA (ISAS) · Japan · Orbiter · 2010 · Radar, and the return Ended

Ended. JAXA lost contact with Akatsuki at the end of April 2024 and ended the mission on 18 September 2025. The silent spacecraft is thought to be still in orbit around Venus. JAXA ended IKAROS's operations on 15 May 2025, and Shin-en's team never heard it after its first day in May 2010.

TL;DR· 21 min read

Akatsuki was Japan's weather satellite for Venus. Its main engine failed during the 2010 arrival, but five years later to the day it entered orbit on its small thrusters and watched the atmosphere until April 2024, finding among other things evidence that waves from the Sun's daily heating keep the winds racing around the planet. JAXA ended the mission on 18 September 2025, and the silent craft is thought to be still in orbit. Its launch companions were the solar sail IKAROS, which flew past Venus in December 2010, and the student probe Shin-en, which its team never heard after the first day.

What happened to Japan's Akatsuki Venus probe?

Akatsuki was Japan's weather satellite for Venus. Its main engine failed as it tried to enter orbit in December 2010, but five years later to the day it got in on its small steering thrusters, watched the clouds until contact was lost in April 2024, and JAXA ended the mission on 18 September 2025. Its cameras found a 10,000 km standing wave above Aphrodite Terra, a jet and a giant cloud front in the lower clouds, and evidence that waves raised by the Sun's daily heating keep the winds near the equator racing around the planet. It has been silent since April 2024, but it was never steered out of orbit and is thought to be circling Venus still. It launched in May 2010 with the solar sail IKAROS, which flew past Venus on 8 December 2010, and the student probe Shin-en, which its team heard only on its first day.

Facts as of 4 November 2026, from the sources listed below.

1,228 secondsburn of four attitude-control thrusters that captured Akatsuki into Venus orbit on 7 December 2015, five years to the day after the main engine failed (ISAS)
8+ yearsof Venus atmosphere observation from orbit, December 2015 to April 2024 (JAXA)
80,800 kmIKAROS's distance from Venus as it sailed past on 8 December 2010 (JAXA IKAROS blog)

UVI images taken 2018-03-30 at 12:01:12 UTC (283 nm, 0.5 s exposure) and 12:04:46 UTC (365 nm), from about 81,000 km. The gallery colours 283 nm blue, 365 nm red and their mixture green; SO2 absorbs at 283 nm, so bluish areas have relatively little SO2, and the unidentified absorber darkens 365 nm. A sideways V pattern crosses the disc. — False colour photograph: Venus's dayside in ultraviolet from Akatsuki's UVI camera, 30 March 2018, built from 283 nm (blue) and 365 nm (red) images, with the cloud tops' sideways V across the disc. PLANET-C Project Team, ISAS/JAXA

Commonly wrong. Reference works stumble over this trio in a handful of recurring ways. (1) Stale status. NSSDCA's Akatsuki record, as archived on 20 September 2026, still says JAXA is trying to re-establish contact; JAXA ended the mission on 18 September 2025. The same record gives 480 kg (and 320 kg in its summary box), 'over 1200 W' and a 263 nm ultraviolet band, where JAXA gives 518 kg, about 500 W and 283 nm. (2) The cause stated as settled. Siddiqi's NASA chronicle writes that salt deposits apparently jammed the valve, the JPL Horizons notes say salt was later determined to be the cause, and the JST Science Portal (February 2026) names ammonium nitrate outright; JAXA's June 2011 report left two candidates standing, salt formation and contamination caught in the valve. (3) A 9 day orbit. Siddiqi and the Horizons notes say a March manoeuvre (dated 2015 in Siddiqi, 2016 in Horizons) cut apoapsis to about 330,000 km and the period to about nine days. Nine days was the plan JAXA announced in December 2015; ISAS reported 360,000 km and 10.5 days before the April 2016 trim and 370,000 km and 10.8 days after it, and JPL's own trajectory file shows no March change at all: its osculating period falls from about 13.7 to about 10.5 days around the periapsis pass of 20 December 2015 and holds there until the April trim. The Horizons notes also print the first orbit as 400 x 44000 km, a digit short of JAXA's 440,000 km. (4) Shin-en's Venus flyby. Siddiqi says the inert probe flew past Venus in December 2010 and McDowell's catalogue gives a pass near 100,000 km, but nothing was heard after its first days, so the pass is an estimate that nobody observed.

Akatsuki was built to do for Venus what weather satellites do for Earth: watch the atmosphere continuously from orbit and work out the physics that drives it, an aim JAXA describes as founding the field of planetary meteorology. The team set five themes: how the super-rotation works (cloud-top winds that circle the planet in about four days, some 60 times faster than the ground turns), the slow overturning between equator and poles, how the clouds form, whether Venus has lightning, and what the surface emits, including any active volcano. Five cameras (UVI, IR1, IR2, LIR and LAC) watched different heights at once, and an ultra-stable oscillator (USO) turned the radio link into an atmospheric probe. A 2026 ISAS retrospective by Yamazaki Atsushi and Satoh Takehiko calls it the dynamical complement to Europe's Venus Express, whose focus was chemistry.

Japan had looked at Venus before. An ISAS history recalls a group in the late 1970s studying a Venus orbiter and a balloon in the Venus atmosphere as candidates for Japan's first probe beyond Earth's gravity; Halley's comet won instead. The planet returned as PLANET-C, proposed at the ISAS space science symposium in January 2001. The proposal planned a 647 kg orbiter on the solid-fuel M-V rocket in February to April 2007, an Earth swing-by in 2008 and arrival in September 2009, with a plasma package for measuring atmospheric escape as an option that never flew. A project history page by Nakamura Masato (copyright 2008 to 2009) gives the later schedule instead, with observations from December 2010.

The M-V was retired, and in August 2007 the mission moved to the H-IIA. The retrospective says the change came when structural design was nearly finished and that the bigger rocket shook harder than Akatsuki had been built for. Taming the vibration needed about 800 kg of extra mass, supplied by the solar sail demonstrator IKAROS with its adapter, a raising adapter and four small satellites. Akatsuki itself cost about 25 billion yen (about €140 million), which the JST Science Portal says includes the launch.

H-IIA flight 17 lifted off from Tanegashima at 21:58:22 UTC on 20 May 2010 (06:58:22 on 21 May, Japan time). After the second stage's first burn it released three small satellites; after its second burn it set Akatsuki free on the Venus transfer path, about 27 minutes 29 seconds after liftoff, then coasted on and, in the planned order, dropped the PAF900M adapter, IKAROS and the university probe UNITEC-1. The second stage escaped Earth's gravity with them, and Jonathan McDowell's catalogue lists it and the adapter in orbit around the Sun.

IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun) was a drum 1.6 m across carrying a square sail 14 m on a side and 7.5 microns thick, with thin-film solar cells bonded to it. It weighed 310 kg according to ISAS, or 308 kg including a 16 kg membrane according to the 2025 report, and had its sail unfurled by 9 June 2010. On 15 June (14 June in the 2025 report and McDowell's catalogue) a camera about 6 cm across, DCAM2, sprang off on a spring and radioed back pictures of the whole sail, and DCAM1 followed on 19 June. By mid July JAXA had confirmed acceleration by sunlight and steering with liquid crystal panels on the sail, achievements ISAS describes as world firsts.

IKAROS reached Venus a day after Akatsuki, flying on autonomous sequences because Akatsuki had priority on the ground antennas. Its team blog says it passed about 80,800 km from the planet at 16:39 Japan time on 8 December 2010 (07:39 UTC). The pass was not aimed low for a strong gravity assist, yet Venus still bent the path and raised the craft's speed. Akatsuki, launched onto almost the same orbit, had approached from the sunward side, while IKAROS passed on the side away from the Sun without entering the planet's shadow, so the team concluded that solar sailing had steered it there. Its ALADDIN dust detector counted 2,789 impacts between the orbits of Earth and Venus.

Its attitude propellant was nearly gone by late 2011 (29 November or December, in different JAXA documents), after which the sail slipped into hibernation whenever its angle to the Sun grew too large. It was heard after four hibernations, the last in April and May 2015, each search testing the team's model of how an uncontrolled sail moves; after a fifth began in May 2015 nothing more came. JAXA carried out the transmitter shut-off operation on 15 May 2025 and stopped tracking IKAROS, which McDowell's catalogue lists in orbit around the Sun.

The third traveller was a student project. UNITEC-1, nicknamed Shin-en ('abyss', in Asif Siddiqi's NASA chronicle), was built by the University Space Engineering Consortium (UNISEC), which called it the world's first university-developed deep space satellite. Its archived project page describes a box about 30 by 35 cm with no attitude control, about 25 W of power and a 5.8 GHz amateur-band transmitter. Its main mission was a survival contest between computers from six universities, selected in August 2009, to see which would keep working longest on the way to Venus. The operations plan set arrival for December 2010, 207 days after launch and some 63 million km from Earth.

It went quiet at once. At a JAXA workshop in August 2010, as reported by Mynavi, Kurahara Naomi of the University of Tokyo said signals arrived 45 minutes after separation and from about 270,000 km, beyond the radiation belts, but nothing came on the second pass (16:00 on 22 May to 02:00 on 23 May, Japan time) or afterwards. The team suspected a low battery and unexpected cold. Siddiqi gives the last communication as 15:43 UTC on 21 May at 320,000 km and adds that a beacon was tracked until 31 May, a detail absent from the Japanese accounts. Nobody observed whether Shin-en passed Venus or how close. McDowell's catalogue estimates a pass around 8 December 2010 at about 100,000 km and flags both values as uncertain.

Akatsuki's own arrival went wrong. The insertion burn, VOI-1, on the 500 N class main engine (OME) burning hydrazine with nitrogen tetroxide, began at 08:49 Japan time on 7 December 2010 (23:49 UTC on 6 December). As planned the craft passed behind Venus 103 seconds later, but when it should have reappeared, just after 09:12, there was no signal. NASA's Deep Space Network found a faint signal at Madrid, Goldstone determined an orbit and Usuda reacquired the craft a day later; the retrospective states that without the DSN the whole spacecraft would have been lost. The data showed a large attitude disturbance about 152 seconds into the burn and shutdown at about 158 seconds, against a planned 720 seconds or so. With a fifth of the braking done, Akatsuki swept past Venus at about 550 km (JPL's reconstruction) and into orbit around the Sun.

An investigation for the Space Activities Commission traced the failure through a fault tree to check valve CV-F on the gas line that pressurised the fuel tank. With the valve blocked, fuel pressure fell and the engine ran oxidiser-rich and too hot, and the June 2011 report judged the most likely damage to be a thruster nozzle broken by excessive heat flux, which set the craft turning and made its autonomous control stop the burn. Of 16 candidate causes of the blockage two survived testing: salt, ammonium nitrate, formed where oxidiser and fuel vapour met in the gas line, and contamination caught in the valve. The report also found that design models had underestimated how oxidiser vapour permeates valve seals. JAXA says the lessons shaped the propellant systems of Hayabusa2, SLIM and later craft.

Everything else aboard was healthy, so the team decided to try again. With some fuel already spent, the retrospective says, they chose a path inside Venus's orbit that would let Akatsuki catch the planet up in late 2015. That orbit came within 0.63 AU of the Sun, where heating was three times that at Earth against a design value of two (the retrospective), or about 30 percent above what had been expected at Venus (the 2018 completion review). It survived nine perihelion passages. Main engine test firings in September 2011 failed to give the expected thrust, so from 30 September to 13 October the team dumped about 65 kg of oxidiser, 99 percent of the load, and from 1 to 21 November fired the attitude thrusters at perihelion three times, for 88.6, 90.6 and 63.8 m/s.

The four attitude thrusters together give a little under 20 percent of the main engine's thrust, wrote trajectory designers Ishii Nobuaki and Hirose Chikako, so with the fuel left the far point had to be about 370,000 km. At that size the Sun's tide swings periapsis between about 400 km and 10,000 km every 56 days, a quarter of Venus's year, and a slightly larger swing could drag it to 200 km, where Akatsuki might burn up. The science team wanted an orbit running with the atmosphere's rotation, to slow the clouds' apparent motion. Countless simulations showed that such a retrograde orbit would not fall if the arrival time, height and place balanced the Sun's raising and lowering effects, and the result was designed to stay clear of the atmosphere for more than five years.

The burn was trimmed for safety. The ideal braking was about 202 m/s, which tolerated only 3 percent of excess thrust, so the team asked for 198 m/s, allowing errors from +5 to -10 percent and fixing the burn at 1,228 seconds. A backup burn stood ready to be commanded across the 8.5 minute one-way light time. On 7 December 2015, five years to the day after the first attempt, the four thrusters on the top face fired for the full 1,228 seconds. On 9 December JAXA reported an orbit of 13 days 14 hours between about 400 km and 440,000 km, inclined about 3 degrees to Venus's orbital plane and running with the planet's rotation. ISAS called Akatsuki Japan's first planetary orbiter, and project manager Nakamura Masato later said that when he was sure of success he told the control room, in English, 'Our dreams will come true.'

The orbit cost the science: against the planned 30 hour orbit, the 2025 report says, radio occultations and lightning observations fell to about a tenth. A 15 second burn on 4 April 2016 lengthened the period to 10.8 days, dodging long apoapsis eclipses due in mid 2018 and stretching the possible observing life from about 800 to about 2,000 days; regular science began that month. On 9 December 2016 the electronics shared by the near-infrared cameras began drawing unstable current, and from the next day IR1 and IR2 could not be switched on. JAXA linked the fault to degradation after five unplanned years in a harsher radiation environment.

On arrival day, 7 December 2015, LIR, an uncooled 10 micron camera that reads cloud-top temperature to about 0.3 °C, saw a bow of warmer cloud tops about 10,000 km from north to south that stayed almost in place until 11 December while the 100 m/s winds blew through it. Mapped onto the ground it sat over the western highlands of Aphrodite Terra, which rise to about 5 km, and a simulation showed that a disturbance in the lower atmosphere sends a gravity wave upward that spreads into a bow at about 65 km (Fukuhara and colleagues, Nature Geoscience, 2017). JAXA calls it the largest mountain wave in the solar system. The retrospective adds that such bows grew only in the local afternoon and evening, which standard Venus meteorology struggles to explain.

IR2's single year of night-side images, in which the lower clouds show as silhouettes against heat from below, kept yielding results long after the camera failed. From July 2016 data Horinouchi Takeshi's group found winds at 45 to 60 km forming a jet near the equator, in a layer thought to be uniform and steady (Nature Geoscience, 2017). Kashimura Hiroki's group at Kobe University found planetary-scale streaks in the lower clouds and explained them by simulation as baroclinic instability, the process behind Earth's mid-latitude storms (Nature Communications, 2019). In 2020 Javier Peralta and colleagues reported a sharp cloud discontinuity near 50 km, sometimes more than 7,000 km long, circling the planet in about five days at some 330 km/h, faster than the wind at that height, and present in images back to 1983. In 2026 a study led by Imamura Takeshi explained it as a hydraulic jump, probably triggered by a planet-sized Kelvin wave; that ISAS release gives its length as about 6,000 km.

In 2020 the team reported an answer to the mission's central question. Using UVI cloud-tracked winds and LIR temperatures, Horinouchi and colleagues showed that at low latitudes the thermal tide, a wave raised by daytime heating of the clouds, carries angular momentum toward the equator and keeps the super-rotation going against the slow overturning circulation that would weaken it, while other waves and turbulence push the other way (Science, April 2020). ISAS noted that uncertainties remain, and a 2026 study gives Kelvin waves a part too. In 2021 Fukuya Kiichi, Imamura and colleagues used two years of roughly hourly LIR images to track cloud-top winds by night as well as by day, finding that the night-time north-south flow runs opposite to the daytime flow and settling what ISAS called a 40 year old puzzle (Nature). Radio occultations from Akatsuki and Venus Express, analysed by Ando Hiroki's team, gave temperature profiles from 40 to 85 km across the globe, with the unstable layer deepening toward the poles, the reverse of Earth (Scientific Reports, 2020).

Lightning stayed elusive. LAC, a camera that samples brightness 30,000 times a second, saw no flash from October 2016 to August 2017, although the estimated chance of catching at least one was 35 percent, a null result that prompted a re-evaluation of Venus lightning rates. In March 2020 LAC recorded one natural flash, which the team weighed as lightning, a meteor or something else, and no second one followed. Over eight years, Akatsuki and Venus Express data showed the super-rotation speed and the ultraviolet albedo varying on scales of years to a decade, with a 'brown noise' spectrum lacking a dominant period, which the retrospective reads as a hint that the variation is self-excited by the planet itself. JAXA approved a third extension in April 2024, to March 2029, to gather more than a full solar cycle and separate internal from external causes.

In 2022 a Keio University team led by Fujisawa Yukiko and Sugimoto Norihiko fed cloud-top winds from an intensive campaign in late 2018 into a Venus circulation model, producing what ISAS calls the world's first objective analysis of the Venus atmosphere; the model's overestimated 150 m/s winds settled near 110 m/s. It was opened to all as ALERA-V version 1.0 in May 2026, and the mission's data sit in JAXA's DARTS archive, with radio science to 25 April 2024. Under a NASA and JAXA agreement, eight and then four US participating scientists joined the team, and India's deep space antenna and Germany's Weilheim station gave several tens of percent more occultations than JAXA's Usuda station could have caught alone. In October 2020 Akatsuki joined BepiColombo and the Hisaki telescope in a campaign around one of the Venus flybys.

The end came quietly. In operations at the end of April 2024 a control mode with low attitude precision persisted for a long time, and communication could no longer be established; JPL's trajectory file shows tracking data ending on 26 April. ISAS announced the problem on 29 May 2024, noting that the craft had long outlived its design life of four and a half years. The team searched for about one Venus year, until August 2025, then carried out the shut-off procedure from 09:00 Japan time on 18 September 2025. With the link already dead, the command went out blind, and JAXA reports only that the procedure was carried out.

Where Akatsuki is now is an open question. No deorbit burn was announced, the craft can no longer be commanded, and none of JAXA's end-of-mission documents, the latest dated 13 July 2026, says what its orbit is today, how long that orbit will last, or whether it will eventually enter the atmosphere. JPL Horizons holds a reconstruction fitted to tracking up to 26 April 2024, then an untracked solution to 23 July 2025 and a prediction to 24 August 2025. In the tracked part the lowest periapsis pass comes to about 250 km, on 15 September 2021; in the untracked part the passes fall to about 350 km in May and June 2024 and climb to about 1,100 km by April 2025. The 2018 review found the orbit stable for at least three more years, and the trajectory designers warned of burn-up at 200 km, but none of the sources checked here forecasts the orbit as it stands. McDowell's catalogue, updated 2 November 2026, still lists Akatsuki in Venus orbit: presumed there, silent and untracked.

Much of Akatsuki lives on elsewhere. Its flat slot-array high-gain antenna passed to probes built to save weight, its ISAS digital transponder flew on IKAROS, Hayabusa2 and BepiColombo's Mio and became a deep space standard, and LIR became the TIR camera on Hayabusa2, with descendants on ESA's Hera and RAMSES, according to the 2025 report. A Japanese research group active since the end of 2021 has made crosslink radio occultation, CROVA, the core of a proposed successor in which mother and daughter craft in Venus orbit sound the air between them; JAXA's July 2026 report still lists it as a plan, with no approval or launch date, and the radio science team has proposed Venus occultations from OPENS-0, a Saturn-bound demonstrator that JAXA was still proposing and studying as of December 2025. The JST Science Portal noted in February 2026 that no Venus probe is active; the missions planned next are EnVision, VERITAS, DAVINCI and India's Venus Orbiter Mission.

Mission facts

Organisations

JAXA Institute of Space and Astronautical Science (ISAS) for Akatsuki and IKAROS; the University Space Engineering Consortium (UNISEC) for Shin-en (UNITEC-1)

Akatsuki: names

Akatsuki (Japanese for 'dawn', the hour when Venus shines most beautifully, per JAXA's naming release of 23 October 2009); project name PLANET-C; also called the Venus Climate Orbiter. COSPAR 2010-020D; JPL Horizons and NAIF ID -5.

Akatsuki: launch

21:58:22 UTC on 20 May 2010 (06:58:22 JST on 21 May), H-IIA flight 17 from Tanegashima; separated about 27 min 29 s after liftoff.

Akatsuki: mass (sources differ)

518 kg at launch (JAXA report to MEXT, December 2025); about 520 kg (JAXA release, September 2025); 500 kg including propellant (ISAS mission page); 517.6 kg (Siddiqi); NSSDCA gives 480 kg in its text and 320 kg in its summary box. The 2001 proposal planned about 647 kg wet, 329 kg dry.

Akatsuki: bus and power

Box 1.0 x 1.5 x 1.4 m (JAXA 2025) or 1.04 x 1.45 x 1.40 m (2018 review), two solar paddles of 1.0 x 1.4 m, 5.1 m tip to tip (ISAS). About 500 W at Venus at end of mission (JAXA); NSSDCA's 1.6 x 1.6 x 1.25 m and 'over 1200 W' disagree with both.

Akatsuki: instruments

UVI (283 and 365 nm; NSSDCA prints 263 nm), IR1 (0.90, 0.97, 1.01 microns), IR2 (1.65, 1.735, 2.02, 2.26, 2.32 microns), LIR (10 microns, uncooled bolometer), LAC (lightning and oxygen airglow), and an ultra-stable oscillator (USO) for X-band radio occultation.

Akatsuki: planned orbit

Periapsis 300 km, apoapsis about 80,000 km, period 30 hours, inclination 172 degrees, about two Earth years of observation (2018 review). The 2001 proposal had 300 km by 10 Venus radii and about 21 hours.

Akatsuki: failed insertion (VOI-1)

Burn began 08:49 JST on 7 December 2010 (23:49 UTC on 6 December). A large attitude disturbance came about 152 s in and the engine stopped at about 158 s, against a planned 720 s or so; only about 20 percent of the braking was achieved. JPL's reconstruction puts closest approach at about 550 km.

Akatsuki: cause of the failure

Blockage of check valve CV-F in the gas line pressurising the fuel tank; of 16 candidate causes, salt (ammonium nitrate) formed from oxidiser and fuel vapour, or contamination in the valve, remained. The oxidiser-rich burn most probably broke the thruster nozzle through excessive heat flux (JAXA investigation, June 2011).

Akatsuki: five years around the Sun

Perihelion about 0.63 AU; nine perihelion passages survived. About 65 kg of oxidiser (99 percent of the load) dumped 30 September to 13 October 2011; three perihelion burns on the attitude thrusters, 1 to 21 November 2011 (88.6, 90.6 and 63.8 m/s), set up the 2015 return.

Akatsuki: orbit insertion (VOI-R1)

7 December 2015 (JST): four top-side thrusters fired for 1,228 s. The braking was set at 198 m/s instead of about 202 m/s so that thrust errors of +5 to -10 percent could be tolerated; one-way light time about 8.5 minutes.

Akatsuki: first orbit

Announced 9 December 2015: period about 13 days 14 hours, periapsis about 400 km, apoapsis about 440,000 km, inclination about 3 degrees to Venus's orbital plane, moving in the same direction as Venus's rotation (a retrograde orbit, because Venus spins backwards).

Akatsuki: working orbit

After a 15 s trim on 4 April 2016: apoapsis about 370,000 km, periapsis 1,000 to 10,000 km, period 10.8 days (before: 360,000 km, 10.5 days). JPL's trajectory file shows true periapsis passes as low as about 250 km (15 September 2021, fitted to tracking) and about 350 km (May and June 2024, untracked solution).

Akatsuki: cameras lost

IR1 and IR2 shared electronics that drew unstable current on 9 December 2016; neither camera could be switched on from 10 December. JAXA paused both on 3 March 2017, citing degradation.

Akatsuki: mission phases

Regular operations December 2015 to March 2018; late operations 1 (April 2018 to March 2021) and 2 (April 2021 to March 2024); late operations 3 approved April 2024 for April 2024 to March 2029, cut short the same month.

Akatsuki: end

Contact lost at the end of April 2024 (Horizons: tracking data to 26 April); search for about one Venus year to August 2025; shut-off procedure from 09:00 JST on 18 September 2025. Design life was 4.5 years from launch. JAXA's latest report (13 July 2026) repeats only the summary orbit of its working years, with nothing on where it is now, how long it will stay or its fate.

Akatsuki: cost and output

About 25 billion yen (about €140 million): JAXA's December 2025 report calls it the total development budget, and the JST Science Portal (February 2026) says it includes the launch; 178 papers by October 2025 (ISAS Cosmos blog); 41 master's and 17 doctoral degrees and 19 awards (JAXA, July 2026; its December 2025 report gave 17 awards). Euro amounts on this page are converted at the European Central Bank rate of 5 November 2026 (177.85 yen to the euro).

IKAROS: spacecraft

Drum 1.6 m across and 0.8 m high; square sail about 14 m a side, 20 m across the diagonal, 7.5 microns thick, with thin-film solar cells. 310 kg (ISAS page) or 308 kg including a 16 kg membrane (JAXA, December 2025). Development budget about 1.5 billion yen (about €8.4 million), built in 2.5 years.

IKAROS: Venus flyby

Passed about 80,800 km from Venus at 16:39 JST on 8 December 2010 (07:39 UTC), on the side away from the Sun and outside the planet's shadow; COSPAR 2010-020E.

IKAROS: end

Attitude propellant nearly exhausted late 2011; heard after four hibernations (last in April to May 2015); silent since entering a fifth hibernation in May 2015; shut-off operation 15 May 2025.

Shin-en (UNITEC-1)

University Space Engineering Consortium probe, COSPAR 2010-020F: a box about 30 x 35 cm with a 5.8 GHz amateur-band transmitter and computers from six universities. Mass 15 kg or less (UNISEC), 20 kg (Siddiqi) or 26 kg (McDowell). Its team heard it only on its first day (Siddiqi adds a beacon tracked to 31 May); its planned Venus pass in December 2010 was never observed.

Mission timeline

Late 1970sAn ISAS group studies a Venus orbiter and a Venus balloon as candidates for Japan's first probe beyond Earth's gravity, then chooses Halley's comet instead.
Jan 2001PLANET-C proposed at the ISAS space science symposium, for an M-V launch in 2007 and arrival in September 2009.
Aug 2007The project switches its launch vehicle from the M-V to the H-IIA.
20 May 2010H-IIA flight 17 launches Akatsuki, IKAROS and UNITEC-1 (Shin-en) toward Venus at 21:58:22 UTC (06:58:22 on 21 May, Japan time).
21 to 22 May 2010Shin-en is heard from about 270,000 km, then falls silent; nothing is received on its second pass.
May to Jul 2010IKAROS unfurls its sail (26 May to 9 June), photographs it with two free-flying cameras (14 and 19 June; JAXA's 2010 release dates the first pictures 15 June) and proves acceleration by sunlight (9 July).
7 Dec 2010VOI-1: Akatsuki's main engine stops after about 158 s of a planned 720 s burn and the craft flies past Venus into solar orbit.
8 Dec 2010IKAROS passes Venus at about 80,800 km, at 16:39 JST.
Sep to Nov 2011Main engine test firings fail; about 65 kg of oxidiser is dumped; three perihelion burns on the attitude thrusters aim Akatsuki at a 2015 reunion.
7 Dec 2015VOI-R1: four attitude thrusters fire for 1,228 s and Akatsuki enters Venus orbit (confirmed 9 December).
7 Dec 2015LIR images a 10,000 km bow-shaped wave standing over Aphrodite Terra on the first day in orbit.
4 Apr 2016A 15 s trim moves Akatsuki to its working orbit of about 10.8 days, ahead of the regular observations JAXA expected from mid April.
9 to 10 Dec 2016IR1 and IR2 fail through their shared electronics (pause announced 3 March 2017).
24 Apr 2020Horinouchi and colleagues publish in Science that thermal tides maintain the super-rotation at low latitudes.
End of Apr 2024Contact is lost after a long spell in a control mode with low attitude precision.
15 May 2025JAXA carries out IKAROS's shut-off operation, ten years after its last signal.
18 Sep 2025JAXA carries out Akatsuki's shut-off procedure from 09:00 JST and ends the mission, 17 months after contact was lost.
May 2026ALERA-V 1.0, the Akatsuki-based Venus weather analysis, is released (21 May), and the cloud front is explained as a hydraulic jump (26 May).

A filled dot marks something that physically happened. A hollow one marks an announcement, a naming or a target.

In pictures

Figure 1 of the bow-wave paper as posted by ISAS: panel a is a LIR brightness temperature image (213 to 233 K) at 2015-12-07 05:26 with the bow traced by a dashed line; panels b to e repeat it from 8 to 11 December; panel f is a UVI 365 nm image at 05:20 on 7 December. ISAS's text says the initial LIR and UVI observations ran from 7 to 10 December; the panel labels run to 11 December 03:52. JAXA's July 2026 report says the bow was seen continuously until 11 December and had vanished by January 2016, which fits the labels. — False colour photograph: thermal infrared views from Akatsuki's LIR camera, 7 to 11 December 2015, showing the bow-shaped standing wave holding its place over several days, beside an ultraviolet view of the same morning. PLANET-C Project Team, ISAS/JAXA (Fukuhara et al., Nature Geoscience, 2017)

ISAS caption: IR2 images taken from 25 to 28 August 2016 stacked to show the giant cloud discontinuity racing through the lower clouds near 50 km, with a series of small images showing its changes from March 2016 to December 2018. The frames are tinted, and the credit line includes NASA's IRTF: IR2 could not be switched on from 10 December 2016, so the later small views must come from telescopes on Earth. — False colour photograph: Akatsuki IR2 night-side images from 25 to 28 August 2016, stacked to show the sharp cloud front in the lower clouds, ringed by smaller views of it up to December 2018, the later ones taken from Earth. PLANET-C Project Team, NASA, IRTF

One of the pictures radioed back to IKAROS by the free-flying separation camera DCAM2, which JAXA says photographed the whole deployed sail on 15 June 2010 JST (the 2025 JAXA report and McDowell's catalogue date the DCAM2 release to 14 June). The drum-shaped body sits at the centre of the square sail. — Photograph: the fully deployed IKAROS solar sail, taken in June 2010 by the small DCAM2 camera after it sprang free of the spacecraft. (c)JAXA

Cosmos blog caption: an imaginary picture of Akatsuki firing to brake into orbit on arrival at Venus, with an infrared night-side pattern drawn onto the planet. It shows the main engine era design; the actual 2015 capture used attitude thrusters. — Artist's concept: Akatsuki braking into orbit at Venus, the planet drawn with an infrared night-side pattern. Ikeshita Akihiro, (c)JAXA

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Translated from the original sources

Passages from the Japanese sources this page draws on, in azmth's translation. Tap Show original to read the original wording.

JAXA press release: end of Akatsuki operations, 18 September 2025

Japanese · 3 passages

JAXA press release: end of Akatsuki operations, 18 September 2025 (Japanese)

What happened to Japan's Akatsuki Venus probe?

01 Mission facts

02 Mission timeline

03 In pictures

04 Translated from the original sources

JAXA press release: end of Akatsuki operations, 18 September 2025

ISAS: communication status of Akatsuki, 29 May 2024

JAXA press release: VOI-1 result, 8 December 2010

JAXA investigation report on the VOI-1 failure

JAXA status report on Akatsuki

Yamazaki and Satoh, Akatsuki retrospective, ISAS, January 2026

JAXA ISAS report to the MEXT space subcommittee on Akatsuki and IKAROS, 16 December 2025

Akatsuki project completion review, MEXT, 2018

JAXA press release: Venus orbit insertion result, 9 December 2015

Ishii and Hirose, ISAS feature: preparing the orbit insertion

ISAS: orbit correction result, 8 April 2016

ISAS: IR1 and IR2 observations paused, 3 March 2017

Akatsuki project site: orbit information gallery

ISAS: giant bow-shaped wave explained, 17 January 2017

ISAS: equatorial jet discovered, 29 August 2017

ISAS: planetary-scale streaks in the clouds, 9 January 2019

ISAS: mechanism maintaining the super-rotation, 24 April 2020

ISAS: night-side circulation, 22 July 2021

ISAS: giant cloud discontinuity, 5 August 2020

ISAS: the cloud front is a hydraulic jump, 26 May 2026

ISAS: thermal structure from radio occultation, 27 February 2020

ISAS: first Venus weather dataset, 12 September 2022

ISAS: ALERA-V released, 21 May 2026

ISAS Cosmos blog: our last presence at Venus has gone silent, 28 October 2025

JST Science Portal: farewell Akatsuki, 25 February 2026

ISAS history: Japan's escape plan

PLANET-C proposal, January 2001

Akatsuki project site: history of the PLANET-C mission

JAXA press release: naming of the Venus probe PLANET-C, 23 October 2009

JAXA: H-IIA F17 launch result, 21 May 2010

JAXA: H-IIA F17 piggyback payloads and separation order

ISAS mission page: IKAROS

JAXA press release: IKAROS separation camera photographs the deployed sail, 16 June 2010

IKAROS blog: Venus pass, 10 December 2010

IKAROS blog: flyby geometry, 12 December 2010

IKAROS blog: daily report, 26 November 2010

ISAS: end of IKAROS operations, 15 May 2025

UNISEC: about UNITEC-1

UNISEC: UNITEC-1 operations plan

Mynavi: JAXA piggyback satellite workshop report, 2 August 2010

JAXA ISAS: results of the Venus probe Akatsuki, report to the Cabinet Office's subcommittee on space science and exploration, 13 July 2026

ISAS mission page: Akatsuki

DARTS: Akatsuki science data archive

05 Sources

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Mission facts

Mission timeline

In pictures

Translated from the original sources

Sources