Farewell, "Akatsuki" — Persistent efforts finally crack Venus's mysteries, solar system exploration continues

The Venus probe "Akatsuki" quietly ended its operations last year. Although it failed its initial orbital insertion in 2010, it succeeded on a second attempt five years later. As Japan's first successful planetary orbiter, Akatsuki made history and yielded significant results, including solving the mystery of a massive atmospheric phenomenon. The English name for "Venus" also refers to the goddess of victory, and this is the story of how that goddess finally smiled upon a spacecraft that refused to give up. We have also highlighted major domestic and international developments related to solar system exploration that are expected in the coming year.

A "Cool Story" Five Years in the Making

"The orbital insertion failed, and the scientists were in a panic. But the engineering leader said, 'Keep quiet, I'll sort it out.' And five years later, he actually did. That's what I call a cool story." This is how Masaki Fujimoto, Vice President of the Japan Aerospace Exploration Agency (JAXA) and Director General of the Institute of Space and Astronautical Science (ISAS), recalled the drama at the Ministry of Education, Culture, Sports, Science and Technology (MEXT) Working Group on Space Development and Utilization on December 16 of last year, a meeting convened to report on the end of Akatsuki's operations and its achievements.

Akatsuki was an orbiter with a solar panel span of 5 meters and a launch mass of 518 kilograms. It carried five types of cameras and a radio oscillator for observation purposes. Its total development costs, including launch expenses, came to approximately 25 billion yen. It departed Earth in May 2010 aboard an H-IIA rocket and attempted to enter Venus's orbit in December of that year. However, the main engine failed. Vapors from the fuel and the oxidizer reacted to form ammonium nitrate, which clogged a valve in the fuel line. Akatsuki then continued to orbit the Sun like a planet.

Rather than giving up, the JAXA team calculated a trajectory for a second attempt and decided to perform the orbital insertion using only the attitude control thrusters instead of the main engine. They jettisoned the oxidizer to reduce the spacecraft's weight and made repeated course corrections along the way.

Then came December 2015, when Venus and Akatsuki drew close again. By skillfully firing the attitude control thrusters, the team successfully placed the spacecraft into an orbit around Venus with a period of just over 13 days (further adjustments were made afterward). This orbit was a much wider, elongated ellipse compared with the originally planned 30-hour orbit. While this meant lower observation resolution, it was also considered advantageous for capturing large-scale phenomena. Coincidentally, the retry fell on December 7, the same date as the original failure, and success was confirmed two days later.

Not Yet a Movie—But Just as Dramatic

The Mars probe "Nozomi" had failed its orbital insertion in 2003. Akatsuki's mission represented a critical test—Japan's first real chance to succeed in planetary exploration—and failure was not an option.

By 2010, Japan had already achieved success with probes to comets, the Moon, and asteroids—but not to planets. When it comes to dramatic spacecraft stories, the asteroid probe "Hayabusa (first generation)" immediately comes to mind, having been turned into multiple films. After overcoming countless hardships, it limped home to Earth that same year, delivered asteroid samples to humanity, and then burned up in the atmosphere—a story no other "actor" can match. Yet Akatsuki, too, told a gripping comeback tale, traveling alone around the Sun for five long years before finally achieving its dream.

When Masato Nakamura, Project Manager (at the time) at JAXA, became certain of their success in the control room, he reportedly said to those around him in English: "Our dreams will come true." He revealed this anecdote at a press conference held right after the successful retry—perhaps alluding to a music band in which there is member that shares his name. His beaming expression, as if to say, "We gave it everything," was truly memorable.

Even before leaving Earth, Akatsuki faced a challenge: a change in launch vehicle. The originally planned solid-fuel rocket "M-V" had been retired, and the replacement H-IIA rocket had different vibration characteristics during flight. To compensate, the solar powered sail demonstration satellite "IKAROS" was flown with it for balance. IKAROS—a "space yacht" that travels without fuel by harnessing the pressure of solar photons on its sail while also generating electricity from its solar cells—itself became the world's first spacecraft of its kind and achieved major results.

Solving the Mystery of the Ferocious Winds on "Earth's Twin"

Venus is a rocky planet orbiting just inside Earth's path around the Sun. With a diameter of 12,000 km (comparable to Earth's 12,800 km), and a mass about 0.8 times that of Earth, the two are often called twins. But their surface conditions could not be more different. Venus is blanketed by a thick atmosphere composed mainly of carbon dioxide, and its surface is hidden beneath clouds of sulfuric acid that block sunlight entirely. The surface is a hellish environment of 90 atmospheres of pressure and a scorching 460 degrees Celsius. Venus rotates in the opposite direction to most other planets, and its rotation period of 243 days is actually longer than its orbital period of 225 days. It also lacks an intrinsic magnetic field, unlike Earth.

One of Venus's great mysteries was the mechanism behind its "super-rotation," extremely strong winds that sweep the atmosphere around the planet far faster than the planet itself rotates. Discovered in the 1960s, this phenomenon covers nearly the entire planet, reaching wind speeds of 100 meters per second (60 times the planet's rotation speed) at an altitude of around 70 km. Like Earth and Mars, Venus has a large-scale north-south atmospheric circulation known as the Hadley circulation, which transports heat from the equatorial region toward the poles. This should cause east-west winds to weaken over time on a global scale, yet on Venus, the east-wind super-rotation somehow persists.

As the world's first planetary meteorological satellite (beyond Earth), Akatsuki tackled this mystery. The research group developed an observation method using ultraviolet cameras to track cloud movements with high precision and successfully mapped detailed wind speeds. Temperatures were also measured using infrared cameras. The results revealed that "thermal tidal waves," periodic temperature variations caused by the atmosphere heating up during the day under solar radiation and cooling at night, are what drive super-rotation. The forces acting on the atmosphere when thermal tidal waves occur push the upper-atmosphere air near the equator in a westward direction, sustaining the ferocious winds.

Akatsuki's mid-infrared camera also discovered a massive bow-shaped feature spanning 10,000 km from north to south, and the research group used simulations and other methods to determine its origin. The feature turned out to be an atmospheric gravity wave, a wave formed by atmospheric disturbance over large surface topography, that propagated upward, reaching an altitude of 65 km where it spread out into the characteristic bow shape.

Akatsuki's observational data was also applied to improving simulations through a technique called "data assimilation." This is expected to continue advancing our understanding of planetary atmospheres going forward.

International Attention Intensifies, Successor Mission Under Consideration

Although Akatsuki's mission was extended repeatedly, contact was lost in April 2024 due to a decline in attitude control precision. Since recovery was deemed impossible and the spacecraft had far exceeded its designed lifespan, JAXA formally ended its operations in September of last year.

Venus exploration had been a stage for the US-Soviet space race since the 1960s, but attention later shifted to Mars. The most recent US mission was the orbiter "Magellan" (operated 1989-94). However, following the European orbiter "Venus Express" (2005-14) and then Akatsuki's success, the scientific importance of Venus's weather and volcanism was rediscovered. In 2021, the United States announced two missions—"DAVINCI," comprising an orbiter and a descent probe, and "VERITAS," another orbiter—while Europe announced the orbiter "Envision." India reportedly has an orbiter planned, and Russia has plans for both an orbiter and a lander. There are no active Venus probes right now, but the future looks promising.

In 2020, phosphine—a gas that could potentially be produced by living organisms—was reportedly detected on Venus, but this was later found to possibly have been a false detection and remains a subject of debate.

Japanese researchers are also considering a successor mission to Akatsuki. The concept would apply a technique called "radio occultation" (in which radio waves are tracked as they refract through Venus's atmosphere, causing a change in frequency) using a parent-and-child spacecraft configuration. Building on the radio occultation approach used by Akatsuki, the successor would map the structure of the atmosphere in three dimensions down to lower altitudes.

At the MEXT Working Group, Director General Fujimoto explained: "Akatsuki's investigation showed us how Venus's atmosphere behaves. Our research theme is the major differences from Earth. The question now is what's happening beyond our solar system—I am thinking of extending our inquiry in that direction." His remarks hinted at the strong interest researchers have in the atmospheric science of exoplanets as well.

A Busy Year Ahead for Solar System Exploration

A series of solar system explorations is expected over the coming year. In Japan: (1) The probe "Hayabusa2" will fly-by the asteroid "Torifune" in July, using its gravitational pull to accelerate while conducting observations. Having successfully returned samples from the asteroid "Ryugu" to Earth in December 2020, the spacecraft is now on an extended mission. After two close passes of Earth in December 2027 and June 2028, it is scheduled to arrive at asteroid "1998 KY26" in July 2031.

(2) The spacecraft of the Japan-Europe joint mission "BepiColombo" will arrive at Mercury in November. It will separate into two probes—the European planetary orbiter "MPO" and JAXA's magnetospheric orbiter "Mio"—and each will carry out their respective roles. Only two US spacecraft have ever been sent to Mercury, and great hopes are riding on the mission to resolve many mysteries about the planet, including its magnetic field and internal structure.

(3) The "MMX" mission probe to retrieve samples from "Phobos," a moon of Mars, is currently scheduled to depart Earth in the coming fiscal year. If it is successful, it will be the world's first round trip between the Mars system and Earth. However, there is concern over the fate of the mission, as the domestic large H3 Launch Vehicle that will carry it failed its launch in December of last year, and the investigation into the cause and corrective measures is still underway.

(4) Following Europe's official approval of the Japan-Europe joint mission "Ramses" in November of last year, preparations are accelerating. The target is the asteroid "Apophis," which will pass within just 32,000 km of Earth in April 2029. Once flagged as a potential Earth impactor, this "troublemaker asteroid" has attracted attention from the perspective of planetary defense—protecting humanity from asteroid collisions. Ramses is scheduled to depart Earth in April 2028 and arrive at Apophis in February 2029. Japan will contribute a thermal infrared camera, solar panels, and the H3 Launch Vehicle for the launch.

Internationally, the most closely watched development right now is the US-led Artemis lunar exploration program, in which Japan is also participating. According to information as of the 24th of this month, four astronauts from the United States and Canada are expected to fly around the Moon in the Orion spacecraft as early as April, which will be the first crewed lunar flyby in 53 years since the Apollo program.

The European probe "Hera," to which Japan contributed a thermal infrared camera, will arrive at the asteroid "Didymos" and its moonlet "Dimorphos" in December.

Internationally, news from China is also particularly prominent. The joint China-Europe probe "SMILE," designed to study the interaction between solar wind and Earth's magnetosphere, is expected to launch as early as April. It is said that the asteroid probe "Tianwen-2" will reach one of its targets, "Kamo'oalewa," in the summer, aiming to return samples to Earth in the manner of Hayabusa and Hayabusa2. And "Chang'e-7," which will land in the region of the lunar south pole, is reportedly set to launch as early as August.

News about the operational "engineering" side of these missions is always exciting. But the scientific knowledge gained from the research that follows exploration and observation is something built up patiently over years and decades. For example, the super-rotation breakthrough from Akatsuki mentioned above was published four and a half years after orbital insertion. The Moon rocks collected during the Apollo program are still stored at NASA facilities more than half a century later, with systems in place to distribute them to scientists. How will the explorations happening now deepen our understanding of the universe over the next 10 or 20 years? It is something to look forward to over the long term, as the textbooks are gradually rewritten.