Akari (satellite)

Lore & Background

AKARI was originally known as IRIS (InfraRed Imaging Surveyor) before its launch. It was launched on 21 February 2006 at 21:28 UTC by an M-V rocket into a Sun-synchronous orbit. Its telescope mirror was made of silicon carbide to save weight, and its designed lifespan for far- and mid-infrared sensors was 550 days, limited by its liquid helium coolant. After launch, a malfunction of the Sun sensor delayed ejection of the telescope aperture lid, shortening the coolant lifespan estimate to about 500 days. However, after JAXA estimated the remaining helium in early March 2007, observation time was extended at least until 9 September 2007.

By mid-August 2006, AKARI had completed about 50% of the all-sky survey. The first all-sky survey (phase-1) finished by early November 2006, and the second (phase-2) began on 10 November 2006. On 11 July 2007, JAXA reported that 90% of the sky had been scanned twice and about 3,500 selected targets observed. The liquid helium coolant depleted on 26 August 2007, marking the end of far- and mid-infrared observations, with more than 96% of the sky scanned and over 5,000 pointed observations completed. A limited 'warm' programme continued with just the near-infrared instrument until May 2011, when a major electrical failure rendered science instruments inoperable in Earth's shadow. The satellite was officially terminated on 24 November 2011 and reentered the atmosphere on 11 April 2023.

Reader's Guide

AKARI's significance lies in its comprehensive all-sky infrared survey, which provided a legacy dataset for astronomers. Its observations covered near-, mid-, and far-infrared wavelengths, enabling studies of star formation, dust processing in supernova remnants, and the detection of molecular gas around active galactic nuclei. Notable results include the first infrared detection of a supernova remnant in the Small Magellanic Cloud and detection of mass loss from red-giant stars in globular cluster NGC 104. The AKARI All-Sky Survey Point Source Catalogues, released on 30 March 2010, remain a key resource. The project also fostered international collaboration, with British and Japanese team members awarded the Daiwa Adrian Prize in 2004. Though its coolant-limited lifespan was shortened by a sensor malfunction, the mission exceeded its initial survey goals, scanning over 96% of the sky. AKARI's data continues to inform infrared astronomy, and it served as a precursor to the proposed SPICA telescope.

Did You Know?

• AKARI's name means 'light' in Japanese.
• The satellite reentered the atmosphere on 11 April 2023 at 04:44 UTC.
• British and Japanese team members were awarded the Daiwa Adrian Prize in 2004 for their collaboration.

Engineering & Design Philosophy

AKARI, originally conceived under the working title IRIS (InfraRed Imaging Surveyor), represented a bold international effort to map the infrared universe from space. Developed by Japan's JAXA in partnership with European and Korean research institutes, the satellite carried a 68.5-centimetre aperture telescope designed to sweep the entire sky across near-, mid-, and far-infrared wavelengths. A key engineering choice was the use of a silicon carbide mirror, a material selected specifically to reduce the spacecraft's overall mass while maintaining structural integrity. The project's budget stood at approximately ¥13.4 billion, roughly equivalent to US$110 million at the time. However, the mission's most defining constraint was thermal: the far- and mid-infrared detectors required a finite supply of liquid helium coolant, capping the designed operational window at 550 days. After its February 2006 launch aboard an M-V rocket into a Sun-synchronous orbit, the spacecraft was christened AKARI, a Japanese word meaning "light," a fitting name for an instrument whose purpose was to reveal the hidden thermal glow of the cosmos.

Operational Challenges & Mission Timeline

The first months of AKARI's flight were marked by an unexpected hardware problem. A malfunction in the Sun sensor delayed the ejection of the telescope's aperture lid, shortening the projected helium coolant lifespan from 550 days to roughly 500. Despite this setback, progress was rapid: by mid-August 2006 the satellite had completed about half of its all-sky survey, and the first full pass was finished by early November. Phase two began on 10 November 2006. In early March 2007, JAXA assessed the remaining helium and confirmed observations could continue at least until 9 September. By 11 July, 90 per cent of the sky had been scanned twice and approximately 3,500 targeted objects observed. On 26 August 2007 the last liquid helium was exhausted, ending far- and mid-infrared capability; more than 96 per cent of the sky had been covered and over 5,000 pointed observations recorded. In December 2007, orbit-correction manoeuvres were needed because boiling helium had raised the satellite's altitude, threatening its solar power supply. A limited near-infrared programme kept the telescope active through 2010, until a major electrical failure in May 2011 rendered the instruments inoperable in Earth's shadow. The mission was officially closed on 24 November 2011, and the spacecraft reentered the atmosphere on 11 April 2023.

Scientific Discoveries & Catalogues

AKARI's infrared observations yielded a remarkable catalogue of firsts and detailed cosmic portraits. Among its most celebrated findings was the first infrared detection of a supernova remnant in the Small Magellanic Cloud, a milestone for understanding the aftermath of stellar explosions in our galaxy's satellite. The satellite also captured evidence of mass loss from relatively young red-giant stars within the globular cluster NGC 104, and identified molecular gas encircling the active galactic nucleus of an ultra-luminous infrared galaxy. In the constellation Vulpecula, AKARI revealed star formation unfolding across three successive generations in the nebula IC 4954/4955. Its 140-micrometre images of Orion and the winter Milky Way, along with views of a star-forming region in Cygnus and the peculiar spiral galaxy M101, provided astronomers with unprecedented thermal snapshots of active star birth. Dust processing within supernova remnants in the Large Magellanic Cloud was another highlight. The full dataset was consolidated in the AKARI All-Sky Survey Point Source Catalogues, released on 30 March 2010, and a dedicated feature issue of Astronomy & Astrophysics (Volume 514, May 2010) showcased the mission's results to the broader community.

International Collaboration & Legacy

AKARI was never a purely Japanese endeavour. From its earliest planning stages, the project brought together JAXA with research institutes across Europe and Korea, a collaboration that was formally recognised in 2004 when British and Japanese team members received the Daiwa Adrian Prize from the Daiwa Anglo-Japanese Foundation for their joint work. The satellite's very name reflects its cultural roots: AKARI (明かり) is the Japanese word for "light," a poetic choice for an instrument designed to illuminate the infrared universe. Before adopting that name, the project had operated under the technical designation IRIS, standing for InfraRed Imaging Surveyor. The mission's scientific legacy extended well beyond its operational years. Its all-sky survey data, released in catalogues in 2010, became a foundational reference for infrared astronomy, and a special issue of Astronomy & Astrophysics dedicated to AKARI's results helped disseminate the findings widely. Looking forward, the proposed SPICA space telescope was conceived as a successor to AKARI, though that project was ultimately not built, leaving AKARI's survey as a singular achievement in mid- and far-infrared sky mapping.