HISAKI (SPRINT-A)

Hisaki, cataloged under NORAD ID 39253 and international designator 2013-049A, is a Japanese ultraviolet astronomy satellite operated by the Japan Aerospace Exploration Agency (JAXA). Launched on September 14, 2013 (September 13 in Eastern Daylight Time), it carries the formal designation SPRINT-A — an acronym for Spectroscopic Planet Observatory for Recognition of Interaction of Atmosphere — reflecting its core scientific focus on the upper atmospheric layers of Solar System planets. Weighing 348 kilograms at launch, Hisaki holds the distinction of being the first spacecraft to fly under Japan's Small Scientific Satellite program and the payload that inaugurated JAXA's Epsilon rocket series. As of the time of this writing, the satellite remains in orbit.

Mission and purpose

Hisaki was designed to conduct extreme ultraviolet (EUV) and far-ultraviolet spectroscopic observations of planetary atmospheres from a vantage point above Earth's own atmosphere, which absorbs much of the ultraviolet spectrum and makes ground-based observations in these wavelengths effectively impossible. By operating in low Earth orbit, the spacecraft could turn its instruments toward neighboring planets and gather spectroscopic data on processes — such as atmospheric escape, ion transport, and plasma interactions — that unfold on timescales ranging from minutes to years.

The primary scientific targets were the outer planets, particularly Jupiter and its complex magnetospheric environment, as well as Venus, Mars, and Io, the volcanically active moon of Jupiter whose sulfur emissions interact dramatically with the Jovian magnetosphere. Tracking these interactions over extended periods was a central aim: because Hisaki orbited Earth continuously, it could monitor a given planetary target for far longer uninterrupted stretches than most space probes dedicated to visiting those bodies. This capability made the satellite something of a long-baseline observatory rather than a conventional planetary flyby or orbiter mission.

The name "Hisaki" refers to a cape on the southern Japanese island of Kyushu, consistent with JAXA's tradition of naming scientific satellites after geographic features of Japan. The SPRINT designation, meanwhile, reflects the programmatic framing of the Small Scientific Satellite series, which was conceived as a means of fielding focused, cost-constrained scientific payloads on relatively short development schedules.

It is worth noting that the current orbital catalog entry lists the mission type and mission status as not publicly recorded. While the scientific objectives described above are well-documented in the public literature, the operational status of the satellite at any given moment — whether it remains in contact with ground stations, whether instruments are still active — is not confirmed in the catalog data on which this entry is based.

Orbit and tracking

Hisaki occupies a low Earth orbit with an apogee of 1,165 kilometers and a perigee of 961 kilometers, giving it a moderately elliptical path around the planet. Its orbital inclination is 29.7 degrees relative to the equatorial plane, meaning it traces a band relatively close to the equator and is visible from tropical and mid-latitude ground stations but does not pass over polar regions. The orbital period is approximately 106.3 minutes, so the satellite completes roughly 13 to 14 full orbits per day.

This orbit places Hisaki in a region that spans the upper boundary of the thermosphere and extends into the inner Van Allen radiation belt. Altitudes in this range are sometimes referred to as medium-low Earth orbit in informal parlance, sitting noticeably higher than the International Space Station's typical operating altitude but well below the medium Earth orbits used by navigation satellite constellations. The relatively high perigee means atmospheric drag is minimal, contributing to orbital longevity; the satellite has remained in orbit for well over a decade since its 2013 launch with no decay or reentry recorded.

For tracking purposes, Hisaki can be followed using its NORAD catalog ID 39253 and COSPAR designator 2013-049A. These identifiers allow observers and researchers to retrieve current two-line element (TLE) sets from space surveillance networks and predict passes over any given location on Earth. Given the inclination of 29.7 degrees, the satellite is accessible to observers located roughly between 30 degrees south and 30 degrees north latitude, as well as somewhat beyond those bounds during certain phases of its orbit.

Design and operator

Hisaki was manufactured by NEC, one of Japan's major aerospace and defense electronics contractors, which has built or contributed to a number of JAXA scientific spacecraft over the decades. At a launch mass of 348 kilograms, it falls within the category of small satellites by the standards of interplanetary or large Earth-observation missions, though it is more substantial than the very small CubeSat-class spacecraft that have proliferated in recent years. The SPRINT program of which it is a part was specifically structured to keep mass, cost, and development time within tightly controlled limits, enabling JAXA to pursue focused scientific objectives without the budget and schedule demands of flagship missions.

The satellite's launch vehicle was the Epsilon rocket, a solid-fueled launcher developed by JAXA and IHI Aerospace as a successor to the M-V rocket. The Epsilon was designed to offer simpler ground operations and a smaller launch infrastructure footprint than previous Japanese scientific launchers, with onboard autonomous health-checking systems reducing the manpower needed at the launch site. Hisaki's flight was the rocket's maiden voyage, so the mission carried the dual significance of validating a new launch system while also deploying a science payload. The launch took place from the Uchinoura Space Center in Kagoshima Prefecture, Japan.

JAXA, the Japan Aerospace Exploration Agency, was established in 2003 through the merger of three predecessor organizations — ISAS, NAL, and NASDA — and has since conducted a broad portfolio of scientific, Earth observation, and human spaceflight activities. The agency's Institute of Space and Astronautical Science (ISAS) division, which traces its heritage to Japan's earliest satellite programs, has been responsible for the scientific satellite program within which Hisaki was developed.

Significance and legacy

Hisaki's scientific contribution lies primarily in the domain of comparative planetary aeronomy — the study of the upper atmospheres of planets and the physical and chemical processes that shape them. By observing Jupiter's plasma torus and aurora in the extreme ultraviolet over extended periods, the spacecraft provided time-series data of a type rarely available from single-flyby missions or from ground-based telescopes operating at other wavelengths. The ability to monitor how Jupiter's magnetosphere responds to variations in solar wind activity, or how volcanic outgassing from Io ripples through the surrounding plasma environment, gave planetary scientists a new observational window.

The mission also demonstrated a programmatic concept: that a small, focused spacecraft with a well-defined instrument suite and a cost-efficient launch vehicle could make genuinely competitive contributions to planetary science. This was significant for JAXA's planning purposes and for the broader international community of space scientists who watched the Epsilon's debut with interest. The success of the Epsilon's maiden flight, carrying Hisaki to its intended orbit, validated the launcher for subsequent missions in the series.

Because the catalog entry does not confirm a current operational status, it is not possible to state with certainty whether Hisaki's instruments are still functioning or whether the satellite is transmitting scientific data. What is clear from the orbital data is that the spacecraft remains in orbit — at altitudes between 961 and 1,165 kilometers — and continues to be tracked by space surveillance networks. Its continued presence in the catalog more than a decade after launch is itself a testament to the stability of the orbit chosen for the mission.

How to spot it

Hisaki is not generally counted among the brightest or most easily observable satellites for casual skywatchers. Its relatively small size — 348 kilograms, with a compact bus typical of the SPRINT program — means it reflects comparatively little sunlight compared to large structures such as crewed stations or rocket bodies trailing long fuel tanks. That said, it is not invisible to observers with patience and appropriate tracking tools.

Because of its 29.7-degree orbital inclination, Hisaki passes overhead only for observers located at relatively low latitudes. Viewers in equatorial regions, the Caribbean, Central America, the Middle East, South Asia, Southeast Asia, and similar latitudes will have the best opportunities for passes. Observers at latitudes above roughly 50 degrees north or south will not see Hisaki pass overhead at all, as the satellite's ground track never reaches them.

To predict a visible pass, use the satellite's NORAD ID 39253 in any reputable pass-prediction tool or satellite-tracking application. Look for passes during the twilight hours — shortly after sunset or before sunrise — when the satellite is still sunlit but the sky is dark enough for it to be visible. Passes directly overhead will offer the best chance of detection, while low-elevation passes near the horizon are harder to observe due to atmospheric extinction and obstructions.