ALOS (DAICHI)

ALOS (Daichi) is a Japanese Earth observation satellite developed and operated by the Japan Aerospace Exploration Agency (JAXA). Assigned NORAD catalog identifier 28931 and international designator 2006-002A, it was launched on January 24, 2006 (January 23 by Eastern Standard Time, UTC-5), and was designed to support land-cover mapping, regional environmental monitoring, disaster response, and resource surveying on a global scale. Though its operational mission ended after roughly five years of service when the satellite lost power and communication with ground controllers, ALOS remains in orbit to this day, continuing to circle Earth as a dormant but trackable object in a near-circular sun-synchronous orbit.

Mission and Purpose

ALOS, whose Japanese name Daichi translates broadly as "land" or "earth," was conceived as a high-capability successor to earlier Japanese land-observation programs. Its core mission centered on producing detailed, consistent imagery of Earth's land surface — a task with wide-ranging applications including cartographic updating, forest and agricultural monitoring, geological surveying, and the rapid assessment of damage caused by natural disasters such as earthquakes, floods, and volcanic eruptions.

The satellite carried multiple remote-sensing instruments intended to work in complementary fashion. One provided high-resolution panchromatic and multispectral optical imagery, enabling fine-grained mapping of surface features. Another was a phased-array L-band synthetic aperture radar (SAR), a particularly powerful tool because radar imaging can penetrate cloud cover and operate regardless of daylight conditions — a critical advantage for monitoring regions frequently obscured by cloud or for responding to disasters that often occur under poor atmospheric conditions. A third instrument offered additional stereoscopic and wide-area coverage. Together, these systems made ALOS one of the more capable land-observation platforms of its generation.

The satellite's data proved valuable internationally, not only for JAXA's own programs but also for agencies and research groups across Asia, Europe, and beyond. During its operational lifetime, ALOS contributed imagery and radar data to disaster response efforts following several major seismic and weather-related events. It participated in data-sharing initiatives that recognized Earth observation as a globally shared resource, with imagery distributed to users who might not otherwise have access to such detailed, frequent coverage of their own territories.

After approximately five years in service, ALOS experienced a serious power anomaly. The satellite lost electrical power, apparently due to a degradation of its solar power generation and storage systems, and communication with the spacecraft ceased. JAXA declared the end of the mission. The satellite was never recovered or deorbited; it continues to orbit Earth in a passive, non-operational state.

Orbit and Tracking

ALOS occupies a sun-synchronous orbit — a specific type of near-polar, low Earth orbit in which the satellite's orbital plane precesses at a rate closely matching Earth's revolution around the Sun. This arrangement ensures the satellite passes over any given location at approximately the same local solar time on each revisit, which is highly desirable for land observation because it minimizes variations in illumination angle between images taken weeks or months apart. Consistent lighting makes it far easier to compare scenes over time, detect changes in land cover, and produce reliable mosaics of large areas.

The orbit is nearly circular, with a tracked apogee of 666 km and a perigee of 664 km above Earth's surface — a difference of just 2 km, indicating almost no eccentricity. At this altitude, the satellite completes one full orbit approximately every 97.9 minutes, meaning it circles Earth roughly 14 to 15 times per day. Its orbital inclination is 98.1°, consistent with the retrograde, slightly past-polar geometry that characterizes sun-synchronous orbits.

Because ALOS is no longer functioning, its orbit is no longer being actively maintained. Low Earth orbit objects at this altitude are subject to atmospheric drag — even at 660-plus kilometers, the residual upper atmosphere exerts a small but cumulative braking force. Without periodic thruster firings to compensate, the orbit will very gradually decay. However, at this altitude the process is extremely slow, and ALOS is expected to remain aloft for a considerable time before any eventual atmospheric reentry. Tracking organizations continue to monitor its orbital position. Its NORAD ID 28931 allows it to be located and followed in real time using standard two-line element (TLE) datasets, and its position can be predicted with good accuracy using publicly available tools.

Design and Operator

ALOS was built primarily by Toshiba under contract to JAXA. With a launch mass of 3,820 kg, it was a large and structurally substantial spacecraft by the standards of the time — its mass reflecting the weight of multiple imaging payloads, large solar panels, onboard data storage, and the structural framework required to maintain precise pointing stability during imaging operations.

JAXA, Japan's national space agency, served as both the mission operator and the primary user of the data returned by ALOS. JAXA was formed in 2003 through the merger of three predecessor organizations and is responsible for Japan's civilian space exploration, satellite development, and launch activities. At the time of ALOS's launch, JAXA was actively expanding Japan's Earth observation capabilities and ALOS represented a flagship investment in that program.

The satellite was launched from the Tanegashima Space Center, Japan's main launch facility, located on a small island off the southern coast of Kyushu. The mission was designated 2006-002A, indicating it was the primary payload of the second orbital launch of 2006 as catalogued internationally.

Beyond the immediate hardware, ALOS carried design heritage from earlier Japanese remote sensing satellites and was intended to demonstrate that Japan could independently develop and operate a world-class land observation system. The technical knowledge gained from designing and operating the satellite — including work with SAR systems, precision attitude control, and large-scale data downlink infrastructure — contributed to subsequent Japanese Earth observation programs.

Significance and Legacy

During its roughly five years of active operation, ALOS generated a large archive of Earth surface imagery and radar data. This archive did not become obsolete when the satellite went silent; it has continued to be used by researchers and institutions for comparative and historical analysis. In particular, the L-band SAR data from ALOS has remained scientifically valuable because it provides a reference baseline against which newer observations can be compared, enabling the study of long-term changes in forests, wetlands, urban areas, and other environments.

The satellite's operational period coincided with growing international emphasis on satellite-based disaster monitoring. ALOS imagery was applied in the wake of significant disasters, and its data contributed to the kind of rapid geospatial analysis that emergency managers and humanitarian organizations increasingly relied upon. This work helped build the case for follow-on missions with expanded capabilities.

JAXA subsequently developed ALOS-2, a successor satellite launched in 2014 that carried an improved L-band SAR with higher resolution and greater operational flexibility. Planning is also well advanced for ALOS-4, with further enhancements. The legacy of the original Daichi mission is thus not only the data archive it generated but the programmatic and technical foundation it established for Japan's continued role in global Earth observation.

Current Status

As of the time this entry was compiled, ALOS remains in orbit in a non-operational state. The satellite has not transmitted data since losing power, and there is no prospect of restoring communication with the spacecraft. It is effectively space debris — a large, inert mass in a near-circular sun-synchronous orbit at approximately 665 km altitude.

Its relatively high altitude means it will not reenter Earth's atmosphere in the near term. Nonetheless, it is tracked continuously as part of the broader effort to maintain an accurate catalog of Earth-orbiting objects. With a mass of 3,820 kg, ALOS is a substantial object, and its continued presence in a densely used orbital band is noted in conjunction with debris-awareness programs.

How to Spot It

ALOS is potentially observable from the ground under favorable conditions. As a large spacecraft — over 3,800 kg, with a significant surface area including solar panels — it is capable of reflecting sunlight and appearing as a moving point of light against the night sky. The best opportunities for visual observation occur shortly after dusk or before dawn, when an observer on the ground is in darkness but the satellite, at roughly 665 km altitude, is still illuminated by the Sun.

Because ALOS is in a sun-synchronous orbit inclined at 98.1°, it has access to nearly all latitudes, meaning observers across a wide range of locations — from equatorial regions to high northern and southern latitudes — may have pass opportunities. The LowEarth tracker uses the satellite's current orbital elements and NORAD ID 28931 to calculate precise pass times, maximum elevation angles, and azimuth directions for any given observer location, making it straightforward to determine when and where to look.