Ajisai (Japanese: あじさい, meaning "

Hydrangea ''Hydrangea'', () commonly named the hortensia, is a genus of over 75 species of flowering plants native to Asia and the Americas. By far the greatest species diversity is in eastern Asia, notably China, Korea, and Japan. Most are shrubs tall, ...

") is a Japanese

satellite A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotope ...

History

EGP was launched from the

Tanegashima Space Center The (TNSC) is the largest rocket-launch complex in Japan with a total area of about 9.7 square kilometers. It is located on the southeast coast of Tanegashima, an island approximately south of Kyushu. It was established in 1969 when the ...

on August 12, 1986 at 20:45 GMT (August 13, 05:45 local time).NASDA
/ref> The launch vehicle was the first H-I rocket. After launch, the spacecraft was given the

International Designator The International Designator, also known as COSPAR ID, is an international identifier assigned to artificial objects in space. It consists of the launch year, a three-digit incrementing launch number of that year and up to a three-letter code repr ...

1986-061A and

Satellite Catalog Number The Satellite Catalog Number (SATCAT, also known as NORAD (North American Aerospace Defense) Catalog Number, NORAD ID, USSPACECOM object number or simply catalog number, among similar variants) is a sequential nine-digit number assigned by the Un ...

16908.

Structure

EGP is entirely passive, and operates by reflecting sunlight or ground-based lasers.Curtis, Anthony R (1989). ''Space Almanac,'' Arcsoft Publishers. The satellite is a 685-kg hollow sphere with a diameter of 2.15 meters, and the surface is covered with 318 mirrors for reflecting sunlight and 1436 corner reflectors for reflecting laser beams.A plate motion study using Ajisai SLR data (Sengoku 1998), from Earth Planets Space 50, 611-627, 1998
Terrapub The mirrors are 10x10 inches, and the corner reflectors are one inch in diameter and grouped into 120 laser reflection assemblies.
/ref>

Orbit

EGP is in a nearly circular orbit at an altitude of approximately 1488 km, close to the (not firmly defined) boundary between

low Earth orbit A low Earth orbit (LEO) is an orbit around Earth with a period of 128 minutes or less (making at least 11.25 orbits per day) and an eccentricity less than 0.25. Most of the artificial objects in outer space are in LEO, with an altitude never mor ...

and

medium Earth orbit A medium Earth orbit (MEO) is an geocentric orbit, Earth-centered orbit with an altitude above a low Earth orbit (LEO) and below a high Earth orbit (HEO) – between above sea level.

. The

orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...

is 116 minutes, and the orbital

inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a Plane of reference, reference plane and the orbital plane or Axis of rotation, axis of direction of the orbiting object ...

is 50 degrees.

Mission

EGP's mission is

geodesy Geodesy ( ) is the Earth science of accurately measuring and understanding Earth's figure (geometric shape and size), orientation in space, and gravity. The field also incorporates studies of how these properties change over time and equivale ...

. The primary objectives were improving Japan's geodetic triangulation network, determining the position of remote islands, and integrating Japan's geodetic network with geodetic systems used in other parts of the world.UN Registry Notes
(as republished by Jonathan McDowell) There are two modes of operation. A ground-based laser can be reflected off the satellite, and light's return time measured to determine the round-trip distance. Alternatively, when the satellite is in sunlight, it can be photographed against the background stars. In both cases, since the satellite's orbit is precisely known, information about the ground-based observer's position can be calculated. EGP's orbital altitude is high enough that

atmospheric drag In fluid dynamics, drag (sometimes called air resistance, a type of friction, or fluid resistance, another type of friction or fluid friction) is a force acting opposite to the relative motion of any object moving with respect to a surrounding flu ...

has no significant effect on the spacecraft trajectory. This is advantageous for maintaining the stable orbit required for geodesy.

Visual appearance

There is a flash produced when the sun's reflection from one of the satellite's mirrors crosses an observer's position on the earth. Due to the satellite's rotation, and the changing geometry as the satellite moves along its orbit, EGP produces several of these flashes per second. EGP can take up to 18 minutes to cross the sky. EGP's flashes are visible in binoculars if the observer is on the nighttime side of the planet, and the satellite is in sunlight while its orbital trajectory takes it above the observer's horizon. These conditions are often met in the hours after sunset, and the hours before sunrise. When EGP enters the Earth's shadow, the stream of flashes abruptly ceases.

View from Earth

The corner reflectors are used to reflect laser beams and are invisible to amateur observers, but the mirrors are spectacular. They are designed to reflect sunlight so the satellite can be photographed by ground stations for precise geodetic surveying measurements. The glints are probably in the third magnitude range but are visible to the naked eye only in very dark skies under good conditions. The brief flashes are too short to be noticed by the naked eye. In binoculars EGP resembles the strobe of an airplane but the flash pattern is more complex than a strobe light. Because of the extremely high orbital altitude of 1,500 kilometres, EGP is often visible closer to midnight than other satellites, and can frequently be seen on as many as four orbits during a single overnight observing session.

Notes

References

Japan Aerospace Exploration Agency (JAXA) - Ajisai EGS
*''TRW Space Log 1957-1991'' (1992), published by TRW Inc., Redondo Beach CA.

External links

Heavens Above
is a website that displays satellite orbits and calculates satellite visibility. This page at Heavens Above shows the current orbital position of EGP.
The Use of Videography in Tracking Earth Satellites
includes a time exposure photograph showing a series of EGP flashes against the background stars (page 48). The file is large and mostly unrelated to EGP, but this image is a good illustration of the satellite's visual appearance from the ground. {{Japanese space program Satellites of Japan Passive satellites Laser ranging satellites