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A Sun-synchronous polar orbit to maintain the cryogenic temperature
Is there also ingenuity in AKARI's orbit?
Heat is the nemesis of infrared observations. In terms of heat sources in space, the Sun obviously springs to mind; however, the Earth is in fact also a tremendous source of heat. As well as reflecting light from the Sun, the Earth also emits large amounts of infrared radiation. If AKARI's telescope were to point at the Earth, not only would the sensitive observational instrumentation be dazzled, but the cooling system would also sustain irreparable damage. AKARI follows a so-called Sun-synchronous polar orbit, which runs close to the boundary between the Earth's night and day. Since the direction of the Sun is always perpendicular to the orbital plane, placing solar shields and solar battery panels on the appropriate side of the satellite affords protection from the heat of the Sun.
Schematic diagram of a Sun-synchronous polar orbit, showing the relative positions of AKARI and the Sun
- Sun synchronous polar orbit
- Orbiting along the boundary between Earth's night and day, while exposing the same surface towards the Sun. AKARI itself is also slowly rotating, so that the telescope continuously points away from the Earth.
Since the satellite is orbiting in such a way that it continuously points away from the Earth, the aperture of the telescope sweeps around in a great circle, allowing observation of the entire sky from North to South Pole. After six months of orbiting in this fashion, following the Earth's rotation, the satellite will have been able to observe every inch of the sky—which is why a Sun-synchronous polar orbit is so well suited for an all-sky survey mission. Incidentally, the Moon is also a significant source of heat (and therefore infrared radiation) and although if the telescope were to pick up the Moon, it would not cause the same irreparable damage to the instrumentation or cooling system that capturing the Sun or the Earth would; the infrared sensors would become saturated (dazzled) and nothing would be visible until the Moon has moved far away from the focus. Since, during the survey, it is not possible to observe the area of the sky around the Moon, the telescope will be pointed at the same area of the sky six months later, to fill in any survey “holes.”
Schematic diagram of a Sun-synchronous polar orbit, showing the form of the AKARI All-Sky Survey
- The AKARI All-Sky Survey
- Since the Earth rotates around the Sun, on each rotation the observation area is shifted slightly. The satellite can scan the entire celestial sphere (i.e., the entire sky) in approximately 180 days (or six months).
Is the All-Sky Survey going smoothly?
The satellite was launched in February 2006, and after some performance test operations, the All-Sky Survey commenced. The first scan of the sky (Phase 1) was completed at the beginning of November 2006, and the second scan of the sky (Phase 2) immediately began thereafter.
As well conducting the All-Sky Survey in the scanning mode operation, AKARI also has a pointed (staring) observation mode, for observations of specific astronomical objects over an extended period of time. The maneuver between the survey mode and the pointed observation mode is driven by reaction wheels onboard the spacecraft rather than the attitude control propulsion jets (or thrusters) that are often employed on satellites.*1 Although the satellite can burn propellant and achieve jet propulsion, this process produces various complex compounds such as ammonia and the resulting gases could pollute the inside of the telescope as they drift along with the satellite. Previous experiments conducted in 1995 with the SFU, indicated that this should not pose a problem over short observation periods of about one month, however, for longer observation periods such as that of AKARI, it was decided that the thrusters would be used as little as possible.
- *1Reaction wheel
The rotation of a heavy disk (flywheel) or rotor is sustained at a uniform rotational frequency using electrical power from the solar batteries. The satellite's direction, known as its “attitude,” is changed using the reaction caused by briefly speeding up and slowing down the speed at which the wheel rotates. AKARI is equipped with four such reaction wheels.
Attitude control in pointed observation mode
- Survey mode and pointed observation mode
- In normal survey mode the satellite scans along the circumference of a great circle; however, there is also a pointed observation mode, which allows the satellite on occasion to look continuously at the same location on the sky. This pointed mode can be used up to three times in one orbit. The satellite takes about 100 min. to complete a single orbit; changing the satellite's attitude takes 7 min. 30 sec., a single pointed observation requires the satellite to stay at a fixed attitude for 15 min., and restoring the satellite's attitude takes another 7 min. 30 sec.
Another solution for avoiding the heat from the Earth is to follow an orbit that lies far from the Earth. Currently, NASA's infrared Spitzer Space Telescope is carrying out pointed observations in such an orbit.*2 However, the long distance from the Earth makes both the launch and propulsion of the satellite much harder. For a scanning telescope like AKARI, a near-Earth Sun-synchronous polar orbit is sufficient and indeed ideal.
- *2The Spitzer Space Telescope (SST)
An infrared telescope with an aperture of 85 cm, launched in August 2003 by NASA in the USA. To protect it from the heat of the Earth, Spitzer was put into a heliocentric (Sun centered) orbit, in which it follows the Earth along its orbit. Spitzer carries 360 liters of liquid Helium for cooling purposes.