The Infrared Astronomical Satellite AKARI and Nikon

Infrared astronomy, began in 1960s, is a very important branch of astronomy. However, since most of the infrared radiation from space is absorbed by the Earth’s atmosphere, it is difficult to make observations from the ground. Although large telescopes such as Subaru are effective, there are still many limitations to earthbound observations. Therefore, in order to observe the entire sky clearly, using long-wavelength infrared light, the only realistic option is to launch a telescope above the Earth’s atmosphere into space. However, infrared observations from space have lagged behind those of X-ray and optical astronomy, because of the many technical challenges such as high performance infrared sensors required and the necessity for cooling equipment. Japanese astronomers have been considering space infrared observations since the late 1970s, and our first small mission was realized in 1995^*2. Thereafter, many of the various technical problems associated with infrared observations from space were resolved, and the ASTRO-F project was born.

At that time, plans were already afoot in Europe and the United States for infrared astronomical satellites that would observe small areas of the sky to great degrees of precision. In Japan, we boldly decided to use the ASTRO-F mission to create a rejuvenated map of the entire sky, using high-sensitivity instruments to establish a legacy and basis for subsequent observations with future telescopes.

In the following 10 years, we solved the various technical issues, and now finally we can be proud to have launched our very own satellite dedicated for infrared observations.

As a space telescope, the improvement is not so apparent—the IRAS satellite had a 60 cm telescope, whereas AKARI has an aperture of 68.5 cm. This is due to constraints of the cargo size and weight limitations of the launch rocket. The telescope system is also quite similar to ground-based telescopes. Since the principal objective of AKARI is to conduct an all-sky survey, the intention behind the optical design is to allow high-quality imagery over a wide field of view (or viewing angle). By improving the optical accuracy of the primary and secondary mirrors, the resolution of the AKARI observations is about one order of magnitude better than that of the IRAS observations. Similar development in infrared detector technology has also produced improved sensitivities.

As a result, fainter, more distant astronomical objects become visible, and their composition can be seen in much finer detail. Astronomy using infrared radiation has various aims. Some researchers wish to see how entire galaxies are formed on global scales, while other researchers endeavor to seek out individual stars being born within a galaxy. More precise size measurements and other data on asteroids within our Solar System are being obtained by observing them in infrared rays. Whereas X-rays are used to observe certain specific astronomical objects under extremely high temperatures, and observations at optical wavelengths are most often used for objects such as fixed stars and bright galaxies, infrared light is used to observe the low-temperature gas of outer space itself, which constitutes the greater part of the volume of our Universe. It is thus a significant step in astronomy that such widely used fundamental data are being revisited with 10 times higher resolution and better sensitivity.

Infrared radiation is emitted from any object that has heat. If the telescope and instruments are warm, they emit infrared light that would interfere with the observations of faint objects in the sky. For this reason, both the telescope and all the observational instruments onboard AKARI are housed within a cryostat and are cooled to an absolute temperature of as low as 6K (−267°C) using liquid Helium. Since AKARI was manufactured at normal room temperatures, the actual operating temperatures in space will differ by 300°C. Therefore, the following challenges had to be overcome in order for the telescope to operate properly at these cryogenic temperatures:

1. The development of a material that would enable the telescope to observe to a high degree of accuracy, even in harsh environments where the temperature undergoes significant changes and variations.
2. Improvements in the efficiency of the cooling equipment so as to maintain these cryogenic temperatures.
3. Attitude and orbit control that would avoid heat from the light of the Sun and radiation from the Earth.

Cooling space instruments requires a great deal of ingenuity. For example, IRAS used 600 liters of liquid Helium for cooling over the course of its ten-month mission. AKARI only carries 170 liters (roughly a quarter of the IRAS amount). A pair of Stirling cycle cryocoolers, which are capable of cooling down to an absolute temperature of 20K (−253°C) in space, are operated to preserve the liquid Helium.^*8 This has enabled an 18-month mission lifetime at the cryogenic temperature (the initial plan called for at least 550 days).

The cryostat is a pressure-tight container and before the launch it is covered by a lid to create a vacuum inside to prevent any heat from the outside reaching the telescope. However, since the lid, made of aluminum, is itself warmer than outer space (which is around 3K), it also emits infrared radiation and thus consumes the liquid Helium. This lid was supposed to have been promptly jettisoned as soon as the satellite was in space.^*9 However, due to unforeseen and unconnected difficulties that arose just after launch, the jettisoning of the lid and the commencement of the observations were delayed for about a month.^*10 As a result, it was feared that there was significantly less liquid Helium available for the rest of the mission than originally planned. Fortunately, later measurements confirmed that the liquid Helium lifetime was almost the same as originally planned, with even the worst-case prediction concluding that it would be possible to continue observations under cryogenic conditions for at least a year.

Even when the liquid Helium has expired, one of the cameras on the IRC instrument (covering the shortest wavelength range), may be able to continue observations, while being cooled solely by the Stirling cycle cryocoolers alone. It is not known how long AKARI can continue observations with this camera, since it is actually the first ever satellite to be equipped with such mechanical coolers with high cooling power (in fact very few satellites have been fitted with their own cryocoolers). Therefore, almost without realizing it, we have found ourselves at the forefront of space cryogenics technology.

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.

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.”

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.^*11 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.

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.^*12 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.

It is predicted that AKARI will run out of liquid Helium in mid 2007. At that point, the observation phase at the cryogenic temperature will end and efforts will shift to in-depth data analysis. After a further year, in 2008, the aim is to construct a comprehensive database of the coordinates and brightness of infrared astronomical objects over the entire sky, optimally to resolutions which are an order of magnitude better than the IRAS observations obtained more than 20 years ago. Over a period of a year we will use this database ourselves, optimizing both the products and the analysis. The data will then be published to the world, incrementally, beginning with the brightest astronomical objects. If possible, eventually we would like to publish the data in image form; although in all honesty, prior to the launch of AKARI it was uncertain whether it would be possible to convert the data into images, and so it was never guaranteed that any image data would be published. When we did experiment with the conversion of data into images, the results appeared to be of excellent quality. Thus, we anticipate that we will also be able to publish image maps of the sky.

Following the initial data release, we plan to proceed with the data analysis of the fainter infrared astronomical objects and correspondingly revise the database. We too have an avid interest in how galaxies came into being in the ancient Universe, and how they evolved to take on their current form, the types of environment in which stars are formed, and the process by which stars are born. AKARI will also open research into the state of partially formed planetary systems, and the physics of the gas and dust in outer space. AKARI will produce comprehensive databases opening many avenues of research and allow researchers all over the world to make use of the fruits of AKARI’s observations. We anticipate this work and research moving full steam ahead.

The development of the AKARI mission required more than an ability in astronomy and optics—it also required fundamental technical capabilities in such areas of expertise as cryogenic temperature material engineering. Although numerous approaches and tenders were made by other companies regarding the development of the telescope, we judged without any doubt that Nikon had the best vision. I believe that, in addition, this project entailed considerable risk for Nikon, since on top of the fact that a completely new material, silicon carbide, was used for the primary mirror construction, in the course of the cryogenic temperature material engineering analysis and experimentation alone, we had Nikon manufacture several hundred different types of materials and structural samples. Working within a limited budget, Nikon pushed itself to the limit. I believe that, thanks to this effort, AKARI has turned out to be a lightweight and compact satellite that will continue to demonstrate its outstanding capabilities over a long period of time.