DARTS for Astrophysics

IRTS

Infrared Telescope in Space (IRTS) on-board the Space Flyer Unit (SFU)

Mission Descriptions

Overview

IRTS (Infrared Telescope in Space) is Japan's first space-based infrared astronomical telescope placed into Earth orbit. Developed by the Institute of Space and Astronautical Science (ISAS), it was designed for high-sensitivity, wide-range infrared survey observations. It was mounted on the Space Flyer Unit (SFU), a versatile space experiment platform. The IRTS observed various targets, including zodiacal light, interstellar matter, cosmic background radiation, and IRAS point sources (such as late-type stars). The observations began on March 30, 1995, and concluded on April 26, 1995, when the superfluid liquid helium used for cooling was depleted.

The SFU was a large satellite developed as a reusable platform for space experiments, a collaborative project by the National Space Development Agency of Japan (NASDA), ISAS, the New Energy and Industrial Technology Development Organization (NEDO), and the Unmanned Space Experiment Recovery System (USER). It measured approximately 4 meters in diameter, 2.8 meters in height, and weighed about 4 tons. In addition to IRTS, it carried several scientific and engineering experiment instruments. The SFU was launched by NASDA's HII rocket on March 18, 1995, into a sun-synchronous orbit at an altitude of approximately 300-500 km and an inclination of 28.5 degrees, orbiting the Earth every 90 minutes. It was recovered by the US Space Shuttle (STS-72) on January 13, 1996.

Observation Instruments

The telescope, a 15 cm Ritchey-Chrétien reflector, housed four instruments on its focal plane, each covering different infrared wavelength bands. The entire telescope was cooled to 2 K using a liquid helium cooling system to minimize thermal radiation and maximize the performance of the observation instruments.

Near Infrared Spectrometer (NIRS)

The NIRS performs spectroscopic observations in the near-infrared range (1.4 μm to 2.5 μm). Utilizing a low-resolution diffraction grating, it covers a broad wavelength range, mainly used for studying interstellar matter and the structure of galaxies.

Mid Infrared Spectrometer (MIRS)

The MIRS conducts spectroscopic observations in the mid-infrared range (4.5 μm to 11.7 μm). Also using a low-resolution diffraction grating, it covers a wide wavelength range, allowing analysis of the chemical composition of interstellar dust and molecular clouds.

Far Infrared Line Mapper (FILM)

The FILM performs spectroscopic observations in the far-infrared range, specifically designed to observe the emission lines of ionized carbon at 158 μm and atomic oxygen at 63 μm. Data from FILM helps analyze the physical state and kinematics of interstellar matter.

Far Infrared Photometer (FIRP)

The FIRP is a four-channel photometer covering the wavelengths of 150 μm, 250 μm, 400 μm, and 700 μm. Equipped with a closed-cycle 3He cryogenic system, it can cool down to 0.3 K. The FIRP can measure the brightness in the far-infrared range with high sensitivity.

Achievements

During its 28-day mission, IRTS observed 7% of the entire sky with high sensitivity. One of the most significant discoveries was the detection of polycyclic aromatic hydrocarbons (PAHs) or organic materials containing such molecular structures, widely and abundantly present in the interstellar space of the Milky Way for the first time. The observational results provide valuable information across various fields, from the study of solar system objects to the structure of the Milky Way and even cosmology.