image: Herschel launched aboard an Ariane 5 rocket from Kourou, French Guiana together with Planck, a mission to map the cosmic microwave background in more spectral bands and with more sensitivity than previous missions. view more
Credit: European Space Agency (Guarniero)
Naval Research Laboratory (NRL) scientists are part of a team working on the Herschel Space Observatory, which was successfully launched by the European Space Agency (ESA) from French Guiana on May 14, 2009. Herschel is the first observatory to cover the full far-infrared to submillimeter spectral range (55 – 670 microns in wavelength / 0.5 – 5 terahertz in frequency) and it hosts a 3.5 meter diameter telescope, the largest single mirror telescope ever launched to space.
The Herschel Space Observatory is the next of ESA's scientific missions and houses a suite of instruments built by a multi-national team which includes partners from Europe, the United States (NASA), and Canada. Herschel opens a new terahertz window on the cold and dusty Universe, enabling its scientific objective: to investigate how planets, stars and galaxies formed and continue to evolve. Terahertz photons are the dominant photons emitted by cold material. They can penetrate through dusty regions that obscure the optical and ultraviolet radiation emitted by young stars and accreting black holes. Also, because our Universe is expanding, the radiation from the very distant galaxies of earlier epochs is Doppler shifted to the lower energy terahertz range.
NRL's Dr. Jackie Fischer, head of NRL's Infrared – Submillimeter Astrophysics & Techniques Section in the Remote Sensing Division, is the Herschel Optical System Scientist and a member of the Herschel Science Team. She has worked on Herschel for the past eight years. She and her colleagues, including Dr. Robert Lucke of the Remote Sensing Division, have carried out laboratory measurements and modeling efforts to better understand how the telescope will perform. Dr. Fischer will use the Herschel Space Observatory to study mergers of galaxies that emit most of their immense luminosity in the terahertz spectral range, and are therefore known as the ultraluminous infrared galaxies (ULIRGs). These galaxy mergers are hypothesized to be responsible for the evolution of galaxies from gas-rich spiral galaxies to gas-poor elliptical ones. Dr. John Carr, also of the NRL Infrared – Submillimeter Astrophysics & Techniques Section, will observe conditions in the cold outer regions of the dusty disks around young stars from which planetary systems are formed.
The Observatory is named after Sir Frederick William Herschel, who discovered at the beginning of the 19th century that there is electromagnetic radiation at wavelengths longer than those visible to the human eye. Herschel placed a thermometer beyond the red part of the spectrum of sunlight dispersed by a prism and observed a rise in temperature due to heating of the thermometer by longer wavelength, infrared radiation. Although infrared radiation is invisible to the human eye, all objects "glow" in the infrared, and the quantity and wavelength distribution of the radiation depend on the temperature, area, and emissivity (surface emitting efficiency) of the object. For objects that are much colder than room temperature, such as the outer planets or interstellar ice and dust, the peak of this thermal radiation shifts from the mid-infrared peak at room temperature, into the far-infrared / terahertz range. Herschel's telescope and instruments are designed to detect and characterize the "cool" Universe.
The Earth's atmosphere both absorbs and emits terahertz radiation. For this reason, the best location for an infrared telescope is in space. The Sun, the Earth, and the Moon are strong terahertz emitters, and this must be taken into account in choosing the Observatory's location in space in order to shield the Observatory. In addition, the colder the telescope and its surroundings, the lower the background radiation and the more sensitive the instruments can be. The Herschel telescope's 3.5 meter-diameter mirror will gather terahertz radiation from some very cool and some very distant objects in the Universe.
The Herschel Space Observatory is a successor to several earlier small cryogenic observatories that covered parts of the terahertz range, including ESA's Infrared Space Observatory (ISO), on which NRL scientists were co-investigators on the Long Wavelength Spectrometer, and NASA's Spitzer Space Telescope, a mission whose cryogens are now depleted. Both hosted small superfluid-helium cooled telescopes, 0.6 and 0.85 meter in diameter respectively. Herschel's three instrument focal plane units will also be cooled by superfluid-helium to temperatures ranging from 1.7 – 10 degrees Kelvin (zero degrees Kelvin is -273 degrees Celsius). However, because of its large size, required to increase the angular resolution and sensitivity of the Observatory, the Herschel telescope was designed for passive (radiative) cooling in space to an operating temperature of approximately 80 K, near the temperature of liquid nitrogen and still relatively cold. Thus, only enough superfluid-helium to cool the instruments and detectors over the lifetime of the mission needs to be carried to space in the cryostat.
Also different from ISO and Spitzer, which were in highly elliptical Earth orbit and Earth-trailing orbit respectively, Herschel will be launched to orbit around the 2nd (of five) Lagrange (L2) point of the Sun-Earth system. Lagrange points are positions where the combined gravitational pull of two large masses provides precisely the centripetal force required to rotate with them. Thus they allow an object to be in a "fixed" position with respect to the two large masses rather than in a position in which the relative positions of the two large masses change. Herschel will be in orbit around a point along the Sun-Earth axis, one million miles more distant from the Sun than the Earth. Because an object orbiting L2 always maintains the same approximate relative orientation with respect to the Sun and Earth, thermal shielding is simpler. Herschel is the second observatory to be located at L2, after NASA's Wilkinson Microwave Anisotropy Probe (WMAP) cosmology mission.
Herschel's telescope will feed a science payload consisting of three instruments:
- Photodetector Array Camera and Spectrometer (PACS), three band camera and a low- to medium-spectral resolution integral field (5 X 5 pixel) spectrometer, covering the 55 – 210 micrometers spectral range. It uses two bolometer detector arrays in the camera and two photo-conductor detector arrays in the spectrometer. The PACS team is led by Germany and includes contributions from Belgium, Austria, France, Italy and Spain.
- Spectral and Photometric Imaging Receiver (SPIRE), a three band camera with bands centered at 250, 350, and 500 microns, and a low- to medium-resolution imaging Fourier transform spectrometer covering the 200 – 600 micrometers spectral range, using five bolometer arrays. The SPIRE team is led by the United Kingdom and includes contributions from the US, France, Canada, Italy, Spain, Sweden, and China.
- Heterodyne Instrument for the Far Infrared (HIFI), a very high spectral resolution spectrometer covering the 157 – 670 micrometers. The team is led by the Netherlands, with contributions from the US, France, Germany, Canada, Ireland, Italy, Poland, Russia, Spain, Sweden, Switzerland, and Taiwan.
The Herschel telescope was constructed almost entirely from silicon carbide (SiC), which is a strong and lightweight ceramic that is as polishable as glass. SiC was chosen because it has relatively high thermal conductivity, important for minimizing thermal gradients, and because it is homogeneous and isotropic, and less hygroscopic than other light-weight materials such as composites. For high reflectivity and low emissivity, its surface was coated with aluminum. As the Herschel Optical System scientist, Dr. Fischer worked with the Herschel telescope engineers to ensure that the telescope's performance would enable it to carry out the science for which the Observatory was intended. She measured the emissivity of Herschel mirror samples at the telescope's operating temperature to quantify the expected telescope background and worked with Dr. Lucke on modeling of telescope secondary mirror reflections of internal instrument radiation causing unwanted baseline ripple for the Herschel heterodyne instrument. She worked with telescope engineers to understand variations in the telescope focus position with temperature, a critically important issue since the Herschel telescope does not have a focus mechanism. She also worked with the engineering team to understand, minimize and finally characterize stray light scattered into and emitted by the optical system surroundings.
Herschel shared the launch with the cosmology mission, Planck, aboard an Ariane 5 ECA rocket. Herschel will commence preparations for its science mission enroute toward its operational orbit around a point in space situated at one million miles away from the Earth. Herschel has been designed to perform routine science operations for a minimum of three years, starting six months after launch. The mission will end when the helium used to cool the focal planes of the three scientific instruments is depleted.