# Submillimetre astronomy

Submillimetre astronomy or submillimeter astronomy (see spelling differences) is the branch of observational astronomy that is conducted at submillimetre wavelengths (i.e., terahertz radiation) of the electromagnetic spectrum. Astronomers place the submillimetre waveband between the far-infrared and microwave wavebands, typically taken to be between a few hundred micrometres and a millimetre. It is still common in submillimetre astronomy to quote wavelengths in 'microns', the old name for micrometre.
Submillimetre observations can be used to trace emission from gas and dust, including the CI, CO, and CII lines. Sources behind this emission include molecular clouds and dark cloud cores, which can be used to clarify the process of star formation from earliest collapse to stellar birth, by determining chemical abundances in dark clouds and the cooling mechanisms for the molecules which comprise them. Other sources include protoplanetary discs, dusty starburst galaxies in the early Universe, immediate environments surrounding AGN, and secondary anisotropies in the cosmic microwave background.
Submillimetre observations have been used to constrain models of planetary, stellar, and galactic formation and evolution. By studying foreground elements of the CMB and environments around SMBHs, submillimetre astronomy can also be used to constrain models of quantum gravity and to investigate the role of gravitational waves and relativistic neutrinos in the early Universe. Notably, the Event Horizon Telescope, which produce the first image of a black hole in 2020 using radio and far-infrared observations, also conducts VLBI observations within the submillimeter regime at 870μm.

## From the ground

The most significant limitations to the detection of astronomical emission at submillimetre wavelengths with ground-based observatories are atmospheric emission, noise and attenuation. Like the infrared, the submillimetre atmosphere is dominated by numerous water vapour absorption bands and it is only through "windows" between these bands that observations are possible. The ideal submillimetre observing site is dry, cool, has stable weather conditions and is away from urban population centres. Only a handful of sites have been identified. They include Mauna Kea (Hawaii, United States), the Llano de Chajnantor Observatory on the Atacama Plateau (Chile), the South Pole, and Hanle in India (the Himalayan site of the Indian Astronomical Observatory). Comparisons show that all four sites are excellent for submillimetre astronomy, and of these sites Mauna Kea is the most established and arguably the most accessible. There has been some recent interest in high-altitude Arctic sites, particularly Summit Station in Greenland where the PWV (precipitable water vapor) measure is always better than at Mauna Kea (however Mauna Kea's equatorial latitude of 19 degrees means it can observe more of the southern skies than Greenland).
The Llano de Chajnantor Observatory site hosts the Atacama Pathfinder Experiment (APEX), the largest submillimetre telescope operating in the southern hemisphere, and the world's largest ground based astronomy project, the Atacama Large Millimeter Array (ALMA), an interferometer for submillimetre wavelength observations made of 54 12-metre and 12 7-metre radio telescopes. The Submillimeter Array (SMA) is another interferometer, located at Mauna Kea, consisting of eight 6-metre diameter radio telescopes. The largest existing submillimetre telescope, the James Clerk Maxwell Telescope, is also located on Mauna Kea.
Some submillimetre telescopes are also used for VLBI.

## From the stratosphere

With high-altitude balloons and aircraft, one can get above more of the atmosphere. The Balloon-borne Large Aperture Submillimeter Telescope (BLAST) and the Stratospheric Observatory For Infrared Astronomy (SOFIA) are two examples, although SOFIA can also handle near infrared observations.

## From orbit

Comparison :
| Name | Year | Wavelength | Aperture |
| Human Eye | – | 0.39 – 0.75 μm | 0.01 m |
| SWAS | 1998 | 540 – 610 μm | 0.55 – 0.7 m |
| Herschel | 2009 | 55 – 672 μm | 3.5 m |
Space-based observations at the submillimetre wavelengths remove the ground-based limitations of atmospheric absorption. The first submillimeter telescope in space was the Soviet BST-1M, located in the scientific equipment compartment of the Salyut-6 orbital station. It was equipped with a mirror with a diameter of 1.5 m and was intended for astrophysical research in the ultraviolet (0.2 - 0.36 microns), infrared (60 - 130 microns) and submillimeter (300 - 1000 microns) spectral regions, which are of interest to those who are interested in which makes it possible to study molecular clouds in space, as well as obtain information about the processes taking place in the upper layers of the Earth's atmosphere.
The Submillimeter Wave Astronomy Satellite (SWAS) was launched into low Earth orbit on December 5, 1998 as one of NASA's Small Explorer Program (SMEX) missions. The mission of the spacecraft is to make targeted observations of giant molecular clouds and dark cloud cores. The focus of SWAS is five spectral lines: water (H$_2$O), isotopic water (H$_2$$^{18}$O), isotopic carbon monoxide ($^{13}$CO), molecular oxygen (O$_2$), and neutral carbon (C I).
The SWAS satellite was repurposed in June, 2005 to provide support for the NASA Deep Impact mission. SWAS provided water production data on the comet until the end of August 2005.
The European Space Agency launched a space-based mission known as the Herschel Space Observatory (formerly called Far Infrared and Sub-millimetre Telescope or FIRST) in 2009. Herschel deployed the largest mirror ever launched into space (until December 2021, with the launch of the near-infrared James Webb Space Telescope) and studied radiation in the far infrared and submillimetre wavebands. Rather than an Earth orbit, Herschel entered into a Lissajous orbit around L$_2$, the second Lagrangian point of the Earth-Sun system. L$_2$ is located approximately 1.5 million km from Earth and the placement of Herschel there lessened the interference by infrared and visible radiation from the Earth and Sun. Herschel's mission focused primarily on the origins of galaxies and galactic formation.