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Summary

HIFI's superb spectral resolution coupled with its ability to observe thousands of molecular, atomic and ionic lines at submillimeter wavelengths make it the instrument of choice to address many of the key  questions in modern astrophysics related to the cyclic interaction of stars and the interstellar medium:

1. HIFI will probe the physics, kinematics and energetics of star forming regions through their cooling lines, including H₂O, the major coolant.
2. HIFI will survey the molecular inventory of such diverse regions as shocked molecular clouds, dense Photon-Dominated Regions (PDRs), diffuse atomic clouds, Hot Cores and proto-planetary disks around newly formed stars, winds from dying stars and toroids interacting with AGN engines.
3. HIFI is uniquely suited to search for low-lying ro-vibrational transitions of complex species such as PAHs and, thus, to investigate the origin and evolution of the molecular universe.
4. HIFI can provide the out-gassing rate of comets through H₂O rotational lines and determine the vertical distribution of H₂O in the giant planets and on Mars.
5. HIFI can measure the mass-loss history of stars which, rather than nuclear burning, regulates stellar evolution after the main sequence, and dominates the gas and dust mass balance of the ISM.
6. HIFI will measure the FIR line spectrum of nearby galaxies as templates for distant, possibly primordial galaxies.

The main reason to build HIFI was because the above sketched science cannot be done from the ground, since atmospheric water lines block all radiation coming from space. On the right plots are given of the atmospheric transmission.

HIFI is optimised to address the astronomical key questions given above. All of these require high spectral resolving powers and sensitivity. Combining the high spectral resolving power of the radio heterodyne technique with quantum-noise limited detection from superconductor physics and state-of-the-art microwave technology, has made it possible to develop an instrument with a continuous frequency coverage from 480 to 1250 GHz in five bands, plus a sixth band providing coverage for 1410-1910 GHz at an unrivalled spectral resolution and ultimate sensitivity. This instrument is able to perform rapid and complecte spectral line surveys with resolving powers from 10³ up to 10⁷ (300 - 0.03 km/s) and deep line observations.

HIFI Band	1	2	3	4	5	6
Coverage (GHz)	480- 640	640- 800	800- 960	960- 1120	1120- 1250	1410- 1910
Half Power Beamwidth (arcsec)	39	30	25	21	19	13
Rec. Noise (DSB) in K Baseline values	110	145	175	210	370	800