Radio astronomy is the observation of astronomical phenomena via the reception of radio waves originating in the cosmos. ITU has identified the frequency bands necessary for these observations based on the physical characteristics of the chemical molecules under observation: hydrogen, water vapour, methanol or carbon monoxide, for example. Radio astronomical measurements are often carried out as part of an international framework involving research laboratories in several countries.
Because they measure radio emissions from celestial objects at cosmic distances from the Earth, radio astronomy receivers are designed to detect extremely weak signals, without comparison with those used in terrestrial applications. There are two types of radio astronomy observations:
- observation of spectral lines, where the radiation detected by the radio telescope is the result of spontaneous emissions (associated with changes of quantum state) by certain atoms or molecules (hydrogen or hydroxyl radical, for example). These lines are characterized by precise central frequencies, determined by the characteristics of and physical changes to the molecules under observation;
- observation of continuum emissions, whether thermal or non-thermal in origin (planetary magnetosphere, for example, or solar flares), for which radio spectrum is wide-band.
To observe these cosmic sources, radio astronomers use either an extremely large antenna providing sufficient spatial resolution to distinguish the various celestial objects under observation, or interferometry systems combining simultaneous measurements by a number of radio telescopes thousands of kilometres apart. These systems achieve resolutions so fine that they are able to study the detailed structure of distant radio sources. Observations made by high spatial resolution interferometry therefore rely on simultaneous reception of the same radio frequency by widely dispersed reception systems, further emphasizing the international scope of the protection afforded to radio astronomy: if just one of the observation systems is affected by interference, all the other international measurements are compromised.
There are four radio astronomy observatories in France: Nançay, the plateau de Bure, Maïdo on the island of Réunion and Floirac.
While radio astronomy studies the cosmos from Earth, satellites, too, can be used to observe celestial objects. Scientific space research is set firmly in a dynamic of international cooperation: costly programmes (astronomical missions such as the Herschel infrared space telescope or the Planck cosmic microwave background mapping mission) are conducted by the European Space Agency (ESA) and financed by a budget to which member states contribute. Onboard instruments are supplied by member states following requests for proposals. France’s participation in ESA is coordinated by the French space research agency, CNES.
In addition to its European initiatives, CNES conducts national programmes (such as the MICROSCOPE project launched in April 2016, designed to verify the principle of equality of gravitational and inertial mass, one of the foundations of the theory of general relativity) and engages in multilateral cooperation (such as the CoRoT satellite carrying a space telescope designed to study the internal structure of stars and search for exoplanets). These programmes are generally based on micro or mini-satellites. For projects such as these, CNES brings in scientific and industrial partners to carry out the space programmes it designs.
Because they are so intrinsically international in nature, space research systems rely solely on frequencies that have been globally harmonised under the ITU Radio Regulations. In France, CNES operates a space research station, based on the Kourou site, in the 8400-8500 MHz band, for the needs of projects such as Mars-Express, Rosetta, Herschel or Planck.