The European Space Agency (ESA) is Europe’s gateway to space. Its mission is to shape the development of Europe’s space capability and ensure that investment in space continues to deliver benefits to the citizens of Europe and the world.
Find out more about space activities in our 23 Member States, and understand how ESA works together with their national agencies, institutions and organisations.
Exploring our Solar System and unlocking the secrets of the Universe
Go to topicProtecting life and infrastructure on Earth and in orbit
Go to topicUsing space to benefit citizens and meet future challenges on Earth
Go to topicMaking space accessible and developing the technologies for the future
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The hydrogen atoms in the Passive Hydrogen Maser emit a microwave frequency frequency as they return to their 'fundamental' energy state. This is detected by an interrogation circuit which tunes an external signal to the 'natural' transition of the hydrogen atoms. The frequency of this microwave signal is known very precisely - it serves as the 'tick' of the Passive Hydrogen Maser clock.