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Cambridge University Science Magazine
European Space Agency's (ESA) Huygens probe is on its way to Saturn's moon Titan, aboard NASA's Cassini spacecraft. The Cassini will remain in orbit around Saturn, investigating the gas giant and its rings, while the Huygens will probe one of the most interesting objects of the Solar System: Titan has a thick atmosphere and it is supposed to resemble the Earth in its early stages. The Huygens was launched in October 1997 and will reach Saturn in 2004.

Huygens on Titan
Vision of Huygens on Titan © ESA

However, tests performed this February and confirmed in September imply that the Cassini is unable to receive signals from Huygens during its landing on Titan. This is due to the Doppler shift of the radio waves sent from the moving probe. The frequency of the signal will be shifted towards the edge of the frequency range that can be received, which will decrease the intensity by about 10dB.

The Doppler effect is most often exemplified by a fast-moving ambulance. The pitch of the siren appears lower when it is driving away. The effect is slightly more complicated for radio waves, yet simple enough to be covered in Part IA Physics. ESA has set up a committee to investigate why such a simple phenomenon should have been ignored.

A possible scenario for recovering the signals is to alter Cassini's orbit in order to reduce its speed relative to Huygens.

Risto A. Paju is an Undergraduate in Physics at Queens'