A number of faithful readers spread around the world replied to ESAs call. After a careful examination of all entries, Anders Nyholm from Sweden was selected as the winner. His reply was elegant and exhaustive, recalling all his scientific assumptions and describing step-by-step the procedure followed to solve the quiz.

Anders Nyholm will receive a Cassini-Huygens mug, a Huygens book and a stylish Huygens table model. All the other participants (*) will receive a Cassini-Huygens mug.

Just as a curiosity: believe it or not, the Cassini-Huygens mug  showing the spacecraft instruments around it perimeter - is often used by scientists and engineers during meetings to figure out a particular attitude of the spacecraft!

Overall comment on the entries

The first question 'How big a telescope would be needed to see Cassini/Huygens in mid-May 2000?' proved to be the most problematic to answer. Many competitors only tried a guess.

According to the basic principles of optics, having a good resolution image of an object in the sky is only influenced by the aperture (diameter of the mirror) of a telescope and not by its focal length. So, how big refers in fact to the diameter of the mirror and not to length of the telescope: some competitors misinterpreted this concept.

Bearing in mind that the observer is interested in the diameter of the mirror, the first step to be taken is evaluating the apparent magnitude of the object we want to observe. The apparent magnitude is a number, which tells us how bright the object looks. It depends on the distance from the object and on its intrinsic brightness. Once the observer has computed the apparent magnitude, a straightforward calculation can be made of the aperture diameter needed to observe the object using the formula that relates the magnitude of the faintest object visible with a telescope of a given aperture. Observing an object means you see it in the sky as a light spot; an observer interested in resolving the shape of the object must take one step further. To do that the observer needs to calculate the apparent size of the object under study applying the Rayleigh principle. The final answer could even be that you would need a telescope with an aperture of several kilometers!

The second question 'At what angle with respect to the Sun-Earth line would you see Cassini/Huygens?' was easier and more than one correct answer was given. A straightforward application of the trigonometry principles, led easily to the answer. The only difficulty was finding the correct data on Cassinis geocentric and heliocentric distances, that could  for instance - be obtained from a Solar System Simulator (see for example the JPL Cassini-Huygens simulator: http://www.jpl.nasa.gov/cassini/today/). It is obvious that during the alignment of the planets the angle between Earth, Cassini and Sun was going to be small.

(*) Other participants: Arnold de Vet, The Netherlands; Simon Stanley, USA; Eduardo Benjamin Caldas Ribeiro, Portugal; Tecbier, Spain; Andrew Cody, USA; Eric Paul Brunner, USA; Dimitrios Oikonomidis, Greece; Peter de Blecourt, The Netherlands

Congratulations, Anders!!