By Claudio Maccone
Read Online or Download Deep Space Flight and Communications: Exploiting the Sun as a Gravitational Lens (Springer Praxis Books Astronautical Engineering) PDF
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Extra resources for Deep Space Flight and Communications: Exploiting the Sun as a Gravitational Lens (Springer Praxis Books Astronautical Engineering)
35 cm HPBW at 550 AU vs. 5 Â 10 À7 HPBW at 800 AU vs. 5 Â 10 À7 HPBW at 1,000 AU vs. , it gets smaller) as long as the distance increases beyond 550 AU. 5 gives angular resolutions for the same three distances at the same ®ve frequencies. Let us take a moment to ponder over these numbers. The best angular resolutions achieved so far, in visible light, were obtained by the European astrometric satellite Hipparcos, launched in 1989, and dismissed from service in 1993. 5. Table showing angular resolution for three spacecraft distances (550 AU, 800 AU, and 1,000 AU), at the ®ve selected frequencies.
Then ask: How far is the minimal focal distance dfocal on the opposite side of the source with respect to the focusing star center? The answer is given by the formula r 2star dfocal : 1:10 4GMstar r 2star À Dsource c2 This is the key to gravitational focusing for a pair of stars, and may well be the key to SETI in ®nding extraterrestrial civilizations. It could also be considered for the magni®cation of a certain source by any star that is perfectly aligned with that source and the Earth: the latter would then be in the same situation as the FOCAL spacecraft except, of course, it is located much farther out than 550 AU with respect to the focusing, intermediate star.
In 1978 the ®rst ``twin quasar'' image, caused by the gravitational ®eld of an intermediate galaxy, was spotted by the British astronomer Dennis Walsh and his colleagues. Subsequent discoveries of several more examples of gravitational lenses eliminated all doubts about gravitational focusing predicted by general relativity. Von Eshleman of Stanford University then went on to apply the theory to the case of the Sun in 1979 . His paper for the ®rst time suggested the possibility of sending a spacecraft to 550 AU from the Sun to exploit the enormous magni®cations provided by the gravitational lens of the Sun, particularly at microwave frequencies, such as the hydrogen line at 1,420 MHz (21 cm wavelength).