Among the curious features of a gravitational lens is the way it focuses electromagnetic waves. Supposing we could build a spacecraft like Claudio Maccone’s FOCAL concept, a vehicle designed to reach the Sun’s gravity focus at 550 AU. From that vantage, the electromagnetic radiation from an object occulted by the Sun (i.e., on the other side of the Sun from the spacecraft), would be amplified by a factor of 108. Such amplification could be exceedingly useful for astronomy at all wavelengths, and even for SETI.
But note this key difference between a gravity focus and its optical counterpart: in an optical lens, the light diverges after the focus. Light focused by the Sun’s gravitational lens, however, stays fixed along the focal axis as you move to distances greater than 550 AU. Quoting Maccone: “It is true that one does not have to stop FOCAL at just 550 AU, because every point along the straight line trajectory beyond 550 AU still is a focal point.”
It was in the 1980s that Alenia Spazio, an Italian aerospace company based in Turin, began looking into a mission that would evolve through successive iterations into FOCAL. The mission was originally conceived as an inflatable radio telescope that was christened Quasat. In regular meetings that explored the concept in the following decade, the uses of inflatable technologies for a mission to the gravity focus were analyzed. Gregory Matloff, for example, studied an inflatable solar sail with reflective metallic layer facing the Sun. Matloff’s baseline sail would have had an area of 10,000 square metres and a total mass of less than 100 kg. Time to reach the gravity focus: about 60 years.
But another way to reduce travel time does exist, as suggested by Maccone. Imagine not one but two flybys of Jupiter. The first close pass slingshots the probe to a perihelion extremely near the Sun, where the sail is deployed for maximum acceleration. The probe then makes the second flyby of Jupiter and receives an even greater boost in velocity. Significantly shortening that 60 year flight time, and the possibility of continuing to make enhanced observations along the focal axis after 550 AU, makes the idea of a FOCAL mission all the more compelling.
It is here that the FOCAL concept recalls earlier NASA studies like the Jet Propulsion Laboratory’s Thousand Astronomical Units (TAU) mission, conceived in the 1980s and designed to double FOCAL’s distance from the Sun. In both concepts, the scientific payoff would be immense in the form of observations of the heliosphere and heliopause as well as the interstellar medium itself. However, FOCAL must be considered an evolutionary step forward from the TAU concept, with a potential for astronomical breakthroughs available to no other technology. It would be the first interstellar precursor mission designed as such, and a symbol of mankind’s commitment to galactic exploration.
Needless to say, FOCAL remains only a concept at this stage, but this is how missions are built, by the labors of scientists producing papers and advocating in the journals and at conferences the scientific payoff that will justify funding their concepts. We will be having a lot more to say about FOCAL. Meanwhile, see Claudio Maccone’s The Sun as a Gravitational Lens: Proposed Space Missions (Aurora, CO: IPI Press, 2002). The Matloff paper is “Solar Sailing for Radio Astronomy and SETI: An Extrasolar Mission to 550 AU,” Journal of the British Interplanetary Society Vol. 47, pp. 476-484 (1994).