A Mission to the Gravity Focus

by Paul Gilster on October 26, 2005

Voyager 1 is, in a sense, our first interstellar spacecraft, with evidence mounting that it has reached the heliopause, that area marking the boundary between the Sun’s outward-flowing particles and the true interstellar medium. The New Horizons mission, scheduled for launch in January, will go on to explore at least part of the Kuiper Belt. But what will our first true interstellar mission be; i.e., when will we launch a spacecraft designed from top down to studying nearby interstellar space?

The answer may well be a mission to the Sun’s gravity focus. Located at 550 AU (3.17 light days), some 14 times farther from the Sun than Pluto, the focus is that point to which the Sun’s gravity bends the light from objects on the other side of it. The effect is to magnify distant images in ways that could be observed using the proper equipment. The effect of gravitational focus, first studied by Einstein in 1936, had already borne observational fruit by 1978 in the discovery of a ‘twin quasar’ image caused by the gravitational field of a galaxy between us and the distant object. Stanford’s Von Eshleman applied the same theory to the Sun in 1979, and suggested sending a spacecraft to 550 AU to take advantage of this magnifying effect at microwave wavelengths like that of the hydrogen line at 1420 MHz.

The SETI potential is obvious, for 1420 MHz (21 cm) is the famous ‘waterhole’ frequency thought by some to be likely for interstellar transmissions. Magnifying such signals holds the clear potential for aiding the search for other civilizations, as Frank Drake pointed out in 1987 at the Second International Bioastronomy Conference. But it was not until 1992 that space scientists turned serious attention to studying the gravity focus in terms of a mission. That took place at the Conference on Space Missions and Astrodynamics in Turin, Italy, led by Claudio Maccone. The proceedings were published in the Journal of the British Interplanetary Society. Maccone would go on to submit a formal proposal to the European Space Agency to fund design for a potential mission.

In the years since, it is Claudio Maccone, a space scientist based in Turin, who has emerged as the champion of the mission he originally called SETISAIL, and now calls FOCAL. His book The Sun as a Gravitational Lens: Proposed Space Missions (Aurora, CO: IPI Press, 2002) contains the key papers that define the mission and suggest its ramifications. Given the length and complexity of FOCAL, it became clear to Maccone early on that the mission could not retain a purely SETI-based focus. Instead, it should broaden its study to consider factors such as the computation of the parallaxes of distant stars and the detection of gravity waves, among other experiments.

On the subject of the gravitational lens itself, Maccone has this to say in his book:

As each civilization becomes more knowledgeable they will recognize, as we now have recognized, that each civilization has been given a single great gift: a lens of such power that no reasonable technology could ever duplicate or surpass its power. This lens is the civilization’s star, in our case, our sun. The gravity of each star acts to bend space, and thus the paths of any wave or particle, in the end creating an image just as familiar lenses do.

The potential of this lens for observation is staggering. Maccone again:

This lens can produce images which could take perhaps thousands of conventional telescopes to produce. It can produce images of the finest detail of distant stars and galaxies. Every civilization will discover this eventually, and surely will make the exploitation of such a lens a very high priority enterprise. One wonders how many such lenses are being used at this moment in time to scan the universe, capturing a flood of information about both the physical and the biological realities of our time.

Given a possible flight time of 50 years or more, will FOCAL ever be flown? Maccone relates the response of French professor Roger Bonnet, who at that time was Director of Scientific Programs for ESA. Bonnet told the physicist that had FOCAL been approved by the agency, it would have provided work not just for ESA’s current generation of employees, but for both their children and their grandchildren. Some would say this disqualifies the mission from serious consideration. Centauri Dreams begs to disagree. FOCAL and other interstellar concepts are a classic case of the need to rearrange our cultural priorities to think and plan for the long-term, to strive for accomplishments begun by one generation and maintained by its descendants. To think, in other words, beyond the limitations of a single human lifetime for the good of the species.

FOCAL thus becomes, in many respects, a way for Centauri Dreams to focus anew on these key issues. We will be examining a number of aspects of the gravitational lens mission in coming weeks in both a scientific and cultural context. Those interested in background material might want to begin with Eshleman’s “Gravitational Lens of the Sun: Its Potential for Observations and Communications over Interstellar Distances,” Science Vol. 205, (1979), pp. 1133-35.

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Luigi Semenzato October 9, 2008 at 16:42

Cute idea, but wouldn’t such telescope be awfully hard to aim? Also, for visible-light observations, how do you get around the problem that you’d be staring at a narrow ring quite close to the sun? This is a non-starter on earth, but maybe blocking the sunlight works better without atmospheric scattering. Finally, how is this “gravity focus” exactly defined? Photons from a distant object will be bent by an amount that varies with the distance from the sun’s surface, so the effect should be available at a range of distances. Perhaps 550 AU is the smallest such distance, for rays that skim the surface, but is it even the optimal distance?

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