Starshot and the Gravitational Lens

Although the idea of a mission to the Sun’s gravitational lens has been in Claudio Maccone’s thinking for a long time, it has never been linked with the financial resources of a concept study like Breakthrough Starshot. The Italian physicist led a conference on mission concepts in the early 1990s and submitted a proposal for an ESA mission in 1993. What’s striking to me is that throughout that time, Maccone has explored aspects of the mission he calls FOCAL that at one point seemed far too futuristic for our era. Could we, for example, do SETI with a FOCAL mission? Could we use it to enhance communications with an interstellar probe?

The answer to both is yes, but the problem was pushing a spacecraft out to 550 AU in the first place, a challenge involving flight times of many decades. Then the Breakthrough Starshot initiative emerged and suddenly Maccone found himself in Palo Alto talking about a well-funded study, one that looked to FOCAL to support interstellar probes both in terms of defining their target and enabling their data return. FOCAL was a bit less theoretical, and all those papers over the years now had ramifications in an ongoing mission design.

Image: The FOCAL mission as described by Claudio Maccone in his 2009 book Deep Space Flight and Communications (Springer).

Power of the Lens

Stanford’s Von Eshleman was probably the first to think about using the lensing properties of mass to do science at the lensing distance and beyond, though Frank Drake and others have pondered the possibilities of boosting reception at the hydrogen line (1420 MHz), the famous ‘waterhole’ for interstellar communications. But most readers will also be familiar with the astronomical studies that have been conducted using the lensing of distant objects. A galaxy located behind an intervening galaxy can reveal itself by the bending of its light, another way of saying that mass shapes spacetime — the light is still following the shortest possible route.

In a similar way, light from an object directly behind the Sun can be ‘bent’ by the Sun’s mass, converging at the gravitational focus some 550 AU out. This can lead to misconceptions, especially the idea that we have to get a spacecraft to a specific distance and then stop there to take advantage of the effect. Not so — there is no focal ‘point’ here but a focal line. As we move through and past 550 AU, we take advantage of the fact that the focal line extends to infinity. Coronal effects from the Sun are diminished as we continue to travel and we have the opportunity to make observations of the object on the other side of our star.

Image: Claudio Maccone in the hotel lobby the evening before the Breakthrough Discuss conference began. That’s Denise Herzing (Florida Atlantic University) on the left.

A working constellation of FOCAL spacecraft could be critical to the success of a fast flyby mission like Breakthrough Starshot. We want to know as much as possible about what is around Alpha Centauri before we send our first probes. An infrastructure that can push a small sail to 20 percent of lightspeed gets us to the gravitational lens within days. Each spacecraft it delivers can then makes continuous observations as it moves away from the Sun in the direction opposite the Alpha Centauri system.

Speaking before Maccone at the Breakthrough Discuss meeting, Slava Turyshev (Caltech) pointed out that the gain for optical radiation through a FOCAL mission is 1011, a gain that oscillates but increases as you go further from the lens. This gives us the opportunity to consider multi-pixel imaging of exoplanets before we ever send missions to them. Lou Friedman, whose sail experience at JPL involved a study of a possible sail mission to Halley’s Comet, spoke of a FOCAL mission as ‘an interstellar precursor for Starshot or other destinations beyond the Solar System. Right now we are brainstorming,” he added. “We are studying spacecraft requirements to fly within the ‘Einstein ring’ and do the necessary maneuvering.”

I mentioned Von Eshleman above — he was the first to suggest using the gravitational lens for communications purposes in a 1979 paper, and as Slava Turyshev noted, this was where the practical application of General Relativity for space missions was truly born. But it has been Claudio Maccone who developed these ideas in a series of recent papers, noting that laser communications are deeply compromised at interstellar distances because of pointing accuracy problems and the need for power levels far beyond what we might expect from a StarChip.

Building Bridges Between the Stars

Is the gravitational lens, then, what Maccone likes to call a ‘radio bridge’? Bit Error Rate (BER) charts the possibilities. It’s the number of erroneous bits received divided by the total number of bits transmitted. A probe in Alpha Centauri space trying to communicate with a NASA Deep Space Network antenna — using parameters Maccone developed for a mission payload much larger than Starshot — suffers a 50 percent probability of errors (see The Gravitational Lens and Communications). But a FOCAL probe exploiting the gravitational lens picks up the signal without error. In fact, we don’t start seeing errors until we’re fully nine light years out.

I don’t have Maccone’s slides from the Palo Alto presentation, but the figure below comes from one of his papers, and it illustrates the same point.

Image: The Bit Error Rate (BER) (upper, blue curve) tends immediately to the 50% value (BER = 0.5) even at moderate distances from the Sun (0 to 0.1 light years) for a 40 watt transmission from a DSN antenna that is a DIRECT transmission, i.e. without using the Sun’s Magnifying Lens. On the contrary (lower red curve) the BER keeps staying at zero value (perfect communications!) if the FOCAL space mission is made, so as the Sun’s magnifying action is made to work. Credit: Claudio Maccone.

But as Maccone told the crowd at Stanford, we do much better still if we set up a bridge with not one but two FOCAL missions. Put one at the gravitational lens of the Sun, the other at the lens of the other star. At this point, things get wild. The minimum transmitted power drops to less than 10-4 watts. You’re reading that right — one-tenth of a milliwatt is enough to create error-free communications between the Sun and Alpha Centauri through two FOCAL antennas. Maccone’s paper assumes two 12-meter FOCAL antennas. StarShot envisions using its somewhat smaller sail as the antenna, a goal given impetus by these numbers.

Now we can start thinking about a galactic communications network. If we can start building out these bridges, we may well be latecomers in the activity. Maccone puts it this way:

The galaxy is a bonanza of stars that can be used as gravitational lenses. There may be civilizations that discovered that fact long ago. Perhaps we are the newcomers. The conclusion is that more advanced civilizations than we might have established sets of radio bridges between stars, a network of radio bridges, a ‘galactic internet.’ If this is true, then the conclusion is that as long as humanity is not capable of reaching the minimal focal distance of our own star, we will remain cut off from rest of galaxy in the sense of SETI.

The conclusion for StarShot: The first FOCAL spacecraft is sent out beyond 550 AU to the region in the sky precisely opposite to Alpha Centauri. This craft acts as our relay satellite, enabling communications between the Earth and any probe reaching our nearest neighbor. The second FOCAL mission is now sent to Alpha Centauri to create the radio bridge. All exploratory missions to come then have robust communications without the need for huge power resources aboard the spacecraft. The gravitational focus is thus our first target.

As Blakesley Burkhart (Harvard-Smithsonian Center for Astrophysics) noted in a follow-up panel, a mission to the gravitational lens contradicts a lot of things astronomers have been taught since their earliest days; specifically, the first thing you learn to do with a telescope is not to point it toward the Sun. FOCAL demands that we do just that, but the rewards are immense, not just in terms of exoplanet imaging and telecommunications, but also in discoveries we can’t anticipate, perhaps involving the Cosmic Microwave Background, itself a wonderful FOCAL target because being isotropic, it removes the need for exquisitely precise targeting.

A Voyager-class spacecraft, said Cornell University’s Zac Manchester, would take 150 years to reach 550 AU, while the Innovative Interstellar Explorer concept, developed in 2003, would reach the gravitational focus in about fifty years, using multiple Jupiter flybys. StarShot’s goal is to move fast enough to reach the lensing area in just a couple of weeks. Manchester noted the need for multiple spacecraft to sample the huge lensed image pixel by pixel. Think in terms of a spacecraft ‘array’ more than one or two craft. Just how we do this in the StarShot framework is something that research teams will be studying for some time to come, given the gradual realization that if you want to do interstellar, you’d better look at FOCAL first.

We’ll also have to take into account Geoffrey Landis’ findings in a paper just now becoming available on the arXiv site. It’s “Mission to the Gravitational Focus of the Sun: A Critical Analysis” (preprint), which looks at problems at realizing the FOCAL concept and in particular at acquiring a workable image. Claudio Maccone’s paper on radio bridges is “Interstellar Radio Links Enhanced by Exploiting the Sun as a Gravitational Lens,” Acta Astronautica Vol. 68, Issues 1-2 (January-February 2011), pp. 76-84 (abstract).