The Gravitational Lens and Communications

If we can get the right kind of equipment to the Sun’s gravitational focus, remarkable astronomical observations should follow. We’ve looked at the possibilities of using this tremendous natural lens to get close-up images of nearby exoplanets and other targets, but in a paper delivered at the International Astronautical Congress in Daejeon, South Korea in October, Claudio Maccone took the lensing mission a step further. For in addition to imaging, we can also use the lens for communications.

The communications problem is thorny, and when I talked to JPL’s James Lesh about it in terms of a Centauri probe, he told me that a laser-based design he had worked up would require a three-meter telescope slightly larger than Hubble to serve as the transmitting aperture. Laser communications in such a setup are workable, but getting a payload-starved probe to incorporate a system this large would only add to our propulsion frustrations. The gravitational lens, on the other hand, could serve up a far more practical solution.

Keeping Bit Error Rate Low

Maccone goes to work on Bit Error Rate, a crucial measure of signal quality, in assessing the possibilities. Bit error rate charts the number of erroneous bits received divided by the total number of bits transmitted. Working out the numbers, Maccone posits a human probe in Centauri space trying to communicate with a typical NASA Deep Space Network antenna (70 meter dish), using a 12-meter antenna aboard the spacecraft (probably inflatable).

Using a link frequency in the Ka band (32 GHz), a bit rate of 32 kbps, and forty watts of transmitting power (and juggling the other parameters reasonably), the math is devastating: we get a 50 percent probability of errors. So much for data integrity as we operate within conventional systems.

But if we send Maccone’s FOCAL probe to the Sun’s gravitational lens at 550 AU, we now tap the tremendous magnification of the lens, which brings us a huge new gain. Using the same forty watts of power, we derive a completely acceptable bit rate. In fact, Maccone’s figures show that the bit error rate does not begin to become remotely problematic until we reach a distance of nine light years, when the increase in BER begins slowly increasing.

Image: Figure 4. 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.

Building a Radio Bridge

Now this is interesting stuff because it demonstrates that when we do achieve the ability to create a human presence around a nearby star, we will have ways to establish regular, reliable communications. A second FOCAL mission, one established at the gravitational lens of the target star, benefits us even more. We could, for instance, create a Sun-Alpha Centauri bridge. The bit error rate becomes less and less of a factor:

…the surprise is that… for the Sun-Alpha Cen direct radio bridge exploiting both the two gravitational lenses, this minimum transmitted power is incredibly… small! Actually it just equals less than 10-4 watts, i.e. one tenth of a milliwatt is enough to have perfect communication between the Sun and Alpha Cen through two 12-meter FOCAL spacecraft antennas.

This seems remarkable, but gravitational lenses make remarkable things possible. Recall that it was only months ago that the first tentative discovery of an extrasolar planet in the Andromeda galaxy (M31) was made, using gravitational lensing to make the observation.

Into the Galactic Bulge

Maccone goes on to work out the numbers for other interstellar scenarios, such as a similar bridge between the Sun and Barnard’s Star, the Sun and Sirius A, and the Sun and a Sun-like star in the galactic bulge. That third possibility takes us into into blue sky territory, but it’s a fascinating exercise. If somehow we could use the gravitational lens of the star in the galactic bulge as well as our own gravitational lens, we would have a workable bridge at power levels higher than 1000 watts.

Image: Bit Error Rate (BER) for the double-gravitational-lens of the radio bridge between the Sun and Alpha Cen A (orangish curve) plus the same curve for the radio bridge between the Sun and Barnard’s star (reddish curve, just as Barnard’s star is a reddish star) plus the same curve of the radio bridge between the Sun and Sirius A (blue curve, just as Sirius A is a big blue star). In addition, to the far right we now have the pink curve showing the BER for a radio bridge between the Sun and another Sun (identical in mass and size) located inside the Galactic Bulge at a distance of 26,000 light years. The radio bridge between these two Suns works and their two gravitational lenses works perfectly (i.e. BER = 0) if the transmitted power higher than about 1000 watt. Credit: Claudio Maccone.

I’m chuckling as I write this because Maccone concludes the paper by imagining a similar bridge between the Sun and a Sun-like star inside M31, using the gravitational lenses of both. We’re working here with a distance of 2.5 million light years, but a transmitted power of about 107 watts would do the trick. This paper is a dazzling dip into the possibilities the gravitational lens allows us if we can find ways to reach and exploit it.

The paper is Maccone, “Interstellar Radio Links Enhanced by Exploiting the Sun as a Gravitational Lens,” presented at the recent IAC. I’ll pass along publication information as soon as the paper appears.