Boosting interstellar communication to a laser pushed sail probe to multiple megabits per second

Michael Hippke has calculated how to have multi-megabyte communication speeds between a laser pushed sail probe and a telescope at our solar systems gravitational lensing point. Previously Hippke had determined how to use stellar photon pressures of the stellar triple I Cen A, B, and C (Proxima) together with gravity assists to decelerate incoming solar sails from Earth.

Hippke works out the problems of interstellar communication and communication using the gravitational lensing points in more detail.

There have been past work on interstellar communication using gravitational lens for improved performance as reported here at Nextbigfuture.

There was a 238 page paper in 2013 (David G. Messerschmitt at Berkeley) , Project Icarus and work by Benford.

Recent technological advances could make interstellar travel possible, using ultra-lightweight sails pushed by lasers or solar photon pressure, at speeds of a few percent the speed of light. Obtaining remote observational data from such probes is not trivial because of their minimal instrumentation (gram scale) and large distances (pc). Michael Hippke derive the optimal communication scheme to maximize the data rate between a remote probe and home-base. The framework includes models for the loss of photons from diffraction, technological limitations, interstellar extinction, and atmospheric transmission. Major noise sources are atmospheric, zodiacal, stellar and instrumental. He examines the maximum capacity using the "Holevo bound" which gives an upper limit to the amount of information (bits) that can be encoded through a quantum state (photons), which is a few bits per photon for optimistic signal and noise levels. This allows for data rates of order bits per second per Watt from a transmitter of size 1 m at a distance of I Centauri (1.3 pc) to an earth-based large receiving telescope (E-ELT, 39 m). The optimal wavelength for this distance is 300 nm (space-based receiver) to 400 nm (earth-based) and increases with distance, due to extinction, to a maximum of a 3 m to the center of the galaxy at 8 kpc.

Hippke applies his framework of interstellar extinction and quantum state calculations for photon encoding to the solar gravitational lens (SGL), which enlarges the aperture (and thus the photon flux) of the receiving telescope by a factor of over 1 billion. For the first time, we show that the use of the SGL for communication purposes is possible. This was previously unclear because the Einstein ring is placed inside the solar coronal noise, and contributing factors are difficult to determine. He calculates point-spread functions, aperture sizes, heliocentric distance, and optimum communication frequency. The best wavelength for nearby (less than 100 pc) interstellar communication is limited by current technology to the UV and optical band. Data rates scale approximately linear with the SGL telescope size and with heliocentric distance. Achievable (receiving) data rates from Alpha Cen are 1-10 Mbits per second per Watt for a pair of meter-sized telescopes, an improvement of a million times compared to using the same receiving telescope without the SGL. A 1 m telescope in the SGL can receive data at rates comparable to a km-class "normal" telescope.

The strongest influence on data rate comes from z and dSGL, and both are nonlinear. While the probe can achieve a data rate of 4.9 Mbits/s at z = 600 au, the rate increases to 10.3 Mbits/s at z = 600 au, rougly a factor of two. An increase in the probe's aperture (1 meter to 10 meter) increases the data rate to 44.7 Mbits/sec (at z = 600 au), roughly a factor of ten.

Hippke has shown, for the first time, that the gravitational lens of our sun can be effectively used for interstellar communication. This had previously been unclear due to the unknown impact of the coronal noise. We have calculated point-spread functions, aperture sizes, heliocentric distance, and optimum communication frequency of a receiving probe in the SGL.

Data rates are higher by a factor of a million compared to equal-sized classical telescopes. A 1 m telescope in the SGL can achieve the same receiving data rate as a classical 9-45 km telescope. If classical telescope sizes are restricted to E-ELT size (39 m), power levels on the transmitter side need to increase from 1W into the MW range to match the data rate of an SGL probe. If data rates at Mbits/s level are required, it might be cheaper to invest into space flight to the SGL. The obvious alternative is to limit data rates to a level achievable with smaller telescopes. A single 39 m telescopes may receive data of order bits per second per Watt from I Cen, sufficient to transmit several high resolution photographs over the course of a year (Hippke 2017). Perhaps this is judged to be sufficient.

In paper III of this series, we will relax technological constraints, mainly on the focusing of short wavelengths (Hippke 2017). This opens our horizon to more advanced civilizations, if they exist, and allows us to examine how they would maximize data rates. If advanced civilizations value data as much as we do, our framework will tell us how they communicate, where we can look for such communication, and how we could join the galactic network.

With today's technology, resolution in the milli-arcsec regime is possible at optical wavelengths, but X-rays are limited to angular resolutions of 20 arcsec, a difference of 4 orders of magnitude. For example, the Swift X-Ray satellite has an angular resolution of 18 arcsec at I = 1 nm (1.5 keV) from a 30 cm aperture, while the diffraction limit would be 1.22I/D = 8 i- 10^a'4 arcsec, so that Qreal/QR = 4 i- 10a'5. Technology is believed to eventually achieve sub-arcsec resolution at X-rays, but at the expense of large designs, with focal lengths of 100000 km.

A more traditional interstellar radio communication design from I Cen has recently been published by Milne et al. (2016). It presents scenarios for antennas with sizes of 1-15 km on both sides, transmitting MW power at 32 GHz, achieving a data rate of Gbits/s (10^9 bits/s). The antenna weight is mentioned as 40, 000 kg, and the total space-ship weight is 10 million kg. Clearly, if such masses and power can be sent to other stars, the question of communication will be trivial in comparison.