Alternative data transfer channels for inter-stellar probes?

Question

The problem how it is possible to receive data from Voyager I at all is dealt in this question: https://physics.stackexchange.com/questions/13227/how-earth-communicates-with-voyager-i

However, there are still problems if we want to go really far away and still be able to send scientific data back to Earth:

1. The receivers on Earth must go really big
2. With more and more precise directional antennas, more and more precision is required to position them, otherwise the signal will simply miss the target
3. Transmission bitrate for Voyager I is quite low, too low to be able to send photos or other big volume data

Are there any alternatives currently developed or conceptualized by space agencies? I can think of 2:

1. Building intermediate receiver stations, which is also problematic, because it would be very hard to keep them in constant distance to probe and Earth
2. Sending back data on something like flash card, every few years, but we have a limited number of such transmissions, we need much power to launch them in the direction of Earth, and they could be quite hard to intercept.

Answer 1

First of all, to solve the receiver strength issue, it is merely a matter of using higher frequencies. The reason this works it it allows for a smaller dish to have a high gain, which allows for more efficient data processing. The main reason the Mars Reconnaissance Orbiter is able to process so much more data than its predecessors is that it uses X band, at 8 GHz. Technology is allowing for higher datarates from spacecraft, from algorithms, frequency, and pointing accuracy, it's all looking better. MRO, if magically placed where the Voyager probes are, would have significantly more data capacity than the Voyager probes have. The key has to be to use more directional pointing to get the dish pointed right at Earth.

It is worth noting that higher frequencies continues to work, and it is frequently proposed that really long distance missions use either lasers, or even higher frequencies, like X-Rays or Gamma Rays. These of course require increasingly accurate pointing, but could theoretically be done.

Alternatively, the pulses could be short in time duration, and achieve a similar affect. This would work best for a beacon of source. We could place one of these beacons to let the satellite know where to point the antenna, and it should be able to find a very precise location. See Optical SETI projects. Still, it is a challenge, but it is one that is getting more solvable every day.

Answer 2

David G. Messerschmitt has been looking at this issue. He argues that "energy consumption should take priority in interstellar communication, as distinct from most terrestrial systems that primarily conserve scarce spectrum." His analysis suggests "transmit signals should have wide bandwidth and consist of energy concentrated sparsely in both time and frequency."

From the abstract to "Power Efficiency in Interstellar Communication":

A major obstacle to communicating with other civilizations at interstellar distances at radio wavelengths is the lack of coordination in transmitter/receiver design. We propose to deal with this by optimization with respect to relevant resource consumption in light of the observable interstellar impairments, which include interstellar propagation effects (noise, plasma dispersion, and scattering) and motion effects. In communication there are two primary resources, the transmitter’s energy requirement for radiated power and the signal bandwidth, and there is a direct tradeoff between the two.

In view of the large distances and the large microwave window available, we argue that energy consumption should take priority in interstellar communication, as distinct from most terrestrial systems that primarily conserve scarce spectrum. The fundamental limit on energy consumption for interstellar communication is a wakeup call that the types of signals currently anticipated in SETI searches are inefficient by multiple orders of magnitude. We briefly review a set of five principles of transmit signal design that collectively can asymptotically approach that fundamental limit. These principles teach us that transmit signals should have wide bandwidth and consist of energy concentrated sparsely in both time and frequency. Although signals with these characteristics will not be discovered by current SETI search methodologies, we review the discovery challenge and discuss how current searches can be modified to seek these energy-conserving signals. Information-free beacons as well as information-bearing signals can be sought simultaneously.

From his 2013 paper, "End-to-end interstellar communication system design for power efficiency":

Abstract
Radio communication over interstellar distances is studied, accounting for noise, dispersion, scattering and motion. Large transmitted powers suggest maximizing power efficiency (ratio of information rate to average signal power) as opposed to restricting bandwidth. The fundamental limit to reliable communication is determined, and is not affected by carrier frequency, dispersion, scattering, or motion. The available efficiency is limited by noise alone, and the available information rate is limited by noise and available average power. A set of five design principles (well within our own technological capability) can asymptotically approach the fundamental limit; no other civilization can achieve greater efficiency. Bandwidth can be expanded in a way that avoids invoking impairment by dispersion or scattering. The resulting power-efficient signals have characteristics very different from current SETI targets, with wide bandwidth relative to the information rate and a sparse distribution of energy in both time and frequency. Information-free beacons achieving the lowest average power consistent with a given receiver observation time are studied. They need not have wide bandwidth, but do distribute energy more sparsely in time as average power is reduced. The discovery of both beacons and information-bearing signals is analyzed, and most closely resembles approaches that have been employed in optical SETI. No processing is needed to account for impairments other than noise. A direct statistical tradeoff between a larger number of observations and a lower average power (including due to lower information rate) is established. The ”false alarms” in current searches are characteristic signatures of these signals. Joint searches for beacons and information-bearing signals require straightforward modifications to current SETI pattern recognition approaches.

There is also video on YouTube of Prof. Messerschmitt's presentation to the Starship Congress this summer:

Answer 3

Sending a memory card (or anything else) back to Earth would add a huge amount of weight to the probe, because of the engine and all the fuel you need to decelerate the memory card to 0, then accelerate it to a decent speed in the direction of Earth. Also, you'd need a heat shield, a radio transmitter so you can find it once back at Earth, which in turn means a power system, etc. etc. This would make the mission hugely more expensive. For the same reason, we've had quite a few spacecraft land on Mars but no return missions yet.

Answer 4

It may be worth noting that physically sending back 'memory cards' (or in this case, film canisters) has been done by the recently declassified KH-9 Hexagon spy satellites, each of which was equipped with 4 to 5 re-entry capsules that were packed with film for recovery. These satellites operated in Low Earth Orbit for only a few months each, so the Delta-V and return vehicle complexity requirements are significantly lower than returning a capsule from a solar escape trajectory.

In addition to the points Hobbes raised, you would also need to consider:

• The return trajectory design of such a vehicle.
• The risks associated with a physical return - remember the Genesis crash? Both Hexagon and Genesis planned mid-air retrievals (using a modified C-130 and a helicopter respectively) of reentry capsules. A long term trickle of data may be preferable to a high-risk, piecemeal return of greater volumes.
• Given that mass constrains are already very tight, it seems unlikely that mass would be spent on a return system given that any such mission would require a capable transmitter/receiver in any case.

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Sources:

Wikipedia: KH-9 Hexagon

Working in the shadows: Phil Pressel and the Hexagon spy camera

Answer 5

You mentioned Voyager. This is actually an example of an alternative data transfer strategy. Voyager was built for flyby missions: it whizzed by a planet at high speed, gathering science data at a much higher rate than could be transmitted. This data was stored onboard and sent to Earth later, during the long, boring cruise to the next planet.
New Horizons will do the same during the Pluto flyby. In fact, it can't transmit during the flyby because it has to point its science instruments at Pluto, which points the radio antenna away from Earth.

Interstellar probes could follow a similar strategy, although it is hard to imagine a probe that has a radio transmitter capable of sending signals over a distance measured in lightyears. The power required boggles the mind.

Answer 6

1. The interstellar mission could consist of a continuous convoy of ships which form a chain of communication. If the interstellar ships receive beamed power from Earth, the ships in such a convoy might also be used to refocus that power beam to the next ship.

2. The gravitational focus of the Sun could be used to lens the radio signals. The gain would be stupendous. We'd probably like to do it anyway in order to telescopically investigate the interstellar destination in detail, and measuring the amount of dust along the way, before launching any voyage there. The focus is a line, not a dot, so the radio focusing communication spacecraft never needs to decelerate. Geometrically it starts 550 AU from the Sun, but because of the corona one might have to go 700 to 1000 AU away (in the opposite direction from the destination) before it works practically. That is nearby compared to the nearest star 270,000 AU away.