Project Icarus: The Interstellar Communication Problem : Discovery News
Project Icarus is an ambitious five-year study into launching an unmanned spacecraft to an interstellar destination. Initiated by the Tau Zero Foundation and British Interplanetary Society, and managed by Icarus Interstellar Inc., a non-profit group of scientists dedicated to interstellar spaceflight, Icarus is working to develop a spacecraft that can travel to a nearby star.
Pat Galea, a software and communications engineer from the United Kingdom, serves as Secretary for Icarus Interstellar.
When we think about the engineering required for an interstellar probe, it's easy to get absorbed in the obvious -- and admittedly very exciting -- challenges involving the propulsion system.
However, we also need to keep in mind that unless we can get scientific data from the probe back to Earth then there is very little point in getting the probe to another star at all. The data are the product that we have paid for; all the other engineering can be seen as the means to that end.
In this article I'll be looking at the communications and telemetry investigations that we have been undertaking for Project Icarus.
Up to now, all spacecraft have communicated with ground control using radio frequency electromagnetic waves, or "radio" for short. The huge advantage this technology has is that it has been around for a long time, so we understand a lot about how to build the electronics and antennas required for long distance transmission.
One of the problems with radio over long distances is that it's hard to generate a radio signal that produces a directional beam that is tightly focused on the target.
The degree to which an antenna directs the energy it produces in one direction is known as the 'gain' of the antenna. The higher the gain, the more the energy is tightly focused in a particular direction, and generally speaking an antenna that has a certain gain for transmission will also have the same gain for reception too.
There are lots of ways to achieve high gain, but the two that are most familiar to people are rooftop TV antennas which use 'director' elements to increase the gain in the direction of the TV transmitter, and parabolic satellite TV dishes which use a dish reflector to focus the energy coming from a particular direction (the satellite) onto a receiver element mounted in front of the dish.
For interstellar distances, we want a very large gain antenna on the craft to ensure that as much energy as possible makes it back to Earth, so that we can (a) maximize the amount of data that we can send per second (the bandwidth), and (b) reduce the transmitter power requirements on the craft.
Big Transmitters, Giant Receivers
The Project Daedalus team in the 1970s came up with an ingenious solution to the problem. Rather than deploy a dedicated antenna dish, which would need to be huge in order to achieve the required gain, they took advantage of a piece of equipment they already had on board that was roughly the size and shape required: the second stage reaction chamber.
This is the forty-meter diameter dome at the back of the second stage of the craft that forms that largest component of the engine. Ideally this dome should be a spherical section, but the Daedalus team realized that if it could be made parabolic then it could serve as an antenna once the engine was no longer needed for acceleration.
So, once the boost phase of the mission is complete (within a few years of launch), the engine is decommissioned. A transmitter is then inserted into a hole at the center of the dish (the hole previously being the place from which fuel pellets were ejected -- pictured right). By aiming the craft at Earth, the dish provides a very high gain antenna for communications with home.
What kind of receiver is needed to pick up the data? Daedalus was designed with a one megawatt (MW) transmitter, but even combining this with the huge gain of the forty-meter dish, the amount of power actually received by the time the signal gets to Earth is very small. The team realized that a substantial receiver antenna would be required.
The solution they came up with was to use a concept from the Search for ExtraTerrestrial Intelligence (SETI). Project Cyclops had, a few years earlier, produced a design for an array of antennas on Earth which would cover a circle with a ten kilometer diameter. The purpose of this was to detect very weak transmissions that might be coming from distant civilizations. Daedalus proposed building such an array for picking up the weak signal from the distant craft.
The Daedalus team's calculations showed that with such a system we could expect to receive over 800 kilobits per second (kbps). That means that Daedalus could transmit a 1 megabyte (MB) file to Earth in about 10 seconds. That would be a disappointing broadband connection, but for interstellar distances it is quite impressive.
We do not yet know whether we'll be using radio for the primary link from Icarus to Earth. Our preliminary calculations suggest that the Daedalus figures may be a little on the optimistic side. However, since the 1970s some interesting advances (such as Turbo Codes) have been made in techniques that allow us to squeeze more data down a narrow comms pipe.
We haven't yet established whether Icarus will have a large dish available for use as an antenna. As Icarus has a requirement to decelerate at the destination star system (a requirement that Daedalus did not have) it may be the case that we cannot rely on having this resource available to us.
One of our students, Brandon Vernon from Arizona, has been looking at the possibility of using a Fresnel Zone Antenna. This is a large flat plate which has concentric metal and transparent rings which are carefully designed to refract the radio waves of a transmitter and focus them in a particular direction. Vernon has produced a computer program containing a mathematical model of the system to see whether this idea can work out in practice.
It looks like it's going to be too inefficient for our purposes right now, but just having done the research is a powerful contribution to our understanding of the options available to us.
The size of antenna required in order to keep the dispersion of the beam to the required angle (the 'beamwidth') is related to the wavelength of the electromagnetic radiation used. The shorter the wavelength (and hence the higher the frequency), the smaller the antenna required for a given beamwidth. If we could use light for the communications system rather than radio, then we ought to be able to get away with smaller antennas.
Light is physically the same 'stuff' as radio; it's just electromagnetic radiation with a much much shorter wavelength. From an engineering point of view, however, the way we create, manipulate and receive light involves very different components. Instead of radio antennas, we use mirrors and lenses.
Although laser communication is not yet commonly used on spacecraft, it has been tested in experiments, and the physics is well understood. Generally speaking, the transmission of data via a laser beam involves switching the laser on and off very rapidly, using a receiver at the far end to detect the flashing light, and decoding the data that is carried in the pattern of flashes.
The biggest challenge for laser communications comes from its biggest benefit. The narrow beamwidth means that we get a lot of the power focused in a relatively narrow beam (narrow relative to the beamwidth for radio, that is). The drawback of this is that we have to point the laser very accurately at the receiver in order to ensure that a reasonable amount of power is being received.
While a radio antenna does have to be pointed accurately, the requirement for the laser is much more severe. The problem is known as Acquisition, Tracking and Pointing (ATP).
In the 1970s, the Daedalus team concluded that lasers were not up to the job of communicating across interstellar distances. The primary reason for this was that they could not see a way to point the laser accurately enough at Earth from several light-years distance, and maintain that direction.
However, in the intervening years, new techniques have been developed that solve many of the problems of ATP. For example, if a ground station directs a laser beam to the craft, then the craft can use this incoming beam to direct its own laser transmitter. Furthermore, the aiming of the laser beam does not require the accurate positioning of the entire laser itself. Sophisticated optics inserted into the path of the light beam can be manipulated carefully to direct the beam onto the required path.
Now the problem for Icarus is that it's not clear whether transmitting an uplink laser beam from Earth will actually help the craft to aim its own laser. The distances involved are so large that the errors may swamp the benefits.
So we are looking into this, as well as other methods such as determining position and direction from analysis of pulsars. Initial calculations on the raw characteristics of laser communications, conducted with the assistance of our student designer Divya Shankar in India, show that the technology holds great promise for delivering respectable amounts of data across large distances.
Location, Location, Location
The Daedalus team, as mentioned above, envisaged the receiving station as being situated on the surface of Earth. However, they also considered off-Earth locations as potential bases for a Solar System Receiving Station (SSRS), as they called it.
The advantage of basing the SSRS on Earth is that it is much easier to make a very large array covering a huge area. Maintenance is relatively simple, as technicians can drive out to the antennas in trucks. The cost of construction is lower too, because shipping massive components around on Earth is much cheaper than launching them into space.
Earth has a couple of disadvantages as a base. First, the atmosphere degrades (or 'attenuates') the received signal to some extent. (The amount of degradation depends on the wavelength of the signal.) Also, the rotation of the Earth means that any antenna on the surface will be pointing at the distant craft for only a fraction of each day. Weather events can further impact the availability of the receiver.
These factors can be offset to some extent. The atmospheric attenuation of the signal can be compensated by making the antenna array bigger than would otherwise be required. The fact that Earth-based construction is cheaper than space construction may make this cost-effective. By having multiple copies of the antenna array in different locations on Earth, weather effects can be mitigated, and a near-continuous link with the craft can be maintained. This might not be cost-effective for one mission, but if the network is being used to cover several missions (perhaps interplanetary craft as well) then it may begin to make economic sense.
Alternatively, the receiving station could be built in space.
Given the parameters of the problem for Icarus, the use of laser communications would result in a requirement for smaller receiver components than for radio. The economics therefore favor laser comms if we have a space-based receiver.
Possible space locations for the receiver include Earth orbit, the surface of the moon (though this too would suffer from the periodic availability problem due to the rotation of the moon), and the Lagrange points (volumes of space where the gravity of the Earth, Moon and Sun are balanced). We are investigating all of these options to see which makes the most sense for Icarus.
Relaying the Message
Given the challenges of transmitting a signal over the long distances of interstellar space, is there another way to tackle the problem by dropping relays along the way? Andreas Tziolas has looked at the use of expended fuel tanks as relays (an artist's impression is shown, right -- click for animation of the expended fuel tanks in action).
The idea is that with a chain of relays between Icarus and Earth, each 'hop' of the signal is a much shorter distance than the whole distance of several light years. So we could, potentially, reduce the transmitter power requirement, or the antenna size on Icarus, or alternatively, increase the data rate that can be sent over the link.
Unfortunately, when we built our mathematical model for a relay system using radio frequencies, it turned out that the number of relays required just to match the performance of the direct link is immense, even given very conservative assumptions about the antenna size and power available to each relay.
At this stage it appears that relays will not help the communications problem. However, that doesn't mean the relays will be abandoned as a concept. They may still have value for other purposes, such as scientific investigations, and there is a possibility that relays may be more economical for laser communications.
Einstein's General Theory of Relativity tells us that massive objects deflect light that is passing near them. For a really massive object like the Sun, the deflection of light is large enough that the Sun itself can be used as a gravitational lens. How does this help the communications problem?
From Einstein's equations, we know that the minimum focal distance from the Sun is about 550 AU. (1 Astronomical Unit -- 1 AU -- is the distance from the Earth to the Sun.) When we consider that the Voyager 1 craft which left Earth in the 1970s is currently only about 120 AU from the Sun, we begin to appreciate that this 550 AU is actually quite a significant distance (though nowhere near the distance to Alpha Centauri, which is about 270,000 AU away).near If we send a receiver craft out to 550 AU from the Sun in the exact opposite direction to Icarus, then the receiver will be able to use the Sun as a huge lens -- effectively a huge antenna -- to boost the signal from Icarus.
The craft can then retransmit the data on to Earth. The potential gain from doing this is immense. The transmitter power on Icarus could be ramped down to much lower levels without impacting the available data rate, or if the power is kept the same, we could be receiving much more data than a direct link would provide.
(It is important to note that gravitational lensing is slightly different to the optical lensing that we are familiar with. In an optical lens, parallel rays are brought to a single focus. That is, we can talk about the focal distance of a lens. For a gravitational lens, we just have a minimum focal distance. Once the craft gets to 550 AU, it can keep going, as the gravitational lens will still be working. We don't have to bring the craft to a halt.)
Now there are a couple of significant challenges to getting this system to work.
First, we have to get a receiver craft out to 550 AU from the Sun. Currently, that's a tough problem, but the kind of civilization that can launch an Icarus-like interstellar probe will probably not find this too difficult.
Second, we have to hold the receiver craft exactly in line with the center of the Sun and the distant Icarus probe. The tolerance here is severe. The receiver cannot drift by more than a few meters off-track before it loses the signal completely.
Keeping the receiver craft on track is very challenging. At the International Astronautical Congress (IAC) in Prague, 2010, we presented a paper on a system we have devised that might be able to achieve the navigation control required. This is a plausibility argument rather than a suggestion for a working system, as we wanted to see whether such a concept was workable.
The idea we came up with was to launch a system of navigational satellites (Navsats) in orbit around the Sun. The distant Icarus probe would emit extremely bright flashes periodically which would travel across interstellar space and be detected by these Navsats. By detecting the tiny differences in timing of the pulse arrivals at the Navsats, the system as a whole could direct the receiver craft at the gravitational focus to maintain its lock on the signal from Icarus. Our calculations showed that with reasonable assumptions the Navsats would need to be in an orbit of radius approximately 10 AU. That's about the same as the orbit of Saturn. So from this perspective, the system seems to be plausible. The problem we have not tackled yet is the thrust requirements of the receiver craft. In order to maintain its lock on the signal, the craft at the focus will need to make adjustments to its position over time. If the distant Icarus probe is in orbit around the target star, then the receiver craft will need to be making a large amount of adjustment for a prolonged period of time. It is not yet clear whether this aspect of the system can be solved in the near term. For now, we are keeping gravitational lensing as an interesting option that could potentially be tested using Icarus, but we are not going to specify it as the primary communications mechanism. Optimism While the physics of interstellar communications is well understood, the engineering challenges are tough. We are optimistic, however, because even in the 1970s it was clear to the Daedalus team that a basic system could be built using the technology of the day: We do not need technological "breakthroughs" in this area; development of the system could begin now. Today, this statement still holds good. We could, indeed, begin developing this system right now.