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When spacecraft return to Earth, one of the tensest parts of the mission is the radio black out that occurs as the vehicle re-enters the atmosphere. Travelling at hypersonic speeds of between Mach 8 and 15, the spacecraft heats and breaks down molecules in the atmosphere causing a plasma to form. It is this plasma sheath that prevents radio communication.

For manned spacecraft re-entering from orbit, the radio black out last a couple of minutes, creating sweaty palms for all concerned.

Over the years, numerous groups have studied various ways around this problem. One idea is to use low frequency signals that are not blocked by the plasma. However, these provide only low data rates.

Another is to shape the craft so that the plasma does not form in certain areas where a radio antenna can be placed. But this means the entire vehicle has to be designed around the communications system, which then cannot be changed.

Yet another idea is place the radio antenna in the nose spike so that it sticks out beyond the plasma. This allows radio communication until the antenna wears away due to ablation.

None of these solutions is ideal. But today, Aleksandr Korotkevich at the Landau Institute for Theoretical Physics in Moscow and a few buddies reveal a new way to get around this problem.

Their idea is to use the properties of the plasma itself to effect transmission “in the same way a judo expert uses the strength and motion of an opponent to defeat him.” This seems a simple and elegant approach.

First some background. Plasmas absorb electromagnetic waves close to a special resonant frequency called the plasma frequency, which depends on the properties of the plasma itself such as its density.

Korotkevich and co point out that any incoming signal close to this frequency is both reflected and absorbed by the plasma. The reflected signal is lost but the absorbed energy sets up a resonating electric field at a certain depth with the plasma.

That’s a crucial point. In effect, this layer within the plasma is acting like a radio antenna, receiving the signal. However, the signal cannot travel further through the plasma to the spacecraft.

The new idea is to zap this layer with radio waves generated from within the spacecraft. These waves will be both absorbed by the plasma and reflected back inside the spacecraft. However, the key point is that the reflected waves ought to be modulated by any changes in the electric field within the plasma.

In other words, the reflected waves should carry a kind of imprint of the original external radio signal. That would allow ground control to communicate with their astronauts.

Korotkevich and pals say the same idea can be used in reverse to transmit signals. In this case, simply blast the plasma sheath from inside the spacecraft with the signal they want to transmit. This transfers the broadcast signal to the plasma’s internal electric field, which then radiates a much weaker signal into the atmosphere.

Korotkevich and co say the weakness of the transmission signal doesn’t matter because ground-based receivers can be made hugely sensitive, certainly much more so than mobile ones.

That’s a clever idea. In effect, they are turning the plasma sheath into a giant antenna that both receives and transmits messages. And they say that it can be achieved with standard transmitters that are available more or less off-the shelf today.

There are some caveats, of course. Wile the study they publish today is interesting, it considers only an idealised case and numerous extra details will need to be taken into account to get a prototype working.

One question, for example, is whether the radio signals will change the aerodynamic behaviour of the plasma, creating instabilities that could endanger the craft.

Korotkevich and co say not, because the radio signals vary much more quickly than any plasma instability. In effect, the plasma can be thought of as frozen at radio frequencies, they say.

However, this does not rule out nonlinear effects that may allow small instabilities to grow rapidly. That’s something that will have to be investigated in more detail.

Of course, the big interest is likely to come from the military. While the radio black-out is little more than an irritant for human missions, it is a serious limitation for military craft such as ballistic missiles or hypersonic planes. Radio black out prevents these vehicles from accessing GPS signals for navigation and does not allow them to be re-targeted or disarmed at the last minute.

If Korotkevich and co have found a practical way to solve this problem, their ideas are likely to be hugely valuable in certain quarters.

Ref: arxiv.org/abs/0704.3103: Communication Through Plasma Sheaths

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