NEW YORK: There's a new way now to maintain communication with spacecraft -- sometimes lost for several moments -- as they re-enter the atmosphere.

"The key lies in tweaking with the antenna," the researchers said.

Even routine communication blackouts with a returning spacecraft can create moments of anxiety as there is no way to know or control the location and state of the spacecraft from the ground.

"When a re-entry vehicle is unable to be connected, the only thing you can do is pray for it," said Xiaotian Gao, a physicist at the Harbin Institute of Technology in China.

Gao and his colleague Binhao Jiang have proposed a new way to maintain communication with spacecraft as they re-enter the atmosphere.

Communication blackouts with hypersonic vehicles occur because as the craft zips along at more than five times the speed of sound, an envelope of hot ionised air -- called a plasma sheath -- surrounds it.

This plasma sheath will reflect electromagnetic signals under most conditions, cutting off connection with anything outside of the vehicle.

However, under certain special conditions, a plasma sheath can actually enhance the radiation from a communication antenna.

Gao and his colleagues reasoned that it would be possible to replicate these special conditions in ordinary hypersonic flight by redesigning the antenna.

The researchers first analysed earlier experiments and found that the special signal enhancement could be explained by a resonance, or matched electromagnetic oscillations, between the plasma sheath and the surrounding air.

They propose adding a "matched layer" to ordinary communication antennas to create the desired resonant conditions during normal hypersonic flight.

The matched layer works because it acts as like a capacitor -- a type of electrical energy storage unit -- in the antenna circuit, Gao explained.

The plasma sheath, on the other hand, acts like an inductor, which resists changes in an electric current passing through it. When a capacitor and an inductor are paired together, they can form a resonant circuit.

"Once the resonance is reached, the energy can be exchanged between them steadily and without any loss, like real capacitance and inductance do in a circuit," Gao said.

"As a result, the electromagnetic radiation can propagate through the matched layer and the plasma sheath like they do not exist," he said.

For the resonance to work, the thickness of the matched layer and the plasma sheath must be smaller than the wavelength of the electromagnetic waves used to communicate, so the approach would be ineffective if the antenna frequency were too high, Gao said.

The approach might also be applied to other hypersonic vehicles such as futuristic military planes and ballistic missiles, researchers said.

The findings were published in the Journal of Applied Physics.

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