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JPL wants to fire a laser at MARS!

No! Mars is peaceful. We have no weapons. You can't possibly...

Four patterns available from the laser: single point, line, cross and dots

A group of JPL scientists, working on improving space-scale laser ranging, believe they could one day measure the distance from Earth to Mars with millimetre accuracy.

If their scheme were adopted, it would have to wait until a new Mars mission takes place to put a laser on the red planet. That's because, unlike Earth-Moon laser ranging, which uses reflectors on the Moon, the new design calls for lasers at both ends of the measurement.

The problem with reaching beyond Earth-Moon distances, the researchers explain, is that the signal falls off by the radius to the power of four – R4 – which puts Mars out of reach. An active scheme only deteriorates by R2, which extends distances “thousands of times”, according to their paper published in Applied Physics Letters.

At the moment, ranging for distances as great as Mars use radiofrequency signals. “Sub-millimetre accuracy has been achieved in real-time active laser ranging for interplanetary distances, providing precision improvement well above three orders of magnitude over the current RF techniques”, the researchers write.

JPL scientists Yijiang Chen, Kevin M. Birnbaum, and Hamid Hemmati say the system can be scaled up by simply increasing the size of the laser telescopes. For an Earth-Mars measurement – or even Earth-Jupiter – a 1 metre instrument on Earth and a 15 cm instrument on a spacecraft should be enough.

The scheme is simple enough: the Earth-bound laser sends a time-tagged pulse to the planetary transceiver, which responds by triggering its time-tagged reply.

“The lasers themselves do not need to be very powerful,” Birnbaum told “Commercially available lasers have enough pulse energy, and the light intensity as it leaves the transmitter can be low enough that it is even eye-safe. The key is to have a very sensitive receiver and a method to pick out the 'signal' photons from all the background light.”

Accurate synchronisation, and short pulses with a low repetition rate, help overcome this, and in test between two labs on Earth, the researchers achieved 0.14 mm accuracy, far better than their target of 1 mm. Next on their list is to demonstrate the laser ranging between the ground and an aircraft, and then with a spacecraft.

More accurate positional measurement could help inform questions such as the atmosphere, interior composition, oceans and ring material of other planets, as well as aiding long-distance experiments looking at tests of gravity. ®

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