Newton never quite conceived of gravity on the lines of "What goes down must come up."

But nothing is quite the same when you are yanked free of Earth's tentacles and can float weightless, strange and free, as if you were aboard the International Space Station (ISS).

The craft that provides this astonishing sensation is Zero-G, a plane designed and built by Europe to simulate the gravity-free conditions of space.

It is a priceless laboratory-in-the-sky, providing training for future astronauts and a testbed for basic science and new technology, with the potential to reshape our lives.

The specially-adapted Airbus A300 flies parabolas, a specific track that gives between 20 and 22 seconds of weightlessness at the top of the curve before the plane pulls out and gravity abruptly resumes.

That may not seem much, but the plane flies 30 parabolas in the course of its two-hour flight, and repeats the mission each day for the next two days -- giving the equivalent of a 30-minute flight orbit.

"This is TERESA's final flight before we put her on the ISS for an experimental mission," says French robotics expert Pierre Vieyres, showing off the invention of that name -- a remote-controlled ultrasound scanner.

A volunteer, the "patient," lies on the aircraft's floor while a colleague places the tiny robot probe, housed in a light metal frame, on his abdomen.

The "doctor" in the experiment looks at a screen nearby, which gives an ultra-sound picture of the internal organs, and toggles a joystick, directing the robot probe towards the area of interest.

"There are fantastic opportunities for this device," says Vieyres.

"It could be used in long-term space flight to help doctors on Earth diagnose a problem if one of the crew falls sick. And of course it could be used in remote places on Earth, too, among people who need expertise that is far away. We plan to test it at a base in Antarctica."

Gravity is one of the fundamental forces in the Universe, an invisible hand that sculpts life, tests manmade materials to destruction, gives birth to planets and sounds the death knell of stars.

It holds out a special challenge for humans if we are ever to leave our home. Long missions aboard the Soviet-Russian space station Mir showed some of the damaging effects of weightless on bone density, muscle tone and the cardiac system.

"If humans are ever to fly to Mars, we have to understand more about this," said Andre Aubert, of Belgium's Catholic University of Louvain. His team has wired up two volunteers to monitor their heart rate, blood pressure and respiration to explore how the heart rhythm slows in zero gravity as it no longer has to fight resistance.

Among the 13 experiments packed into last week's Zero-G flights was an attempted simulation of the early Solar System -- when gas and dust glommed together to form the kernel of the future heavenly bodies.

"We are growing planets in a teapot," quipped team leader Michael Rost of the Technical University of Braunschweig, as he showed off a machine, swathed in coils to neutralise Earth's magnetic field, that fires particles of microscopic particles into a chamber to see how they cluster.

Zero G is owned by Novespace, a French firm which is commissioned by the European Space Agency (ESA) and France's National Centre for Space Research (CNES) to carry out gravity-free trips.

On average, it flies three times a year for ESA, but the loss last year of the US space shuttle Columbia, which mothballed much scientific work on the ISS, caused a surge in demand and four three-day "campaigns" have been scheduled for 2004 already, in addition to two for the CNES. Each "campaign" cost around 400,000 euros (half a million dollars).

Only two other space powers, the United States and Russia, have a "Vomit Comet" as these exceptional planes are known. China used Russia's planes to prepare its astronauts for its own manned programme.

Each mission is carefully prepared, from exacting medical tests to rigorously observed safety procedures.

At the controls are three French test pilots who carry out the parabolas with utter precision, angling the aircraft at 47 degrees to the horizontal and timing the curve to the very second.

The climb exerts a force nearly twice that of gravity. Unseeable fingers pull at your heart and stomach, tug your facial muscles downwards and turn your legs into liquid concrete.

Then comes weightlessness.

With just the slightest push against the aircraft's floor, you are suddenly floating upside down against the padded ceiling, or can emulate the grace of the finest dancer, your arms and legs twirling slowly in mid-air.

The altered perception is almost indescribable.

The feeling is euphoric, a sensation of escaping lifelong shackles. For a tiny while, at least.

SPACE.WIRE