The only way to get away from the sensation and effects of gravity is to give in to it. That is why the 'Zero-G' Airbus A300 that ESA uses for parabolic flights has to 'freefall' through the air - with no force other than gravity acting on it - to generate 20 seconds of weightlessness at a time. It would be a lot easier if we could just flick a switch.
Except that gravity cannot be turned off. Science fiction writers might casually invent gravity-reducing gadgets with a swish of the pen, but real life is tougher.
This is a fact of life that is frustrating for many scientists, because the force of gravity distorts the underlying physical processes they attempt to study. Pinned down at the bottom of Earth's 'gravity well', we're hardly conscious of it, but gravity changes the way matter and energy interact, how gases and liquids behave, even the way that fire burns.
Gravity still reaches up to spacecraft in Earth orbit. It inexorably pulls them downward but they fly so fast - over 25 times the speed of sound - that they constantly bend around the curve of Earth instead of crashing into it. Centrifugal force cancels out gravitational force and the spacecraft remains in continuous freefall. The result is 'weightlessness', also called microgravity.
But it remains a difficult and expensive task to accelerate a payload up to orbital velocity. There are other means of creating microgravity without going so fast and so far – at least for a very short while.
In one of his famous 'thought experiments' back in the early 1900s Albert Einstein realised there would be no way for a person in a sealed elevator in freefall to tell whether they were plunging down to the ground or floating in space (until the elevator landed, of course).
Gravity is a force with downward acceleration that everything is always subject to. This includes objects dropping in a state of freefall - but because gravity is already making them drop, their local weight drops to zero. So everything inside our falling elevator becomes weightless - because the downward plunge already caused by the force of gravity cancels out any extra effect.
Today, scaled-down versions of just such a runaway elevator are the single most accessible way for scientists to experiment with weightlessness. Tall airless shafts called drop towers (such as the 146-metre Bremen Drop Tower in Germany) have experimental packages dropped in them so they undergo a few seconds of microgravity. And shooting them up to the top of the tower in the first place using a catapult adds a few additional seconds.
But the duration of microgravity is frustratingly brief, the size of the packages very limited, and experimenters certainly do not get to ride along with their experiments. Which is where the 'Zero-G' Airbus A300 comes in. Operated for ESA by the French company Novespace, the aircraft is specially flown so that its passengers and cargo experience longer periods of freefalling weightlessness - but in a survivable fashion.
Operating out of Bordeaux-Mérignac Airport, the aircraft usually flies up to a specially assigned air corridor above the Gulf of Gascogne. Flying level at about 6000 metres the weightlessness manoeuvre – known as a parabolic arc – begins when the aircraft is sent into a steep 45 degree climb on full engine thrust.
At this point the passengers experience an interior gravity of about 1.8g, because the acceleration of the engines adds to the normal acceleration of gravity. It is as if our elevator is moving up to the top of its shaft at high speed.
After about 20 seconds of this, at an altitude of 7600 metres, the Airbus crew throttles back the engines almost to zero – just enough to compensate for air resistance. All forces other than gravity are cancelled out and the plane is in freefall. The pilots radio back the warning of 'injection' and the 20-second period of microgravity begins.
Just like the way a ball curves thrown into the air, its remaining momentum carries the aircraft up to the top of a parabolic arc. The Airbus reaches the apex of the arc at 8500 metres up then its nose traces the parabola round and it freefalls downward.
When the angle of the aircraft points 45 degrees below horizontal the engines are started up again at full power – the passengers once again experiencing 1.8g gravity – and the aircraft levels off horizontally, back to 6000 metres, ready for the next parabola.
Each parabolic arcs generates about 20 seconds of microgravity. That does not sound like much but it quickly adds up. The Airbus flies 31 parabolas per day, with three flights per ESA parabolic campaign and three ESA campaigns a year (two professional campaigns and one student campaign) the annual microgravity achieved equals an entire 90-minute orbit of Earth.
Parabolic flights are very demanding manoeuvres. The 'Zero-G' Airbus has been specially strengthened and each flight carries five pilots, all from the French Test Flight Centre. Three of these pilots fly the Airbus at any one time, each one overseeing a different axis of movement.
ESA has been running parabolic flight campaigns since 1984, first on a NASA KC-135, then from 1988 using a CNES Caravelle and most recently, from 1996, on Novespace's 'Zero-G' A300 Airbus – the largest aircraft ever utilised for parabolic flights.
"Nowadays we often have experimenters returning for flight after flight to carry out sustained research," said ESA Parabolic Flight Co-ordinator Vladimir Pletser. "The parabolic flight campaigns are in the middle of a spectrum of access to microgravity, that starts with drop towers and ends with flight aboard the International Space Station."