The only way to get away from the sensation and effects
of gravity is to give into it. That’s why the ‘Zero-G’
Airbus A300 that ESA uses for parabolic flights has to
free-fall through the air — with no force other than
gravity acting upon it — to generate 20 seconds of
weightlessness at a time. It would be a lot easier if
we could just flick a switch.

Except gravity can’t be turned off. Science fiction
writers might casually invent gravity-reducing gadgets
with a swish of the pen, but real life is tougher.

That’s a fact of life that’s frustrating for many
scientists, because the force of gravity distorts the
underlying physical processes they attempt to study.
Pinned down at the bottom of the 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 — upwards of 25 times the speed of sound —
that they constantly bend around the curve of the
Earth instead of crashing into it. Centrifugal force
cancels out gravitational force and the spacecraft
remains in continuous free-fall. 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
free-fall to tell whether they were plunging down to
the ground or floating in space (until the elevator
landed, of course).

Gravity is a force of downward acceleration that
everything is always subject to. This includes objects
dropping in a state of free-fall — but because
gravity is already making them drop, their local
weight drops to nil. 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 don’t 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 free-falling 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 engines.

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’s 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 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 free-fall. The
pilots radio back the warning “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 free-falls 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 doesn’t 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 the 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 CEV. 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.”

Related articles

* ESA’s free-fall laboratory poised to escape gravity’s
grip
http://www.esa.int/export/esaCP/SEMDWX8YFDD_Life_0.html

* Flying fish in 0g
http://www.esa.int/export/esaCP/ESAJIXZK0TC_index_0.html

* Floating students at work
http://www.esa.int/export/esaCP/ESAFDMPV16D_Life_0.html

Related links

* Next ESA Parabolic Flight Campaign
http://www.spaceflight.esa.int/users/file.cfm?filename=miss-parafl-next

* Student Parabolic Flight Campaign
http://www.estec.esa.nl/outreach/parabolic/

* Novespace
http://www.novespace.fr

[NOTE: Images supporting this article are available at
http://www.esa.int/export/esaCP/SEM8WZ8YFDD_FeatureWeek_0.html ]