IN A thought experiment in 1907, Albert Einstein realised that there would be no way for a person in a runaway lift to tell whether they were falling towards the ground, or whether they were floating far from any gravitational fields in deep space. Einstein's observation paved the way for his theory of general relativity. But it also opened the doors to the growing industry of space simulation. Scientists still use runaway lifts (or more precisely, evacuated tubes) to mimic the microgravity of space. But even the tallest terrestrial towers provide fewer than ten seconds of free-fall. A longer period of zero gravity is achievable by flying a jet steeply upwards on full power then throttling back to almost nothing. The plane continues along a special trajectory called a parabola—like a shell shot from a cannon—with passengers on board experiencing 20 to 30 seconds of weightlessness before the pilot needs to pull up again.

The manoeuvre is usually repeated dozens of times with periods of stomach-churning acceleration in between, earning NASA's parabolic jet the nickname "the Vomit Comet". For decades, space agencies used rollercoaster flights like these to train their astronauts and to test equipment destined for orbit. Now, commercial companies are betting that thrill-seeking members of the general public will also pay for a short taste of space (and possibly last night's dinner). For the first time, European taxpayers can now buy a ticket to ride the Airbus A300 Zero-G jet used for over a decade by the European Space Agency (ESA). Taking off from Bordeaux, the A300 Zero-G has just 40 seats and a huge 200 cubic metre padded area for free-floating acrobatics. It joins an increasingly crowded microgravity marketplace. The Zero G Corporation in America has completed over 450 parabolic flights in a Boeing 727-200F since 2004, hosting such geeky megastars as Stephen Hawking and Buzz Aldrin. Parabolic flights are also available aboard an Ilyushin II-76 jet in Russia and, sporadically, aboard smaller planes elsewhere in the US and South America.

The procedure for each flight is similar. Passengers are briefed on the ground, encouraged to take motion sickness medication and then flown to around 20,000 feet. Warm-up parabolas simulating Martian (0.38g) and lunar (0.16g) gravities are followed by a dozen or so fully weightless manoeuvres before returning to Earth. The flights appeal to more than just thrill-seeking space buffs. They are proving popular with commercial space start-ups, schools and even individuals who want to conduct research in microgravity without the expense of achieving orbit. The Zero G Corporation can fly simple experiments for just a few thousand dollars, and NASA is funding small scale research and education under its Flight Opportunities programme https://flightopportunities.nasa.gov/.

Passenger prices are only a little higher. Tickets range from under $4,000 in Russia to over $7,500 in France. By comparison, a seat on board Virgin Galactic's sub-orbital SpaceShipTwo—which has yet to actually fly—currently costs $250,000. Moreover, a parabolic flight typically accumulates the same duration of microgravity (five to eight minutes in total) as SpaceShipTwo will experience when it begins commercial operations in early in 2014. Although Virgin Galactic astronauts will enjoy spectacular views through their portholes, budget space tourists aboard a zero-g airline can console themselves that with their eyes closed, even Einstein himself would not have been able to tell the difference.



