In the new thriller “Gravity,” Sandra Bullock plays a reluctant astronaut who wants to complete her space walk to repair an instrument as quickly as possible. Her plans are foiled by a series of terrifying events. The stunning 3-D cinematography gives viewers a sense of being in space and fuels the astronaut’s feeling of helplessness. But how realistic are these dangers? Here are some challenges that Bullock’s character faces (spoiler alert!) and their likelihood of happening, according to NASA experts:

Russia blew up its own satellite and created a tsunami of space junk!

Possible, but . . . In 2007, China used a guided missile to destroy one of its aging weather satellites , creating hundreds of thousands of smaller pieces of space junk. But the danger the debris posed was to other satellites, not astronauts, says Mark Uhran, who retired last year as NASA’s director of the international space station (ISS). Most satellites are in geosynchronous orbits about 22,000 miles above Earth, well above the roughly 250-mile altitudes of the ISS and most manned flights. Lower-space junk can present a threat; NASA tracks hundreds of thousands of objects in this zone, and the station’s thrusters can change its orbit slightly to avoid collisions.

I got disconnected from my ship!

Extremely unlikely. NASA can’t prepare for every nightmare scenario, so it relies on a “failure modes and effects analysis” to determine an event’s likelihood. Uhran says NASA calculated the chance of an astronaut coming untethered as infinitesimally small. Just in case, astronauts perform space walks in pairs, and they wear thruster belts that can provide an emergency boost, though not a powerful one. No spacewalkers, or their steel tethers, have ever been struck by debris, though NASA astronaut Mike Massimino says astronauts practice returning to a vehicle if a spacesuit is punctured.

We can’t get home!

Extremely unlikely. NASA requires all missions to have a backup means of return, usually an escape vehicle. The ISS has two Russian Soyuz capsules. During Massimino’s last mission to upgrade the Hubble Space Telescope, NASA had a backup shuttle ready to launch in case of trouble. “You always have to have a lifeboat,” he says.

The space station is on fire!

Possible. “Fire is one of the greatest hazards that can occur on the space station,” Uhran says. In 1997, an oxygen canister ignited on the Russian station Mir, unleashing smoke and three-foot-high flames. Cosmonaut Valery Korzun managed to douse the fire. Had he failed, half the crew might have died because the fire blocked access to one of the two Soyuz escape capsules. Uhran says that materials on the space station have high ignition temperatures so that small fires won’t spread.

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