Critical Analysis Of The Film Gravity
Gravity is a 2013 film starring Sandra Bullock (Dr. Ryan Stone) and George Clooney (Lieutenant Matt Kowalski) as American astronauts who are stranded in space after the mid-orbit destruction of their Space Shuttle, and their attempt to return to Earth. The triggering incident in Gravity occurs when Russia launches a missile to destroy one of its own satellites, accidentally creating a chain reaction that demolishes most of the communications satellites orbiting the planet. It portrays Kessler Syndrome and how debris from a satellite collision can have a cascading effect with other satellites. Unlike shrapnel here on Earth, which travels for a distance before gravity pulls it to the ground, each piece of collision debris in space just keeps moving, circling the planet in its own independent orbit until it hits something, or eventually burns up in the Earth's atmosphere. The satellite disaster puts them in the path of a cloud of space debris that whips around the planet every 90 minutes. They are hit, ejected from the shuttle, and eventually try to grab onto the International Space Station. They fail to grab on, but Stone’s leg gets entangled in some parachute cords. She grabs a strap on Kowalski's suit, but it becomes clear that the cords will not support them both. Kowalski detaches himself from the tether to save Stone from drifting away with him. Stone is pulled back towards the ISS, while Kowalski floats away.
The scene contradicts Newton’s first law of motion, which states that an object remains at rest, or moves in a straight line at a constant speed, unless acted upon by a net outside force. Once Stone and Kowalski are attached to the taut parachute cord, they should all be moving at the same speed and in the same direction. There is simply no reason that there should even be any tension in the parachute cord, since if they’re all experiencing the same constant motion, there’s no acceleration, and hence there’s no force. Yet, when Kowalski is released, he drifts away. However, there are outside, external forces. There is the force of gravity from the Earth and a slight but non-negligible drag force from the atmosphere at those high altitudes that causes a constant loss of orbital energy. This is why satellites in a low orbit need boosts occasionally, or they deorbit, re-enter, and burn up in the atmosphere.
The ISS is more massive than Stone and Kowalski, and so experiences a greater gravitational force. However, this should not matter because of Newton’s second law, which tells us that F = ma, which tells us that the acceleration of the ISS, of Stone and of Kowalski should all be the same, even though their masses are different. The drag force depends on an object’s surface area and its physical size/geometry (FD=1/2*ρu2CDA). A person would experience a larger relative drag force than the ISS, and would slow down a little more easily in an orbit. However, it would not slow down by enough to cause the effect shown in Gravity. The density of air at ISS altitudes (orbital height around 254 miles) is so sparse (around 3. 32 x 10-12 kg/m3 according to the MSISE-90 Model of Earth's Upper Atmosphere) that it would take months for Kowalski to drift away. A simple tug might propel him towards the spacecraft, rendering the whole tether scene problematic. The scene shows that Stone and Kowalski are still decelerating with Stone's leg caught in the parachute cords.
The cords stretch as they absorb her kinetic energy. The director wanted to portray how Kowalski thinks the cords are too weak to absorb their combined kinetic energies, and that he must release the tether to give Stone a chance of stopping before the cords fail. However, this scene is completely inaccurate because it eventually shows the pair not accelerating and having the same angular momentum. When Stone’s leg got tangled in the cord, both her and Kowalski’s velocity relative to the space station eventually became zero. His velocity relative to her was also zero (both stopped). Recall that within a frame of reference, only the relative motions between objects have any meaning. Kowalski also gains momentum from some invisible force and is accelerated into space when he unclips, which is physically impossible. All she had to do was give the tether a gentle tug and Kowalski would’ve been pulled toward her.
One of the movie posters showing this scene invites us to think about whether the tether is a purely linear system. In the poster, Kowalski is at an angle to Stone, who clearly is at an angle to the ISS. This could happen in space if the entire spacecraft is rotating (collision prior in location other than the center of mass of the ISS). Although the rotation could be incredibly slow (barely perceptible in camera shot), it could keep the tether taut and increase the risk of a more massive weight at the end breaking the tether. If the weight were to become detached (Kowalski lets go), it would move off from its own inertia, away from the tethered masses. There does need to be some form of acceleration in order to cause the tether to be taut, for the mass of the people to risk breaking the cord and for Kowalski, when he lets go, to actually move away. This acceleration could be caused either by an external force, which leads to a change in speed, or by a rotational motion, which leads to a change in direction. Based on the movie itself, it is probably a change in direction (small but sufficient to cause what the movie displayed). The rotation of the ISS could have been greater than the camera angles showed, making it impossible for Kowalski to pull himself in. Unless there is some sort of acceleration (rotation) there’s no reason Kowalski should have died.
The filmmakers took artistic and plot-related liberties at the expense of scientific accuracy. Given Gravity’s large budget and advisory from Kevin Grazier (physicist), the primary reason for the inaccuracy was dramatic value. Plot trumps science, and due to the importance of Kowalski’s death, the creators opted to neglect basic laws of motion. No serious changes could make the science right, as it would be impossible in the context of the work due to the dramatic effect being predicated on Kowalski’s death. They could modify the death, but it would reduce the gravity of the moment. They could show Stone decelerating from the cords while Kowalski continued to accelerate, followed by their realization that the cords would not absorb their kinetic energies, followed by Kowalski unclipping himself before he began to decelerate from the tether and break the cords. This would cut the scene short, as Kowalski would need to unclip while moving fairly quickly. It would be difficult to film their realization that the cords would break, and the audience would be confused if Kowalski suddenly unclipped himself. The climax of the movie is the brief conversation that Kowalski and Stone have while stationary, and adhering to physics would have ruined this moment.
