Tuesday, January 10, 2012
Hollywood Films Get Space Travel Wrong
As the space ship in the 1986 film Aliens moves towards a mysterious planet, the cosmonauts aboard gaze suspiciously at the foreign body with their feet firmly pressed against the ship’s floor. The sci-fi adventure has barely begun, and already, the laws of physics have been ignored due to a common imbalance in Hollywood: too much fi, too little sci.
I want to be clear about something: I am an avid fan of science fiction movies, whether or not they are set in space. In fact, I was thoroughly entertained by Aliens, the sequel to Alien - so much so that I watched it recently for the second time.
As the film is set in the distant future, it is sensible that several technological leaps have been made by mankind. I am comfortable with the intelligent robot that is a part of the crew. I am intrigued by the sensitive motion detection equipment that they use. What makes me queasy is when the most basic law of motion is inexplicably defied.
When advances in technology occur, they lead to more robust tools. However, these tools, no matter how fantastic they are, must operate within the constraints of the universe. Otherwise put, technology is dynamic, but exists within an operational framework that is static.
When a space travelling Sigourney Weaver stands up on the surface of a ship, this implies that there is a set of contact forces between her feet and the surface. A force, commonly referred to as the normal force, is pushing up onto her feet. Unless you are currently undergoing a vertical loop on a jet airplane, or happen to be reading this article while in a space orbit, the normal force is currently acting on you.
If you are currently standing, as were many aboard the space ship, then the normal force is pressing up onto the surface of your shoes, and compressive pressure exists throughout your legs. If you happen to be sitting, as was the commander aboard the ship, then the majority of the contact force exists at the chair to rear end interface (over extended periods of time, this evenly distributed normal force may produce a flatter butt, while inactivity as a whole tends to cause a more ample one).
This normal force should not have manifested between the space travellers and the surface of the ship. When they looked upon the distant planet, they were orbiting either it or some other large body like a star. In a nutshell, here is how interplanetary travel within our solar system plays out...
Stage 1: Earth surface to Earth orbit (Impulse 1)
Stage 2: Maintain Earth orbit
Stage 3: Earth orbit to Sun orbit (Impulse 2)
Stage 4: Maintain Sun orbit
Stage 5: Sun orbit to other planetary orbit (Impulse 3)
Stage 6: Maintain other planetary orbit
Stage 7: Other planetary orbit to planetary surface (Impulse 4)
The odd stages (1, 3, 5, and 7) above require that external forces act on the spacecraft. The action of some external force onto the spacecraft is referred to as an impulse manoeuvre. A typical mission within our solar system lasts a number of years, however the total amount of time spent undergoing impulse manoeuvres is a matter of minutes. Virtually the entire interplanetary mission is spent in orbit of the Sun or some planet. In such instances, the volume inside the ship is a zero-g environment, where all passengers feel a sensation known as weightlessness.
The term weightlessness is not well understood by most people. The common public perception of why astronauts float around the International Space Station (ISS) is that they are far from Earth, and are thus unaffected by the Earth’s gravitational field - this is not true. As Isaac Newton determined in the late seventeenth century, the gravitational pull of large masses on other masses decreases as the distance between them grows; it decreases, but does not simply vanish.
The ISS orbits the Earth some two hundred miles above its surface. The radius of the Earth is a shade less than four thousand miles. As such, the distance from the center of the Earth to you at this instant is 4,000 miles, while that from the Earth’s center to the ISS and all of its contents is about 4,200 miles. The difference in the force of gravity acting on you and the ISS is minor: about 10%.
The reason why the passengers aboard the ISS may be floating freely at this moment, and you are not, is because you are stationary on the surface of the Earth, and they, along with the ship that surrounds them, are falling. That’s right, there is nothing high-tech about a spacecraft in orbit. It is a falling body, much like a thrown tennis ball the moment after it leaves your hand.
More specifically, there are two real differences between a ball that you toss and a spacecraft orbiting a celestial body. The first difference is that the ball travels through a low density medium of air, and as such, experiences some drag (air friction), which slows it down. A spacecraft moves through a vacuum, and therefore maintains its velocity.
The second, and perhaps more important difference between the orbiting spacecraft and the tossed ball is just how fast the two are moving. The fastest baseball pitch is just over 100 miles per hour. The ISS orbits the Earth at over 17,000 miles per hour.
The pitch follows a parabolic path that would eventually strike the ground if the catcher moved out of the way. It is falling. The ISS, while moving 170 times faster than the ball, also follows a bent path. It is falling too, but it is moving so fast that its path does not coincide with the Earth. Its path maps out an elliptical shape that we call an orbit. If the pitcher threw the ball much, much faster, it too would circle the Earth and strike the pitcher dead in the back of the head (in the absence of an atmosphere and unsmooth terrain).
When the Aliens crew orbit the planet that they intend to land on, they are “falling” around it. And yet, they stand upright as though they were standing in a field. If the crew were in an elevator that was plummeting to the ground, I think the audience would be perplexed if they stood on the floor unaffected while in free fall. That is because human beings live on the surface of the Earth, and, other than a select few, have never spent the day in orbit. We are accustomed to how the basic laws of science apply to our living environment. An audience of astronauts would find both scenes equally laughable.
What does weightlessness feel like? Fortunately, we do not need to enter a spacecraft to answer this question. Anyone who has ever been tossed in the air as a child has felt the sensation for a brief time. A roller coaster that follows the shape of an upside down letter ‘U’ gives its riders the sensation for a short time as well. Thrill-seekers yearn for the zero-g experience: just ask bungee jumpers and sky divers.
If you would like to feel weightless for an extended period of time, it will cost you a pretty penny. A high speed airplane heading upwards can shut off its engines for about a minute while everyone inside floats around. This is exactly how the space scenes from Apollo 13 were filmed. This Hollywood film is the exception to the rule, where space travel is accurately depicted.
Had James Cameron, the director of Aliens, offered a plausible explanation for why the space travellers do not float around, I would have been more than satisfied. Two characters could have had a quick exchange of dialogue:
Ripley: “Wow, these new charged space suits are so comfortable.”
Commander: “Ya, and we’ve finally configured the charge of the floor so that it feels just like home.”
After all, the film gets so much science right. There are detailed discussions about the chemical composition of the planet’s atmosphere, as well as its surface gravity. I am just amazed that space travel is the item that was overlooked.
I kind of feel bad singling out Aliens for its scientific shortcomings, but I think that I chose it because it is otherwise scientifically sound. Had I chosen to examine the scientific accuracy of Independence Day, I would really not have known where to start. I mean, yes, the extra-terrestrial ships do just hover inexplicably in the air, but then, is that really more unbelievable than the rest of the film? Any visiting civilization that manages to arrive at our planet with the intent of destroying life here will undoubtedly succeed. Their plan will not be thwarted by the Fly and the Fresh Prince of Bel-Air.