As the space ship in the 1986 film Aliens moves towards a mysterious planet, the cosmonauts aboardgaze suspiciously at the foreign body with their feet firmly pressed againstthe ship’s floor. The sci-fi adventurehas barely begun, and already, the laws of physics have been ignored due to acommon imbalance in Hollywood: too much fi,too little sci.
I want to be clear about something: I am an avid fan ofscience 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 itrecently for the second time.
As the film is set in the distant future, it is sensiblethat several technological leaps have been made by mankind. I am comfortable with the intelligent robotthat is a part of the crew. I am intriguedby the sensitive motion detection equipment that they use. What makes me queasy is when the most basiclaw of motion is inexplicably defied.
When advances in technology occur, they lead to more robusttools. However, these tools, no matterhow fantastic they are, must operate within the constraints of the universe. Otherwise put, technology is dynamic, butexists within an operational framework that is static.
When a space travelling Sigourney Weaver stands up on thesurface of a ship, this implies that there is a set of contact forces between herfeet and the surface. A force, commonlyreferred to as the normal force, ispushing up onto her feet. Unless you arecurrently undergoing a vertical loop on a jet airplane, or happen to be readingthis article while in a space orbit, the normal force is currently acting onyou.
If you are currently standing, as were many aboard the spaceship, then the normal force is pressing up onto the surface of your shoes, andcompressive pressure exists throughout your legs. If you happen to be sitting, as was thecommander aboard the ship, then the majority of the contact force exists at thechair to rear end interface (over extended periods of time, this evenlydistributed normal force may produce a flatter butt, while inactivity as awhole tends to cause a more ample one).
This normal force should nothave 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 interplanetarytravel 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 (Impulse4)
The odd stages (1, 3, 5, and 7) above require that externalforces act on the spacecraft. The actionof some external force onto the spacecraft is referred to as an impulsemanoeuvre. A typical mission within our solar system lasts a number of years, however the total amount of time spentundergoing impulse manoeuvres is a matter of minutes. Virtually the entire interplanetary missionis 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 mostpeople. The common public perception ofwhy astronauts float around the International Space Station (ISS) is that theyare far from Earth, and are thus unaffected by the Earth’s gravitational field- this is not true. As Isaac Newtondetermined in the late seventeenth century, the gravitational pull of largemasses on other masses decreases asthe distance between them grows; it decreases, but does not simply vanish.
The ISS orbits the Earth some two hundred miles above itssurface. The radius of the Earth is ashade less than four thousand miles. Assuch, the distance from the center of the Earth to you at this instant is 4,000miles, while that from the Earth’s center to the ISS and all of its contents isabout 4,200 miles. The difference in theforce of gravity acting on you and the ISS is minor: about 10%.
The reason why the passengers aboard the ISS may be floatingfreely at this moment, and you are not, is because you are stationary on thesurface of the Earth, and they, along with the ship that surrounds them, arefalling. That’s right, there is nothinghigh-tech about a spacecraft in orbit. Itis a falling body, much like a thrown tennis ball the moment after it leavesyour hand.
More specifically, there are two real differences between aball that you toss and a spacecraft orbiting a celestial body. The first difference is that the ball travelsthrough a low density medium of air, and as such, experiences some drag (airfriction), which slows it down. Aspacecraft moves through a vacuum, and therefore maintains its velocity.
The second, and perhaps more important difference between the orbiting spacecraft andthe tossed ball is just how fast the two are moving. The fastest baseball pitch is just over 100miles per hour. The ISS orbits the Earthat over 17,000 miles per hour.
The pitch follows a parabolic path that would eventuallystrike the ground if the catcher moved out of the way. It is falling. The ISS, while moving 170 times faster thanthe ball, also follows a bent path. Itis falling too, but it is moving so fast that its path does not coincide withthe Earth. Its path maps out an ellipticalshape that we call an orbit. If thepitcher threw the ball much, much faster, it too would circle the Earth andstrike the pitcher dead in the back of the head (in the absence of anatmosphere and unsmooth terrain).
When the Alienscrew orbit the planet that they intend to land on, they are “falling” aroundit. And yet, they stand upright asthough they were standing in a field. Ifthe crew were in an elevator that was plummeting to the ground, I think theaudience would be perplexed if they stood on the floor unaffected while in free fall. That is because human beings live on thesurface of the Earth, and, other than a select few, have never spent the day inorbit. We are accustomed to how thebasic laws of science apply to our living environment. An audience of astronauts would find bothscenes equally laughable.
What does weightlessness feel like? Fortunately, we do not need to enter a spacecraftto answer this question. Anyone who hasever been tossed in the air as a child has felt the sensation for a brieftime. A roller coaster that follows theshape of an upside down letter ‘U’ gives its riders the sensation for a shorttime as well. Thrill-seekers yearn forthe zero-g experience: just ask bungee jumpers and sky divers.
If you would like to feel weightless for an extended periodof time, it will cost you a pretty penny. A high speed airplane heading upwards can shut off its engines for abouta minute while everyone inside floats around. This is exactly how the space scenes from Apollo 13 were filmed. ThisHollywood film is the exception to the rule, where space travel is accuratelydepicted.
Had James Cameron, the director of Aliens, offered a plausible explanation for why the spacetravellers do not float around, I would have been more than satisfied. Two characters could have had a quickexchange of dialogue:
Ripley: “Wow, these new charged space suits are socomfortable.”
Commander: “Ya, and we’ve finally configured the charge ofthe floor so that it feels just like home.”
After all, the film gets so much science right. There are detailed discussions about thechemical composition of the planet’s atmosphere, as well as its surfacegravity. I am just amazed that spacetravel is the item that was overlooked.
I kind of feel bad singling out Aliens for its scientific shortcomings, but I think that I chose itbecause 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 unbelievablethan the rest of the film? Any visitingcivilization that manages to arrive at our planet with the intent of destroyinglife here will undoubtedly succeed. Their plan will not be thwarted by the Fly and the Fresh Prince of Bel-Air.