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.
3 comments:
Right on. As a volunteer for the Spaceward Foundation I cannot begin to count the number of times I have been asked "How high do you have to go to get out of Earth's gravity?" As you so properly point out, the answer is "You can't." and the real question is "How fast do you have to go?"
-- Vern McGeorge
Author of "All Fall Down" a
sci-fi techno-thriller about
the space elevator
Vern, what you're telling me is that there is a sci-fi techno-thriller about the space elevator that I have yet to read??? I'd better get on that...
Well, a good time to get on it would be now. All Fall Down is free for download this weekend (7/4-7/8/12).
I hope you like it and tell all your friends - preferably on Monday.
-- Vern McGeorge
Author of "All Fall Down" a
sci-fi techno-thriller about
the space elevator
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