Finding Physics in ‘The Martian’

By: Dr. Aaron Titus, chair of the High Point University Department of Physics

 

The Martian movie posterIn our quest to answer the question, “Are we alone in the Universe?” my First Year Seminar (FYS) class at High Point University recently read the book “The Martian” by Andy Weir. With NASA announcing evidence for water flowing on present-day Mars just three days ago, and with today’s release of the film “The Martian” starring Matt Damon, this is the perfect time to engage students and the public with Martian science (which is the same science as everywhere else in the Universe).

In a contemporary, yet classic, survival story, “The Martian” documents the struggle of astronaut Mark Watney to withstand the barrage of life-ending threats thrown by Mars.

“The Martian” is like a combination of “Apollo 13” and “Cast Away.” With in-depth description of science, from physics and astronomy to chemistry, botany, and geology, Weir’s book grabs both the mind and soul.

When the main character, Mark Watney, hits the big screen later tonight, our FYS class will be there to enjoy both the science and science-fiction. Here’s a primer on a few of the topics that make “The Martian” so engaging.

 

Travel to Mars

If you travel to Mars, there are three legs of your journey: (1) the launch from Earth; (2) the orbit around the Sun; and (3) the landing on Mars. During the launch and landing, you will use rocket fuel. But during the orbit, you will drift through space with Newton and the Sun driving.

To reach Mars, the rocket must launch from Earth at just the right time, speed, and direction so your orbit around the Sun is on a collision course with Mars. This window of opportunity only comes around every 26 months, which is the time it takes for Mars, Earth, and the Sun to align (though you will certainly not travel in a straight line). If you miss your flight, you will have to wait 26 months for the next one! (Click here to run a simulation that shows the 26 month interval between alignment of Earth, Mars, and the Sun.)

Earth’s escape speed is 11 km/s. To leave Earth and not return, your rocket will have to reach a speed of 11 km/s shortly after takeoff. Once your rocket’s fuel is exhausted, your orbit around the Sun will take you to Mars. Depending on your particular orbit, it can take from 130 to 330 days to reach Mars. For NASA’s Mars missions, the average travel time is approximately 225 days, or about eight months. This is a long time, so you should definitely pack snacks and movies. The Mayflower took just over two months to cross the Atlantic. The journey to Mars is about four times longer than the Pilgrims’ journey.

I’ve created a simulation where you can launch a rocket to Mars using accurate positions of the planets as of Aug. 29, 2015. In this simulation, the travel time is 211 days, with the astronauts traveling from Dec. 22, 2015 to July 20, 2016. If you want to play the role of Mission Director, try this simulation where you can control the day you launch the rocket and the direction. You can see that there is a small launch window that will give an orbit where the rocket’s path intersects Mars.

 

Weight on Mars

I have a great idea for a new weight loss plan called “The Martian Diet.” It requires no exercise and no change in your eating habits. Just go to Mars, and you will lose 60 percent of your weight without losing any mass! Isn’t that wonderful? Mars’ gravity is only 40 percent of Earth’s gravity, so Mars pulls me downward with a force that is 60 percent less than on Earth. I weigh 200 pounds on Earth. On Mars, I weigh 80 pounds.

You can also lift objects of greater mass on Mars compared to Earth. Suppose you can bench press a maximum of 100 pounds on Earth, how much can you bench press on Mars? If you apply the same force to a barbell on Mars, you can lift 2.5 times more mass on Mars than you can on Earth. Thus, on Mars you can benchpress a mass that would weigh 250 pounds on Earth. Even though you’re not stronger on Mars, you can lift 2.5 times more mass on Mars because Mars does not pull as hard as Earth.

 

Solar Power

Solar energy is a critical source of energy for Watney’s survival in “The Martian.” However, Mars is farther from the Sun. Thus, a 1 square-meter solar panel on Mars receives less solar energy in one second than the same solar panel on Earth.

At Earth’s distance from the Sun, a 1 square-meter solar panel pointed directly at the Sun (with no atmosphere affecting the sunlight) receives 1400 watts (W) of power. Mars is 1.5 times further from the Sun than Earth. So, Mars only receives 600 W, or 4/9 of the Sun’s power, on the same solar panel.

Due to inefficiencies of solar panels, absorption by the atmosphere, and the location where you live (both latitude and time of year), on Earth a one square-meter solar panel might give you 100 W of electrical power. This is enough to continually light a 100-W incandescent lamp, but it will hardly satisfy your energy needs. Thus, you need a LOT of solar panels on Earth. And whatever you need on Earth, you will need 2.25 times as many solar panels on Mars.

 

Water on Mars

After discovering he is alone on Mars, one of Watney’s first problems to solve is creating drinking water. By burning hydrogen (which he obtains from rocket fuel) in his oxygen-filled Hab in a controlled way, Watney produces water. But could Watney have obtained liquid water from Mars itself?

The polar caps of Mars contain water ice, but surviving on Mars is hard enough. It’s not worth landing on the polar caps. Is there liquid water elsewhere on Mars? There is one primary condition that prevents liquid water from existing on Mars — very low atmospheric pressure, about 0.6 percent of Earth. If Watney ejected a water ice cube from Earth’s atmospheric pressure in his Hab out into the Martian atmosphere, the ice cube would quickly sublimate, turning to a vapor, even on the warmest summer Martian day.

But as everyone who drives on salty winter roads can attest, saltwater behaves differently than freshwater. Saltwater can exist in the liquid state in the low atmospheric pressure on Mars, and that’s what NASA recently found. Salty water flows in some locations on present-day Mars during the Martian summer. Therefore, if our hero, Mark Watney, had to reclaim Martian water, he’d definitely have to desalinate it. Making it from rocket fuel is definitely more efficient.

 

NASA Johnson Space Center 2

HPU physics students test their device at the Johnson Space Center with NASA engineers and divers

Summary

In our PHY 2010 course, we use the book Matter and Interactions and the programming language VPython to teach students how to create simulations of orbits and rockets. In August, a team of five High Point University physics majors traveled to the Johnson Space Center as part of NASA’s Micro-g NExT program to test a tool they designed for chipping and collecting samples of an asteroid, in preparation for a future manned-mission to an asteroid. While “The Martian” may be science fiction, the book and movie are appealing because of their connection to what we do every day in the Department of Physics at High Point University.

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