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'The Martian' gets the science right

Posted January 12, 2016
Updated January 14, 2016

NASA's Multi-Mission Space Exploration Vehicle (MMSEV) inspired the movie's Mars Ascent Vehicle (MAV) (Credit: NASA/JSC, Fox)

“The Martian” was released on Blu-ray and DVD on Tuesday after winning Golden Globes for Best motion picture, musical or comedy (I don't get it either) this week. What began as a blog on author Andy Weir’s personal website went on to become a best selling Random House book and ultimately Ridley Scott Fox film with a $100 million-plus budget.

Weir, a computer scientist by trade, describes himself as “a lifelong space nerd and a devoted hobbyist of subjects like relativistic physics, orbital mechanics, and the history of manned spaceflight,” a description I can relate to. What sets "The Martian" apart is how much of the science it got right and how effective good science can be in good story telling. As the Honest Trailers YouTube channel put it: "Science fact makes science fiction ... kind-a boring."

The orbital mechanics in the book and movie are sound, based on a 1990 paper by NASA engineer Robert Zubrin known as “Mars Direct.” The plan gets humans to Mars faster, for a longer stay with fewer resources from Earth as humans live off the land through in-situ resource utilization. The flight plan that gets the Ares II crew to Mars is based on one proposed by Walter Hohmann in 1925. The Hohmann transfer orbit takes advantage of a positioning of Earth and Mars which occurs about every 26 months to minimize fuel required.

The Ion propulsion system portrayed in the Hermes spacecraft is also based in fact. The real Dawn spacecraft currently studying the dwarf planet Ceres is making use of its ion drive, which has accumulated 25,000 mph of velocity change over five years of continuous acceleration.

While it is clear that the story takes place in the future, neither the book nor the film mention the year, but the author left clues. The 124-day trip from Earth to Mars, Thanksgiving falling during the planned month long mission, and the 11-minute communications delay on Watney’s 96th sol (80th Earth day) on Mars provide the information needed.

I wrote a quick Python script which found 2020, 2032 and 2035 as the years meeting this criteria. 2020 is too soon for the spacecraft in "The Martian" to be ready for launch, and a Hohmann transfer orbit isn’t available in 2032, leaving 2035 as the answer. Weir confirmed he'd built the story around a launch date of July 7, 2035 and arrival on Nov. 7, 2035 on his Facebook page.

Production designers on the film got a lot of the visuals right as well. The rover Matt Damon’s character covers a lot of Martian area in is based on NASA’s Multi-Mission Space Exploration Vehicle. My favorite detail in the movie was the rover wheels and their spiral, vibration absorbing design. Production designer Arthur Max was inspired by the design he saw on the Mars Exploration Rovers Opportunity and Spirt during a tour NASA's Jet Propulsion Laboratory early in the film's pre-production.

Astronaut Mark Watney uses his botany skills on Mars to grow food. Astronauts on the International Space Station (ISS) have been growing and eating red romaine lettuce grown aboard since last year. The Ares 3 crew also recovers water using the Water Recovery System technology in place on the ISS since 2008. No drop of sweat, tears, or even urine goes to waste. This is critical to Watney's survival.

For all the science fact that drives the story, there is fiction in the opening pages of the book and scenes of the movie.

Massive dust storms are very real on the red planet. “Once every three Mars years (about 5.5 Earth years), on average, normal storms grow into planet-encircling dust storms.” according to Michael Smith, a planetary scientist at NASA’s Goddard Space Flight Center. The Viking 1 lander observed winds topping 150 mph. Wind speed wasn’t what forced the crew to abort the the mission. Limits on wind pressure on the Mars Ascent Vehicle (MAV) needed to get off Mars are what stranded Watney.

Pressure exerted by the wind is proportional to atmospheric density and the wind speed squared. With an atmospheric density 81 times less than Earths, the 165 mph winds that forced the Ares III team to abort could produce the 8600 Newtons on the massive cylindrical MAV … on Earth. In the thin Martian atmosphere, that wind speed would produce wind pressures equivalent to what the Beaufort Wind Scale labels a “Fresh Breeze” here on Earth, just over 18 mph.

Both Weir and filmmakers have acknowledge cheating a bit here. They needed a mechanism to strand Watney.To produce forces which threaten to destroy the crew's only way home, filmmakers had to choose between an implausible wind speed approaching 1,500 mph or ignoring atmospheric density using wind velocities closer to reality.

I think they made the right choice, moving the story along in a way that is completely excusable given the pains taken elsewhere in the film to get the science right. NASA agreed during the movie's debut inviting Damon to NASA's Jet Propulsion Laboratory in Pasadena, Calif. to where his character used some historic hardware to communicate with Earth. Weir was also invited to tour mission control at the Johnson Space Center in Houston.

Tony Rice is a volunteer in the NASA/JPL Solar System Ambassador program and software engineer at Cisco Systems. You can follow him on twitter @rtphokie.


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