The Gravity Question

March 2016

Author Marianne Dyson’s Science Snacks Newsletter

Hello, and a special welcome to those of you who subscribed after my visits to NASA Glenn’s AIAA section and the International Women in Aviation & Space Museum in Ohio in February. I look forward to sharing space stories with you, and seeing some of you again at upcoming appearances (listed below). I hope your brain enjoys its science “snack” this month!


Caption: I played Tour Guide for the American Engineering Geologists’ visit to Space Center Houston February 13.

The Gravity Question

Later this month, I’ll be doing the activity on pages 60-61 in Welcome to Mars (coauthored with Buzz Aldrin) comparing Earth with Mars with 60 girls attending a spring break STEM camp at the Frontiers of Flight Museum in Dallas. The activity compares the two worlds by diameter, volume, and mass. Considering Mars is one half as wide, one sixth as big, and one tenth as massive, why then does a person on the surface weight about one third as much as on Earth?

The answer is because weight is a measure of gravity’s downward pull, and gravity depends on two things: mass and (the square of) the distance from the center of mass. To avoid any messy math, I suggest using “Earth” units. In other words, the radius of Mars is simply half the radius of Earth versus 4,221 miles; and the mass is simply one tenth that of Earth versus 7 followed by 20 zeroes in tons!

Fortunately, calculating gravity does not require any fancy equations, just multiplication and division. Gravity equals a constant (G) times the mass (M) of the world divided by the square of the distance (R)(R) from the center of mass. Gravity of Earth=GM/R². So gravity of Mars in Earth units equals G (0.1M)/(0.5R)(0.5R). If we then divide that by Earth’s gravity, the G, M’s, and R’s all cancel out to 1, and we’re left with 0.1/(0.5)(0.5)=0.4. The actual value (in the “The Facts” table on page 61) is 0.37 which is most often rounded to one third.

Note that the gravity of the Moon (which is one hundredth the mass and a quarter of the radius) is (0.01M)/(0.25R)(0.25R) which is 0.16, about one sixth of Earth.

To feel how much something weighs on Earth versus on Mars or the Moon, fill three plastic bags or Easter eggs with 100 pennies or nickels for Earth, 37 for Mars, and 16 for the Moon. I wonder if shoes would last three times longer on Mars? Or would we just decide to go barefoot?

Writing about Space

The anthology, Touching the Face of the Cosmos: On the Intersection of Space Travel and Religion that contains my story, “The Right of Interference” is now available in paperback. If you order after using the link from my website, I get a credit from Amazon and you get a big THANK YOU!

Speaking about Space

Look for me at the following events. Watch my website Contact page for updates and local (Houston) events.

Monday, March 7, 6:15 pm, Reitz Union Room 2365. Speaking to the UF. Society of Women Engineers, Gainesville, FL.

Tuesday, March 15. Welcome to Mars workshops as part of Spring Break Camp for Girls, Frontiers of Flight Museum, Dallas Love Field.

Saturday, April 9, 11 am to 4:15 pm. Selling/signing books at Reach for the Stars STEM Festival, Rice University. Free, but preregistration required.

April 19-21. Staffing booth of Houston Authors, Texas Library Association Annual Conference, GRB Convention Center, Houston.

Saturday, April 23. Featured Author, Ohioana Book Festival, Columbus, Ohio.

Saturday, May 28. Speaking at Gulf Coast Mensa Regional Gathering, Crowne Plaza Northwest-Brookhollow, 12801 Northwest Freeway, Houston. Open to public.

Saturday, June 18, panels 10-11 am and 1-2 pm. Comicpalooza, GRB Convention Center, Houston.

Author: Marianne

Marianne Dyson is an award-winning children's author, science fiction writer, and former NASA flight controller. To invite her to speak or order her books, visit her website,