Slowing Down from Space

Marianne Dyson May, 2018

Many people mistakenly think that there is no gravity in space, and thus all that’s needed to reach space is to attain a certain altitude. But this is NOT true! Earth’s gravity at the surface is defined as 1g. A simple calculation* shows that the gravity at 200 miles (320 km) altitude is 90 percent of what it is on the surface, or 0.9g.

The reason spacecraft stay in orbit is not because there isn’t any gravity, but because they have attained the speed necessary to balance gravity’s pull. They must go up high enough to avoid running into mountains and the upper atmosphere which would slow them down.

A bit of algebra** proves that the velocity required to stay in an orbit does not depend on the mass of the object, only the mass of Earth and the object’s distance from the center of Earth. For orbits of 200 miles up, the speed is 17,500 mph (28,200 kph). (Note, it takes more energy to speed up a larger mass, but the speed that must be attained is the same.)

To drop to a lower orbit or return to Earth, a spacecraft doesn’t just “step off” a platform in the sky: it must slow down. This happens naturally over time because, though thin, the Earth’s atmosphere extends far into space. This is what happened to Skylab and more recently, to the Chinese space station. To stay in orbit, spacecraft must be periodically boosted.

The key to a safe return from space is to slow down, and slow down gradually.

Slowing down begins by flipping the spacecraft so that the engines are pointed forward, into the direction of travel. To slow down completely would require about the same amount of fuel as it took to reach orbital speed in the first place. On an airless world like the Moon, that is the only way to slow down. But because of Earth’s atmosphere, spacecraft need only fire their engines enough to slip into the atmosphere and then let friction do the rest.

But friction between objects at high speed produces a lot of heat. (Try rubbing your thumb and finger together slowly and then faster and note the difference in heat.) Meteors enter the atmosphere at very high speeds and quickly turn into fireballs. To avoid a similar fate, spacecraft use heat shields to protect the hull and crew. Heat shields can be made of what are called ablative materials such as used during Apollo that burn off and take heat with them; or they may take the form of insulating tiles such as were used on the space shuttles. (Damage to the heat shield is what caused the destruction of Space Shuttle Columbia.)

space shuttle
The space shuttle orbiter was covered with tiles to insulate the aluminum skin underneath from the high temperatures produced by the friction of passing through the Earth’s atmosphere at high speeds. Image: Lockheed Martin.

Once the spacecraft has passed through the upper atmosphere and lost much of its speed to heat, it is still going very fast. Unless it slows down more, it will hit the surface like a speeding car crashing into a wall. To slow down further, winged craft like the space shuttle increase their time in the lower atmosphere by executing “S” turns and then deploying parachutes after touchdown. Capsules like the Russian Soyuz use parachutes while still in the air and fire retrorockets just before touchdown.

So when it comes time to return to Earth from your trip in space, remember to slow down!

Prove that heavy objects do NOT fall faster than light ones, but compact objects DO. Take two identical plastic containers with lids. Put a flashlight battery in one. Seal and drop them both. They hit the floor at the same time. Then take two identical sheets of paper. Crumple one and leave the other flat. Drop them. The compact one hits first. If you did this experiment on the Moon, they would strike at the same time. (Watch an Apollo demonstration.) The speed of falling does not depend on mass. In an atmosphere, objects with more surface area fall more slowly than compact objects.

*The equation for gravity is g=G x M/D² where G is a constant, M is mass, and D is the distance. GM/D² for the surface divided by GM/D² for 200 miles up ends up with all terms except D cancelling out, i.e. 4000×4000/4200×4200=0.9.

**For calculating orbital velocity (v=√GM/r) see Gravitation Calculating Orbital Velocity of a Satellite, Step-by-Step Science.



Writing about Space

Analog readers, watch for my guest editorial on Gender Parity in the July/August issue.

My next book, coauthored with Buzz Aldrin, To the Moon and Back: My Apollo 11 Adventure, a pop-up book from National Geographic, is available for preorder now from Amazon. Look for it in stores in October.

Speaking about Space

I offer programs for school-aged children up through senior citizens, as well as science workshops for students and teachers. Please consider me for Author Visits.

Nevada Space Center
I was inducted into the Nevada Space Center Hall of Fame on May 5, 2018. Prior to the evening event, I received a T-shirt from Challenger Center Flight Director Jenny McFarlane and former NASA Flight Director Paul Dye. Photo: Nevada Space Center.

Saturday, May 12, 1:00-4:30 PM. Speaking on “How to Publish a Book” at Houston Writers House. Event sold out, but watch for another session to be scheduled.

Tuesday, May 15, 9-10 AM Passion for Space, 10-11 AM, Children’s program, Laredo Public Library.

Tuesday, May 22, author visit to The Westview School.

Friday, May 25, panelist, Comicpalooza, George Brown Convention Center, Houston.

Thursday, June 7, vendor fair participant, Setting the Trend, Librarians as Leaders conference. Clear Falls High School, 4380 Village Way, League City, TX.

September 21-23, Science GOH at FenCon XV in Dallas. See their website for program details. Writer GOH is Larry Niven.

See my website’s contact page for a complete appearance schedule.

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,