A Message to Dr. Neil deGrasse Tyson re: Zero Gravity

Dr. Neil deGrasse Tyson, if you’re reading this, I respect you greatly.  I watched all of the Cosmos series, I follow you on Twitter, I will be visiting the Hayden Planetarium this summer, and I think that you’re excellent as a spokesperson and role model for future scientists.  But on the subject of zero gravity . . . with all due respect, sir . . . you are absolutely, 100% wrong.

Star Talk is Dr. Tyson’s new television program on the National Geographic Channel.  His guest on a recent episode was astronaut, Mike Massomino.  Seemingly out of nowhere, the topic of microgravity or zero-gravity came up.  Dr. Tyson (who I respect greatly) went on a tirade about how microgravity is not accurate to describe astronauts in orbit in the shuttle or space station and that it is actually zero gravity.  He went on to give an example that if you are standing on a scale in an elevator and he cuts the cable, you cannot weigh yourself on the way down.

Dr. Tyson (who fascinates me every time I hear him speak), you are wrong. 

I should point out that Dr. Tyson has a Bachelor’s Degree in physics from Harvard, and a Masters Degree and Doctorate in astrophysics from Columbia.  I hold a Bachelor’s Degree in physics from San Diego State University.  But degrees do not make either of us necessarily right or wrong.  Evidence, data, and scientific consensus do.

The fact that one cannot weigh oneself does not mean that there is no gravity.  You might call this weightlessness (although I’ll argue against that, too), but you CANNOT call it zero gravity.  It is abundantly clear from numerous lines of evidence that everywhere near the surface of the earth, gravity exists.  Whether you are falling down an elevator shaft or not has no impact on whether there exists gravity in said elevator shaft.  Satellites exist that can measure the gravitational field near the surface of the earth and I absolutely, 100% guarantee you that they would measure a gravitational field inside of the elevator shaft as it is falling.  It is also abundantly clear that there must be a gravitational field between the Earth and moon because the gravitational attraction between the Earth and moon is what keeps the moon in orbit.  So, there is no evidence at all to support the notion that the astronauts orbiting the Earth are not under the influence of gravity.  In fact, Dr. Tyson said on the Star Talk episode that the reason that the astronauts are experiencing this apparent weightlessness is because they are falling.  What is causing them to fall?  Gravity.

Here is a map of the surface gravity of the Earth as determined by satellites.  There is no area on or near the surface of the planet that is devoid of gravity.

http://www.popsci.com/science/article/2010-06/esa-satellite-maps-earths-gravity-3-d

or this one

Maybe what you meant to say is that some people call this “apparent weightlessness” and you were saying that it is “actual weightlessness.”  That is not correct either.  Weight is the gravitational attraction between two objects.  Any two objects that have mass and are sufficiently close to each other have a gravitational attraction to each other and could be said to have weight.  The equation for weight is the mass of the object times the acceleration of gravity.  An astronaut is made of matter and thus has mass and as demonstrated above, there is a gravitational field at the distances where astronauts orbit, therefore, the astronaut has weight whether our earthly spring scales can measure it or not.
Or maybe what you meant to say is that the farther one is from the Earth's surface, the smaller the force exerted by gravity.  Although this relationship is absolutely true, the effect on astronauts orbiting in the space station or in a space shuttle is minimal.  At space station height (~400 km), the acceleration of gravity only decreases about 12%.  This website allows you to enter the height above Earth's surface and find out what fraction of surface gravity exists there.  The value for the space station is 88%.  http://hyperphysics.phy-astr.gsu.edu/hbase/orbv.html 

Dictionary.reference.com defines weight as, “Physics. the force that gravitation exerts upon a body, equal to the mass of the body times the local acceleration of gravity.”  Clearly, there is a force that gravitation exerts upon the body, because as you said, the astronaut is falling.  The reason that the astronaut is falling is because gravity is pulling on him/her.  By definition of orbiting, there MUST be gravity.  Hence, anything that is orbiting another object must be under the influence of its gravitational field and therefore cannot be said to be in zero gravity.  Objects do not fall if gravity is not acting on them.  By definition, all objects have mass.  By the definition given above, an object with mass (read: all objects) in a gravitational field (like all objects in orbit are) have weight.

In the episode, Dr. Tyson argues that astronauts are in orbit because they are falling and travelling horizontally at the same time.  He gives the traditional example of a cannon ball launched from a mountain-top.  If the cannon ball is launched fast enough, it will never hit the ground.  Excellent example and completely true, but also completely irrelevant to this topic.  All that this shows is that at a certain speed, the Earth’s surface is curving away from the cannon ball faster than it is falling, therefore, it will never hit the ground.  Certainly, Dr. Tyson is not saying that the ball does not have mass.  Certainly, he is not saying that there is no gravitational field at the top of a mountain or that the cannon ball is falling due to some influence other than gravity.  So, certainly, it can neither be said that there is zero gravity nor that the ball is weightless.

To summarize:

1. Astronauts are falling
2. They are falling because of gravity
3. Therefore, one cannot say that astronauts are in zero gravity
4. We cannot measure the weight of an astronaut in orbit with a scale
5. We can calculate the weight of an astronaut in orbit
6. Astronauts have mass and are in a gravitational field (12% smaller than surface gravity)
7. Astronauts, then, also have weight
8. Therefore, one cannot say that astronauts are weightless

Did I mention how great I think that Dr. Tyson is regardless of this slip up?  Here, to show that I still appreciate him, here is a link to his book that I like the most: