# Apollo 11's velocity is dropping. Why? **URL:** \ **Category:** Factual Questions\ **Created:** [July 17, 2009, 1:48pm UTC](https://boards.straightdope.com/t/apollo-11s-velocity-is-dropping-why/503332 "2009-07-17T13:48:06Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![Redsland](https://avatars.discourse-cdn.com/v4/letter/r/9de0a6/32.png) [@Redsland](https://boards.straightdope.com/u/Redsland)\ **Post date:** [July 17, 2009, 1:48pm UTC](https://boards.straightdope.com/t/apollo-11s-velocity-is-dropping-why/503332/1 "2009-07-17T13:48:06Z")
I’m following the Apollo 11 mission at [http://www.wechoosethemoon.org](http://www.wechoosethemoon.org) and I’ve noticed something that has surprised me. I have always heard that, due to the lack of friction in space, once a vehicle gets going out there, you can shut the engine off and the vehicle keeps right on going. But what I’ve noticed since the Command and Service Modules left Earth orbit yesterday is that the velocity has dropped. As I type this, it’s at 5,356 feet per second. So what is causing the decline in speed? I doubt the vehicle is using propellant to slow itself at this point in the journey. There isn’t supposed to be any friction out there to slow it down. Earth’s gravity maybe? But that’s supposed to be awfully weak at a distance, and we’re talking about a small ship 100,000 miles from the planet. Thanks in advance for your input. ---
**Author:** ![DSYoungEsq](https://avatars.discourse-cdn.com/v4/letter/d/c6cbf5/32.png) [@DSYoungEsq](https://boards.straightdope.com/u/DSYoungEsq)\ **Post date:** [July 17, 2009, 1:54pm UTC](https://boards.straightdope.com/t/apollo-11s-velocity-is-dropping-why/503332/2 "2009-07-17T13:54:23Z")
How much of a drop in velocity? Over what time period? ---
**Author:** ![Earl\_Snake-Hips\_Tucker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/earl_snake-hips_tucker/32/3269_2.png) [@Earl\_Snake-Hips\_Tucker](https://boards.straightdope.com/u/Earl_Snake-Hips_Tucker)\ **Post date:** [July 17, 2009, 1:54pm UTC](https://boards.straightdope.com/t/apollo-11s-velocity-is-dropping-why/503332/3 "2009-07-17T13:54:35Z")
That big honkin’ sphere called ‘The Earth’ is still just a-steady tugging on it. At a certain point (maybe 40K miles from the Moon?), the Moon will have a greater influence, and it will start accelerating again. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [July 17, 2009, 1:56pm UTC](https://boards.straightdope.com/t/apollo-11s-velocity-is-dropping-why/503332/4 "2009-07-17T13:56:37Z")
Yes, gravity. The size of the ship is irrelevant. The velocity will continue to drop until the point where the Moon’s gravitational attraction is stronger than the Earth’s, and then it will speed up again. ---
**Author:** ![chrisk](https://avatars.discourse-cdn.com/v4/letter/c/6de8d8/32.png) [@chrisk](https://boards.straightdope.com/u/chrisk)\ **Post date:** [July 17, 2009, 2:01pm UTC](https://boards.straightdope.com/t/apollo-11s-velocity-is-dropping-why/503332/5 "2009-07-17T14:01:35Z")
> [@Redsland](#): > > Earth’s gravity maybe? But that’s supposed to be awfully weak at a distance, and we’re talking about a small ship 100,000 miles from the planet. I’d put my money on this. Gravity does get weaker at distances, but not that weak. Acceleration due to Earth’s gravity (or decceleration, if you’re going away from it,) at 100,000 miles distance, would be 0.015 of a meter per second squared, if I’ve used this calculator right: [Gravity Equations Formulas Calculator - Gravitational Acceleration](http://www.ajdesigner.com/phpgravity/gravity_acceleration_equation.php) That would add up to 1296 meters/second, or 4252 feet/second, over the course of 24 hours. (Of course, it’s not going to be at that distance for 24 hours, but I think it gives you an idea.) ETA: Beaten to the point, but I brought the numbers. 😉 ---
**Author:** ![Duhkecco](https://avatars.discourse-cdn.com/v4/letter/d/8491ac/32.png) [@Duhkecco](https://boards.straightdope.com/u/Duhkecco)\ **Post date:** [July 17, 2009, 2:14pm UTC](https://boards.straightdope.com/t/apollo-11s-velocity-is-dropping-why/503332/6 "2009-07-17T14:14:50Z")
This how I got about the same rate of acceleration: 1. Distance from Earth’s center at the Earth’s surface is about 4,000 miles. 2. The rate of acceleration due to gravity at Earth’s surface is about 10m/sec squared. 3. 100,000 miles is 25 times this distance. 4. 1/25 squared = 1/625. 5. The rate of acceleration at 100,000 miles is about 1/625 of what it is on the earth’s surface. 6. 10/625 = 0.016. This doesn’t take into account the fact that the gravitational force between the Moon and the ship is increasing the closer the ship gets to the Moon. ---
**Author:** ![Malacandra](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Malacandra](https://boards.straightdope.com/u/Malacandra)\ **Post date:** [July 17, 2009, 2:22pm UTC](https://boards.straightdope.com/t/apollo-11s-velocity-is-dropping-why/503332/7 "2009-07-17T14:22:28Z")
And this is all by design, because they’ve given that ship just enough of a boost to match orbits with the Moon - they don’t want it to go whistling past, or have to do a big burn to slow down when it gets there. “Small ship” is irrelevant - its lesser weight is exactly counterbalanced by its lesser momentum, and if it had been an Imperial Star Destroyer at the exact same speed, it would have been decelerating just the same. Think cannon balls and Leaning Towers. ---
**Author:** ![Duhkecco](https://avatars.discourse-cdn.com/v4/letter/d/8491ac/32.png) [@Duhkecco](https://boards.straightdope.com/u/Duhkecco)\ **Post date:** [July 17, 2009, 2:26pm UTC](https://boards.straightdope.com/t/apollo-11s-velocity-is-dropping-why/503332/8 "2009-07-17T14:26:27Z")
> [@Malacandra](#): > > Think cannon balls and Leaning Towers. Oh, do you mean Galileo’s thought experiment? Or the experiment that never happened: [Galileo's Leaning Tower of Pisa experiment - Wikipedia](http://en.wikipedia.org/wiki/Galileo%27s_Leaning_Tower_of_Pisa_experiment) ---
**Author:** ![MrFloppy](https://avatars.discourse-cdn.com/v4/letter/m/43a26b/32.png) [@MrFloppy](https://boards.straightdope.com/u/MrFloppy)\ **Post date:** [July 17, 2009, 2:43pm UTC](https://boards.straightdope.com/t/apollo-11s-velocity-is-dropping-why/503332/9 "2009-07-17T14:43:27Z")
I just have to say that [www.wechoosethemoon.org](http://www.wechoosethemoon.org) is very cool. It’s a shame that the weekend gets in the way. I was enjoying it playing in the background in my cubicle. And I’ll be on the road on Monday ☹ ---
**Author:** ![Sam\_Stone](https://avatars.discourse-cdn.com/v4/letter/s/ecccb3/32.png) [@Sam\_Stone](https://boards.straightdope.com/u/Sam_Stone)\ **Post date:** [July 17, 2009, 2:49pm UTC](https://boards.straightdope.com/t/apollo-11s-velocity-is-dropping-why/503332/10 "2009-07-17T14:49:35Z")
Just imagine what would happen if Apollo didn’t quite have escape velocity. It would slow down, stop, then fall back to Earth. If it has just slightly over escape velocity, it will slow down, almost stop, then be captured by the moon’s gravity and start accelerating again. ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [July 17, 2009, 3:29pm UTC](https://boards.straightdope.com/t/apollo-11s-velocity-is-dropping-why/503332/11 "2009-07-17T15:29:50Z")
> [@Redsland](#): > > …I have always heard that, due to the lack of friction in space, once a vehicle gets going out there, you can shut the engine off and the vehicle keeps right on going. Only in the absence of external forces, such as gravity. > [@Redsland](#): > > …Earth’s gravity maybe? But that’s supposed to be awfully weak at a distance, and we’re talking about a small ship 100,000 miles from the planet. Absoutely it’s Earth’s gravity. And while the force of gravity does get weaker with distance, it has an infinite range. Realize that the Moon is about 240,000 miles from the Earth, and gravity keeps it in orbit around the Earth. ---
**Author:** ![KneadToKnow](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kneadtoknow/32/3999_2.png) [@KneadToKnow](https://boards.straightdope.com/u/KneadToKnow)\ **Post date:** [July 17, 2009, 3:43pm UTC](https://boards.straightdope.com/t/apollo-11s-velocity-is-dropping-why/503332/12 "2009-07-17T15:43:04Z")
nevermind ---
**Author:** ![Cliffy](https://avatars.discourse-cdn.com/v4/letter/c/59ef9b/32.png) [@Cliffy](https://boards.straightdope.com/u/Cliffy)\ **Post date:** [July 17, 2009, 3:45pm UTC](https://boards.straightdope.com/t/apollo-11s-velocity-is-dropping-why/503332/13 "2009-07-17T15:45:01Z")
> [@Redsland](#): > > Earth’s gravity maybe? But that’s supposed to be awfully weak at a distance, and we’re talking about a small ship 100,000 miles from the planet. Note that Earth’s gravity is strong enough to hold on to the moon itself, and so far 11 isn’t nearly that far away. > [@Duhkecco](#): > > Oh, do you mean Galileo’s thought experiment? > Or the experiment that never happened But [this one](http://video.google.com/videoplay?docid=6926891572259784994) did. –Cliffy ---
**Author:** ![Icerigger](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/icerigger/32/432_2.png) [@Icerigger](https://boards.straightdope.com/u/Icerigger)\ **Post date:** [July 17, 2009, 4:36pm UTC](https://boards.straightdope.com/t/apollo-11s-velocity-is-dropping-why/503332/14 "2009-07-17T16:36:17Z")
BTW The **New Horizons** space probe launched to Pluto a few years ago left earth with the highest velocity of any previous spacecraft around 36,000 MPH. It shot past the moon in less than 8 hours.