# First People in Space, Gravity **URL:** \ **Category:** Factual Questions\ **Created:** [May 12, 2015, 10:07pm UTC](https://boards.straightdope.com/t/first-people-in-space-gravity/719890 "2015-05-12T22:07:03Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Robot\_Arm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robot_arm/32/18280_2.png) [@Robot\_Arm](https://boards.straightdope.com/u/Robot_Arm)\ **Post date:** [May 13, 2015, 3:10am UTC](https://boards.straightdope.com/t/first-people-in-space-gravity/719890/21 "2015-05-13T03:10:49Z")
> [@cmyk](#): > > The Moon, which is about 240,000 miles away is kept in orbit by Earth’s gravity. No one has been past the orbit of the Moon (that is 12 men from the Apollo missions), and rest only orbit about 250 miles above the surface of the Earth. A couple of the Gemini missions went fairly high, as such things go; Gemini X (408 nautical miles) and Gemini XI (739 nmi). > [@Johnny\_L.A](#): > > Sort of. The Moon is going just a _little_ too fast to stay in orbit. It’s moving away from Earth at a rate of 3.78 cm (1.5 in.) per year. If the moon had sufficient energy to leave orbit now, it would. If it’s moving away from Earth, it must be gaining energy (or something else is going on). Maybe someday it’ll be going too fast to stay in orbit, but it ain’t there yet. > [@cmyk](#): > > Actually, that’s 24 men from the Apollo missions. :smack: Can’t forget the ones who made to Lunar orbit, but never walked on the surface. I went to double check your count, and you’re right. It’s not as simple as it sounds, though. Three crew members per mission, six missions landed, two went to the moon without planning to land, and Apollo XIII which went around the moon on its return trajectory to the Earth. That’s 27, except Cernan, Lovell, and Young each went twice. ---
**Author:** ![Robot\_Arm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robot_arm/32/18280_2.png) [@Robot\_Arm](https://boards.straightdope.com/u/Robot_Arm)\ **Post date:** [May 13, 2015, 3:16am UTC](https://boards.straightdope.com/t/first-people-in-space-gravity/719890/22 "2015-05-13T03:16:31Z")
> [@Ranger\_Jeff](#): > > What medicos didn’t know were the effects of weightlessness on humans. Read Tom Wolfe’s _The Right Stuff_ - the Mercury astronauts were NOT selected because of their engineering or piloting skills; they were selected as medical test subjects. It’s also worth pointing out that both the U.S. and USSR sent animals into space before humans, in part to make sure that anything could survive in prolonged weightlessness. For example, would blood still circulate without some force pulling it downward to then be returned to the heart?Yes. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [May 13, 2015, 9:14am UTC](https://boards.straightdope.com/t/first-people-in-space-gravity/719890/23 "2015-05-13T09:14:23Z")
> [@Johnny\_L.A](#): > > Sort of. The Moon is going just a _little_ too fast to stay in orbit. It’s moving away from Earth at a rate of 3.78 cm (1.5 in.) per year. > [@Robot\_Arm](#): > > If the moon had sufficient energy to leave orbit now, it would. If it’s moving away from Earth, it must be gaining energy (or something else is going on). Maybe someday it’ll be going too fast to stay in orbit, but it ain’t there yet. The moon is moving away from Earth, and it is gaining energy due to tidal forces. The steadily increasing orbital radius means it has a steadily decreasing orbital speed. And it’s not going to gain so much energy it can’t stay in orbit. ---
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [May 13, 2015, 9:21am UTC](https://boards.straightdope.com/t/first-people-in-space-gravity/719890/24 "2015-05-13T09:21:03Z")
> [@cmyk](#): > > Actually, that’s 24 men from the Apollo missions. :smack: Can’t forget the ones who made to Lunar orbit, but never walked on the surface. Only 22 men have been in Lunar Orbit. ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [May 13, 2015, 9:36am UTC](https://boards.straightdope.com/t/first-people-in-space-gravity/719890/25 "2015-05-13T09:36:03Z")
> [@AK84](#): > > Only 22 men have been in Lunar Orbit. Wiki has a [by-name list](http://en.wikipedia.org/wiki/List_of_Apollo_astronauts#Apollo_astronauts_who_flew_to_the_Moon_without_landing) of all 24 men who have made it around the moon and back. Are you just not counting Swigert and Haise beause it wasn’t really an orbit? ---
**Author:** ![Colophon](https://avatars.discourse-cdn.com/v4/letter/c/f05b48/32.png) [@Colophon](https://boards.straightdope.com/u/Colophon)\ **Post date:** [May 13, 2015, 10:20am UTC](https://boards.straightdope.com/t/first-people-in-space-gravity/719890/26 "2015-05-13T10:20:13Z")
Others have touched on it, but it’s important to note that there really isn’t “zero gravity” in space. At the height of the International Space Station (say 400km above the Earth), the pull of gravity from the Earth is not much less than it is on the Earth itself. How much less? On the ground you are about 6,400km from the centre of the Earth. On the ISS you are about 6,800km from the centre of the Earth. Gravity is proportional to 1/r^2, so the gravity on the ISS is 9.8 x (6400^2)/(6800^2) = 8.68 m s^-2 or less than 13% weaker than on Earth. So if the ISS was somehow held stationary, at the same altitude that it orbits, you would only be 13% lighter than you are on Earth. You probably would barely notice. ---
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [May 13, 2015, 10:26am UTC](https://boards.straightdope.com/t/first-people-in-space-gravity/719890/27 "2015-05-13T10:26:44Z")
> [@Bear\_Nenno](#): > > Wiki has a [by-name list](http://en.wikipedia.org/wiki/List_of_Apollo_astronauts#Apollo_astronauts_who_flew_to_the_Moon_without_landing) of all 24 men who have made it around the moon and back. Are you just not counting Swigert and Haise beause it wasn’t really an orbit? Yes. They never orbited the moon. Lovell of course did on Apollo 8. ---
**Author:** ![Donnerwetter](https://avatars.discourse-cdn.com/v4/letter/d/b2d939/32.png) [@Donnerwetter](https://boards.straightdope.com/u/Donnerwetter)\ **Post date:** [May 13, 2015, 10:34am UTC](https://boards.straightdope.com/t/first-people-in-space-gravity/719890/28 "2015-05-13T10:34:04Z")
They did reduced gravity flights as early as the 1950s to simulate weightlessness and prepare the astronauts: → [Reduced-gravity aircraft - Wikipedia](http://en.wikipedia.org/wiki/Reduced_gravity_aircraft) > [@](#): > > Project Mercury astronauts on board a C-131 Samaritan flying as the “vomit comet”, 1959. Edit: I just saw that this has already been mentioned. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [May 13, 2015, 11:44am UTC](https://boards.straightdope.com/t/first-people-in-space-gravity/719890/29 "2015-05-13T11:44:56Z")
> [@Robot\_Arm](#): > > If the moon had sufficient energy to leave orbit now, it would. If it’s moving away from Earth, it must be gaining energy (or something else is going on). Maybe someday it’ll be going too fast to stay in orbit, but it ain’t there yet. This. See [tidal acceleration.](http://en.wikipedia.org/wiki/Tidal_acceleration) The moon’s tidal forces on the earth cause the earth to distort/bulge toward the moon on its near side, the earth’s rapid rotation carries that bulge “ahead” of the moon, and the gravitational effects of that bulge accelerate the moon along its orbital path. The related effect is that the earth’s spin is slowing down (this is where the energy comes from to accelerate the moon). ---
**Author:** ![MonkeyMensch](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@MonkeyMensch](https://boards.straightdope.com/u/MonkeyMensch)\ **Post date:** [May 13, 2015, 4:46pm UTC](https://boards.straightdope.com/t/first-people-in-space-gravity/719890/30 "2015-05-13T16:46:21Z")
> [@Knorf](#): > > Yep. An orbit is just free fall where you are high enough and fall fast enough to miss the ground. And so Douglas Adams’ idea of how to fly was right all along. Kinda… ---
**Author:** ![Knorf](https://avatars.discourse-cdn.com/v4/letter/k/58956e/32.png) [@Knorf](https://boards.straightdope.com/u/Knorf)\ **Post date:** [May 13, 2015, 5:07pm UTC](https://boards.straightdope.com/t/first-people-in-space-gravity/719890/31 "2015-05-13T17:07:46Z")
> [@MonkeyMensch](#): > > And so Douglas Adams’ idea of how to fly was right all along. Kinda… Absolutely! The difficult bit is the way air tends to slow you down. ---
**Author:** ![Robot\_Arm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robot_arm/32/18280_2.png) [@Robot\_Arm](https://boards.straightdope.com/u/Robot_Arm)\ **Post date:** [May 13, 2015, 5:15pm UTC](https://boards.straightdope.com/t/first-people-in-space-gravity/719890/32 "2015-05-13T17:15:24Z")
> [@naita](#): > > The moon is moving away from Earth, and it is gaining energy due to tidal forces. The steadily increasing orbital radius means it has a steadily decreasing orbital speed. And it’s not going to gain so much energy it can’t stay in orbit. > [@Machine\_Elf](#): > > This. See [tidal acceleration.](http://en.wikipedia.org/wiki/Tidal_acceleration) The moon’s tidal forces on the earth cause the earth to distort/bulge toward the moon on its near side, the earth’s rapid rotation carries that bulge “ahead” of the moon, and the gravitational effects of that bulge accelerate the moon along its orbital path. The related effect is that the earth’s spin is slowing down (this is where the energy comes from to accelerate the moon). I figured the moon must be gaining energy if it’s getting farther away. I didn’t know where it could be getting the energy from, and I know the Earth is still accreting dust and such, so I wasn’t sure what that would do to the equations. And looking it up just now I found a cite that says the Earth is losing mass each year, mostly due to escaping hydrogen and helium. ---
**Author:** ![DrCube](https://avatars.discourse-cdn.com/v4/letter/d/a3d4f5/32.png) [@DrCube](https://boards.straightdope.com/u/DrCube)\ **Post date:** [May 13, 2015, 5:49pm UTC](https://boards.straightdope.com/t/first-people-in-space-gravity/719890/33 "2015-05-13T17:49:43Z")
> [@Machine\_Elf](#): > > The related effect is that the earth’s spin is slowing down (this is where the energy comes from to accelerate the moon). Isaac Asimov informed me that with the observed rate of slowing down, the Earth must have had a 13 hour day when it originally formed. The day has been getting longer ever since, almost doubling to its current 24 hours. Eventually the moon and Earth will be tidally locked, so the moon will only be visible on one side of the Earth and its orbit will stop increasing. I don’t remember what the length of the day will be then, but the 36-hour range sounds vaguely familiar. ---
**Author:** ![Frazzled](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/frazzled/32/3368_2.png) [@Frazzled](https://boards.straightdope.com/u/Frazzled)\ **Post date:** [May 13, 2015, 6:28pm UTC](https://boards.straightdope.com/t/first-people-in-space-gravity/719890/34 "2015-05-13T18:28:29Z")
> [@DrCube](#): > > Isaac Asimov informed me that with the observed rate of slowing down, the Earth must have had a 13 hour day when it originally formed. The day has been getting longer ever since, almost doubling to its current 24 hours. Eventually the moon and Earth will be tidally locked, so the moon will only be visible on one side of the Earth and its orbit will stop increasing. I don’t remember what the length of the day will be then, but the 36-hour range sounds vaguely familiar. If the Earth and Moon were tidally locked, then a day on Earth would be the same length as one orbit of the moon, about 30 days now - and much longer by the time the 2 actually are locked. ---
**Author:** ![buddha\_david](https://avatars.discourse-cdn.com/v4/letter/b/ee7513/32.png) [@buddha\_david](https://boards.straightdope.com/u/buddha_david)\ **Post date:** [May 13, 2015, 6:49pm UTC](https://boards.straightdope.com/t/first-people-in-space-gravity/719890/35 "2015-05-13T18:49:27Z")
> [@DrCube](#): > > Eventually the moon and Earth will be tidally locked… Given enough time, this would be true. However, the sun will go nova and incinerate the Earth & Moon long before that happens. ---
**Author:** ![Biffy\_the\_Elephant\_Shrew](https://avatars.discourse-cdn.com/v4/letter/b/e47c2d/32.png) [@Biffy\_the\_Elephant\_Shrew](https://boards.straightdope.com/u/Biffy_the_Elephant_Shrew)\ **Post date:** [May 13, 2015, 7:20pm UTC](https://boards.straightdope.com/t/first-people-in-space-gravity/719890/36 "2015-05-13T19:20:02Z")
> [@Johnny\_L.A](#): > > Sort of. The Moon is going just a _little_ too fast to stay in orbit. It’s moving away from Earth at a rate of 3.78 cm (1.5 in.) per year. I \*thought \*it was looking smaller lately. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [May 13, 2015, 7:31pm UTC](https://boards.straightdope.com/t/first-people-in-space-gravity/719890/37 "2015-05-13T19:31:04Z")
> [@Biffy\_the\_Elephant\_Shrew](#): > > I \*thought \*it was looking smaller lately. Indeed. A very long time ago (in living-thing terms) the Moon would block out the entire Sun during an eclipse. Now they appear about the same size. In the far future, the Moon will not cover the Sun during an eclipse. Oh, about the tidally-locked Earth-Moon system. I wouldn’t worry about it. By the time that happens, the Earth will long have been burnt to a crisp by the Sun’s red giant phase. ---
**Author:** ![DrCube](https://avatars.discourse-cdn.com/v4/letter/d/a3d4f5/32.png) [@DrCube](https://boards.straightdope.com/u/DrCube)\ **Post date:** [May 13, 2015, 7:43pm UTC](https://boards.straightdope.com/t/first-people-in-space-gravity/719890/38 "2015-05-13T19:43:44Z")
> [@Johnny\_L.A](#): > > Indeed. A very long time ago (in living-thing terms) the Moon would block out the entire Sun during an eclipse. Now they appear about the same size. This is a very interesting coincidence. Human civilization has existed during the small window of time where we get both annular (the moon is smaller than the sun and leaves a ring of light around it) and total eclipses (the moon blocks out the sun entirely) depending on the time of the year. Pretty soon (anybody know the time frame on this?) there will be no more total eclipses. ---
**Author:** ![DrCube](https://avatars.discourse-cdn.com/v4/letter/d/a3d4f5/32.png) [@DrCube](https://boards.straightdope.com/u/DrCube)\ **Post date:** [May 13, 2015, 7:50pm UTC](https://boards.straightdope.com/t/first-people-in-space-gravity/719890/39 "2015-05-13T19:50:14Z")
Time of the _month_, I should say. Whether an eclipse is annular or total depends on how close the moon is to the earth. EDIT: More research tells me it’s both. The sun’s apparent size changes over the course of a year and the moon’s apparent size changes over the course of the lunar month. Therefore both time of year and time of month determine whether an eclipse will be total or not. ---
**Author:** ![Blue\_Blistering\_Barnacle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/blue_blistering_barnacle/32/3386_2.png) [@Blue\_Blistering\_Barnacle](https://boards.straightdope.com/u/Blue_Blistering_Barnacle)\ **Post date:** [May 13, 2015, 10:05pm UTC](https://boards.straightdope.com/t/first-people-in-space-gravity/719890/40 "2015-05-13T22:05:53Z")
> [@DrCube](#): > > Isaac Asimov informed me that with the observed rate of slowing down, the Earth must have had a 13 hour day when it originally formed. The day has been getting longer ever since, almost doubling to its current 24 hours. Eventually the moon and Earth will be tidally locked, so the moon will only be visible on one side of the Earth and its orbit will stop increasing. I don’t remember what the length of the day will be then, but the 36-hour range sounds vaguely familiar. Asimiv told me, too. I thank him for this as it helped me to understand Io, Eropa, etc, as we learned more about them. In my head- “Of course, tidal energy, just like Asimov said.” There is fossil evidence of a shorter day in ancient seashells (due to daily and seasonal variations in calcium deposition). I don’t have a cite for this. I’ll try to find it. Here is a description of a different method, looking at “tidalites”. [http://www3.scienceblog.com/community/older/1996/A/199600486.html](http://www3.scienceblog.com/community/older/1996/A/199600486.html) [Previous page](https://boards.straightdope.com/t/first-people-in-space-gravity/719890.md?page=1) [Next page](https://boards.straightdope.com/t/first-people-in-space-gravity/719890.md?page=3)