# Lowest Possible Earth Orbit **URL:** \ **Category:** Factual Questions\ **Created:** [March 13, 2004, 4:51pm UTC](https://boards.straightdope.com/t/lowest-possible-earth-orbit/234084 "2004-03-13T16:51:59Z")\ **Posts on this page:** 16\ **Page:** 1
**Author:** ![The\_Great\_Zamboni](https://avatars.discourse-cdn.com/v4/letter/t/4bbf92/32.png) [@The\_Great\_Zamboni](https://boards.straightdope.com/u/The_Great_Zamboni)\ **Post date:** [March 13, 2004, 4:51pm UTC](https://boards.straightdope.com/t/lowest-possible-earth-orbit/234084/1 "2004-03-13T16:51:59Z")
Whatis the lowest possible altitude at which Earth can be orbited? Would atmospheric density determine this, or is there a point where not enough speed could be maintained to stay above the surface? All the astronomy books and magazines I read over the years have never mentioned this. You would think I would see at least one article. ---
**Author:** ![herman\_and\_bill](https://avatars.discourse-cdn.com/v4/letter/h/f1d935/32.png) [@herman\_and\_bill](https://boards.straightdope.com/u/herman_and_bill)\ **Post date:** [March 13, 2004, 5:10pm UTC](https://boards.straightdope.com/t/lowest-possible-earth-orbit/234084/2 "2004-03-13T17:10:42Z")
Rutan and Yeager orbitid the earth non-stop in nine days at an average altitude of 11000 feet in 1986. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [March 13, 2004, 5:18pm UTC](https://boards.straightdope.com/t/lowest-possible-earth-orbit/234084/3 "2004-03-13T17:18:23Z")
First we need a better definition of “orbit.” Jets have flown complete circumnavigations of the globe without landing. This is obviously an orbit, but probably not what you mean. I assume what you are looking for is the minimum height at which an object that has reached sufficient speed can maintain its trajectory without requiring additional expense of fuel and without burning up from atmospheric friction. I’m going to say that rocket designers already use minimum orbit heights to conserve fuel and maximize useful life. (And yet satellites still burn up even from that minimum friction over time.) The lowest orbits appear to be about 200 mi (320km) above the earth and take a near-minimum 90 minutes to circumnavigate the earth. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [March 13, 2004, 5:20pm UTC](https://boards.straightdope.com/t/lowest-possible-earth-orbit/234084/4 "2004-03-13T17:20:02Z")
The exact altitude would depend on the density of the atmosphere, with increased solar activity causing the atmosphere to billow up to greater altitudes. But for working purposes, NASA counts “nominal reentry” at 400,000 feet or ~75 miles. And the lowest orbit I’ve ever read about was specialized reconnaisance satellites at 80 miles. So somewhere in there. ---
**Author:** ![The\_Great\_Zamboni](https://avatars.discourse-cdn.com/v4/letter/t/4bbf92/32.png) [@The\_Great\_Zamboni](https://boards.straightdope.com/u/The_Great_Zamboni)\ **Post date:** [March 13, 2004, 5:45pm UTC](https://boards.straightdope.com/t/lowest-possible-earth-orbit/234084/5 "2004-03-13T17:45:02Z")
> [@Exapno Mapcase](#): > > Jets have flown complete circumnavigations of the globe without landing. This is obviously an orbit, but probably not what you mean. No, it’s not. Those jets expensed fuel and required the atmosphere to maintain lift on the wings. That is just a very long normal flight. > [@Exapno Mapcase](#): > > The lowest orbits appear to be about 200 mi (320km) above the earth and take a near-minimum 90 minutes to circumnavigate the earth. Is the velocity needed to maintain this minimum speed and altitude the cheapest and simplest to attain, i.e., the most efficient use of fuel and fastest way to orbit? I assume to maintain a lower orbit, a sattelite or Space Shuttle would have to use more fuel to gain a higher speed, which is expensive. Because more fuel is also needed to get to a higher orbit and escape velocity, then is that 200 miles-320 km a “happy medium?” > [@herman\_and\_bill](#): > > Rutan and Yeager orbitid the earth non-stop in nine days at an average altitude of 11000 feet in 1986. 11,000 feet is very low. Is that a typo meant to be 110,000 feet? At 11,000 feet, they would have hit a few mountains. ---
**Author:** ![MonkeyMensch](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@MonkeyMensch](https://boards.straightdope.com/u/MonkeyMensch)\ **Post date:** [March 13, 2004, 7:22pm UTC](https://boards.straightdope.com/t/lowest-possible-earth-orbit/234084/6 "2004-03-13T19:22:06Z")
> [@The Great Zamboni](#): > > Is the velocity needed to maintain this minimum speed and altitude the cheapest and simplest to attain, i.e., the most efficient use of fuel and fastest way to orbit? I assume to maintain a lower orbit, a sattelite or Space Shuttle would have to use more fuel to gain a higher speed, which is expensive. Because more fuel is also needed to get to a higher orbit and escape velocity, then is that 200 miles-320 km a “happy medium?” The higher the orbit, the more energy is needed to reach it. It’s true that the orbital speed drops with altitude, but the potential energy of the body in a higher orbit more than makes up for the decreased kinetic energy needed. You might also think about what orbit means. Even at a few hundred miles above the Earth bodies are not “orbiting” perfectly. At those altitudes there is enough atmosphere present to degrade orbital energy; that is to say, the object is spiraling in. Skylab was originally launched at 250 miles, or so, altitude and we all know what happened to it. Compared to intergalactic vacuums the atmosphere at hundreds of miles above the Earth is positively _thick_. ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [March 13, 2004, 7:43pm UTC](https://boards.straightdope.com/t/lowest-possible-earth-orbit/234084/7 "2004-03-13T19:43:47Z")
> [@The Great Zamboni](#): > > 11,000 feet is very low. Is that a typo meant to be 110,000 feet? At 11,000 feet, they would have hit a few mountains. Psst, **Zamboni** : He’s talking about the flight of the [Voyager.](http://www.centennialofflight.gov/essay/Explorers_Record_Setters_and_Daredevils/rutan/EX32.htm) ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [March 13, 2004, 7:59pm UTC](https://boards.straightdope.com/t/lowest-possible-earth-orbit/234084/8 "2004-03-13T19:59:50Z")
Which leads to a related question: if the Earth were billiard-ball smooth and utterly lacking an atmosphere, what velocity would be required to orbit at an altitude of a few feet? I don’t recall a good formula for orbital velocity vs altitude, and my quick Google isn’t coming up with good hits. Naturally it depends on exactly what value you take for the planetary mass & radius, but I’m looking for a ballpark number here. ---
**Author:** ![MonkeyMensch](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@MonkeyMensch](https://boards.straightdope.com/u/MonkeyMensch)\ **Post date:** [March 13, 2004, 8:10pm UTC](https://boards.straightdope.com/t/lowest-possible-earth-orbit/234084/9 "2004-03-13T20:10:57Z")
> [@LSLGuy](#): > > Which leads to a related question: if the Earth were billiard-ball smooth and utterly lacking an atmosphere, what velocity would be required to orbit at an altitude of a few feet? I don’t recall a good formula for orbital velocity vs altitude, and my quick Google isn’t coming up with good hits. > > Naturally it depends on exactly what value you take for the planetary mass & radius, but I’m looking for a ballpark number here. Well, for a ballpark figure use the orbital speed of low Earth orbit objects. It’s around 18,000 mph. Remember that orbital speed varies with distance from the Earth’s center, not its surface. So the orbital speed of an object right at the Earth’s surface (4000 miles from the Earth’s center) isn’t going to be much higher than an object in low Earth orbit (4150 miles from the Earth’s center.) And the Earth _is_ pretty much billiard ball smooth. Well, at least bowling ball smooth. Those scratches and irregularities on a bowling ball would correspond nicely with a scale model of the Earth’s mountains and ocean trenches. ---
**Author:** ![herman\_and\_bill](https://avatars.discourse-cdn.com/v4/letter/h/f1d935/32.png) [@herman\_and\_bill](https://boards.straightdope.com/u/herman_and_bill)\ **Post date:** [March 13, 2004, 8:15pm UTC](https://boards.straightdope.com/t/lowest-possible-earth-orbit/234084/10 "2004-03-13T20:15:56Z")
> [@MikeS](#): > > Psst, **Zamboni** : He’s talking about the flight of the [Voyager.](http://www.centennialofflight.gov/essay/Explorers_Record_Setters_and_Daredevils/rutan/EX32.htm) Yep, I considered an orbit was just going around the globe ---
**Author:** ![panamajack](https://avatars.discourse-cdn.com/v4/letter/p/47e85d/32.png) [@panamajack](https://boards.straightdope.com/u/panamajack)\ **Post date:** [March 13, 2004, 11:00pm UTC](https://boards.straightdope.com/t/lowest-possible-earth-orbit/234084/11 "2004-03-13T23:00:59Z")
Jerome Bixby’s [The Holes Around Mars](http://isfdb.tamu.edu/cgi-bin/pw.cgi?541900) posits a third moon just above the surface. I don’t recall if it gives any details (or even how such a thing could happen), and I think he assumes less atmosphere there than what we know now. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [March 14, 2004, 1:23am UTC](https://boards.straightdope.com/t/lowest-possible-earth-orbit/234084/12 "2004-03-14T01:23:46Z")
> [@The Great Zamboni](#): > > Is the velocity needed to maintain this minimum speed and altitude the cheapest and simplest to attain, i.e., the most efficient use of fuel and fastest way to orbit? It depends on purpose, the amount of time you want to have your satellite stay safely in orbit. I haven’t heard of spy satellites with an orbit as low as 80 miles, as **Lumpy** mentioned, but if you have a particular need and don’t care whether the satellite burns up in record time, then put it as low as possible. If you need it to stay up for years and years, even 200 miles is probably too low. There’s no one answer. ---
**Author:** ![umop\_ap\_sdn](https://avatars.discourse-cdn.com/v4/letter/u/dbc845/32.png) [@umop\_ap\_sdn](https://boards.straightdope.com/u/umop_ap_sdn)\ **Post date:** [March 14, 2004, 2:19am UTC](https://boards.straightdope.com/t/lowest-possible-earth-orbit/234084/13 "2004-03-14T02:19:20Z")
> [@LSLGuy](#): > > if the Earth were billiard-ball smooth and utterly lacking an atmosphere, what velocity would be required to orbit at an altitude of a few feet? The orbit would take about 82 minutes instead of 90. The square of the period is proportional to the cube of the semi-major axis and inversely proportional to the mass of the center. Technically, there are many examples of objects near the ground which are put into temporary (imperfect) orbits. Bullets, baseballs, golf balls - even thrown stones. Ignoring wind resistance, anything thrown will obey the same laws of gravity that keep satellites in orbit. All 6 orbital elements are there, but the velocity is much too low to maintain a positive altitude, meaning the semi-major axis is less than the radius of the Earth (and the eccentricity is very high) so the projectile eventually returns to the ground. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [March 14, 2004, 4:05am UTC](https://boards.straightdope.com/t/lowest-possible-earth-orbit/234084/14 "2004-03-14T04:05:56Z")
> [@Exapno Mapcase](#): > > I haven’t heard of spy satellites with an orbit as low as 80 miles, as **Lumpy** mentioned, but if you have a particular need and don’t care whether the satellite burns up in record time, then put it as low as possible. Ok, I looked in my old (1981) edition of TheIllustrated Encyclopedia of Space Technology. Chapter 5, Military Space Systems has a table of Selected Military Satellites. It lists a Titan IIIB- Agena D reconnaissance satellite and gives the orbit as 84x205 miles (135x330 km). So apparently 135 km is the lowest perigee they could dip a satellite down to for high resolution photos. I vaguely recall reading somewhere about tethered satellites where the lower of two connected satelllites was as low as 80 miles, but I can’t find a cite. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [March 14, 2004, 5:28am UTC](https://boards.straightdope.com/t/lowest-possible-earth-orbit/234084/15 "2004-03-14T05:28:53Z")
Interesting. I hadn’t considered highly elliptical orbits. Also interesting that apogee is just about at the 200 mile point. ---
**Author:** ![The\_Great\_Zamboni](https://avatars.discourse-cdn.com/v4/letter/t/4bbf92/32.png) [@The\_Great\_Zamboni](https://boards.straightdope.com/u/The_Great_Zamboni)\ **Post date:** [March 14, 2004, 5:00pm UTC](https://boards.straightdope.com/t/lowest-possible-earth-orbit/234084/16 "2004-03-14T17:00:54Z")
> [@MikeS](#): > > Psst, **Zamboni** : He’s talking about the flight of the [Voyager.](http://www.centennialofflight.gov/essay/Explorers_Record_Setters_and_Daredevils/rutan/EX32.htm) Okay, thanks for the link. I had never heard of this accomplishment. I had never thought of elliptical orbits, either. Could the elliptical orbit of that sattelite imply that perigee could be even lower? I would imagine that is about the limit, though. Atmospheric drag would probably defeat orbital velocity. > [@LSLguy](#): > > if the Earth were billiard-ball smooth and utterly lacking an atmosphere, what velocity would be required to orbit at an altitude of a few feet? The Moon would be a good example of this possibility. Apollo astronauts orbited as low as 60 mi. They may have been able to orbit much lower if NASA had desired. Could they have gone as low as 30 mi, or 15 mi?