Jonathan Wojack has the right idea but his accuracy is off somewhat. The speed of a satellite in orbit is related to it's altitude. The higher the orbit, the slower the speed around the Earth relative to the center of the Earth. Earth satellites never escape the Earth's gravity. In fact, Earth's gravity is the only force keeping them in a curved orbit around the Earth. A satellite in orbit never has a speed of zero because then it would fall directly toward the center of the Earth. Any satellite must have tangential velocity (lateral velocity) so that the two bodies won't hit each other. Even the Moon, a Earth orbiting satellite, must have lateral velocity around the Earth. Satellites in geostationary orbit must keep moving or they'll crash into the Earth. For a satellite to appear stationary in the sky to a ground observer two conditions must be met. The period of rotation of the satellite must equal the period of rotation of the Earth. The plane of rotation of the satellite must coincide with the plane of rotation of the Earth (i.e., the satellite must remain over the Equator at all times. Then, assume the following: Satellite's mass is negligible compared to Earth's mass The orbit is perfectly circular (they never are) The mass of the Earth is homogeneous (it isn't) The radius of the Earth's surface is the same in all directions (Earth is not a perfect sphere) Perturbations (fluctuations)caused by the solar wind, the gravitational attraction from the Moon, Sun and other planets are very small. The Earth takes exactly 24 hrs to rotate on it's axis (it doesn't) With these assumptions in effect, Wojack's logic then can be applied. Given: Earth's radius to the surface at the Equator = 3,963.19556 miles Period of rotation = 24 hrs. Altitude of a geostationary orbit is about 22,300 mi above the surface of the Earth Then: v = circumference of orbit /period of rotation v = 2 pi x (radius of Earth + alt of sat)/period v = 2 x 3.1416 x (3,963 mi + 22,300 mi)/24 hr v = 163,671.01 mi /24 hr v = 6,819.625 mi/hr = 10002.117 ft/sec = 1.89434 mi/sec Look's like Jonathan Wojack was just about right. That's a lot faster than any rifle bullet. A satellite may be in a geostationary orbit, but it must move fast to keep up with the Earth's rotation. Otherwise the satellite will appear to the ground observer to move across the sky. Jeff Barker ARINC Sr. Space Instructor US Army Command and General Staff College Fort Leavenworth, Kansas -----Original Message----- From: Jonathan T Wojack [mailto:tlj18@juno.com] Sent: Thursday, October 11, 2001 3:06 PM To: SeeSat-L@satobs.org Subject: Re: Newbie GEO question > I'm new to the list and have a question regarding GEO sat's. > At what speeds do they travel? > I realize that relative to the earth at 22,000 miles they travel 0, > but > let's say bring their orbit down to LEO, how fast would they be > traveling? > 17,500? > I was able to observe one in my homemade 10" telescope, very cool. > Stars > move, Satellite doesn't. I'll theorize: If we assume that GEO sats orbit at 22,000 miles (I think it's closer to 25,000 miles, but I'm not sure), then I theorize that we can find their speed (relative to what I don't know). I think we can just extend the radius of the Earth 22,000 on paper to discover the satellite's speed: Diameter = Pi*d Diamteter = Pi*(22,000+22,000+8,000[Diamter of Earth]) Diameter = Pi*(52,000) Diameter = 163,362 miles. Now, this theoretical distance is traveled in approximately 24 hours: Speed = s[Distance]/t Speed = 163,362/24 Speed = 6,806 miles per hour This translates into 1.89 miles per second, which translates into the perferred 3 kilometers per second. [This was all a big guess. I hope it's right! : - )] ------------------------------ Jonathan T. Wojack tlj18@juno.com 39.706d N 75.683d W http://www.angelfire.com/stars2/projectorion 4 hours behind UT (-4) ________________________________________________________________ GET INTERNET ACCESS FROM JUNO! Juno offers FREE or PREMIUM Internet access for less! Join Juno today! For your FREE software, visit: http://dl.www.juno.com/get/web/. ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www.satellite.eu.org/seesat/seesatindex.html ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www.satellite.eu.org/seesat/seesatindex.html
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