An Orbit in Your HandA nice model for orbital motion is a rubber stopper tied to a string. Swing it overhead. Compare its speed while changing the length of the string.
The string represents the inward force of gravity. When you let go of the string, the stopper will fly off in the direction it was moving when released (see diagram). This demonstrates Newton's first law of motion: A body moves at a constant speed in a straight line unless a net force acts upon it. While you were swinging the stopper overhead, the force exerted by the string caused the stopper to move in a circle. When you let go, the stopper moved off in a straight line.
For more details on this activity, see the ASP's The Universe at Your Fingertips resource notebook, activity C-5.
(c) 1996 Astronomical Society of the Pacific
For most folks, the space program means a program on television or a space capsule in a museum. But did you know you can witness space travel from your own backyard?
With so many satellites now in orbit, it is likely that you have seen an artificial satellite zipping across the night sky. As orbiting spacecraft have grown in size, their visibility has increased. The Russian Mir space station, the American space shuttle, and the Hubble Space Telescope are large enough and bright enough to be spotted easily.
Perhaps the most conspicuous space bird is the Mir. The station is about 19 by 26 meters (63 by 85 feet) and orbits at an altitude of 300 to 400 kilometers. During favorable passes, it shines at zero magnitude and has occasionally and suddenly brightened to -3.5, brighter than any star and as bright as Venus. Because Mir's orbit is inclined 51.6 degrees to the equator, it sweeps out latitudes between 51.6 degrees north and 51.6 degrees south -- making it visible, at least in principle, to observers lower than about 65 degrees latitude. Hubble, whose orbit is inclined 28.5 degrees to the equator, can only be seen in and near the tropics.
In December 1997, the first pieces of the International Space Station will be launched into an orbital inclination of 51.6 degrees. As assembly of the station continues through 2002, the station will grow ever bigger and brighter.
requires a data file with the "orbital elements'' for each satellite.
These elements, a mathematical description of the space bird's orbit, are
computed by the U.S. Space Command and are available on the World Wide Web
ROB LANDIS is the program coordinator of the Project to Re-Engineer Space Telescope Observing at the Space Telescope Science Institute in Baltimore. As the human link between Hubble and astronomers, Landis is responsible for the development and implementation of observing programs. His email address is firstname.lastname@example.org. For more information on space bird watching, visit http://learn.jpl.nasa.gov/spotsat.htm.
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