## Up, Up, and Away

A 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.

## Orbital Inclination

This activity requires a small round balloon, a metric ruler, paper, a marker, scissors, and a protractor. Blow up the balloon to about 20 centimeters (8 inches) in diameter. Use the marker to draw a circle around its middle representing the equator. Cut a paper ring about 6 millimeters (a quarter-inch) wide to just fit around the balloon's equator. Fit the ring around the equator and then change its inclination from 0 degrees (equatorial orbit used by geosynchronous satellites) to 28.5 degrees (space shuttle orbit) to 51.6 degrees (space station orbit) to 90 degrees (polar orbit used by spy satellites).

## Orbital Ornithology

by Rob Landis, Space Telescope Science Institute

(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.

### Finding Space Birds

Three basic rules can help the fledgling bird watcher:

### Observing Tips

Roger Mansfield of the Astronomical Data Service has four helpful observing hints to bear in mind:
• Be certain to have the exact time. The National Bureau of Standards operates a talking clock at 303-499-7111 and, for those with world-band radios, at 5 and 10 megahertz. The recording provides you with UTC, or Coordinated Universal Time, also called Greenwich Mean Time. If you're on Eastern Standard Time, subtract 5 hours from UTC; for Central Standard Time subtract 6 hours; Mountain Standard Time, 7 hours; Pacific Standard Time, 8 hours.
• Get to your observing site early. Find the constellations and bright stars that the satellite will pass through or near. If the sky is hazy or cloudy and you cannot see the stars, then you won't see the satellite either. If the Moon is bright, finding the satellite will be challenging.
• Bring along a planisphere (star chart), red flashlight, and binoculars. A planisphere with the predicted path of the satellite across the sky penciled in can be a great help for beginning birders. To draw the path, plot the positions given by the computer program.
• Prepare for acquisition. You should be looking exactly where the satellite is due to appear. If you miss the satellite at the first possible acquisition point, try again along the sky trace at another point.
Becoming a space ornithologist requires skill and patience. You may need to look at your watch, the star chart, and the night sky several times in rapid succession without losing your night vision (the reason for the red flashlight). Familiarity with the night sky can help enormously. With a little experience, you can look up and see the shuttle or space station with their human cargoes, silently drifting amongst the stars.

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 landis@stsci.edu. For more information on space bird watching, visit http://learn.jpl.nasa.gov/spotsat.htm.

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