Posted by James R. Frysinger on December 31, 1999 at 20:46:11:
In Reply to: weigt of Hubble posted by Bad Girl on December 31, 1999 at 11:53:16:
> My prof of physics gave us the assignment to look up the weight of the Hubble telescope. He wiil probably ask it on the exam of physics in January. So if anyone can help me, let me know the weight of The Hubble.
> Thanks a lot!!
> Bad Girl
Can't be all THAT bad if you're taking physics! ;-)
I'm afraid that Bill and John have misadvised you. The Hubble Space Telescope (HST) orbits Earth at an altitude of about 590 km. That's from the GSFC page
http://hstsci.gsfc.nasa.gov/orbit.html
but the bad news is that almost all of that information is in non-metric units. I got the value of 590 km by multiplying their figure of 320 nautical miles by 1.852, giving proper regard to significant figures.
At that altitude (590 km above Earth's surface) HST is about 6960 km from the center of the Earth; you are about 6370 km from the center. So up there, the HST experiences about 84 % of the gravitational attraction that it would have on the surface (by Newton's inverse square law of gravitation).
I couldn't find the HST's mass in just a short search, but the JSC status reports have been using the term "tons" to give its mass as "12.5 tons". Those are probably the American or short tons of 2000 pounds each. So the value in proper metric units would be 11 300 kg. In common usage "weight" is used to mean mass, so in that sense, HST's weight is about 11.3 metric tons (of 1000 kg each).
But in physics we try to be more specific about our terminology. So we would multiply that by the acceleration due to gravity that the HST experiences. This gives us a weight of
W = m g = (11 300 kg)(9.81 m/s2)(0.84) N.
Notice that I multiplied the normal value for g by that 84 % we calculated earlier. That gives us a weight of 93.0 kN. Remember that weight (as the term is used in science) is actually a force. That weight (93.0 kN) the force that keeps pulling the HST around the Earth. This centripetal force is what keeps it from flying off into space in a straight line. So you can see that the HST is not weightless because its path circles the Earth! Of course, the astronauts working up there felt weightless because they were in "freefall" around the earth. It's like being in an elevator with a cut cable -- but one which is moving sideways at about 28 200 km/h.
I hope this has helped. Post a followup message if you have any questions about it, please.
regards,
Jim Frysinger
Dept. of Physics and Astronomy
College of Charleston