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Name: Deb
Status: other
Grade: other
Location: NV
Country: N/A
Date: N/A


Question:
As part of a class project, we are trying to determine how long a trip to Uranus would take via automobile, airplane, and space ship. I have searched many web sites but cannot come up with any answers other than to take the approximate distance between Uranus and Earth (1.69 billion miles) and then divide that distance by the approximate general speed each vehicle would go. Am I on the right track or is there something I am missing? If I am on the right track, what is the approximate speed of a space/rocket ship?


Replies:
A spacecraft's velocity depends on how quickly it was launched, then on a combination of the gravity of the Sun, Earth, and whatever body it is approaching. Often a spacecraft will speed up as it slingshots close by a planet as it is on its way to another planet. New Horizons will speed up as it passes Jupiter on its way to Pluto.

David Levy


Deb,

As a first approximation you are on the right track, and if all you really want is to get some numbers indicating the difference it would take to travel that long distance at different speeds then what you are doing is good enough. I do not know if you need to add the complications that come from a more "accurate" or realistic measurement. Here are some of the complications that come immediately to mind: (1) the speed of the vehicle must necessarily change over the course of the travel, the speed will be different when boosting out of the Earth's gravity, in space - since the vehicle is no longer being boosted by rockets and is essentially using whatever remaining forward momentum it had to get to Uranus - it is continually being slowed by the decreasing effect of the Sun's gravity; this makes calculation of the vehicle's speed difficult; (2) the trajectory of the vehicle is not straight between Earth and Uranus, not only are both planets moving; the Earth's momentum is transferred to the vehicle, but the target landing may not be the shortest distance between the two planets or the two orbits; (3) the vehicle may be "slingshot" through another planet like Jupiter or Saturn, using the gravitational effect of this planet to boost the vehicle, and so on. I leave it up to you to decide how much of these real world complexity you want your students to consider.

Greg (Roberto Gregorius)



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