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pi is exactly 3
2003-May-23, 09:58 PM
If the earth is moving at roughly 100000 km / hour ( I hope i calculated this right) how do space craft over come this speed when launched. If it is launched with the movement of the planet does this mean it is moving 100000+ km / hour? and if it launches against the movement of the earth. How does it get anywhere. Since no spacecraft can travel faster than 100000km / hour (I think) whould they actually be moving backwards if launched in the opposite direction of the earth? Please help a confused highschool student. :-?

daver
2003-May-23, 10:45 PM
Most of the Earth's speed can be ignored (the speed around the sun, the sun around the galaxy, the galaxy towards wherever it's heading). The other speeds can be ignored because the rocket also has their speed (it's the change in speed that's important).

However, one bit can't be ignored--the speed of the earth rotating on its axis. The earth is roughly 24,000 miles around at the equator, it rotates roughly once every 24 hours, so the rotational speed at the equator is on the order of 1000 mph.

Orbital velocity is on the order of 18,000 mph. So, if you launch the same direction the earth is rotating, you only have to gain 17,000 mph. If you launch the other direction, you have to gain 19,000 mph. That ends up being pretty significant; it's why most satellites are launched to the east (Israel launches the other direction, because of range safety and itchy tirgger finger issues).

Bob
2003-May-23, 11:06 PM
Daver is generally correct in his reply if we're dealing with earth-orbiting satellites, but it seems to me that the original post asked about spacecraft leaving earth orbit. The earth does indeed orbit the sun at a bit more than 100,000 km/hr. To leave the vicinity of the earth for an outer planet, a spacecraft must reach earth orbit and then, at just the right time add a bit more velocity to achieve an orbit which will rendezvous at apehelion with the destination outer planet. To reach an inner planet, a bit of velocity is subtracted. Do a google search for "hohmann orbit" for lots more information.

ToSeek
2003-May-24, 12:41 AM
To reach an inner planet, a bit of velocity is subtracted.

Or a lot of velocity: I think Mercury is supposed to be one of the hardest places in the solar system to get to because so much delta-v is required. (It's going to take the MESSENGER mission something like five years to lose enough delta-v to go into orbit.)

daver
2003-May-24, 02:04 AM
Yep. I've seen statements that it's cheaper to reach Mercury by way of Jupiter than by launching directly to Mercury. This statement was written before VEGA orbits were used; i'm guessing that slingshotting off of Venus and Earth would be cheaper than a Jupiter rendezvous.

OK, i dredged up my hohmann ellipse calculator (i wrote it, so the results are pretty suspect). The statement about being cheaper to go to Mercury by way of Jupiter doesn't appear to be correct--it requires a change of about 7.5 km/sec to enter into a hohmann transfer ellipse to mercury, and about 9 km/sec to enter into a hohmann transfer to jupiter. It takes about 2.5 km/sec to enter into a transfer to Venus.

To get back to the original question, Earth's orbital velocity is around 30 km/sec. So to launch into a Venus transfer orbit, the rocket needs to lose about 2.5 km/sec. It would start off orbiting the sun slower than the earth--about 27.5 km/sec. If it were going to mars, it would have to launch the other way--in the direction the earth was orbiting the sun, with a speed, once it's left the earth's general vicinity, about 1.5 km/sec faster than the earth--about 31.5 km/sec. Earth's escape velocity complicates the issue somewhat; i'm trying to ignore that for now.

daver
2003-May-24, 02:14 AM
Daver is generally correct in his reply if we're dealing with earth-orbiting satellites, but it seems to me that the original post asked about spacecraft leaving earth orbit. The earth does indeed orbit the sun at a bit more than 100,000 km/hr. To leave the vicinity of the earth for an outer planet, a spacecraft must reach earth orbit and then, at just the right time add a bit more velocity to achieve an orbit which will rendezvous at apehelion with the destination outer planet. To reach an inner planet, a bit of velocity is subtracted. Do a google search for "hohmann orbit" for lots more information.

My bad; i should have realized what the 100,000 km/hour was referring to. Thanks for the correction.

pi is exactly 3
2003-May-24, 11:52 AM
Thanks for your help guys. I think I have it straight in my head now.
This is a really fast forum. I got my first reply in a few minutes. I know I'll be using this Bulletin board in the future. Thanks for helping. :wink:

DStahl
2003-May-25, 06:44 AM
Geez--you guys are so savvy! Of course it would be almost as hard to go to Mercury as to go to Jupiter, I just never thought of it in terms of delta-V and heliocentric orbital velocities. Nice explanations.