@ 17. Neil Haggath Says:

A challenge to astrologers… They say Mercury will “enter Libra” on 10 October. So… I will personally give one million dollars to any astrologer who can photograph Mercury, on 11 October, among the stars of the constellation Libra. Of course, I don’t have a million dollars, but I don’t think I need to worry!

Be careful there – there’s always photoshop!

You’d be well advised to stress a genuine, unfaked and confirmed photo. Then agian I wonder how many astrologers could even find Libra in the sky or name any of its stars and their spectral types.

That’s actually a much better description of orbital ballistics than Heinliens, since the closer you are to the sun, the faster you have to move in order to remain in orbit but it’s also true that to drop your orbit closer to the sun you have to slow down from current velocity. The thing is, as your orbit decays(gets closer to the sun) your velocity WILL increase. It’s much more about ones potential energy(height above the gravity source times mass times the acceleration) being converted to kinetic energy(velocity^2 times mass/2). We start at a high potential energy with some kinetic energy,ie,earths velocity is about 30 km/sec and we’re about 150 million km above the sun, so we have both potential and kinetic energy. That’s what makes it so darned counter intuitive to figure out how to go where we want to go. To get closer to the sun, we reduce our orbital velocity, drop down and our Velocity then increases while potential energy decreases. If you twirl a weight around on a string, pull it in closer to you(pretend you’re the sun) it will increase the objects velocity. If you reduce the velocity, it’s distance from you will decrease. Not the greatest model, because it’s also affected by earths gravity but maybe it will give some insight.
Jus an addendum: Mercurys velocity is about 48 km/sec.

GAry 7

Or, as one of my physics profs used to say, in orbital mechanics you have to go forward to go up, go up to go back, go back to go down, and go down to go forward.

(Forward to go up: forward acceleration shifts you into a higher orbit. Up to go back: going straight up causes you to lose angular velocity because you have greater distance per orbit. Back to go down: reverse acceleration causes your orbit to decay. And down to go forward, dropping down increases your angular velocity because the orbit is smaller but your linear velocity stays similar.)

Real-time orbital corrections can be highly non-intuitive.

Thanks for the kind words, but truth is I messed up big time as I glossed over the potential energy loss before acquiring the orbit. Luckily toasterhead listened to saner advice.

I like your argument, because then I don’t need to analyze orbital motion for its effect on energy balance.

Yet another way to see that a round orbit is the least energy state is to realize that this is the reason for hydrostatic spherical shape of planets. [More generally, this condition must mean stability of this type of field, or, say, planets would split and sunder.] So the probe going in another trajectory at the moment it hits Mercury orbit (acquires the same potential energy) must have higher kinetic energy in the balance, and needs to bleed off speed in the gravity assist.

Merkur, der kleinste und sonnennächste Planet des Sonnensystems, gehört zu den am wenigsten erforschten Planeten des Sonnensystems. Mit knapp 4900 Kilometer Durchmesser ist er etwa anderthalb Mal so groß wie der Erdmond, und mit seiner …

;p