Mercury has never been orbited by a spacecraft before. That willchange this month

SCIENTIFIC AMERICAN MAGAZINE: Journey to the Innermost Planet
Mercury has never been orbited by a spacecraft before. That will change
this month

http://www.scientificamerican.com/ar...AT_BS_20110225

On March 18 MESSENGER finally enters into orbit around Mercury and
begins a one-year orbital mission. Just getting there has been hard,
given the planet’s high orbital speed and proximity to the sun.

On Feb 25, 6:03*pm, Sam Wormley quoted, in part:
Just getting there has been hard,
given the planet s high orbital speed and proximity to the sun.

Even Scientific American gets the science wrong sometimes.

The planet's proximity to the Sun certainly made it hard to get there.
Had a spaceship been sent there by a straightforward Hohmann orbit, a
large delta-V would have been required. Instead, to save on fuel,
complicated gravity assists were used.

And, of course, its proximity to the Sun also makes it hard to keep
the probe in working order.

However, Mercury's high orbital speed makes it easier to send a
spaceship there - and it would have been easier still if Mercury's
orbital speed was higher.

To see why, as an oversimplification, let's suppose that after a
gravity assist from Venus, the probe was travelling to Mercury in a
Hohmann orbit for a Venus to Mercury trip. (In real life, it might
have been going faster than that. But then it slowed down with further
gravity assists from Mercury itself.)

This would mean that when it encountered Mercury, it would have been
travelling around the Sun *faster* than Mercury was, as its orbit
would have taken it back out to the orbit of Venus again.

So the fact that Mercury is moving swiftly around the Sun isn't making
it harder to get there, rather, it's the fact that it isn't moving
swiftly *enough*.

John Savard

John Savard wrote:
So the fact that Mercury is moving swiftly around the Sun isn't making
it harder to get there, rather, it's the fact that it isn't moving
swiftly *enough*.

Perhaps--but if it *were* travelling that fast, its orbit would
be close to Venus's (possibly even extending beyond it, if
eccentric enough) and probably unstable as a result.

I'm not sure you can explain just why it's tricky to position
the satellite in orbit (around any planet, really) without
spending quite some time explaining basic things about
orbits and conservation of energy.

--
Brian Tung (posting from Google Groups)
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