[meteorite-list] DAWN drives up to Vesta

From: MexicoDoug <mexicodoug_at_meteoritecentral.com>
Date: Sun, 17 Jul 2011 00:36:45 -0400
Message-ID: <8CE125244644135-2338-2FB41_at_webmail-m048.sysops.aol.com>

Hi everyone with a bit of Vesta fun,

This is a different kind of encounter to visit Vesta, not like we're
used to hearing about anyway from visits to the Moon, Mars. At the
moment, it's about 13,000 km (9,000 miles) to Vesta and like a nice
Chevy Camaro (or a Ford Mustang in a pinch) DAWN is cruising along the
interplanetary highway (route I-5 in honor of Vesta's soon to be
crowning as a dwarf planet, the fifth planet, considering all roads
lead to and from Earth). Get ready to promote all of your HED
meteorites ;-), even the moon isn't planetary according to the IAU ...
because where the rock is matters to them for some fool reason.

Is there really much risk to the Vesta orbital insertion? I'd say no,
nothing to hold your breath over. Does anyone recall the Six-Million
Dollar Man - he pretty much could could run the approach to Vesta -
heck even we could, so I'm imagining DAWN tooling along in slow motion
just as he would, for the effect of speed (of course by slowing down -
I need a psychologist to explain why we are now all conditioned from
television to feel speed when the film is slowed down with interesting
sound effects).

The real risk, I'm guessing has already been made and we are kind of
stuck with it and most depends on the assumption of Vesta's mass
barring mechanical steering failure which is very unlikely during this
critical maneuver considering the long track record and minimum of
moving parts and that it would have to be for a much longer time than a
conventional propellant motor. If the target is an initial orbit
around Vesta at 100 km altitude, for example, I'm thinking how close
they will get to it since changing the course significantly on these
incredibly weak ion engines is like trying to turn the Titanic on a
dime - you can't. The real risk would be to basically know the mass of
Vesta (or the product of G*M). I'm not sure how accurate that it was
known and I'm betting that a lot of work has gone into refining the
number for the purpose of navigation on this mission. It really wasn't
that well determined before! So that is already one benefit of the
mission - Vesta's mass.

I don't know the initial altitude insertion target, let's assume it is
between 100 km and 500 km and somewhat circular (though an eccentric
orbit might be chosen), the spacecraft will accelerate to around 294 to
377 mph (31 m/s to 168 m/s ; or, 474 to 606 km/h ) and the speed will
determine the initial altitude. So as long as the spacecraft is within
that speed range, I think, and has decent pointing capability (which is
all happening in slow motion due to the nature of the engines), now, it
is a done deal, whoever figured out a mass good enough to navigate to
Vesta is a real hero here and deserves the congratultations - as the
main power must be coming from the gravitational acceleration into
Vesta now and the ion engine mostly secondary and primarily for (slow)
steering manuevers.

The other detail I see is that since Vesta isn't perfectly round, if
they don't pick an "iso-radial" or whatever you call a constant
diameter great circle in Vesta that happens to not be oblong, I think
that even a circular orbit could be fun, like a ride at an amusment
park - the ground could actually be going up and down below you 65 km
during each orbit - at a 100 km orbit, that would be pretty awesome.
And also a reason not to start with an orbit too low right away for
fear of getting Vesta whipped. Being in a circular orbit but having it
feel elliptical without doing anything! Charlie Sheen could make a
better joke than I can here ...

All this assumes no atmosphere. That seems like a good assumption
since many spectra have been taken of Vesta - but, there theoretically
could be a a few patches of dust floating around. With those huge (and
beautiful work of art) solar panels that conceivable could be a
consideration and would be a first instance of
"micro-meteoroid-braking" a.k.a., sand-blasting.

Lots of fun things to think about Vesta while they are still not
confirmed, which won't last long ... just thought I'd note these
thoughts in case anyone was interested while we all take a vicarious
walk to Vesta and route for the home team. I guess I need a "blog"

Best wishes
Doug
Received on Sun 17 Jul 2011 12:36:45 AM PDT