[meteorite-list] DAWN drives up to Vesta

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Sun, 17 Jul 2011 01:12:47 -0500
Message-ID: <C4F3D5DFBC3A4A13BCEF3C8355C8890E_at_ATARIENGINE2>

Doug, List,

I suggest the very detailed "Dawn Journal" postings
by Dr. Marc D. Rayman, Chief Engineer. Of course,
he's busy right now! The last Journal log was June
23, 2011, but the earlier extensive Journals have a
lot of information. They can be found at:
http://dawn.jpl.nasa.gov/mission/journal.asp

On June 1, Dawn was closing at 540 mph. By
June 23, about 250 mph. Currently, it's within
your local speed limit 55-65 mph. Hands on
the wheel and eyes on the road.

    "The spacecraft will glide into a very high orbit
in late July and continue thrusting, gently as always,
until early August, when it will arrive in its survey
orbit at an orbit at an altitude of about 2700
kilometers (1700 miles)."

You will note that Dawn is AHEAD of schedule
now, gaining it more time at Vesta. Ion drive is
like a video game -- play it right, you get bonus
points.

    "In survey orbit, the probe will be about 2700
kilometers (1700 miles) above the surface. During
the approach phase, navigators will measure the
strength of Vesta's gravitational tug on the spacecraft
so they can compute the giant asteroid's mass with
much greater accuracy than astronomers have yet
been able to determine it. (The mass is calculated
now using observations of how Vesta perturbs the
orbits of other asteroids and even of Mars.) That
knowledge will allow them to refine the survey orbit
altitude, and they may target it to be somewhat higher
or lower, depending on whether Vesta is more massive
or less massive than the current calculations show.
The sequences for acquiring science data are being
designed to accommodate a reasonable range of masses.
    Dawn will be in a near-polar orbit. Its trajectory
will take it over the north pole (which will be in
darkness, because it will be northern hemisphere
winter at that time), then over the terminator (the
boundary between the illuminated and unilluminated
sides), down over the equator, over the south pole,
and then across the terminator again to pass over
Vesta's night side. Such an orbit allows the spacecraft
to have a view of virtually every part of the lit surface
at some time. Each revolution in survey orbit will take
2.5 to 3 days to complete. While this may seem like a
leisurely pace, the spacecraft will be busy the entire time.
    The primary objective of survey orbit is to get a broad
overview of Vesta with color pictures and with ultraviolet,
visible, and infrared spectra. The camera will obtain views
with 250 meters (820 feet) per pixel, about 150 times
sharper than the best images from the Hubble Space
Telescope. The mapping spectrometer will reveal much
of the surface at better than 700 meters (2300 feet)
per pixel."

Actually finding the Pole (so you can line up for a polar
orbit) has been a problem. North? South? East? West?
Front? Back? Which pole? Vesta's irregularity poses a
limit on "How low can you go?" Interplanetary Limbo
can be hazardous to your spacecraft...


Sterling K. Webb
--------------------------------------------------------------------------------
----- Original Message -----
From: "MexicoDoug" <mexicodoug at aim.com>
To: <Meteorite-list at meteoritecentral.com>
Sent: Saturday, July 16, 2011 11:36 PM
Subject: [meteorite-list] DAWN drives up to Vesta


> 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
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Received on Sun 17 Jul 2011 02:12:47 AM PDT