[meteorite-list] DAWN drives up to Vesta

From: Galactic Stone & Ironworks <meteoritemike_at_meteoritecentral.com>
Date: Sun, 17 Jul 2011 09:34:04 -0400
Message-ID: <CAKBPJW_YeWhhV+hDWYzLiD4GRdCOvX5QUp+CDfGnVQfaxtTATw_at_mail.gmail.com>

Hi List,

I fell asleep last night before 2:30am. So what is the good word on
Dawn? Do we have orbit?

As has it captured a photo yet of a mile-wide patch of diogenite
exposed through the regolith? ;)

Best regards,

MikeG

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On 7/17/11, Sterling K. Webb <sterling_k_webb at sbcglobal.net> wrote:
> 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 09:34:04 AM PDT