[meteorite-list] DAWN drives up to Vesta

From: MexicoDoug <mexicodoug_at_meteoritecentral.com>
Date: Sun, 17 Jul 2011 12:29:06 -0400
Message-ID: <8CE12B5C3F58137-24C8-1841C_at_webmail-d147.sysops.aol.com>

Hi Mike,

Clearly, orbit was achieved at least by yesterday, at some point
probably while we were chattering about this. Since they basically
drove up to Vesta and we don't know the outer limit with super accuracy
there can be no real time congratulation announcement.

So a few more significant digits will need to be calculated on the mass
of Vesta and then some engineer will rubber-stamp with hindsight an
"official time" of capture. But they will need to do a little more
communication with the spacecraft to determine precisely where it is to
get that super precise mass, and from that back out when orbit actually
started which might only be an estimate with the best data, anyway. So
that is why this is a different moseying up to Vesta capture that will
be successful under a wide range of mass assumptions.

I don't know if this is being maintained accurately to reflect the
maneuver into orbit, here's the link though:

http://neo.jpl.nasa.gov/orbits/fullview4.jpg

seems to be a simulation noting the relative speed between the probe
and Vesta as well as the distance. It shows ~50 m/s (104 mph) and
13,100 km 8,100 miles), and those numbers haven't changed much. That
sounds like a fast approach already so if the proposed two week descent
is still the plan and the image data accurate, at this point we are
well past the capture and have already started the orbital transition
to lower altitude (the two week decent into the first scientific
orbit).

Unless I'm doing something wrong, a normal satellite at that 13,100 km
distance would have an orbital speed of 82 miles/h (132 km/hr) and in a
circular orbit no relative speed toward Vesta instead of the 100 mph.

The image numbers were 700 km further out and 3-5 km/h slower 10 hours
ago so it doesn't exactly add up, but it shows the ion engine is
currently thrusting and still approaching Vesta. Since the initial
target orbit was 15,000 km and they are around 13,000 km now from Vesta
according to this you might also imagine that either they decided to
come in more closely, or Vesta was more massive than calculated.

Best wishes
Doug



-----Original Message-----
From: Galactic Stone & Ironworks <meteoritemike at gmail.com>
To: Sterling K. Webb <sterling_k_webb at sbcglobal.net>
Cc: Meteorite-list at meteoritecentral.com; MexicoDoug <mexicodoug at aim.com>
Sent: Sun, Jul 17, 2011 9:34 am
Subject: Re: [meteorite-list] DAWN drives up to Vesta


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
>
-------------------------------------------------------------------------
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> ----- 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 12:29:06 PM PDT