[meteorite-list] Rosetta gravity assist flyby

From: lebofsky at lpl.arizona.edu <lebofsky_at_meteoritecentral.com>
Date: Tue, 13 Nov 2007 08:39:04 -0700 (MST)
Message-ID: <2972.71.226.60.25.1194968344.squirrel_at_timber.lpl.arizona.edu>

Hi Sterling, et al.:

The "asteroid" (Rosetta) was discovered near midnight on November 7 and
was confirmed the next night at 2 other sites.

I did a calculation of size vs magnitude for the "asteroid" at discovery.

Its H magnitude (how bright it would be at 1 astronomical unit, 1 AU, from
the Earth) was 26.3. That would make it 23 meters in diameter with a 10%
reflectivity (gray). The darkest asteroids reflect 5% of the light the
hits them which would give a diameter of about 30 to 35 meters.

At the time of discovery, it was 0.04 AU from the Earth (about 6,000,000
km) was magnitude 19.7 (about 1,000,000 times fainter than the faintest
stars one can see with the naked eye) and was moving at a little less than
2 arc-minutes a day (mostly north to south).

The diameter of the Moon is 30 arc-minutes (1/2 degree) for comparison. It
turns out that the main belt asteroid Ceres was in the same area of the
sky and was moving about 1/2 as fast north to south, but 15 times faster
west to east at this time. Why the difference? Ceres is moving in its
orbit around the Sun while Rosetta was aiming right at the Earth (nearly
so), so even though is was much closer to Earth, it was going almost
directly toward us! (if an object is getting brighter but with no apparent
motion, duck!)

The whole idea behind discovering Earth-approaching asteroids is to find
them not when they come by the first time (not much you can do about them)
but to get an "early warning" for when it might be coming by the next
time, as in the case of Apophis. For comparison, I think that Apophis was
moving at several degrees a day at the time of discovery. At that point
you have a chance to do something about it (beyond just running for
cover).

So, to answer Sterling's question, VN84 was not discovered because of its
fast motion OR brightness, but more for how slow it was moving west to
east relative to its north-south motion! I will try to track this down.

Larry



On Mon, November 12, 2007 10:53 pm, Sterling K. Webb wrote:
> Hi, Larry, Doug, and other members
> of the Target Population,
>
> While my first reaction was to be pleased:
>
>
>> There's something re-assuring about the notion
>> that we could detect a "potential impactor," even if we didn't
>> immediately recognize that it's one of ours!
>
> I'm beginning to reconsider. The MPC Circular
> was Nov. 8th, so the detection would seem to have been the night of Nov.
> 7-8, or since that seems to be
> cutting things close, perhaps Nov. 6-7. But the first "we" knew of the
> potential event was Nov. 8th. The flyby is 20:57 UT on Nov. 13th, which
> for the US is the early morning hours of the 14th.
>
> That means that effectively there was 5.5 days
> of lead time for a potential impact. When we thought it was a small, dark
> (and hence unreflective) asteroid,
> it was estimated to be a 20-meter object (instead of a small shiny
> spacecraft) which, if it had stuck the Earth's atmosphere, would not have
> survived the encounter and would have been no threat. (Or not, if stone,
> it's about 10,500 tons, with an airburst of a trivial 150,000 tons of
> TNT.)
>
>
> It would interesting to know if 2007 VN84 was
> detected because of its "brightness" or because of its high proper motion.
> If it was because of brightness,
> we would have 11 days of warning for a 40-meter object, 23 days of warning
> for a 80-meter object, 47 days of warning for a 160-meter object...
>
>
> That last one, the 160-meter stone, weighs in at
> 5,120,000 tons, and I, for one, am willing to take
> that seriously (75 MegaTons of TNT and a one-and- a-half mile crater 1500
> feet deep). OK, I've got 47 days. What do I do? Practically, I think the
> only thing that I could do is to calculate exactly where is a good place to
> watch the show when it hits.
>
> It's probably significant that the CSS (Catalina Sky
> Survey) was the one to catch it; in 2005 they found
> the greatest number of NEO's of all the searchers. Their methodology, as I
> understand it, is based on detection by large proper motion, but the
> object has to be bright enough to be noticed first. A really "dim" object
> shifting its relative position rapidly wouldn't get caught... until it
> brightened.
>
> Our problem (as a species and civilization) is that
> we're still in the phase of trying to find all those sneaky potential
> impactors. We haven't been promoted to the grade level where all we have
> to do is keep track of them and roll out the laser cannons in case of
> emergency!
>
> In this event, however, we have encountered the first
> known occasion where part of the problem fell under the jurisdiction of the
> keeping-track department (you know, the one we don't have yet). And, by
> pinpointing that problem, I think we've made another advancement, assuming
> we do something about getting that department.
>
>
>
> Sterling K. Webb
> -------------------------------------------------------------------
>
>
>
>
>
>
>
> ----- Original Message -----
> From: <lebofsky at lpl.arizona.edu>
> To: "mexicodoug" <mexicodoug at aol.com>
> Cc: <meteorite-list at meteoritecentral.com>
> Sent: Monday, November 12, 2007 8:33 PM
> Subject: Re: [meteorite-list] Rosetta gravity assist flyby
>
>
>
> Hello Doug:
>
>
> I take exception to your comments that this was either a screw-up or a
> joke. These are hard-working dedicated people, most of whom I have known
> for 20-30 years.
>
> I do not know all of the details, but when a "discovery" is made, the
> discoverers have access to a very large database of Small Solar System
> Bodies (asteroids and comets). Generally, things in orbit around the
> Earth
> have distinct enough orbits so that they are easily recognized. Not so for
> objects in heliocentric orbits (orbiting the Sun). In this case, an
> object was seen that appeared to be a Near-Earth Object that was about to
> make a close approach to the Earth and for which the database did not have
> the orbital elements. Thus, it was at first considered to be a new
> discovery.
>
> There are nearly 500,000 known asteroids (many with poorly known orbits)
> and about 5000 new ones are being discovered every month! Maintaining this
> database is not an easy task.
>
> Obviously, someone fairly quickly realized that this was not an asteroid,
> but Rosetta, but not before the alert went out for astronomers to make
> observations. The system worked!
>
> What did not work, as was pointed out by the Minor Planet Center, was
> that unless there is someone who is in a position to provide them with the
> orbital elements of Rosetta, there is no way that they can put this into
> their database. This is where the system failed. Actually it is
> impressive that the Catalina Survey people did see this "incoming
> asteroid" and shows how well they are covering the sky in order to locate
> any asteroids heading toward the Earth.
>
> However, Doug, Pluto and the IAU decision is another story that we should
> discuss over beers sometime.
>
> Larry Lebofsky
>
>
> On Mon, November 12, 2007 6:51 pm, mexicodoug wrote:
>
>> Hi Darren,
>>
>>
>>
>> It certainly was an actual screw-up by the IAU. The joke I meant was
>> by Catalina Sky Survey, no matter what they say. You deserve a medal.
>> Just
>> tell us you didn't look in the back of the book (or leave a Google crumb
>> path)! Clyde Tombaugh is is snickering in his grave at the foolish
>> bureaucracy that was arrogant enough to strip a true astronomer of his
>> life's crowning achievement to play word footsies...
>>
>> Best wishes,
>> Doug.
>> ----- Original Message -----
>> From: "Darren Garrison" <cynapse at charter.net>
>> To: <meteorite-list at meteoritecentral.com>
>> Sent: Monday, November 12, 2007 6:13 PM
>> Subject: Re: [meteorite-list] Rosetta gravity assist flyby
>>
>>
>>
>>
>>> On Fri, 9 Nov 2007 12:35:28 -0600, you wrote:
>>>
>>>
>>>
>>>> Someone has a sense of humour, especially the flying couch comment
>>>> !
>>>>
>>>>
>>>>
>>>
>>> Looks like it might have been an actual screw-up, not just a joke.
>>>
>>>
>>>
>>> http://blogs.smh.com.au/sit/archives/2007/11/alarm_astronomers_in_a_s
>>> pi n_ov.html
>>>
>>> http://www.space.com/businesstechnology/071112-technov-asteroid-mista
>>> ke .html
>>>
>>>
>>>
>>> http://www.dailymail.co.uk/pages/live/articles/technology/technology.
>>> ht ml?in_article_id=493152&in_page_id=1965
>>> ______________________________________________
>>> Meteorite-list mailing list
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>>>
>>>
>>>
>>
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>>
>>
>
>
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Received on Tue 13 Nov 2007 10:39:04 AM PST