Hi Bill -- I guess the next question I have is the same one I have for similar objects: how does
a minor planet get into such an orbit where its aphelion is far inside Mars, and its perihelion
is far outside Venus? I understand how resonances with Jupiter will increase the eccentricity
of an object originally in the Main Belt, eventually leading to the orbit being Mars-crossing
and perhaps eventually Earth-crossing, but some mechanism has to bring down that
aphelion (lower the eccentricity) to get it into an orbit like that of 2019 UB4. It's the same
problem for us putting a spacecraft into orbit around Venus or Mercury, except in our
case we have the benefit of being able to specifically target particular orbital keyholes
(gravity assist) and provide our own delta-V at critical moments. It seems like a very unlikely
series of events must happen for such a thing to come about naturally. --Rob
-----Original Message-----
From:
find_orb@yahoogroups.com <
find_orb@yahoogroups.com>
Sent: Thursday, October 24, 2019 8:35 AM
To:
find_orb@yahoogroups.com;
find_orb@...
Subject: EXTERNAL: [find_orb] Re: 2019 UB4: manmade or lunar ejecta?
Hi Rob,
(CCed to the replacement list... you'll all get an "invite" e-mail, but here's a reminder that you can sign up for the new list at
https://projectpluto.com/mailman/listinfo/find_orb_projectpluto.com
I expect to cross-post to both lists for as long as the Yahoo! list exists. Which looks like December.)
2019 UB4 has a bit more speed relative to us than you'd think, almost entirely due to the nearly 0.1 eccentricity.
If you click on the orbital elements shown in Find_Orb, you get the 'alternative information' (not to be confused with 'alternative facts!') such as MOIDs with various planets, state vectors, Tisserand values, etc.
This will also tell you that the Earth encounter velocity is 2.91 km/s, and the Barbee-style encounter velocity is 3.33 km/s. Both are safely above the 2.5 km/s or so level you'd use as a rough cutoff for lunar ejecta and junk.
The concept of "encounter velocity" (also known as v_inf) seems to be a little weird and not totally defined, which is why Find_Orb shows v_inf computed according to two different definitions. The 2.91 km/s corresponds to a somewhat "traditional" method that assumes the earth (or whatever object you're considering) is in a circular orbit; the formula for it doesn't include our eccentricity at all, only that of the asteroid/ejecta/junk. Since we're moving about a full kilometer a second faster at perihelion than we are at aphelion, this is not a minor difference, and I've never been particularly happy with this concept of v_inf.
Brent Barbee suggested an alternative definition of encounter velocity : first, you find the MOID. You then know the points along the (unperturbed) orbits where the objects would have to be if they were as close together as they can get. Now, compute their relative velocity if they actually were at those points.
In general, this does appear to give a better indication of how fast an object is moving relative to us and whether it's junk/ejecta or a "normal" NEO. I think it could be considered as essentially perfectly correct for a MOID of zero. But as the MOID increases, it gets to be less reliable (and the concept of "encounter velocity", for two orbits that don't really encounter each other, becomes more and more fuzzy; what, for example, is our "encounter velocity" with Pluto?)
-- Bill
On 10/24/19 2:55 AM, 'Matson, Rob D.' robert.d.matson@... [find_orb] wrote:
> Hi Bill,
>
>
>
> Don't know if MPML is still operating, so figured I'd just send this
> your way since it's of more potential interest to you than most. This
> object is probably either lunar ejecta or manmade.
> Mean motion nearly the same as that of earth, low inclination, and low
> eccentricity. Earth encounter velocity must be very low. --Rob
>
> https://www.minorplanetcenter.net/mpec/K19/K19UC1.html
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Posted by: Bill Gray <pluto@...>
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