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