Alan Harris Oct 24 1:36 PM
To add to Bill's discussion and answer directly Rob's question of how an object with perihelion outside of Venus and aphelion inside of Mars can get into such an orbit, the answer is previous "scattering" encounters with the Earth. A low-v encounter with the Earth (at v_enc less than the Earth escape velocity, ~11 km/sec) can result in a re-direction of the relative velocity by a nearly arbitrary change. Thus, in some past encounter of 2019 UB4 with the Earth, it apparently came away with a relative velocity nearly perpendicular to the Earth's orbital motion, and thus ended up in an orbit with substantial eccentricity and some inclination, but essentially a semimajor axis of almost exactly 1.0. Such an orbit has a substantial Earth encounter velocity, around 3 km/sec, even though it doesn't go far in or out from the Earth's orbit. Now suppose in some future encounter the opposite happens, and the "scattered" direction is almost aligned with the Earth's orbital motion. It turns out that an object moving along the Earth's orbital track with a total velocity of 33 km/sec (Earth orbital v plus 3) will go out well past the orbit of Mars, and likewise at 27 km/sec (Earth minus 3) will go well inside of Venus. So it works both ways, an object can get into the present UB4 orbit from Venus or Mars crossing through an encounter from nearly parallel to the Earth's motion to roughly perpendicular to it, and someday can return to Venus or Mars crossing by a reverse encounter, from nearly perpendicular to nearly parallel relative velocity (if it doesn't hit the Earth first). To illustrate this, my little program that computes encounter velocity looks like this:
Enter semimajor axis
of reference body: 1.0
Enter a, e, i: 1.005 .0909 .796
Encounter velocity = 2.76 km/sec
Minimum heliocentric distance = .70 A.U.
Maximum heliocentric distance = 1.48 A.U.
Following the encounter velocity, the program returns an estimate of the minimum heliocentric perihelion and the maximum heliocentric aphelion possible for a body with the given encounter velocity, corresponding to relative velocities aligned, forward and backward, with the Earth's orbital motion.
This also illustrates the origin of that magic threshold of around 2.5 km/sec. At lower relative velocity, an object scattered exactly parallel to the Earth's velocity does not go in as far as Venus orbit or as far out as Mars orbit, so it is more or less trapped in Earth orbital space. To be sure, over hundreds of thousands to millions of years, encounters with the Earth and Moon can pump up the relative velocity slowly (from effects of Earth orbital eccentricity plus the separate Earth-Moon gravities), so such objects, such as lunar ejecta, will eventually escape, although on such long time scale and low velocity, impacting the Earth becomes a more likely fate, thus we get lunar meteorites.
Cheers,
Alan
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-- Alan Harris harrisaw@... 4603 Orange Knoll Ave. 818-790-8291 La Canada, CA 91011