I'll claim minor ignorance on this... so someone please correct me if I'm wrong.
I think that the Epoch it's talking about is the associated date/time where those orbital elements are based.
If you're wanting to add various comets and asteroids into Universe Sandbox do this:
Click on this Horizon System link just about the applet:
http://ssd.jpl.nasa.gov/horizons.cgi?find_body=1&body_group=sb&sstr=106P%20[1999]
Make these changes:
Ephemeris Type = 'Vector Table'
Coordinate Origin = @sun
Under Table Settings Change:
output units = km & km/s
Change time span to:
start = 2008-01-01
stop = 2008-01-02
For this comet it should look like this:
Ephemeris Type [change] : VECTORS
Target Body [change] : Comet 106P/Schuster [1999] (SAO/1999)
Coordinate Origin [change]: Sun (body center) [500@10]
Time Span [change] : Start=2008-01-01, Stop=2008-01-02, Step=1 d
Table Settings [change] : output units=KM-S
Display/Output [change] : default (formatted HTML)
Press the Generate Ephemeris button.
Find this section in the output results:
*******************************************************************************
JDCT
X Y Z
VX VY VZ
LT RG RR
*******************************************************************************
$$SOE
2454466.500000000 = A.D. 2008-Jan-01 00:00:00.0000 (CT)
-3.696330150114028E+08 1.876239664454978E+08 1.481976483689495E+08
-1.651474509485226E+01 -9.403602827061283E+00 2.488145223257269E+00
1.468416667434924E+03 4.402202420984842E+08 1.069644369078651E+01
2454467.500000000 = A.D. 2008-Jan-02 00:00:00.0000 (CT)
-3.710577443435432E+08 1.868104099691846E+08 1.484117649337029E+08
-1.646512075064017E+01 -9.428678541826788E+00 2.468272783599756E+00
1.471498000230135E+03 4.411440024310767E+08 1.068688672266089E+01
$$EOE
*******************************************************************************
The numbers you want are the 1st two lines of 3 numbers each:
-3.696330150114028E+08 1.876239664454978E+08 1.481976483689495E+08
-1.651474509485226E+01 -9.403602827061283E+00 2.488145223257269E+00
This is the x,y,z position and x,y,z velocities for that body at that date.
Copy those numbers into your XML file and make it look like this:
<PositionX>-3.696330150114028E+08</PositionX>
<PositionY>1.876239664454978E+08</PositionY>
<PositionZ>1.481976483689495E+08</PositionZ>
<VelocityX>-1.651474509485226E+01</VelocityX>
<VelocityY>-9.403602827061283E+00</VelocityY>
<VelocityZ>2.488145223257269E+00</VelocityZ>
You can drop these values into any of the "Our Solar System" simulations since they all use this date (January 1, 2008).
Let me know if you need more help.
If you want to do more than 5 or 6 of these there's a hidden feature in Universe Sandbox to automate the extraction of this data into the XML format. Let me know and I'll share the instructions.
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