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 Post subject: C/2017 U1
PostPosted: Thu, 26-10-17, 0:54 GMT 
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Hello, everybody.

Comet C/2017 U1 may be the first incoming (outging now, actually) object discovered with an interstellar trajectory (that, or it´s Rama ;) ).

http://www.minorplanetcenter.net/mpec/K17/K17UI1.html

Celestia does not plot its orbit nor its name, because Celestia does not like hyperbolic orbits :( . Will Celestia.Sci do?

Guillermo

# Comet C/2017 U1 (Panstarrs)
# Source: http://www.minorplanetcenter.net/mpec/K17/K17UI1.html
# Orbital elements:
# Epoch 2017 Sept. 4.0 TT = JDT 2458000.5
# T 2017 Sept. 9.41719 TT MPCW
# q 0.2515404 (2000.0) P Q
# z -0.7541603 Peri. 241.01670 -0.63536548 +0.68733697
# +/-0.0181483 Node 24.61531 +0.49903801 +0.71329677
# e 1.1897018 Incl. 122.32770 -0.58929769 -0.13702411
# From 34 observations 2017 Oct. 18-24, mean residual 0".5.
#
# Guillermo Abramson (g.abramson@gmail.com)

"C2017 U1" "Sol"
{
Class "comet"
Mesh "roughsphere.cms"
Texture "asteroid.jpg"
Radius 5.0 # guess
RotationPeriod 5.0 # guess
Albedo 0.2 # guess
EllipticalOrbit
{
Epoch 2458000.5 ##2013-Nov-28.86962143
# SemiMajorAxis # a=1/z
PericenterDistance 0.2515404 # q: more natural than semimajor axis for hyperbolic orbit
Eccentricity 1.1897018 # e
Period 1.5268783711297478595937466090309 # P = (q/(e-1))^1.5 for hyperbolic orbit (https://www.classe.cornell.edu/~seb/cel ... meris.html)
Inclination 122.32770 # i
AscendingNode 24.61531 # Node
ArgOfPericenter 241.01670 # Peri
MeanAnomaly 0.000000
}
}

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Guillermo Abramson
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 Post subject: Re: C/2017 U1
PostPosted: Thu, 26-10-17, 14:13 GMT 
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Hi Guillermo,

it's good you raised the question about hyperbolic orbits in celestia.Sci. It's now in my "todo" notes.

I don't understand the physical meaning of a Period P in case of hyperbolic orbits, since in this case the comet never returns...Without an entry for P and of SemiMajorAxis, there is no output at all about C2017 U1 like you also found. So I employed two alternatives:

  • I commented out P but instead used SemiMajorAxis a = 1/z ~ -1.326. Note that always a<0 in the hyperbolic case. This results in a reasonable display.

  • I used just a very large (dummy) value of P = 10 000 to make it (hopefully) drop out in the code for elliptical orbits .... In fact, the corresponding display looks also reasonable:

    [click on image by all means]
    Attachment:
    c2017_U1.jpg
    c2017_U1.jpg [ 112.12 KiB | Viewed 198 times ]

But also with your value P = q/(e-1) ~ 1.52 I get a sensible display, presumably meaning that there is no strong P dependence in our case...

I went again through the code in orbit{cpp,h} and parseobject.cpp which should not be too different from the corresponding Celestia files. While hyperbolic orbits may be formally obtained simply by analytic continuation from 0< e < 1 to e>1, computer code is highly unsuited for this process ;-)

Clearly, with relatively little effort, I will be able to code a more solid dedicated routine for hyperbolic orbits. As written, this task is now in my notes.

Thanks,
Fridger

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 Post subject: Re: C/2017 U1
PostPosted: Thu, 26-10-17, 15:34 GMT 
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Thanks, Fridger. Good to know.

Yes, I also don't understand the meaning of the period, that's why I put the reference to Selden's notes where I got it. I didn't check the code to see if it's meaningful. Anyway, the comet is also displayed correctly in Celestia 1.7 with my ssc. What is not shown is the orbit (nor the label). I had already seen this years ago with comet ISON, which had an eccenctricity very slightly above 1.

G

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 Post subject: Re: C/2017 U1
PostPosted: Thu, 26-10-17, 16:08 GMT 
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Guillermo, I completely agree with your findings about the missing orbit and label displays in case of hyperbolic orbits. For elliptical orbits everything looks fine over here...This shouldn't be hard to fix. -> also this job is now in my notes ;-)

Fridger

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 Post subject: Re: C/2017 U1
PostPosted: Sun, 29-10-17, 14:19 GMT 
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https://en.wikipedia.org/wiki/A/2017_U1

Initially thought to be a comet, it was reclassified as an asteroid a week later.

not Ice(water) => A/XXXX XX

yes Ice(water) => C/XXXX XX

26.4 km/s

;)


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 Post subject: Re: C/2017 U1
PostPosted: Sun, 29-10-17, 14:34 GMT 
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yes, I have added a post abbout the new asteroid interpretation some days ago, here:

viewtopic.php?f=11&t=919

Fridger

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