[neo_followup] {MPML} Tianwen-1 guesstimated orbit
Bill Gray
pluto at projectpluto.com
Sat Jul 25 09:55:07 EDT 2020
Hi Patrick,
Sorry for the late reply! Time zones work against us here (I'm
two hours east of you); I'm apt to be sleeping while you're observing.
For future reference... there are two ways you could go about
getting topocentric positions. One would be to take the geocentric
ephemerides and aim slightly closer to the horizon. At the time you
observed the object, it was about twice as far from us as the moon,
and could have been as much as half a degree closer to the horizon
than the geocentric ephems would tell you.
The geocentric ephems will put you on the right spot in azimuth;
it's just the altitude that will need to be decreased.
Since it was about 60 degrees above your horizon, the "drop" was
actually about cos(60) = 1/2 of that, or about a quarter degree
closer to the horizon... which would have put it just outside
your 18' x 26' FOV; did you have to hunt around a little?
The generalized version of this would be : if it's at the
distance of the moon (about 400000 km) and rising or setting,
drop one degree closer to the horizon. If it's further or closer,
adjust accordingly; for example, at ten times that distance,
you only have to drop a tenth as much, or 6'.
Few of us would try to observe at the horizon, so the "drop
distance" would be multiplied by the cosine of your altitude
above the horizon.
You can also generalize this to say that, with your FoV,
if the object is more than about a million km away, you can
probably use geocentric ephems and still find the object;
even closer than that if the object is near the zenith.
The alternative, usually easier method will be to copy the
observations from the pseudo-MPEC or other source, go to the
on-line Find_Orb, paste them in, and generate ephemerides for
your particular location and desired time span.
-- Bill
On 7/25/20 8:10 AM, Patrick Wiggins wrote:
> Here’s what I got this morning despite lots of clouds. (I’ve posted an animation made from the images to my Facebook page https://www.facebook.com/patrick.wiggins.71 )
>
> patrick
> 718
>
> Tianwen1 C2020 07 25.44727 00 52 28.46 +16 07 18.0 16.9 718
> Tianwen1 C2020 07 25.44766 00 52 28.30 +16 07 18.1 16.8 718
> Tianwen1 C2020 07 25.44883 00 52 27.80 +16 07 21.0 16.8 718
> Tianwen1 C2020 07 25.44922 00 52 27.64 +16 07 21.9 16.9 718
> Tianwen1 C2020 07 25.44960 00 52 27.50 +16 07 22.5 16.7 718
> Tianwen1 C2020 07 25.44999 00 52 27.33 +16 07 23.6 16.8 718
> Tianwen1 C2020 07 25.45117 00 52 26.85 +16 07 25.6 16.8 718
> Tianwen1 C2020 07 25.45155 00 52 26.68 +16 07 26.5 16.8 718
> Tianwen1 C2020 07 25.45194 00 52 26.54 +16 07 26.9 16.9 718
> Tianwen1 C2020 07 25.45350 00 52 25.87 +16 07 30.3 16.7 718
> Tianwen1 C2020 07 25.45389 00 52 25.70 +16 07 31.2 16.8 718
> Tianwen1 C2020 07 25.45427 00 52 25.56 +16 07 31.8 16.9 718
> Tianwen1 C2020 07 25.45466 00 52 25.39 +16 07 32.7 16.7 718
> Tianwen1 C2020 07 25.45584 00 52 24.95 +16 07 34.5 16.9 718
> Tianwen1 C2020 07 25.45623 00 52 24.76 +16 07 35.6 16.7 718
> Tianwen1 C2020 07 25.45662 00 52 24.59 +16 07 36.1 16.8 718
> Tianwen1 C2020 07 25.45701 00 52 24.42 +16 07 37.2 16.8 718
> Tianwen1 C2020 07 25.45817 00 52 23.93 +16 07 39.3 16.7 718
> Tianwen1 C2020 07 25.45856 00 52 23.77 +16 07 40.5 16.7 718
>
>
>> On 23 Jul 2020, at 12:06, Bill J. Gray <pluto at projectpluto.com> wrote:
>>
>> Hi folks,
>>
>> Cut-and-paste the following synthetic "observations" into
>> Find_Orb, and you should be able to get ephemerides for this
>> object which, I hope, will not be too laughably wrong.
>>
>> I'm working from limited data here. I know when the object
>> was launched, and I am reasonably certain that it went into an
>> orbit with an inclination of about 30 degrees. At the southern
>> end of that orbit about 40 minutes later, it then accelerated
>> into an orbit that should get it to Mars in mid-February 2021
>> (the orbit below corresponds to arriving on 2021 Feb 17).
>>
>> Given that guesswork, ephemerides computed from this may be
>> right on the nose. Or they could be several degrees off; I
>> really have almost no way of knowing, given the usual lack of
>> information from the China National Space Agency.
>>
>> I added a magnitude to give us an H=27 object, which should
>> match the booster pretty well; the spacecraft itself is apt
>> to be several magnitudes fainter.
>>
>> -- Bill
>>
>> Tianwen1 C2020 07 23.90000000 34 35.409+27 16 00.63 SynthJ95
>> Tianwen1 C2020 07 24.00000000 36 06.923+27 39 55.95 SynthJ95
>> Tianwen1 C2020 07 24.10000000 36 15.306+27 56 26.04 SynthJ95
>>
>> Tianwen1 C2020 07 24.20000000 39 33.283+27 53 19.52 Synth703
>> Tianwen1 C2020 07 24.30000000 40 19.638+28 11 24.54 Synth703
>> Tianwen1 C2020 07 24.40000000 39 50.006+28 24 18.52 Synth703
>> Tianwen1 C2020 07 24.50000000 38 45.602+28 29 27.62 Synth703
>>
>> Tianwen1 C2020 07 23.70000000 30 34.371+26 59 43.86 SynthC42
>> Tianwen1 C2020 07 23.80000000 32 10.944+27 35 22.19 SynthC42
>> Tianwen1 C2020 07 23.90000000 32 03.001+27 55 08.90 SynthC42
>>
>> Tianwen1 C2020 07 23.70000000 31 32.802+27 09 40.91 SynthN55
>> Tianwen1 C2020 07 23.80000000 33 16.512+27 47 35.50 SynthN55
>> Tianwen1 C2020 07 23.90000000 32 51.951+28 09 23.93 15.9 V SynthN55
>>
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