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Attempt to View 2021 NA Last Night July 2
#1
I've got to get better at this. I thought I might have had a chance to snag 2021 NA whizzing by last night (July 2 21:00-23:00CDT). I fetched MPC NEA Today (July 2) and did the ephem for time and date when close and bright. Showed best and apparent at 22:00 July 2. This small rock was moving over 900"/min but near the limit of scope plus there were some clouds moving in and out. Did not find it. Just now compared ephem with one from Horizons and position was different. Enough that I could have easily been looking 1.5° too far SE. I need to get better at this. It takes a lot of patience to track down and chase one of these faint fast movers.
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#2
Hi BMG,

I noticed in your thread where you posted the osculating elements that you were getting different values for the elements for every hour. Am I interpreting that correctly? The values seemed to change in the 3rd or 4th decimal place for several elements. Wow! It's a wonder you can find anything at all under those conditions.

Phil S.
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#3
(2021-07-03, 06:20 AM)bigmasterdrago Wrote: I've got to get better at this. I thought I might have had a chance to snag 2021 NA whizzing by last night (July 2 21:00-23:00CDT). I fetched MPC NEA Today (July 2) and did the ephem for time and date when close and bright. Showed best and apparent at 22:00 July 2. This small rock was moving over 900"/min but near the limit of scope plus there were some clouds moving in and out. Did not find it. Just now compared ephem with one from Horizons and position was different. Enough that I could have easily been looking 1.5° too far SE. I need to get better at this. It takes a lot of patience to track down and chase one of these faint fast movers.

Wow, that's a fast one! I'd definitely use Horizons orbits, and be sure to get the orbits off their server as close to when you go out as possible, so they reflect the most recent observations.
Clear skies,

Greg
Head Dude at Skyhound
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#4
Yep. It was ripping along. And even much faster as the phase angle changed quickly and the magnitude also caved. So my window was very small to attempt when possibly at the edge of my scope and skies. I tend to almost always use Horizons as a cross check but cannot print finder charts from there. ST4 makes awesome charts. It had only 2 days of observations so was a last minute opportunity for a fast mover.

(2021-07-03, 04:13 PM)PMSchu Wrote: Hi BMG,

I noticed in your thread where you posted the osculating elements that you were getting different values for the elements for every hour. Am I interpreting that correctly? The values seemed to change in the 3rd or 4th decimal place for several elements. Wow! It's a wonder you can find anything at all under those conditions.

Phil S.

As you know the elements change rapidly as we encounter an especially close small rock from space. That's why we need those up to the moment element sets. To know where the gravitational perturbations of the Earth/Moon system have redirected its path. I think with this really close rock being so small, it would have behooved me to get elements and positions by the minute! Crazy stuff. This small rock really got thumped when it passed over and outside our planet. I stuck a small video of the impact on the orbit of 2021 NA at https://vimeo.com/manage/videos/570736817. At its closest point to my backyard, it was moving 2152.98"/m (~0.59°/min)! At that moment, ~5° up in the NW and ~18th magnitude due partly to a low phase angle (15%). But really close (64415Km).

You probably know how to get those Osculating Elements. I'll refresh for others. Fill in the form at https://ssd.jpl.nasa.gov/horizons.cgi  Make sure that Elements Type is set to Elements. Fill in your other info. You can also get positions by choosing Observer Table for Elements Type. This will give you everything you need to edit the Osculating Elements within ST.
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#5
Thanks BMG. I haven't used the Horizons website for elements, but have looked at it in the past. The user interface is confusing to me. Are they doing perturbation calculations including the effects of earth-moon gravity 'on-the-fly' so to speak? Then showing the modified elements?Awesome, as is your video. The orbit really changed after the close approach.

Cool stuff for sure.

Phil S.
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#6
Horizons does each step calcs and creates new (osculating) elements prior to next step, so yes, takes into account the gravity of the perturbing objects. It is not a two-body calc as most of our software does (MegaStar, The Sky X, Stellarium). With really close small fast movers, we need to update elements rather often (each step) if we want good positions. There was a very old piece of software that did that called "Dance of the Planets". Doesn't run well on modern PCs. There are some online that you can create your own simulation as well as some modern software that will simulate gravity. Do a search for solar system gravity simulators. Nothing I've seen that can display/print tracks with such ease like SkyTools 4. You just need to stay close to the epoch of elements. And if done correctly, the positions in SkyTools completely agrees with Horizons. The most tricky part is determining when the best viewing window is for your location as these fast movers run the sky quickly. I'm still struggling with that a bit. Occasionally, the magnitude will be wildly off in the generated ephemeris (ST) - example - 2008 GO20 close on July 24, 12:13UT - big rock (92-210 meters) shows obvious difficulty, 5.2 mag but not shown on IA (most likely due to low altitude).
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#7
One thing to add that you guys may not be aware of: SkyTools keeps multiple orbital elements for a single object and then picks the one with an epoch closest in time to when you are calculating the position. So for close approaches I normally have HORIZONS output elements every few hours and then pick one from the beginning of the close approach, one from closest approach, and one from after the close approach and add all three sets of elements.

For very fast movers, I add sets of elements every hour or so throughout.
Clear skies,

Greg
Head Dude at Skyhound
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#8
Thanks Greg,

I knew that the MP database kept element data for multiple epochs, but was not aware that additional elements could be added at multiple epochs as you described for MPs whose osculating elements change rapidly during a close approach. I've never tried to input elements from HORIZONS as BMG does.

Good to know. 

Phil S.
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#9
I too thought that only edits could be performed not sets of individual epochs. Some experimenting needed. Playing with it currently.

When I try adding 2021 MC as a new rock and paste from Horizon, add the name back, and correct H, I get a not responding after clicking OK. Then a program close when trying any other mouse click.

So it appears that I need to edit

BUT.... if I edit, the original elements get replaced. It does not add another epoch.

FYI, I pruned and the program still crashes when I try to add another set of element with a different epoch.
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#10
When ever I right click on an asteroid number in the drop down list after a search. It brings up the add elements. If I then paste from Horizons a different epoch set of elements, make sure the name is right, and correct the H, click OK, it crashes after a not responding warning

I'm adding it at this prompt
   

Did I mention I pruned to only the 200 day epoch (s).
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