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2021 RG19
The fast mover 2021 RG19 is predicted to reach ~16 mag on the nights of Sep 27 & 28 2021. It's moving ~2'/min. 

Here's a screenshot of the path using elements from 2021 Sep 24: [attachment=2018]

I'm currently getting updated elements from MPC & will post a path using elements for today's epoch & a file of elements from HORIZONS at 4 hr intervals.

Hope someone can get a look at it,

Phil S.

Here's the path predicted using elements for 2021 Sep 27: [attachment=2019]

The file of osculating elements calculated by HORIZONS at 4 hr intervals is posted below.

The HORIZONS page for calculating the elements is here:
Horizons System (

I hope the link works.

Phil S.
Hi Phil,

Thanks for posting this (also to @bigmasterdrago for other threads). I have a general question: how do you find out about the fast movers? I see that MPC has a list of "close approaches" on its front page; do you click on each entry to get the details and check the "mean daily motion" field to identify which move quickly? Or is there a more efficient way?

Hi razvan,

I use the Database Power Search (DBPS) to search for MPs that are predicted to pass within 0.05 AU 2 days in the future. Greg doesn't recommend using the DBPS for this application, but I'm retired & consider it a fun endeavor  Wink.

Nearly all the hits are too faint, but every so often a good one shows up. Using the CNEOS website to check for close approaches doesn't give the brightness or other necessary observing conditions that ST4 provides, so I like it. You do need to keep your MP database updated frequently - especially the NEA data at Today's Epoch from MPC.

After the DBPS I calculate ephemerides for each NEA to find max brightness & min earth distance (Re) to see what's observable in my 13" scope. I post the good ones here. BMD like fast movers, so the closer they get, the faster their relative motion.

Have to go now, but I'll provide more info if you like later.

Hope this helps,

Phil S.
Thank you, I was interested in the workflow, this is fun, I'll do this as well. 2021 RG19 moves fast!
Yes, its maximum motion is ~2'/min. BMD found one in July or August that was moving at 900"/min!

I have the full MP database from the MPC, but they only provide elements at epochs ~6 months apart. ST4 can download MP elements of the 'NEAs at Today's Epoch' which you need for close approaches.

What I've been doing is:
1) Download the full MP data from MPC every 10-14 days (doing this will slow down the calcs for the NEAs),

2) Download the NEA at Today's Epoch every 2-3 days,

3) Run the DBPS for 2 days in the future looking for MPs that come within 0.05 AU (this gives you some advance warning that something is coming - also check the CNEOS website for the upcoming close approaches) There are usually ~20 NEAs found by this search.

4) Run the ephemeris from ~5 days before to ~20 days after the present day.

5) Look for the max brightness & create an ST4 Quick Log entry with this info for this time. I note the Re in the log comments too.

6) Assign a Star Rating to each MP. I use 1 star if its too faint, only visible in daylignt, from the wrong hemisphere, etc. 3 stars if its only detectable via imaging. 4 or 5 stars if its bright enough to be visible in the telescope.

7) For 4&5 star objects, save more info about the pass in the log.

8) Download element data from HORIZONS around the time of the close approach to update the ST4 MP DB with the elements right at the approach time. (This is complicated because ST4 only seems to keep one set of elements like this. I tried to add elements at T-6, T0 & T+6 hours from close approach, but only one set was saved in the DB. It's also very time consuming to input one set of elements at a time then have ST4 perform all the pre-calcs that it does to speed up plotting of the MPs each time.)

Most of the MPs get 1 star & many were detected after close approach (hence the reason to calculate the ephemeris a few days in the past to see what we missed :-).

As Greg suggests, you want to use elements that are calculated right before the approach because they're the most accurate. Another complication is that these are osculating elements & their values change during the pass. They're never right, just close enough to find the MP. BMD has run into this problem repeatedly when he tries to observe these things. Even the elements provided by HORIZONS can change as more observations of the MP are made to refine the motion. Celestial mechanics is a complex field of study.

Hope this helps, let me know if you have any questions,

Phil S.
Thanks again! One thing that I wasn't sure about: apart for the fun factor, is there any advantage of using DBPS instead of the list from CNEOS to identify upcoming close approaches? I understand the rationale for using ST4 afterwards. 

I'll definitely keep a closer eye of what you and BigMasterDrago post, I hope to see a fast mover one day (or image, rather, as my largest scope now is only a 120mm refractor).

One downside of the CNEOS list is objects drop from the list after close approach. A bright object might still be visible after close approach although it won't be moving as fast. I looked at the Mean Daily Motion field, but it doesn't seem to correspond to the really rapid motion that occurs at close approach. Check out 2021 RG19 & 2021 RM5. 2021 RM5 has n=0.73°/day, but moves 11"/min at close approach, while 2021 RG19 has n=0.21°/day, but moved at ~120"/min.  Approach distance plays a big factor too. Lots of factors affect the apparent motion. High relative velocity + small Re = Fast Motion.

An alternative approach to calculating an ephemeris in ST4 would be to open the Object Information dialog at the time of close approach & see how fast it's moving. As BMD has noted, brightness is affected by phase angle which changes rapidly during the pass, so brightness can drop just when the MP is closest to earth. Frustrating.

Good luck hunting,

Phil S.
Thanks again. Indeed, I think the mean daily motion is, well, the mean over the entire orbit. 

I'll probably use ST4 to plot the positions, also because I need to check where in the sky the asteroid is.
A similar and not outdated list can be found at and by clicking on the name, it's easy to quickly run a fast ephemeris for your location spanning the 48 hours (1 hour steps) near close approach. I use that info to best determine the time and interval to run an ephem in ST, etc. to get best times. And discover that the best apparition for 2021 RG19 is not at closest (Sep 28, 17:01 CDT moving 3.9'/min) but in dark time on the 27th between 20:00 and 23:45 CST for me. Moving ~1.5'/min, alt 31°-57° at NE azimuth moving from Andromeda into Cassiopeia. And very little difference in apparent magnitude due to the phase angle change 84% vs 56%@closest approach. At closest approach it is actually 8° below my NW horizon! I did not even know of this rock as I was out of town. good for those with imaging 4"-6" but too faint for my visual 13".

In the produced ephemeris at Horizons, use the column deldot to determine when the object is changing from coming to going and then look at the illu% for % phase angle.
Thank you, good tips!

One more question if you don't mind. I suppose that in the past used to show the apparent magnitude of the object as the legend below the table contains "MAG is the visual magnitude of the asteroid on the date of closest approach" even though there's no MAG column. I found recent magnitude measurements via MPC  (eg but no estimation about the expected magnitude in the near future at various moments.

Do you use the ephemeris calculated by HORIZONS for the latter? If yes, it seems to be something to be calculated by the user. We get the "approximate apparent airless visual magnitude" and the atmospheric "visual magnitude extinction" (a value, in magnitudes, to be substracted, so the apparent visual magnitude would be APmag-mag_ex) . I'm asking not because I cannot substract the numbers :-), just to be sure I'd do the correct thing. 

Thanks again,

'a-mass mag_ex' =
    RELATIVE optical airmass and visual magnitude extinction. Airmass is the
ratio between the absolute optical airmass for the targets' refracted CENTER
point to the absolute optical airmass at zenith. Also output is the estimated
visual magnitude extinction due to the atmosphere, as seen by the observer.
HORIZON.  Units: none (airmass) and magnitudes (extinction).

'APmag  S-brt' =
  The asteroids' approximate apparent airless visual magnitude and surface
brightness using the standard IAU H-G system magnitude model:

  APmag = H + 5*log10(delta) + 5*log10® - 2.5*log10((1-G)*phi_1 + G*phi_2)

  For solar phase angles >90 deg, the error could exceed 1 magnitude. For
phase angles >120 degrees, output values are rounded to the nearest integer to
indicate error could be large and unknown. For Earth-based observers, the
estimated dimming due to atmospheric absorption (extinction) is available as
a separate, requestable quantity.


Date__(UT)__HR:MN Azi_(a-appr)_Elev a-mass mag_ex  APmag 

2021-Sep-29 00:00  8.5344  -1.3300  n.a.  n.a.  16.569 
2021-Sep-29 01:00  16.8881  -1.8994  n.a.  n.a.  16.718 
2021-Sep-29 02:00  25.8320  -1.1728  n.a.  n.a.  16.878 
2021-Sep-29 03:00  35.1346  0.9918 23.463  6.595  17.046 
2021-Sep-29 04:00  44.5732  4.6259 10.661  2.997  17.221 
2021-Sep-29 05:00  54.0028  9.6522  5.721  1.608  17.401 
2021-Sep-29 06:00  63.4057  15.9074  3.594  1.010  17.583 
2021-Sep-29 07:00  72.9221  23.1689  2.525  0.710  17.766 
2021-Sep-29 08:00  82.8839  31.1690  1.926  0.541  17.950 
2021-Sep-29 09:00  93.8913  39.5822  1.566  0.440  18.132 
2021-Sep-29 10:00 106.9797  47.9628  1.345  0.378  18.311 
2021-Sep-29 11:00 123.8652  55.5913  1.211  0.340  18.488 
2021-Sep-29 12:00 146.7066  61.1958  1.140  0.321  18.660 
2021-Sep-29 13:00 175.1863  62.9367  1.122  0.315  18.829 
2021-Sep-29 14:00 202.8923  59.7962  1.156  0.325  18.994 
2021-Sep-29 15:00 224.3186  52.8236  1.254  0.352    19. 
2021-Sep-29 16:00 239.9649  43.6543  1.446  0.407    19. 
2021-Sep-29 17:00 252.0321  33.3527  1.814  0.510    19. 
2021-Sep-29 18:00 262.1551  22.4990  2.595  0.729    20. 
2021-Sep-29 19:00 271.3757  11.4395  4.894  1.376    20. 
2021-Sep-29 20:00 280.4221  0.4308 27.626  7.766    20. 
2021-Sep-29 21:00 289.9089 -10.2780  n.a.  n.a.    20. 
2021-Sep-29 22:00 300.4639 -20.3833  n.a.  n.a.    20. 
2021-Sep-29 23:00 312.7928 -29.4580  n.a.  n.a.    20. 
2021-Sep-30 00:00 327.6099 -36.8806  n.a.  n.a.    20. 
2021-Sep-30 01:00 345.2430 -41.8212  n.a.  n.a.    21. 
2021-Sep-30 02:00  4.8873 -43.4559  n.a.  n.a.    21. 
2021-Sep-30 03:00  24.4619 -41.4555  n.a.  n.a.    21. 

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