Minor Planet (52768) 1998 OR2 Close approach

[PMSchu]

The minor planet (52768) 1998 OR2 is about to make a close approach of earth over the next several weeks. It's currently in Lynx at 14.5 mag. Closest approach (0.04 AU) occurs on 29 Apr when it will be in Hydra at 10.9 mag.

Phil S.

[theskyhound]

I will add it to the minor planets lists.

[bigmasterdrago]

Greg, just curious. You say you will add it to the minors planets list. What minor planets list are you referring to? Sorry for being uninformed. Is not it already in the astorb or MPCORB files? Or for that matter in the Unusual or NEA files?

[PMSchu]

Kenneth,

I think that he will add it to the 'Current Minor Planets' list that can be uploaded when ST4 starts. That way you won't need to mess with the MPCORB or ASTORB downloads. Pretty convenient! Just be sure that you have the box for MPs checked in the 'Subscriptions' dialog. Since I have the MPCORB downloaded, I don't usually download the "Current MP' file, but it's a good idea to update it occasionally incase Greg adds something like this to the list.

Hope this helps,

Phil S.

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Make that 'downloaded' not 'uploaded.  

Phil S.

[bigmasterdrago]

The files that I see the elements for 1998 OR2 are unusual and MPCORB. If I understand it correctly, they have different epoch dates. Unusual is 2017 Sept 4, MPCORB is 2020 May 31. I seem to think that the ephemeris "start at" date should be near or at the epoch date for best position calculation. So it makes since to start at May 31, 2020 and run the ephemeris back to April 29th. Do I have this right? Or just run the ephemeris from Sept 4, 2017 using the elements in the unusual file. A rather long list.

[theskyhound]

The files that I see the elements for 1998 OR2 are unusual and MPCORB. If I understand it correctly, they have different epoch dates. Unusual is 2017 Sept 4, MPCORB is 2020 May 31. I seem to think that the ephemeris "start at" date should be near or at the epoch date for best position calculation. So it makes since to start at May 31, 2020 and run the ephemeris back to April 29th. Do I have this right? Or just run the ephemeris from Sept 4, 2017 using the elements in the unusual file. A rather long list.

Yes. The MPC has made changes that result in epochs far from when we are observing and I am not happy with it. Their standard epochs aren't close enough in time either. This is why ASTORB is often a better choice, but I understand it may not be updated at the moment.

The focus of the MPC is on near earth asteroids like this one, and I think if you asked them about it they would say that for other minor planets the inaccuracies are acceptable, and for NEAs they have special lists that are updated daily. So what you want to do is to download the MPC NEA Today list. I already used it when I added the most recent orbit to the Current Minor planets list.

Unfortunately, what we really would want to do in this case is to use the Single Minor Planet Download. But they use the "standard" epochs for those and the orbits can be up to 6-months off the current date. This ticks me off, to be honest. I have plotted asteroid position in Astrometrica and had them all over the place because of this.

[bigmasterdrago]

I ran an ephemeris on 1998 OR2 from March 31 00UT for 40 days. It looks to be using an epoch of May 30. The asteroid position on Apr 29 00UT shows 10h37m31.1s -20 17 14 from my location. This position compares favorably with that computed by Horizons of 10 37 21.83 -20 14 38.7 also for my location.
Even if this were a faint rock from space, with a speed of ~17"/min, the error in DEC of 3 arc minutes and 10 seconds of RA should not create a big issue in locating it.

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Would not the greatest accuracy come from running the ephemeris from the epoch date of what ever elements we choose (get) and let the software work out the gravitational perturbations of the moon, earth and any other planets effecting the osculating elements? Don't those elements change somewhat with every time step?

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Thanks Greg for the explanation.

[theskyhound]

Would not the greatest accuracy come from running the ephemeris from the epoch date of what ever elements we choose (get) and let the software work out the gravitational perturbations of the moon, earth and any other planets effecting the osculating elements? Don't those elements change somewhat with every time step?

That's literally the Minor Planet Center's job. It is also a big expensive computation, especially if you do it for more than one minor planet. Why should everyone be sitting at their own devices waiting for individual calculations when the MPC could do it for everyone once? This is why we have ASTORB, but it isn't always reliable these days. Again, I believe that the MPC is letting the community down in this regard. They could at least adopt a shorter time period between their standard epochs.

[theskyhound]

By the way you don't have to guess what epoch is bring used. SkyTools always uses the closet available to the date of interest. Each minor planet can have more than one set of elements at different epochs. To see what is available on your device:

1. Open the Minor Planets database from the Data menu
2. Type the ID of a minor planet into the search area at the top right and press enter
3. Open the tree to show the orbits available, listed by epoch.

[PMSchu]

Ah, but to calculate the perturbations, you need to know the positions of all of the potentially perturbing bodies and their motions. The osculating elements are supposed to be a simplification of the orbital mechanics problem and make use of the Keplerian approximations. These approximations are 'good enough' when used within a sufficiently close time of the epoch of the osculating elements. We're not trying to hit these things with a spacecraft & even NASA has to employ midcourse corrections to pull that off .

Phil S.