Large asteroid ephemerides are incorrectly computed

[mjuric]

When Sorcha (internally via assist) computes asteroid positions, it includes top N most massive asteroids as perturbers. While this is desirable, if the orbital elements of one of the perturbers itself are given as input Sorcha will incorrectly compute its position.

For example, for May 15, 2025, Horizons places Vesta at (ra, dec) ~ (219.8, -3.8). Sorcha computes it to be at (~212.6, -3.1). I suspect we'll have this type of problem with all perturbers & should somehow detect (or at least warn) about it.

@devanshi-s04 just bumped into this trying to Sorcha (via another tool) for observation planning with a small (0.5m) telescope.

[matthewholman]

I know this is an ongoing annoyance. I'll try to make time to develop a good solution.

…

On Thu, May 15, 2025 at 8:10 PM Mario Juric ***@***.***> wrote: *mjuric* created an issue (dirac-institute/sorcha#1148) <#1148> When Sorcha (internally via assist) computes asteroid positions, it includes top N most massive asteroids as perturbers. While this is desirable, if the orbital elements of one of the perturbers itself are given as input Sorcha will incorrectly compute its position. For example, for May 15, 2025, Horizons places Vesta at (ra, dec) ~ (219.8, -3.8). Sorcha computes it to be at (~212.6, -3.1). @devanshi-s04 <https://github.com/devanshi-s04> just bumped into this trying to Sorcha (via another tool) for observation planning with a small (0.5m) telescope. — Reply to this email directly, view it on GitHub <#1148>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AA5CVWMKDQBG5HA5MDV5TPL26UUF3AVCNFSM6AAAAAB5HNHOU6VHI2DSMVQWIX3LMV43ASLTON2WKOZTGA3DONRSGQ3DQMA> . You are receiving this because you are subscribed to this thread.Message ID: ***@***.***>

-- Matthew J. Holman, PhD Senior Astrophysicist Center for Astrophysics | Harvard & Smithsonian 60 Garden Street, MS #51 Cambridge, MA 02138 (617) 496-7775

[mschwamb]

We warn about this in the documentation

[mschwamb]

We warn here and in the paper that is linked at the start of the docs

[mschwamb]

I'll also add the warning to What Sorcha Does Not Handle docs page to try and catch all users, because maybe there's a greater chance the user might read that documentation page

[mjuric]

Wontfix is a bit harsh... I know this will come as a news to most of us, but I worry that people don't read documentation 😉.

There's a principled way to fix this. Massive asteroids (and planets) are not point masses. At the start of the simulation we can compute the distance of the radius vector of the object to all objects in the DE+perturber ephemerides -- anything coming in lower than the radius of the planet/perturber Sorcha (or, better, ASSIST) should either refuse to integrate or should return the ephemerides from DE and not the n-body simulation (which would be what the users expect).

As people start using Sorcha with a broad set of telescopes, this will be a serious (and -- worse -- silent) bug.

[mschwamb]

Fine. I do know most people don't read the documentation. We likely need additional human effort to fix this.

[mjuric]

❤️. Go humans!