When you first try to add asteroid perturbations in the current version of Find_Orb, you will get the following message:
No asteroid ephemeris data is available, so
asteroid perturbations cannot be included. You can
fix this by downloading the necessary files. See
http://www.projectpluto/ast_pert.htm for details.
Previous versions of Find_Orb would include the perturbations of the "Big Three" asteroids (Ceres, Pallas, and Vesta) and ignored everything else. Now, Find_Orb can include a user-specified number of asteroids from the 300 given in the BC-405 asteroid ephemeris. You can click here for details as to why this was added, and on how to control the number of asteroid perturbers Find_Orb uses.
You can get details about BC-405 here. Basically, Jim Baer and Steve Chesley solved simultaneously for the orbits and masses of the 300 asteroids used as perturbers for the JPL DE-405 ephemeris, and provided masses and orbits at 40-day intervals covering the years 1800 to 2200. Find_Orb can use the results to get good positions for those 300 asteroids, and can use the masses determined in BC-405 to add suitable perturbations.
To get BC-405 working in Find_Orb (and to eliminate the above error message), download this file to the Find_Orb folder:
(about 131 MBytes). You'll be asked to provide a user name ("anonymous") and password (use your e-mail address... actually, anything with an '@' in it should work). Put the file in your Find_Orb folder, then run Find_Orb and (once you've gotten an orbit for the object of interest using just planets) turn asteroid perturbers On, and do a full step or two.
If you've problems accessing the file, you should click here for some hints on how to access MPC's anonymous ftp site. And if that fails, you can access the file here (about 60 MBytes, decompressing to the same 131 MBytes as the MPC's file.)
Be warned that, on the first full step, Find_Orb will pause for a few seconds to process the BC-405 file from ASCII to binary. Fortunately, it only has to do the conversion step once. But even after that, 'full steps' will be noticeably slower when asteroids are turned on as perturbers. It takes some computational horsepower to add in 300 new perturbers. Find_Orb does do some tricks to speed things up, though, and as described above, you can get some speed back by using only some of the 300 asteroids in BC-405.