Hi Ben,

As Denis and Ireneusz have suggested, the different asteroid
catalogues have different values of H.

I'd expect this to happen, because asteroid absolute magnitudes
tend to be _really_ poorly determined. The raw data that is sent to
the Minor Planet Center (and generally used for orbit determination)
gives the time, RA/dec, and magnitude for each observation. People
usually try very hard to get the RA/dec right, and sometimes try hard
to get the time exactly right. (Getting the time exactly right matters
greatly for fast-moving objects, mostly near-earth ones.)

But the magnitude data is a hodgepodge. Some people submit carefully
measured photometry. In other cases, the asteroid magnitudes are
more of a wild guess. And the photometric band mentioned on the observation
report may be all wrong, and the calibration may be to a not-very-good
star catalog... anyway, those H values are broad guidelines, not
exact measures.

So MPC may apply one approach, perhaps "average all the data, giving
each equal weight"; while Lowell Observatory may filter out outliers,
and JPL might set weights for each observatory. So you get different
estimates of H.

-- Bill