Absolute Magnitudes in MPCORB.dat

I have a general question about MPCORB data, specifically about the "absolute magnitude" number in MPCORB.dat records.

In MPCORB.dat, the field labeled "H" follows the "Designation" field in the data record. In the MPC description (at http://cfa-www.harvard.edu/iau/info/MPOrbitFormat.html) it's called "Absolute magnitude." In the .dat file, the values of H range from something like 3.34 down to 23 or more.

I want to estimate the apparent magnitude of objects as they come near Earth. Does someone know what the reference distance is for the absolute magnitude?

For stellar astronomy, absolute magnitudes are referenced to a distance of 10 parsecs, or 32.6 light-years. This clearly is not the reference distance for MPCORB objects -- if it were, most would outshine the Moon when in the vicinity of the Earth, say at 1 AU. So it's got to be something different other than 10 parsecs.

Pretty sure minor planet reference distance (scaling from abs to vis magnitudes) is 1 AU. However, remember that you have to account for both sun-to-minorplanet distance and minorplanet-to-observer distance.

Should be able to find the definitive answer at minorplanetcenter.net.

Hi Eric,

Thanks for the info. I would have thought 1 AU would be the standard distance. And I could not find any information about this other than the page I cited at the Minor Planet Center. It's probably a matter of not knowing exactly what to search for. I'll keep looking.

I think the relation you're looking for is apparent mag = abs mag + 5 * log(Sun-to-MP in AU) + 5 * log(MP-to-observer in AU).

The 5 comes from the 2.5 in the definition of magnitude, except twice that because of the square relation of illumination to distance.

Eric, yes you're correct. I found this paper online, pointed to in a reference at ProjectPluto.com. The paper describes how to determine H and G based on that phase angle and a measurement of apparent magnitude. I can invert the equations to calculate V using H and G from an MPCORB.dat or NEA.dat file, and the predicted phase angle. I'm not trying to get hundredth or even tenth magnitude accuracy - I just want to be sure that some NEA that I'm chasing will likely have an apparent magnitude I can detect in a CCD image. Thanks for your help.

It's good you don't need tenth-magnitude accuracy, as asteroids rotate and many or most have a lightcurve of significant amplitude. Best one can get from these formulae is a good estimate of the *average* apparent magnitude.

Thanks Eric!!! I would not have known how to answer.