In the JPL small body browser, orbital elements are all provided with uncertainties and even a covariance matrix, such as for 2013 LA2. Very good. But underneath, the absolute magnitude is reported with a sigma of "n/a". Why would the uncertainty for an absolute magnitude be not applicable? Is it not a measurement but somehow given by definition?

  • $\begingroup$ Googling suggests the absolute magnitude is used to compute diameter by assuming albedo (cneos.jpl.nasa.gov/tools/ast_size_est.html), but not that absolute magnitude itself is computed. cneos.jpl.nasa.gov/glossary/h.html may be a hint. The absolute magnitude is defined by the asteroid being at "zero phase angle". It's possible that there are only one or two measurements at zero phase angle, so no point in publishing a sigma. However, this is just a wild random guess on my part. $\endgroup$
    – user21
    Jul 10, 2018 at 14:31
  • $\begingroup$ @barrycarter Even if true, even when there are only one or two measurements, there is still a measurement uncertainty; it just can't be estimated as the standard deviation from repeated measurements. $\endgroup$
    – gerrit
    Jul 10, 2018 at 14:43


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