This query concerns the use of two ostensibly very similar databases
used to predict the positions of asteroids - but which are giving very
different results - at least for one asteroid.

I use Guide to predict asteroid positions. Over the weekend I requested
and received (from the MPC) an asteroid identity suitable for monitoring
within my parameters. Asteroid 1998 RB55 was listed at mag 16.5. Using
MPCorb, its position was computed and I drove the telescope (254mm LX200
with SX CCD) and quickly located the region. Perhaps due to mist,
despite reaching mag 16-17, I could not locate the asteroid, though
positively identified several similar magnitude stars in the region. I
switched to Guide's own database and was amazed to find the same
asteroid predicted to be many minutes of arc away. I checked out that
position as well but there was no sign of the asteroid.

On this occasion, my mag limit might have been due to mist, but I would
appreciate any comment on the above, particularly regarding the
positional discrepancy between the two databases. Maybe Guide's is a
little dated and newer elements may be in the MPCorb base?

I did locate c/1999 S4 LINEAR so all was not lost!

Lawrence Harris
[943 Peverell, Plymouth, UK]