Asteroid troubles

Bill J Gray Apr 7, 2002

Hi Stephen,

I've been asked about this one a few times now, which means I really
ought to post a reply on the FAQ. Which I've just done; the following
now appears at

http://www.projectpluto.com/faq_curr.htm#missing_asteroids

The switchover is at exactly noon TD (Dynamical Time), because
asteroid orbital elements are always expressed in terms of TD. For
example, the file \ASTEROID\ASTEROID\2450400.AST contains elements
for the date JD 2450400.5 Dynamical Time, which occurs a bit before
noon in UT.

Bob, about the odd MPCORB behavior: you may recall that, once
upon a time, drawing asteroids with MPCORB was uniformly slow, taking
several seconds per redraw every #$!* time. I got around that by having
Guide compute limits, in RA/dec and magnitude, for each asteroid,
covering the current day. It does this _once_, and on subsequent redraws,
can say the equivalent of, "This asteroid isn't on the current screen,
or doesn't come up to the current magnitude limit; ignore it."

Change the current time into the next or preceding day, and Guide has
to toss out all that data and recompute it for the new day. That takes
a few seconds. After that, all is well again.

-- Bill

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Q: Asteroids vanish for dates before 9 October 1995 12:00 UT, and
dates after 9 May 2005 12:00 UT.

A: This happens in Guide 8. The problem was one of CD-ROM space.
Previous versions showed, at most, 32,000 asteroids. Guide 8 shows
a bit over 158,000, reflecting the fact that the number of known
asteroids climbed sharply in the interim. So I had to cut down on
the time span that was covered.

Some of the elements for more distant eras were put onto the second
disk. If Guide has access to that, the range over which asteroids are
displayed runs from 6 Mar 1960 to 24 Feb 2023 (though over much of that
range, only the 30,000-odd numbered asteroids are shown.)

I also did one other thing which will be a problem for some people.
In Guide 7, elements near the current date were stored at 50-day intervals,
so that at most, 25 days worth of perturbations were ignored. At distant
dates, the element spacing switched to 200 days. In Guide 8, all elements
are at a 200-day interval. That drastically reduced the storage requirements.
But it does mean that up to 100 days worth of perturbations are sometimes
ignored. (The solution to this will be to have Guide numerically integrate
the orbits when ultra-high precision is required.)

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