APSF
//- Find the closest approach of two objects (obviously at least one of them must be a solar system object).
//- After the script ends, the plan will have the date/time fixed to that of the closest approach, within
//- the time precision specified.
//- The plan must have at least two objects in it. Certain assumptions are made:
//- 1. If there is more than one close approach during the specified period, any one of those close 
//-    approaches will be found.
//- 2. If no close approach is found during the period, the closest approach will be reported, which
//-    will be at one of the period end points.
//- 3. No check is made to see that at least one of the objects is a solar-system object.
//-
//- Inspired by the article on Page 72 of the March 2008 issue of Sky & Telescope.
//-
//- Paul Rodman, Feb 2008
//-
//- Version 1.0  24 Feb 2008
//-   Initial release

//------------------------------------------------------------------------
function DistanceOn(d as double, first as integer, second as integer) as double
  PlanLocalDateTime=d
  return AngleBetween(obj(first),obj(second))
end function

//------------------------------------------------------------------------
function FindMinimum(startDate as double, endDate as double, periods as integer, byref iprogress as integer, _
     first as integer, second as integer) as double
  dim d,dist,mindist,mind as double, i as integer
  
  for i=0 to periods
    if UpdateProgress(iprogress) then return -1.0
    iprogress=iprogress+1
    d=startDate+i*(endDate-startDate)/periods
    dist=DistanceOn(d,first,second)
    if i=0 or dist<mindist then
      mindist=dist
      mind=d
    else
       iprogress=iprogress+(periods-i-1)
       return mind
    end if
  next
  return mind
end function

//------------------------------------------------------------------------

dim startDate, endDate, midDate, dist, period, delta as double
dim precision,iprogress,i,n,isel,osel(-1),first,second as integer
dim ids(-1),s as string

if nObj<2 then
  print "Must be at least two objects in the plan"
  return
end if

for i=1 to nObj
  s=Obj(i).ID
  if Obj(i).Name<>"" then s=s+" ("+Obj(i).Name+")"
  ids.Append s
next

startDate = currentDate
endDate = startDate+31.0*3600.0*24.0

SetCheckListParameter("Select two objects",osel,ids,true,10)
SetDateParameter("Period start",startDate)
SetDateParameter("Period end",endDate)
SetChoiceParameter("Precision",precision,"1 day","1 hour","1 minute","1 second")

if not EditParameters("Closest Approach") then return

osel=GetCheckListParameter("Select two objects")
first=-1
second=-1
for i=0 to ubound(osel)
  if osel(i)=1 then
    if first<0 then
      first=i+1
    elseif second<0 then
      second=i+1
    else
      print "Only select two objects!"
      return
    end if
  end if
next
if first<0 or second<0 then
  print "Choose two objects!"
  return
end if

startDate=GetDateParameter("Period start")
endDate=GetDateParameter("Period end")
precision=GetChoiceParameter("Precision")

midDate = (startDate+endDate)*0.5
period = abs(startDate-endDate)*0.1
select case precision
case 0
  delta=3600.0*24.0
case 1
  delta=3600.0
case 2
  delta=60.0
case 3
  delta=1.0
end select

n=0
while true
  n=n+11
  if period<delta then exit
  period=period*0.1
wend

period = abs(startDate-endDate)*0.1
StartProgress("Find close approach",true,n)
iprogress=0

isel=SelectedObject
while true
  midDate=FindMinimum(midDate-period*5.0,midDate+period*5.0,10,iprogress,first,second)
  if midDate<0.0 then exit
  dist=DistanceOn(midDate,first,second)
  if period<delta then exit
  period=period*0.1
wend
StopProgress
bleep

if midDate>=0 then
    print "Close Approach at "+DoubleToDate(midDate)+" "+DoubleToTime(midDate)+" Distance = "+format(dist,"0.0000")+DegreeSymbol+" ("+format(dist*3600.0,"0.000")+""")"
end if
if isel>0 then obj(isel).Selected=true