Thanks for the comments/suggestions. It looks like the NAIF kernels and software are right and the reference to "It came within 570,000 km of the planet's cloud tops", which may come from a JPL Voyager web page is wrong. The Wikipedia article for Voyager 2 detailed "Timeline of Travel" says "1979-07-09 - 22:29 Jupiter closest approach at 721,670 km from the center of mass", which matches the NAIF/SPICE values. By the way, when I tried running the script with target JUPITER instead of JUPITER_BARYCENTER I got an error "Insufficient ephemeris data has been loaded to compute the position of 599 (JUPITER) relative to -32 (VOYAGER 2) at the ephemeris epoch 1979 JUL 08 00:00:50.183."

Hi Brian

Thanks for posting this example, it has been very helpful to me. However, when I tried plugging in the variables to look at the Voyager 2 encounter, I don't get the right distances. Any idea why?

>>> import math
>>> import spiceypy as spice
>>>
>>>
>>> # utc time range for Voyager 1
... #utcStart = '1979-03-01'
... #utcStop = '1979-03-10'
...
>>> # utc time range for Voyager 2
... utcStart = '1979-07-05'
>>> utcStop = '1979-07-13'
>>>
>>> # target and observer
... target = 'JUPITER BARYCENTER'
>>> #observer = 'VOYAGER 1'
... observer = 'VOYAGER 2'
>>>
>>>
>>> def et2str(et):
... "Convert an ephemeris time (seconds after J2000) to a UTC string."
... formatStr = "ISOC"
... prec = 0
... s = spice.et2utc(et, formatStr, prec, lenout=256)
... return s
...
>>> # load leap second data
... spice.furnsh('naif0012.tls')
>>>
>>> # load voyager data
... #spice.furnsh('Voyager_1.a54206u_V0.2_merged.bsp')
... spice.furnsh('Voyager_2.m05016u.merged.bsp')
>>>
>>> # get ephemeris time (seconds since J2000)
... etStart = spice.str2et(utcStart)
>>> etStop = spice.str2et(utcStop)
>>>
>>> # get time range
... nsteps = 50
>>> etTimes = [i*(etStop-etStart)/nsteps + etStart for i in range(nsteps)]
>>>
>>> # get vectors from observer to target
... # see http://naif.jpl.nasa.gov/pub/naif/toolkit_...e/spkpos_c.html
... frame = 'J2000'
>>> abcorr = 'NONE' # abberation correction
>>> positions, lightTimes = spice.spkpos(target, etTimes, frame, abcorr, observer)
>>>
>>> # get distances
... distances = [math.sqrt(x**2+y**2+z**2) for x,y,z in positions]
>>>
>>> # Voyager 1's closest approach to Jupiter occurred March 5, 1979
... # Distance 349,000 km
... # Voyager 2's closest approach to Jupiter occurred on July 9, 1979.
... # It came within 570,000 km of the planet's cloud tops.
...
>>> for i, distance in enumerate(distances):
... print "%s %.0f" % (et2str(etTimes[i]), distance)
...
1979-07-05T00:00:00 5101847
1979-07-05T03:50:24 4963408
1979-07-05T07:40:48 4824321
1979-07-05T11:31:12 4684556
1979-07-05T15:21:36 4544084
1979-07-05T19:12:00 4402875
1979-07-05T23:02:24 4260894
1979-07-06T02:52:48 4118107
1979-07-06T06:43:12 3974475
1979-07-06T10:33:36 3829961
1979-07-06T14:24:00 3684521
1979-07-06T18:14:24 3538113
1979-07-06T22:04:48 3390691
1979-07-07T01:55:12 3242209
1979-07-07T05:45:36 3092620
1979-07-07T09:36:00 2941877
1979-07-07T13:26:24 2789938
1979-07-07T17:16:48 2636766
1979-07-07T21:07:12 2482335
1979-07-08T00:57:36 2326640
1979-07-08T04:48:00 2169709
1979-07-08T08:38:24 2011626
1979-07-08T12:28:48 1852573
1979-07-08T16:19:12 1692903
1979-07-08T20:09:36 1533246
1979-07-09T00:00:00 1374686
1979-07-09T03:50:24 1219135
1979-07-09T07:40:48 1069943
1979-07-09T11:31:12 933190
1979-07-09T15:21:36 819242
1979-07-09T19:12:00 743424
1979-07-09T23:02:24 721909
1979-07-10T02:52:48 760496
1979-07-10T06:43:12 849068
1979-07-10T10:33:36 970787
1979-07-10T14:24:00 1111683
1979-07-10T18:14:24 1262874
1979-07-10T22:04:48 1419237
1979-07-11T01:55:12 1577894
1979-07-11T05:45:36 1737225
1979-07-11T09:36:00 1896315
1979-07-11T13:26:24 2054649
1979-07-11T17:16:48 2211944
1979-07-11T21:07:12 2368051
1979-07-12T00:57:36 2522903
1979-07-12T04:48:00 2676485
1979-07-12T08:38:24 2828807
1979-07-12T12:28:48 2979899
1979-07-12T16:19:12 3129800
1979-07-12T20:09:36 3278556
>>>