> <http://nssdc.gsfc.nasa.gov/nmc/spacecraftSearch.do>

spacecraftSearch.do contains a session number. you can't just put that url
into httrack. 

Treat the previous page as a form log in and let httrack capture the form
results

or cut and paste the search result links into httrack.  The Explorer 1 link
is:
<http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1958-001A>