SMAP products can be downloaded via HTTPS. You have to register an account with NASA’s Earthdata portal. Instructions can be found here.
After that you can use the command line program
smap_download and your username
and password to download data between 2 dates.
The following command would download all available h5 files of the latest SMAP SPL3SMP data into the folder
~/workspace/smap_data. For more options on other available parameters
mkdir ~/workspace/smap_data smap_download ~/workspace/smap_data --username *name* --password *password*