NSSDCs three-year archive plans summarize expected data inflows to NSSDC from various NASA missions and discipline-specific "active archives." The latest plan, covering 2000-2002, is now available at http://nssdc.gsfc.nasa.gov/nssdc/arch_plan/2000.html.
This plan shows a total inflow ranging from about two terabytes (TB) in 2000 to four TB in 2002. Many of the data are in "backup mode" for active archives or are low-processing level data from principal-investigator-mode missions. Neither of these data types is readily independently usable if received directly from NSSDC.
On the order of 0.5 TB per year is the amount of independently usable data to be received by NSSDC that will not be network-accessible from other NASA space science active archives. NSSDC will make these data network-accessible in part via basic FTP and in part by higher functionality systems such as CDAWeb.