user: Question: For the payload of Van Allen Belts exploration what's the serial number?
Columns: ['Date and time( UTC )', 'Type', 'Serial no.', 'Launch site', 'Payload', 'Type of payload', 'Orbit', 'Outcome']
Types: ['text', 'real', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['August 21, 2002, 22:05', 401.0, 'AV-001', 'CCAFS SLC-41', 'Hot Bird 6', 'Commercial communications satellite', 'GSO', 'Success']
SQL:

assistant: SELECT "Serial no." FROM "Table" WHERE "Type of payload" = 'Van Allen Belts exploration';