A week after successfully launching its huge Falcon Heavy rocket, SpaceX is set to blast off another test of a long-awaited new product. 

More than three years ago we learned Elon Musk and his rocket company were working on developing satellites to provide low-cost internet access around the world. The first pair of demonstration satellites for the company's "Starlink" service will finally be launched into orbit aboard a Falcon 9 on Saturday, according to correspondence between the company and the Federal Communications Commission.

The main payload for the launch from Vandenberg Air Force Base in California will be the Spanish government's "Paz" satellite, designed to capture imagery of the Earth down to the single-meter scale. But there have been unconfirmed reports for several weeks now from space industry sources like NASASpaceFlight.com that a secondary passenger on the flight is rumored to be the Starlink demonstration setup.

SpaceX itself has been relatively mum about the debut of its Starlink satellites, and about the entire program itself. However, a letter from SpaceX to the FCC made available on the FCC website Monday makes it pretty clear what will be aboard the Falcon 9 when it launches Saturday.

The letter refers to two satellites called Microsat-2a and Microsat-2b that will be launched as a secondary payload on the Paz mission. The FCC granted SpaceX a license in November to launch this pair of satellites as part of a test mission. In its application, the company describes the test objectives:

"In addition to proving out the development of the satellite bus and related subsystems, the test program for the Microsat-2a and -2b spacecraft will also validate the design of a phased array broadband antenna communications platform."

Putting that all together: SpaceX is testing internet broadband satellites that will be launched Saturday along with the Paz satellite.

A release from Vandenberg says the launch is scheduled for 6:17 a.m. PT and confirms "multiple smaller secondary payloads will also launch on the Falcon 9 rocket."  

SpaceX declined to make an official comment.