The Search for Intelligent Life Project Shuttles (SILPS) is the NASA project governing the spacefaring Earthling characters in the first trilogy of Space Voyages. In reality, SILPS was a contrivance thrown in at the last minute by the author to make Thaddeus's return to Earth in Book III relevant to the overall plot of the trilogy; he became the "intelligent life" the SILPS mission was originally intended to discover. Up until this point, the entire mission had been referred to as STS-127 or simply "the shuttle mission" (this is in fact a misnomer, since it is made clear that SILPS uses Apollo-like expendable space capsules).
From what little is mentioned about the SILPS program itself, it seems that it is some hybridization of the old Apollo capsule system and the then-current space shuttle program, since it still uses the "STS" numbering system for its missions (it is also implied in Book IV that the normal shuttle program, specifically Atlantis, is still operational). It is interesting to note that there may actually be something comparable in operation in 2018 with the Space Launch System currently in development. According to the Space Voyages Complete Guide, SILPS was first proposed in September 2017, and that the candidates were chosen in January of the following year.
What NASA would hope to accomplish in regard to any "search for intelligent life," which is often carried out over many years or decades by ground-based radio telescopes, is beyond any kind of speculation. Indeed, the funding that would be needed for such a mission relative to its objective is truly outrageous. Then again, choosing three teenagers to go on a space mission would seem equally ill-advised.
The history of the SILPS program as explained in Space Voyages only consists of the two missions, STS-127 and STS-127B, both launched ("just a couple miles apart!") on June 14, 2018. It is assumed that they are identical spacecraft, although how a second vehicle was prepared within a matter of days as a result of the tie in mission testing is not known. The nominal mission was to last 54 orbits, but contact was lost with both spacecraft immediately following their launch.