E.C. Tubb 
Seetee Alert! 
Cap Kennedy Book 6

FREE ACTING TERRAN ENVOYF.A.T.E.Seetee is the most dangerous substance in the universe — bring it into contact with the most stable of normal material and it explodes with enormous energy and destruction.When Captain Kennedy and his crew, the top men of F.A.T.E. — Free Acting Terran Envoys — accidentally brush against some Seetee they not only thank their lucky stars for their escape, but realise it signals an emergency of the highest order.Because such a swarm as they have encountered could cause the destruction of an entire solar system. And thy have reason to believe it is directed at out Sun — including particularly the Earth!