Orphanage. Jason Wander series book 1

When earth needs heroes, who will we call to valour? When flares streaking across the sky herald massive destruction, who will defend us? When mankind's enemy is beyond our worst imagination, we will need champions. And will we pick the brightest and the toughest - or the ones with nothing left to lose?WAR IS AN ORPHANAGEMankind's first alien contact tears into Earth: projectiles launched from Jupiter's moon, Ganymede, vaporize whole cities.
Under siege, humanity gambles on one desperate counterstrike. In a spacecraft scavenged from scraps and armed with twentieth-century weapons, foot soldiers like eighteen-year-old Jason Wander - orphans that no one will miss - must dare to embark on man's first interplanetary voyage and invade Ganymede. They have one chance to attack, one ship to attack with, Their failure is our extinction.