I read this book when I was really little. I am about to start reading it again.
I spent the last hour turning my mental wheels and this is how I got to right now:
Apollo 18 →
history of Apollo missions →
Planetary Grand Tour →
New Horizons →
radio waves →
speed of light →
Alpha Centauri →
aging while traveling speed of light →
twin paradox →
Hafele-Keating experiment →
A Brief History of Time