This just blew my mind, from the fifth image: "Vela Junior was discovered just 20 years ago, although this object is so close to Earth that remains of this exp

This just blew my mind, from the fifth image: "Vela Junior was discovered just 20 years ago, although this object is so close to Earth that remains of this explosion were found in polar ice cores." Aside from being some serious Hurcule Poirot meets Kip Thorne crossover stuff, I want to ask "How do we know?" but I'm pretty sure I wouldn't understand the answer.