John, Zeph, Black Root ship's hull radiation is 72 hours old, but radiation's half-life makes that an impossible call. Radioactive isotopes decay at a rate unique to each isotope. C14 half- life is 5730 years, Strontium81 is 22 minutes, Uranium238 is 4.47 billion years. The time it takes for half of the isotope to decay is the half-life. If you know how much radio-isotope you started with and how much you have now, you can easily figure the elapsed time; this technique is very basic and very usefull in geology and paleontology.
Both Kelly McCormack (Zeph) and Sara Waisglass (Quin, the blind girl who helps the protagonists at the Hullen training camp), have roles in the 2021 Netflix series, Ginny and Georgia. Conversely, Kelly plays a minor role as Georgia's sister in that show while Sara, who plays a one-off character in Killjoys plays a major character in Ginny and Georgia: Maxine, the member representing "M" in that show's main coterie, "MANG."