A previously unseen asteroid the size of a truck flew about 4,350 miles (7,000 kilometers) over the Pacific Ocean on Monday, making it one of the closest passes by our planet on record. Astronomers had no notice of asteroid 2020 JJ's existence, as it was discovered using the Mt. Lemmon Survey in Arizona right around the time it reached its closest point to us. Had 2020 JJ actually struck Earth, most of it probably would have burned up in the atmosphere. In other words, this space rock wasn't any sort of existential threat, but it did fly closer than many...