Debris actually pelts the ISS all the time, and noticeable dents and cracks line the exteriors. But should something fully breach the station, cabin atmosphere will seep into the vacuum of space and alarms will go off. Pressure gauges will confirm to astronauts that the station has, almost certainly, been hit, and the speed of the seepages may indicate how much time the crew has to respond. According to one NASA estimate, a 0.6-centimeter-wide hole leaves 14 hours to plug the leak. A 20-centimeter hole leaves less than a minute.