Imagine planning for a long sailing voyage. Your survival depends upon the sturdiness of your craft, planning and skill. Stowed onboard must be all the provisions, tools and hardware you’ll need. Your knowledge and judgment about how to navigate through wind, weather and waves will be crucial to staying afloat. You’ve learned from the successes and misfortunes of previous voyages. You won’t make the same mistakes, but you know you may encounter some new challenges. When something breaks, you will need to be able to work around it. Beneath the surface may lurk something unexpected. Within the bowels of your sailing craft, there may be a weakness or a flaw that won’t make itself known until later. And it may get you in the end. Vigilant, wary, you’re ready for the best and prepared for the worst, for the things you don’t know will happen. In a way, space engineers say, that’s a little of what it’s like to work on a mission to Mars.