This is because congress has decided that it serves the public interest that certain non-profit institutions be exempt from taxation, just as congress is persuaded that it serves the public interest to make certain institutions and activities exempt from the antitrust laws. As an attorney who has first hand experience in legal practice and appeals before regulatory agencies and then in the federal courts of appeals, the author fears any bureaucracy with the authority to decide what is and is not taxable. If congress makes the Mars mission companies explicitly exempt, then the companies only have to worry if they engage in activities which are arguably non-mission related. These activities will surely arise, but such disputes will not occupy all the company's time if the company has an express congressional tax exemption. In law, as with taxes and antitrust law, tort law is related to economics.
Most noneconomic losses are capable of being translated into dollars.
When plaintiff sues in tort for an injury which he has sustained, he must recover all of his damages arising from that injury, past and future, in a lump sum and in a single lawsuit. At its extremes the law of torts permits certain harms to be knowingly inflicted, irrespective of evil intent of the inflictor of the harms, and at the other extreme places absolute risk of certain transactions on the person engaging in them, irrespective of blameworthiness in any sense. The underlying notion is to financially compensate someone who has been wronged in the eyes of the law. A tort exemption would encourage companies to work on a Mars mission. This does not mean that the companies would be exempt from no-fault insurance claims, such as workers compensation claims.