The suggestion is to leave this and introduce some sort of game over mechanic, that takes care of this scenario. There is nothing wrong with being able to go bankrupt. I could also think of a one time check from the government to enable the player to try another mission. If it happens again, a game over screen should tell the player that he has done goof.
Requirements for the game over screen:
* The player is unable to afford another rover or lander
* The player is unable to complete any further mission objectives with rovers or landers that are still active.
* The player is unable to afford another rover or lander and all previous landers or rovers have been abandoned.