Why Can't We Go to Mars or Other Planets Every Time We Want?

This is because we want to place our satellite in orbit where we will spend the least amount of fuel to go from Earth to Mars or other planets. Although we reach our goal later by choosing such trajectories, we can carry "payloads" more like measuring devices rather than using our limited launch mass for fuel. To do this, we need to wait for the time when Earth and Mars are in a certain direction relative to each other. This particular direction between Mars and Earth, both of which make a circular motion around the Sun, is repeated only once every 780 days (which is called the "conjunction" or "synodic period"). In summary, we can go to Mars whenever we want, but if we want to spend less fuel and do more scientific research, we have to wait for the right time.