[uj8efdkjfdshf]

Because Mars and Earth takes different amounts of time to complete an orbit around the Sun, launching at the right point in time is absolutely essential to ensure that your transfer orbit takes you to the distance of Mars' orbit at the exact point in time that Mars will happen to be there. You can mess around with your transfer orbit parameters to give you a bit of leeway, but ultimately limits on available delta V and on maximum relative velocity on encounter limit the window of time that you have to launch your mission. See https://trajbrowser.arc.nasa.gov.

They have until mid August to launch the mission[0]. If not, the additional cost of keeping personnel employed and hardware maintained for the next 2 years or so while waiting for Earth and Mars to return to the same relative positions can easily cost $150m, if the InSight mission delay is any guide. [1]

[0] https://spacenews.com/mars-2020-launch-slips-three-days/ [1] https://spacenews.com/insight-delay-adds-150-million-to-miss...

[surfsvammel]

Wouldn’t the people mainly work on other projects? Do NASA utilise a lot of consultants or are they mainly permanent employees?

[uj8efdkjfdshf]

Well, yes, NASA does rely a lot on external contractors due to the budgetary flexibility it entails. While there is some cross-communication between teams, it's generally limited by the extreme degree of specialisation needed. Also, as a general rule almost all of the hardware for any given mission is custom built for that mission, so the necessary knowhow to operate it is both unique and irreplaceable.

That said, some of the extra cost is due to scope creep, or due to changes in launch vehicle availability requiring the mission to be redesigned.