An Atlas 5 rocket has blasted off from the United States, sending a Mars orbiter on its way to study how the planet lost its water.
The Mars Atmosphere and Volatile Evolution Mission, or Maven, will scan and sample what remains of the thin Martian atmosphere and watch how it is being destroyed by solar radiation.
Upon arrival, Maven will drop into an elliptical orbit, meaning it will fly as close as 65km to the planet, and as far as 6,000km. It will gather air samples on its close passes, before travelling further out to measure how much and what types of radiation are sweeping past the planet.
The point of the project is to determine how much of the atmosphere is being lost to space today, and then extrapolate back in time to calculate what happened in the past.
The information is also expected to help scientists understand when in Mars' history it may have been most suitable for life to evolve.
In the 49 years since NASA's Mariner 4 spacecraft flew by Mars for the first time, an increasingly more sophisticated
series of orbiters, landers and rovers have amassed solid evidence that the planet had liquid water.
There are two options for where the planet's missing water and atmosphere went: down into the ground or up into space.
Scientists know some of the planet's carbon dioxide ended up on the surface and joined with minerals in the crust. But so far, the ground inventory is not large enough to account for the early, thick atmosphere Mars would have needed to support water on its surface.
Maven is designed to explore the other option, that the water and atmosphere were lost into space, a process that began about four billion years ago.
"The sun, the solar wind can drive processes that remove gas from the top of the atmosphere. We want to understand whether the sun was able to remove gas from the top of the atmosphere and how much," said lead scientists Bruce Jakosky.
Maven is due to reach Mars on Sept 22 next year.