Gerstenmaier said that a manned mission to Mars would weigh around twenty times what previous rovers have weighed. “So it’s a twenty-fold increase in capability,” he said, likely meaning a much higher cost.

Lawmakers allocated NASA a budget of $19.5 billion for the 2017 fiscal year, which equates to less than half a percent of the overall federal budget.

The agency has not produced a specific figure of the cost of a manned mission to Mars, and estimates vary depending on sources. In 2012, the head of NASA’s Jet Propulsion Laboratory, Brent Sherwood, estimated that the project could cost up to $100 billion over the course of 30 or 40 years. More recently, Pascal Lee, the director of the Mars Institute—a nonprofit research group funded partially by NASA and based at a NASA research center in Silicon Valley—said in May that a human mission to Mars could cost up to $1 trillion over 25 years.

Private organizations that are working on their own missions to Mars have estimated lower costs. Mars One, a Dutch-Swiss organization aiming to establish a permanent settlement on Mars, aims to bring four people to Mars at a cost of $6 billion. SpaceX founder Elon Musk, who has said he wants to send humans to Mars in the early 2020s, put the cost at $10 billion per person in 2016.

Landing on Mars poses numerous threats to a manned mission. The spacecraft must angle its entry into the Martian atmosphere correctly: If it is too steep, the craft may burn up, and if too shallow the craft may miss the planet altogether. Astronauts must use reverse thrusters and parachutes to slow the spacecraft down so that it is not destroyed upon impact with the surface. The craft must also locate a safe landing surface on the rugged terrain of Mars, parts of which are peppered with gigantic craters.

And while research has shown that liquid water once flowed on Mars, a recent study found that the soil is toxic to bacteria —one of the simplest forms of living organisms—and thus may also pose problems for sustaining human life.