Colonizing Mars has long captivated the human imagination, and NASA is no exception.

The American space agency has made landing humans on Mars a high priority of its exploration programs and under bipartisan 2010 legislation pledged to develop the capabilities to send humans to the planet bythe 2030s.

But there remains a major problem standing between mankind and the red planet: money.

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The head of NASAs program on human exploration of space, William Gerstenmaier, said on Wednesday that with its current budgetthe agency simply cannot afford the cost of propelling a manned spacecraft to Mars.

This image released August 27, 2003 captured by NASA's Hubble Space Telescope shows a close-up of the red planet Mars when it was just 34,648,840 miles (55,760,220 km) away. NASA/Getty

Through this horizon, through the 2030s, I cant put a date on humans on Mars, said Gerstenmaier on Wednesday, in response to a question at a propulsion meeting of the American Institute for Aeronautics and Astronautics in Georgia.

Read more:Skintight space suits are the order of the day for astronauts who hope to survive life on Mars

At the budget levels weve describedits roughly a 2 percent increasewe dont have the surface systems available for Mars. That entry, descent and landing is a huge challenge for us for Mars.

NASA has landed several unmanned exploratory vehicles on Mars in the past. The Curiosity rover, which landed on Marsh in August 2012 and will soon be celebrating its five-year anniversary exploring the planet, cost around $2.5 billion.

This handout provided by NASA/JPL-Caltech/MSSS on January 1, 2015, shows a self-portrait of NASA's Curiosity Mars rover shows the vehicle at the 'Mojave' site, where its drill collected the mission's second taste of Mount Sharp. NASA/JPL-Caltech/MSSS via Getty

Gerstenmaier said that a manned mission to Mars would weigh around twenty times what previous rovers have weighed. So its 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 NASAs 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 Institutea nonprofit research group funded partially by NASA and based at a NASA research center in Silicon Valleysaid 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 arepeppered 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 organismsand thus may also pose problems for sustaining human life.

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NASA Can't Afford to Put Humans on Mars - Newsweek