We've all heard how U.S. space agency NASA was going to put humans on Mars in the 2030s.
NASA named the mission "Journey to Mars" and put a detailed explanation about it on its official website.
But the agency's chief of human spaceflight acknowledged on Wednesday that it did not have enough money to carry it out as such.
"I can't put a date on humans on Mars, and the reason really is the other piece is, at the budget levels we described, this roughly 2 percent increase, we don't have the surface systems available for Mars," said William H. Gerstenmaier during a meeting of the American Institute for Aeronautics and Astronautics, or AIAA.
Gerstenmaier did not provide a new timetable for the mission, and said "entry, descent and landing is a huge challenge for us for Mars."
Is there a back-up plan?
Instead of talking more about the trip to Mars, Gerstenmaier mentioned the possibility of another lunar mission.
Americans have been there before, so why are they heading for Moon again?
"If we find out there's water on the Moon, and we want to do more extensive operations on the Moon to go explore that, we have the ability," Gerstenmaier explained.
Water is critical for lifeforms on Earth, like humans, to survive. So while this plan sounds solid, it's not as attractive as the Journey to Mars, which NASA said will enable humans to "get, land and live" on the planet.
Competitors in the way
Another reason why a lunar mission is not so attractive: others are trying to get there as well.
U.S. tech giant Google has offered millions of U.S. dollars to encourage private companies worldwide to do moon missions.
At least four teams from various countries are planning to go to the moon before the end of this year, including U.S. startup Moon Express, whose ambition is to do mining on the Moon.
China is also building a spacecraft, named Chang'e-5, to bring rock and soil from the moon back to Earth.
