Future missions to Mars will likely expose astronauts to a staggering amount of radiation during the trip which equates to about what a human can handle in a lifetime, NASA scientists said Thursday.

Curiosity's Radiation Assessment Detector (RAD) is the first instrument to measure the radiation environment on Mars. The instrument found even if astronauts remain in spacecraft, they are still exposed a lifetime limit of radiation.

The findings, which are published in the May 31 edition of the journal Science, indicate radiation exposure for human explorers could exceed NASA's career limit for astronauts if current propulsion systems are used.

"In terms of accumulated dose, it's like getting a whole-body CT scan once every five or six days," said Cary Zeitlin, a principal scientist at the Southwest Research Institute (SwRI) in San Antonio and lead author of the paper on the findings. "Understanding the radiation environment inside a spacecraft carrying humans to Mars or other deep space destinations is critical for planning future crewed missions."

That dose is enough to raise the astronaut's cancer risk by about three percent. "It is clear that the exposure from the cruise phases alone is a large fraction of (and in some cases greater than) currently accepted astronaut career limits," the study says. "Time spent on the surface of Mars might add considerably to the total dose."

 The information provides space mission designers with clues on how to build a spacecraft with sufficient protection for future occupants.

NASA aims to send a crew to orbit the red planet by the mid-2030s. Private outfits like Inspiration Mars - backed by NASA engineer-turned-space tourist Dennis Tito - are seeking volunteers for a Mars flight.

The research team comprise up of NASA researchers and scientists at the Southwest Research Institute in Colorado. They measured radiation levels on the Mars Science Laboratory, the spacecraft that transported the Curiosity rover to Mars in 2012. A subsequent study will measure Curiosity's radiation levels while the rover has been on Mars's surface.