(Getty Images)

As civilian space travel becomes a reality, doctors worry that not everyone who can pay for a ticket is healthy enough to head to the final frontier.

Since governments have been sending crews to space for years, the possible physical impacts of space travel are well-known. The human body responds differently to gravity in space, which translates into "motion sickness, nausea, sinus congestion or dizziness," S. Marlene Grenon, a vascular surgeon at the University of California,tells NPR.

The health threats don't end when the body returns to earth. Astronauts can come back with weaker immune systems, making them more vulnerable to infections.

Keep in mind that up until recently, the only people in space were astronauts who met strict government guidelines. You can imagine why doctors are concerned about regular folks heading into space. NPR covers more about how doctors are responding to space tourists on their website.

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Comet ISON

NASA's Hubble Space Telescope captured this image of Comet C/2012 S1 (ISON) on April 10, 2013. The comet was 394 million miles from Earth. (Courtesy: NASA/Hubble Space Telescope)