Astronauts need to demonstrate excellent vision — and should also have low risk of deteriorating vision. In most cases, there is no disadvantage to candidates with glasses, contacts or those who have undergone corrective surgery. However, some space agencies may consider those corrections as visual defects if there is a risk they could deteriorate.

This article focuses on eyesight rather than medical issues related to the eyes (those are outlined in the article: Medical selection disqualifiers).

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Why is eyesight a consideration for spaceflight?

Many of the astronauts from different space agencies have to have their eyesight corrected, either with glasses or contact lenses. This is not an issue, as long as it can be corrected to 20:20 vision. That is, the ability to see an object from 20 feet away, or on a Snellen chart (the chart where you have to read the letters from a distance) the ability read the fourth row from the bottom.

Visual disturbances are common within spaceflight, and often your vision changes soon after being exposed to a microgravity environment. This is known as Spaceflight Associated Neuro-ocular Syndrome (outlined here by NASA), where for many astronauts, the brain and eye structure changes as a result of being exposed to a microgravity environment. Here, blood and cerebrospinal fluid shifts to the head, causing an increase in pressure.  This causes a flattening of the eye shape.

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What if I’m color vision deficiency?

Color vision will be assessed via the use of the Ishihara test or Nagel's anomaloscope test, which is a pass/fail. JAXA and ESA fail individuals as a result of them being colorblind. NASA disqualifies for a color vision deficiency greater than mild deficiency on red-green or blue-yellow color vision testing.

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Why are the eyesight requirements?

The requirements for major space agencies are outlined below.

United States of America (USA): National Aeronautics and Space Administration (NASA)

Distant and near visual acuity must be correctable to 20/20 in each eye. The use of glasses is acceptable. Corrective eye surgery operations, PKR and Lasik, are permitted. If your vision is no worse than 20/400 and correctable to 20/20 you can be considered. If your vision is no worse than 20/800 and LASIK can put you into the “20/400 correctable to 20/20” category then you are also good to go. To qualify, 2 years must have passed between the surgery and your application to become an astronaut.

Canada: Canadian Space Agency (CSA): Applicants who have undergone refractive laser surgery (PRK or LASIK) are eligible. The CSA does not recommend that applicants undergo refractive laser surgery for the sole purpose of applying for employment as an astronaut. For those applicants under final consideration, an operative report on the surgical procedure will be requested. Applicants must be free from visual and other complications after undergoing this type of surgery.

Countries that are ESA Member States and Associate Members: European Space Agency (ESA)

The applicant must have visual acuity in both eyes of 100% (20/20 vision) naturally or after correction with glasses or contact lenses. In terms of initial visual range requirements, the medical standards for pilots apply and the initial application includes eye sight to the standard of the European Union Aviation Safety Agency (EASA) medical requirements. The main tests applicants must pass include visual acuity, colour perception and 3D vision. Wearing glasses or contact lenses is not a reason for disqualification in itself, but would be evaluated if a visual defect is known to progress rapidly and could be a reason for disqualification. Minor visual defects that require corrective lenses to achieve perfection are acceptable. Some surgical interventions to correct visual acuity can lead to disqualification, while other surgeries are acceptable — each case is evaluated on an individual basis.

Japan: Japan Aerospace Exploration Agency (JAXA)

Visual acuity of 1.0 or more in both eyes. Corrected visual acuity 0.7 or more in both eyes.