A mission to study below the surface of Mars will have to wait another two years, NASA announced Wednesday.

The InSight was postponed in December, when NASA realized that leaks in a vacuum enclosure housing a key instrument were too serious to be fixed by the scheduled March 2016 launch date. NASA will wait until Earth and Mars are close enough in orbit to allow for a short six-month trip by the InSight.

The new launch window begins May 5, 2018 with a Mars landing expected Nov. 26, 2018, according to NASA. An estimate of the cost of delaying the launch two years is being assessed and is expected to be released in August.

InSight’s primary goal is to understand how rocky planets—like Earth and Mars—came to be. The mission is part of NASA’s ambitious plans for Mars, including ultimately sending humans to the Red Planet.

“The science goals of InSight are compelling, and the NASA and CNES plans to overcome the technical challenges are sound,” John Grunsfeld, associate administrator for NASA’s science mission directorate in Washington, wrote in a statement. “The quest to understand the interior of Mars has been a longstanding goal of planetary scientists for decades. We’re excited to be back on the path for a launch, now in 2018.”

Get our Space Newsletter. Sign up to receive the week's news in space. Please enter a valid email address. Sign Up Now Check the box if you do not wish to receive promotional offers via email from TIME. You can unsubscribe at any time. By signing up you are agreeing to our Terms of Use and Privacy Policy . This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply. Thank you! For your security, we've sent a confirmation email to the address you entered. Click the link to confirm your subscription and begin receiving our newsletters. If you don't get the confirmation within 10 minutes, please check your spam folder.

Contact us at letters@time.com.