Hawaii's supreme court has ruled in favour of building the Thirty Meter Telescope (TMT) atop the mountain of Mauna Kea. The decision clears the last legal hurdle for the US $1.4-billion project to resume construction on Hawaii's Big Island.

For years, the next-generation astronomical observatory has been mired in public protests and legal challenges. Some Native Hawaiians say that building the mega-telescope would further desecrate a sacred mountain that is already home to multiple observatories. In April 2015, protesters blocked the road to Mauna Kea's summit as construction of the TMT was set to begin. That December, the state supreme court revoked the project's construction permit , saying that the state government had granted it before opponents of the telescope had their full say.

Hawaii's Board of Land and Natural Resources issued a fresh construction permit in September 2017 , which opponents appealed. The latest ruling upholds that permit.

A separate legal issue, involving the University of Hawaii's sublease of land atop Mauna Kea for the TMT site, was resolved in August. The state supreme court ruled in the project's favour in that case as well.

TMT officials have been considering an alternate site for the telescope, in the Canary Islands , in case they cannot resolve the legal and political obstacles to building in Hawaii. It could take months before project leaders decide whether to go ahead in Hawaii, now that they have the supreme court's backing. Among the issues they face is how to restart construction on Mauna Kea, given the protests that broke out the last time they tried to do so.

"We remain committed to being good stewards on the mountain and inclusive of the Hawaiian community," said Henry Yang, chairman of the TMT International Observatory board of governors, in a statement.

The TMT is one of three planned mega-telescopes that would push astronomy into a new era of ground-based observing. The other two — the Giant Magellan Telescope and the European Extremely Large Telescope — are both under construction in Chile. The TMT is a multinational project involving two universities in California, plus the governments of Canada, China, India and Japan.