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The reason Europa can keep water in a liquid state is that it follows an oval-shaped orbit around Jupiter. The giant planet's gravity stretches and relaxes the moon, and that friction warms Europa's deep underground salt water, keeping it liquid. The warmth from that process could also allow the moon to harbor deep-sea ecosystems.

SpaceX is becoming a NASA favorite

SpaceX, the rocket company Elon Musk founded in 2002, is not in the business of studying other planets. But it is in the business of launching things for NASA, and the agency is awarding the company more and more opportunities to do so.

SpaceX's Crew Dragon spaceship flew NASA astronauts to the International Space Station last year. It was the first time the US has launched its own astronauts since the Space Shuttle Program ended in 2011. SpaceX is now regularly ferrying astronauts to and from the space station.

In April, NASA awarded SpaceX a contract to turn its in-development Starship megarocket into a lunar lander. The agency said Starship is set to land astronauts on the moon in 2024 (though that timeline may be unrealistic). That would be the first human moon landing since the Apollo missions ended in 1972.

The decision prompted challenges from competing rocket makers Blue Origin and Dynetics since the original plan was for NASA to pick two of the three companies for lunar-lander contracts. The protests required NASA to order that SpaceX stop work on the lunar lander.

SpaceX didn't win its new Europa Clipper contract without contest, either. According to Eric Berger, a senior space editor for Ars Technica, Congress has spent years urging NASA to launch the mission aboard its own Space Launch System (SLS) rocket. But legislators finally relented due to delays in the launch system's development, its high cost, and a recent technical issue that would require $1 billion to correct, Berger reported.

According to Berger, NASA could save nearly $2 billion by launching the mission aboard Falcon Heavy instead of SLS.