In the 4.6 billion years since our solar system formed, life could have emerged on several of its worlds. Aside from Earth, however, Jupiter's moon Europa seems to be the most likely to host it today. Early Venus and Mars probably had abundant liquid water, the essential elixir for life as we know it, but one became a hot hell and the other a frozen globe. Saturn's moon Enceladus also has a substantial reservoir of liquid water, but the U.S. scientific community, in its most recent decadal survey, prioritized studying Europa, which is nearer. Europa's ocean, with perhaps twice the water of Earth's oceans, is believed to have been liquid since the moon's formation. On Europa, life might have had time to evolve.

Scientists believe that Europa's ocean lies directly atop a sizable rocky world, putting water in contact with the other elements and minerals essential for life. As the moon orbits Jupiter, tidal flexing heats the world from within, keeping the vast ocean liquid and likely powering volcanic activity. Rich ecosystems exist on our own planet's seafloor, where volcanic rifts create hydrothermal vents. The same might be true on Europa.

We have sent probes to Europa before. During its mission to Jupiter in the late 1990s, NASA's Galileo spacecraft observed the moon, all but confirming that Europa harbored a 100-kilometer-deep ocean covered by a relatively thin, icy shell. Tidal forces regularly break the ice, allowing water from the depths to well onto the surface, leaving stains that provide evidence of the ocean's chemistry. Observations by the Hubble Space Telescope in 2012 suggest that water plumes periodically erupt from Europa's surface. If such plumes are present, a spacecraft could sample the chemistry of this potentially life-bearing ocean by flying through them.

Galileo's vintage instruments (designed at the same time as the Apple II computer) could not determine what exactly stains Europa's surface or just how thick its icy crust may be—crucial for learning whether Europa is habitable. NASA crashed Galileo into Jupiter in 2003, and ever since, scientists and groups such as the Planetary Society have been urging NASA to send a follow-up mission. NASA's proposed Europa Clipper mission could do the job. It would orbit Jupiter, not Europa, and would dip into the planet's intense radiation belts 45 times to fly just above the moon's surface, retreating each time to safer locales to transmit its data home. The Clipper would characterize Europa's ocean, explore its chemistry and study its geologic processes. It would also scout locations for a future mission to land.

For most of the past 15 years, a mission to return to Europa has lingered in a perpetual early-study phase, trapped between the shifting and often conflicting priorities of successive Congresses and presidents. This year that changed, as both Congress and President Barack Obama officially endorsed a mission. This nascent plan, though, comes without a firm launch date or target cost. The Clipper mission would cost approximately $2 billion, but the White House is proposing just $185 million to fund the first four years of developmental work.

By comparison, when NASA began work two years ago on its next Mars rover, the project had a firm launch date (2020), a target budget ($1.5 billion) and substantial funding ($775 million for its first four years). Although the Clipper's engineering team members believe they could be ready to launch in 2022, the White House speaks of a mid-2020 launch. The proposed Europa funding stream punts the commitment to develop and launch a mission to a future president and Congress.

Europa is a compelling target, but a dedicated mission to deeply study it is at risk of slipping into the indefinite future. NASA has a credible mission concept that addresses the key scientific questions at an affordable cost. The White House and Congress should commit funding to move this mission toward launch as soon as possible.