Triton – the largest moon of the planet Neptune – was named after the Greek god of the sea Poseidon and the goddess of the sea Amphitrite. According to mythology, he lived peaceably with his parents in a golden palace on the bottom of the sea. He is represented as a merman, with the upper body of a human and the tailed lower body of a fish. In English literature, Triton is portrayed as the messenger or herald for his father and was often depicted as having a conch shell that he would blow like a trumpet.

It keeps one face oriented toward the planet at all times, as its equator is almost exactly aligned with its orbital plane. Yet the distant planet’s moon, the seventh largest in the Solar System, is an “odd ball” some 4.5 billion kilometers away from Earth as it orbits in the opposite direction to all the other moons of Neptune. The first Neptunian moon to be noticed, it was discovered in 1846 by English astronomer William Lassell. 

If life does exist outside of Earth in our Solar System, it could be hiding in subterranean oceans flowing under the surface of icy moons. And Triton is one of the most promising candidates for such an underground liquid body. 

All detailed knowledge of the surface of Triton was obtained from a distance of 40,000 km by NASA’s unmanned spacecraft Voyager 2 during a single flyby in 1989, when it found surface temperatures of −235 °C and also discovered it had active geysers spewing liquid nitrogen. As Neptune orbits the Sun, Triton’s polar regions take turns facing the Sun, resulting in seasonal changes as one pole, then the other, moves into the sunlight. 

Because of its eccentric orbit, some think it points to an origin outside our Solar System: The moon may be an intruder trapped long ago by Neptune’s gravitational field. The Weizmann Institute of Science in Rehovot and the Israel Space Agency (ISA) in Israel’s Science and Technology Ministry are part of a team planning a mission to Triton. The planned mission, named Trident for Neptune’s three-pronged spear, is due to be launched in 2026. As a finalist in NASA’s Discovery Program, it joins three other proposals, each of which will receive $3 million in the coming year to develop the plans before two are finally selected for missions. The four were chosen from 22 proposals put forward.

If approved, Trident would be the first earthly visitor to Neptune and its moons in over 30 years. Voyager 2 sent back photos taken from 40,000 kilometers away from the moon. Despite the distance, planetary scientists were able to observe that the surface of Triton is one of the most youthful seen in the Solar System – just a few million years – suggesting intense geological activity. And photos shot of the moon’s south pole revealed geysers. This hinted at the possible existence of large underground reserves of liquid, as well as revealing the direction of winds and an atmosphere on the moon.

The mission Trident will also be flying over – but from a distance of only 500 kilometers Prof. Yohai Kaspi of the Weizmann Institute’s earth and planetary sciences department is part of the science team of this proposed space mission, led by Dr. Louise Prockter, head of the Lunar and Planetary Institute in Houston Texas. 

The proposed space mission wants to investigate whether Triton does indeed have a subterranean ocean, study the properties of this ocean if it exists, and investigate why the surface looks so youthful and what makes up its atmosphere. The clock is so accurate that it will lose less than a second in 10 million years and be built to withstand the rigors of space travel over at least 15 years. 

The two Israelis on the team, including Dr. Eli Galant in Kaspi’s group, are there for their experience in analyzing radio signals arriving from distant spacecraft, which in this case will be interpolated with a special clock they designed that will be carried on the spacecraft.

The clock will be built in Jerusalem by the firm Accubeat and funded by the ISA in Israel’s Science and Technology Ministry. Kaspi and Galanti will use the clock to calculate the properties of Triton’s atmosphere, based on the Doppler displacement of radio waves beamed back to Earth from the spacecraft. The Israeli-made clock will assist them in measuring minute oscillations in these waves.

A similar clock is already in advanced stages of preparation at Accubeat: It will be launched aboard the European Space Agency’s JUICE mission to the moons of Jupiter, set for 2022. Calling it the most accurate clock of its kind, Kaspi says: “There are more exact clocks than the one we will be launching, but these are very large and stationary.”

ISA’s director-general Avi Blasberger added: “This unique clock, developed for an ESA flagship mission, has raised interest with other mission teams. It is proof that Israeli industry and research have much to contribute to international space exploration.”

In addition to the clock – which will be operated by the Israeli team, the Trident craft, if chosen, will carry a magnetometer for seeking out the underground ocean, two kinds of spectrometers – one infrared to examine the surface and the other plasma for determining the makeup of the atmosphere – and two kinds of cameras that will also be trained on regions of Neptune that have not before been observed. 

As part of the Discovery Program, the Trident mission is intended to be smaller, cheaper, chancier and shorter-term than the large missions that take off once or twice a decade. Unlike other large missions that may be chosen on the political or leadership level, the Discovery missions are all proposed, planned and selected by researchers.

Galanti added that the mission is dependent on being chosen as one of the two finalists in a year: “The launch date is fixed by window of opportunity in which the configuration of the planets would enable a craft from Earth to reach Triton in just 12 years. The next window of opportunity will not come around until 2038.”