Jupiter mission selection garners excitement, questions

A trip to Jupiter and its moons will be Europe’s next large science mission, the European Space Agency (ESA) announced on 2 May. The Jupiter Icy Moons Explorer (JUICE) beat out proposals for an x-ray mission and a gravitational wave observatory.

Since spring 2011, all three missions were scaled back to go solo instead of as joint missions with NASA, once it became clear that NASA couldn’t come up with the money to participate on ESA’s timeline (see PHYSICS TODAY, May 2011, page 23 , and June 2011, page 22 ). The mission teams were given less than a year to trim their plans to €850 million ($1.1 billion) and not rely on other space agencies. JUICE is scheduled for launch in 2022.

The original Jupiter mission had involved two craft, one by ESA to focus on Ganymede and one by NASA to probe Europa. In JUICE, only the Ganymede probe remains, but its trajectory was revised to include two flybys past Europa, and also to study Callisto. Athena Coustenis, a project science leader based at the Observatory of Paris-Meudon, says JUICE “is going to look at the whole system—the planet, the magnetosphere and its interactions with the moons and with the solar wind, and the moons. It addresses fundamental questions about how planets are formed, their atmospheres and surfaces, the emergence of habitable worlds.” The planetary science community, she adds, “has been waiting for such a large mission for a long time.”

Downsizing the x-ray and gravitational-wave observatories was more complicated. The Advanced Telescope for High-Energy Astrophysics, or ATHENA, would have fewer instruments with less angular resolution than originally envisioned. Even so, says project working-group chair Xavier Barcons of the Institute of Physics of Cantabria in Spain, ATHENA “managed to retain most of the science objectives.” And the New Gravitational-wave Observatory (NGO) would have two, rather than three, interferometric arms, and they would be shorter than originally planned. “It’s still grand science,” says Karsten Danzmann of the Max Planck Institute for Gravitational Physics. “The only thing we really lose is the very high redshift universe.”

Not surprisingly, the losing teams and their supporters are disappointed. Based on what he heard from “trustworthy sources,” Cardiff University astronomer B. S. Sathyaprakash, who is not involved in any of the missions but whose scientific focus aligns most closely with the NGO, wrote a widely circulated letter to Alvaro Giménez, ESA director of science and robotic exploration. Sathyaprakash asked whether it’s true that ESA’s Space Science Advisory Committee (SSAC) gave the NGO the highest scientific rank, why the cost estimate for the NGO was suddenly raised before the selection meeting, and other questions. He is not questioning the merit of any of the projects, he says. “I am asking about the process.” As of press time, he had no answers from ESA. In addition, some 1500 people have signed a letter supporting ATHENA. Their concern is that astrophysics will be left without an x-ray observatory in the 2020s.

Referring to the choice of JUICE, one NGO scientist says, “There was a time when space was about visions, going to where no one has gone before, let us get a man on the Moon before the end of the decade, that stuff. Now it seems to be more about risk avoidance.”

Any mission that gets to this stage of competition “should be expected to be very good,” notes Fabio Favata, who heads ESA’s science planning office. “And any discussion in the SSAC belongs to the committee. The important thing is their recommendation.” The SSAC recommendation for JUICE was unanimously accepted by the Science Programme Committee, which consists of representatives from ESA member states.

Both ATHENA and the NGO will have a chance to compete again, with a foreseen launch date in 2028. Says Danzmann, “This timing will give NASA time to reassemble and contribute.”