Europa, a Jovian moon, is in half shadow in this color-enhanced image from the Galileo spacecraft. The cracks in the icy surface are thought to be caused by heated up-wellings of the water below. Microbes similar to those found in the Earth's ocean floor may exist on Europa.
NASA's newly issued budget has lowered a flagship mission of exploration to half-mast. Backed by scientists and study groups, a mission to Jupiter's moon Europa is missing in action within the pages of NASA's Fiscal Year 2007 budget unveiled yesterday.
The smallest of Jupiter's Galilean satellites--about the size of Earth's Moon--Europa has a facade of white and brown colored water ice. Hidden under that frozen crust, Europa may well harbor a global ocean of liquid water. And coupled to the prospect for a subsurface ocean comes the tempting thought of life.
"NASA's robotic exploration program is being flat-lined, setting aside a mission to Europa to search for its ice-covered ocean and perhaps for life itself," Planetary Society Executive Director, Louis Friedman, said in a statement released Monday.
Both the National Academy of Sciences and internal NASA advisory committees have endorsed Europa exploration as the highest priority solar system objective after Mars.
Last year, the U.S. Congress directed NASA to plan a fiscal year 2007 start on a Europa mission. "If the proposed budget is adopted, that directive will be ignored, and no Europa mission will be planned," the Planetary Society statement noted.
So many false starts
"I am disappointed that after so many false starts over the last decade, it looks like a mission to Europa is slipping once again," said Ronald Greeley, a leading planetary scientist in the Department of Geological Sciences at Arizona State University in Tempe. He also chairs a Europa focus group of scientists keen on furthering the study of the Jovian moon.
Greeley told SPACE.com that the hope had been that a serious study of a Europa mission would be completed this year in anticipation of a NASA new start very soon.
"The planetary community remains essentially unanimous in setting Europa as the highest priority large mission to the outer solar system," he said, in confirmation of both external study groups and NASA committee recommendations.
Europa as well as Titan "are of extremely high scientific interest," said Jonathan Lunine, Professor of Planetary Sciences and of Physics at the University of Arizona in Tucson.
Like the Galileo spacecraft that bolstered the hunger to plumb the mysteries of Europa, the Cassini/Huygens probe has revealed Titan to be a captivating world demanding more scrutiny.
Missions to both worlds should be flown "as the cornerstones of a vigorous effort to explore astrobiologically interesting bodies in the outer solar system," Lunine said. "The technology is there. All that is needed is the will to exercise it," he added.
The most recent debacle in the pursuit to explore Europa was the NASA Prometheus effort--making use a nuclear reactor to power a set of ion thruster engines. A first flight mission was dubbed the Jupiter Icy Moon Orbiter--JIMO for short.
JIMO was to search for evidence of global subsurface oceans on Jupiter's three icy Galilean moons: Europa, Ganymede, and Callisto. The mission was ballyhooed as setting the stage for the next phase of exploring Jupiter and opening the rest of the outer solar system to detailed exploration.
The fire was extinguished on Prometheus. It flamed out given budget and technical stresses.
NASA chief, Mike Griffin, told a U.S. Senate subcommittee in May 2005 that JIMO was in his opinion, "too ambitious to be attempted."
Griffin said JIMO "was not a mission, in my judgment, that was well-formed" and the mission's original purpose was to execute a scientific mission to Europa, "which is extremely interesting on a scientific basis," he told lawmakers.
"It remains a very high priority, and you may look forward, in the next year or so, maybe even sooner, to a proposal for a Europa mission as part of our science line," Griffin testified. "But we would not--we would, again, not--favor linking that to a nuclear propulsion system."
Fresh look needed
A fresh look at a Europa mission based on existing technologies is deserved, suggested Torrence Johnson, Chief Scientist, Solar System Exploration Programs Directorate at the Jet Propulsion Laboratory (JPL) in Pasadena, California.
Johnson said that using the technology developed during past work on a Europa Orbiter, as well as the JIMO studies, helps justify going forward on plotting out a new and highly-productive Europa mission.
"We have continued to study what can be done with existing technology to make a viable Europa mission with a great data return possible," Johnson told SPACE.com. He noted that his personal and professional opinions on the matter are his own and do not, in any way reflect the opinion or policy of JPL.
Robert Pappalardo, Assistant Professor in the University of Colorado at Boulder's Department of Astrophysical and Planetary Sciences and Laboratory for Atmospheric and Space Physics said the outer planets community of scientists has stamped priority "#1" on exploring Europa. A leading look at a spacecraft for that mission is tagged the Europa Explorer. Imbued with radiation hardening technology for a longer stay-time in orbit around Europa, as well as Earth-to-Europa trajectory pluses, a mission to this moon is far more compelling now than in previous times, he said.
"We've spent quite a bit of time and effort trying to understand was Mars once a habitable environment. Europa today, probably, is a habitable environment," Pappalardo advised. "We need to confirm this...but Europa, potentially, has all the ingredients for life...and not just four billion years ago...but today."
A nuclear-powered Europa Explorer would be loaded with scientific gear.
For example, the Europa orbiting spacecraft could be outfitted with ice-penetrating radar to detect shallow water, or partial melt. If the moon's icy face is thin, or thin in spots, that radar could possibly penetrate through the ice to an ocean.
Also onboard would be a camera system, infrared sensor hardware, as well as equipment to discern the crunching, cracking, creaking, and overall strength of Europa's ice shell - to help validate that the moon really does have a global liquid water ocean, Pappalardo explained.
One additional payload on Europa Explorer: a simple lander.
Pappalardo said a lander is still being bandied about, but carrying what kind of technology and at what cost are questions awaiting answers.
"We're not going to search for life with this mission. But just like the Mars rovers in their search for habitable environments...we're going to characterize the habitability of Europa," Pappalardo said.
An orbiter to the moon of Jupiter would allow a now sketchy view to become sharp as to how this world works, Pappalardo concluded. This mission, he said, has compelling science and broad community support and "we're ready to go."
Works in progress
While NASA's new budget carries dire news, there are other works-in-progress in terms of Jupiter and the study of Europa.
In June of last year NASA announced that a mission to fly to Jupiter will proceed to a preliminary design phase. That program is called Juno, and it is the second in NASA's New Frontiers Program - of which the now en route New Horizons mission to Pluto is the first in this category. JUNO would conduct a first-time, in-depth study of the planet Jupiter. It must be ready for launch no later than June 30, 2010, within a mission cost cap of $700 million.
Also, the European Space Agency (ESA) is currently studying the Jovian Minisat Explorer (JME). The JME focuses on exploration of the Jovian system and particularly the exploration of its moon Europa. The ESA study is also looking into deploying a compact microprobe onto Europa to perform on-the-spot measurement of the moon's ice crust.