The unmanned probe scored a major discovery in 2005 when it found geysers erupting from what appears to be a subsurface sea on the moon Enceladus that scientists believe could harbor some form of exotic life. Cassini also mapped the surface of the huge moon Titan, which has a dense atmosphere, and lakes, and rivers, though there's no water - the liquid is made of hydrocarbons such as methane and ethane, as if the place had been designed by the oil and gas industry.
Fuel is running low on Cassini, but there's enough for another four years of maneuvering. Technicians at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., have mastered the art of using Titan's gravity to steer Cassini into new, interesting orbits. NASA hopes to send the spacecraft diving inside the majestic rings of Saturn to study their composition. The extended mission would cost about $60 million a year.
But that money has not materialized in the NASA budget. If there is no funding, NASA will have to end the Cassini mission next year. For robotic spacecraft, the greatest hazard in the solar system turns out to be the NASA budget.
The mere possibility that such luxury-class missions could be shut down reveals the budgetary stress at NASA and calls into question whether the agency in coming years will be able to go forward with some of the big, ambitious exploratory programs that scientists have made their top priority.
NASA cannot simply abandon Cassini, because it could crash someday into Enceladus and could contaminate the hypothetical biosphere with Earth microbes that are lurking aboard it. Instead, the navigators at JPL would be forced to aim the $3.5 billion spacecraft directly at Saturn, which is presumably lifeless, and let it disintegrate as it enters the atmosphere.
"I think it would be the height of folly to terminate such a profoundly successful mission when we're not done yet," said Carolyn Porco, a planetary scientist at the Space Science Institute in Boulder, Colo., and the leader of the imaging team for Cassini.
The bet within NASA is that the Obama administration and Congress will find a way to keep Cassini flying. And it's virtually certain that they will scrape together the money to extend the operation of the Curiosity rover on Mars.
But earlier this month, NASA Administrator Charles Bolden popped into a meeting of a NASA science advisory committee and made an unexpected announcement: There will be no new flagship-class missions.
Those are projects that cost $1 billion-plus. Flagships include Cassini, Curiosity, the Hubble Space Telescope and such legendary spacecraft as Viking and Voyager.
Bolden's comments, first reported by space policy reporter Marcia Smith, landed in a delicate period in which the agency's fiscal 2015 budget is being drafted by the president's Office of Management and Budget, in consultation with NASA.
Will Cassini get funding? Will other high-priority programs? The scientists who depend on this funding are anxious. They have made their to-do list for the coming decade, and it includes some multibillion-dollar proposals. For example, scientists are eager to send a probe to Jupiter's icy moon Europa, another potential abode of extraterrestrial life.
NASA officials tried to settle everyone down a few days later by disseminating a new statement from Bolden saying that "NASA remains committed to planning, launching and operating flagship missions." The gist is that NASA cannot start a new flagship mission right now, but perhaps the fiscal situation will improve in the future.
The Obama administration argues that NASA is being forced to fit 20 pounds of programs into a 10-pound bag. Officials note that Congress began squeezing NASA's budget in 2010, and then the sequester trimmed it further, leaving the agency about $2 billion short of where it had been in 2009. NASA had to find a way to absorb those cuts, even as cost overruns on certain science missions made officials wary of the jumbo, multibillion-dollar projects. The administration essentially wants to go back to the "faster, better, cheaper" philosophy that had been the NASA mantra in the 1990s.
Agency officials emphasize that they continue to push forward with a long list of science missions, most of them costing less than a billion dollars. They include the Maven probe that is on its way to study the atmosphere of Mars and the Osiris REx spacecraft that is supposed to fly to an asteroid, grab a tiny sample and bring it back to Earth. Among already approved flagships are the James Webb Space Telescope, scheduled for a 2018 launch, and a 2020 Mars rover that will be a virtual duplicate of Curiosity.
But the space science community, which includes astrophysicists and Earth scientists, feels it is facing a new era of limits even as the universe itself is screaming to be explored.
One of the biggest space stories of the year was the announcement in November, based on an extrapolation from observations by NASA's Kepler space telescope, that our Milky Way galaxy alone is likely to have about 40 billion planets that are roughly the size of the Earth and in orbits that could potentially allow water to exist at the surface.
But even as scientists feel optimistic that the galaxy hosts a multitude of habitable worlds, NASA is struggling to come up with the money for exploration, whether through telescopes, robotic probes or human spaceflight.
Space beckons, but it is a hostile and unforgiving environment, with fierce head winds.
Congress and the White House tell NASA what to do (the agency is now under orders to visit an asteroid by 2025 and put humans in orbit around Mars by the 2030s), but there is a chronic mismatch between aspirations and dollars. The result is a lot of contortions.
In 2013, NASA had about $17 billion to work with. The agency has been commanded to build a new, heavy-lift rocket and a new space capsule - that's $3 billion a year. The agency has also been told to operate the international space station, develop advanced aerospace technologies, study the Earth from orbit and funnel money to a new "commercial" launch industry to help get that off the ground. And also explore distant planets with robotic probes. And launch space telescopes to peer into the depths of the cosmos.
A problem in one part of NASA can have cascading effects across the agency. That is what happened when the James Webb Space Telescope was delayed by seven years and ran disastrously over budget. There is no consensus on what the telescope was supposed to cost - at one point it was projected at about $2.5 billion - but NASA now puts the price tag at $8.8 billion. The Mars Science Laboratory mission, which put Curiosity on Mars, also ran nearly a billion dollars over its $1.6 billion budget.
These overruns drain money from other science missions. They also make the budget writers nervous. They wonder: Can the scientists be trusted with flagship-class spacecraft?
Jim Bell, an Arizona State University planetary scientist who has worked on the Curiosity mission, acknowledged that the science community did not help itself with the cost overruns, but he noted that they are doing customized, experimental work for which costs are hard to estimate. "They've never been done before," Bell said. "They're one-offs."
The budget for NASA's planetary division fluctuates as big missions such as Curiosity ramp up and ramp down, but in the past couple of years it has plummeted. With Curiosity's costs declining, the Obama administration requested $1.217 billion for planetary science in fiscal 2014, a sharp drop from the $1.5 billion budget in 2012.
Planetary science depends on a steady and reliable stream of NASA funding; there is little private-sector investment in solar-system exploration. The entrepreneurial "New Space" people want to build rocket ships, but they are not as focused on space science. Knowledge is powerful, but not always profitable.
John Grunsfeld is the astronaut-scientist who has been to space five times and made three trips to the Hubble telescope to fix and replace instruments. Now, as the head of NASA's science directorate, he would like to see a fundamental change in the agency.
"I believe getting a human on the surface of Mars is a top priority, but I want that person to be a scientist, to be an astrobiologist or a planetary scientist," he said. "A science-led strategy would actually be more successful."
NASA remains an agency driven by its human spaceflight division. If NASA has an original sin, it is that the agency was spawned by the Cold War and its early exploration activities were a technological battle against the Soviets for high-ground superiority; science was essentially a fringe benefit of this quasi-military enterprise.
In the spring, NASA proposed what is known as the Asteroid Redirect Mission (ARM), which would send a robotic spacecraft to grab a small asteroid and haul it back to lunar orbit. Then two astronauts would visit in the new Orion crew capsule, spacewalking their way to the captured asteroid and taking samples.
There are obvious challenges for such an unusual mission. The target rock has not been found. The leading candidate, used as a reference in feasibility studies, was a small asteroid, named 2009 BD, that was spotted four years ago by ground telescopes. But then in October, scientists tried to get a better look at the asteroid with the Spitzer Space Telescope and could not detect it at all. NASA has concluded that 2009 BD is too small for the asteroid mission, and the agency is still searching the night sky for a suitable rock.
The asteroid mission creates a focal point for numerous existing programs, including the Space Launch System rocket and the Orion capsule, whose destination has been uncertain. Critics suggest that the asteroid mission was dreamed up to give SLS and Orion somewhere to go and something to do. The rocket and capsule have political protection in Congress. The asteroid rendezvous, however, needs additional funding from Congress if it is going to turn into a real mission.
Lori Garver, the No. 2 person at NASA until she left in September for a job in the private sector, said when asked about the NASA budget that the agency lacks the funds to do everything it is supposed to do - "a situation that was exacerbated by the agreement to develop SLS/Orion, while continuing all other programs."
NASA spokesman David Weaver said in response to Garver's comment: "Every agency in the federal government has fiscal challenges. The American people expect a lot of their government, right? . . . But no agency in the federal government does a better job of managing the resources that we get and executing an extraordinary program than NASA does."
One prominent flagship-class mission that cannot seem to get off the ground is the Europa Clipper. The target is Jupiter's moon Europa, which shows signs of a global subsurface ocean.
That evidence grew stronger earlier this month with the announcement that, just like Saturn's moon Enceladus, Europa appears to have geysers of water erupting from its south pole. Astronomers used the Hubble to detect concentrations of atomic hydrogen and oxygen from what seem to be plumes of water vapor shooting more than 100 miles off the moon's surface.
Europa is covered with a shell of ice. It has an internal source of heat from the tidal forces exerted by Jupiter. Europa races around Jupiter in an elliptical, 3.5-day orbit. When the moon is farthest from Jupiter and the gravitational squeeze of the giant planet eases up, the icy shell decompresses just enough to let subsurface water find a path to the surface and vent into space, scientists believe. They say the subsurface ocean could be 50 miles deep at least, containing more water than in all the oceans on Earth.
The geysers are squirting not only water into space, but also whatever else is in that subsurface ocean. Landing on Europa is too expensive, as is sending a probe down through the ice into the water below, but a spacecraft can learn much about the planet through high-resolution stereo imagery, ice-penetrating radar and other instruments. It could also could fly through the plumes and, with an instrument called a mass spectrometer, analyze the ejected material.
"This is huge," said JPL's Robert Pappalardo, who has worked on plans for the Europa Clipper. "It now means that we could effectively sample Europa's interior from flybys."
Initial estimates for a Europa orbiter put the cost at $4.7 billion. That's expensive even by flagship-mission standards. Getting a spacecraft into orbit around Europa is tricky, because it's close to Jupiter and at the bottom of the planet's deep gravity well. Jupiter also emits intense radiation, and the spacecraft's instruments would need to be covered in costly lead shielding.
So engineers went to a Plan B. Rather than orbiting Europa, the spacecraft would go into an orbit around Jupiter, spending most of its time outside the planet's radiation field, and then swoop in repeatedly, with 34 flybys of Europa and nine of the moon Ganymede.
At this point the Europa Clipper is just a "concept under study," and it is not clear when or if it will graduate and become a real mission. What is at stake in the Europa decision is not just the exploration of this one intriguing moon: The engineers at JPL say they need to keep flying outer-planet missions to maintain a skill set that has been honed over decades.
They developed the software programs and navigational techniques that make it possible to guide spaceships around distant worlds. To fly in the Jupiter system they have to make tiny adjustments, using engines and thrusters, because of the gravitational influences of Jupiter's many moons, particularly the four huge moons (Io, Europa, Callisto and Ganymede) discovered four centuries ago by Galileo. Those moons will jerk any spaceship around, but they are viewed as a blessing because the technicians can use the gravity of the moons to change a spaceship's velocity and trajectory.
"Europa looks like a propellant depot to us," said Al Cangahuala, a JPL deep-space navigator.
The JPL navigators managed to fly Cassini within 546 miles of the surface of Titan. They sent Cassini within 16 miles of the surface of Enceladus. Cangahuala said they can figure out to within a few miles the trajectory of a spaceship when it buzzes a moon. That is a skill that cannot be learned in a textbook or downloaded from the Internet.
Pappalardo is not pleased with the no-new-flagships strategy:
"It's shortchanging the science and shortchanging the American people," he said. "Imagine if that had been said in the '70s - 'No, we're not going to send Viking to Mars, and we're not going to send Voyager to the outer solar system or Galileo to Jupiter or Cassini to Saturn.' Where would we be in our understanding of the solar system?"
Mark Sykes, chief executive of the Planetary Science Institute, a nonprofit corporation in Tucson, said his colleagues who study moons and planets and depend on NASA funding are increasingly gloomy.
"People are contemplating what other careers they should pursue," he said.
Science can veer into esoteric territory. It is not yet clear, for example, how the discovery of the Higgs particle by physicists in Geneva might someday lead to a practical application. Cynics might argue the same thing about planetary science. It is not obvious why anyone needs to know what is going on at the south pole of Enceladus. If there is life there, it is not likely to engage humans in conversation, or be tasty in a salad.
But the search for extraterrestrial life is also a search for a theory of life. There is no greater mystery in all of science. Finding and studying life on another world is a requirement if we are to understand, at a fundamental level, what life is.
Space science - whether it is the detection of planets around distant stars or the discovery of the faint traces of an ancient lake on Mars - invariably turns our attention back to Earth, and its unusual nature, and its preciousness.
The Spaceship Earth cliche is well worn by this point, but it is physically accurate - the planet cruises through a hostile environment, protected from the vacuum and the harsh space radiation by a thin layer of atmosphere and a magnetic field. Every time scientists examine a distant planet or a moon, humanity sees an alternative world, a case of what might have been. In every case so far, we have seen worlds that are not like Earth at all and that are at least superficially lifeless.
We've seen nothing as rambunctious as a bacterium.
We've seen no green worlds, no blue worlds. Not yet.
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