NASA could soon be facing an awful choice. The agency, feeling a budgetary squeeze from Congress, might not be able to fund all its robotic planetary exploration missions after next year.

This year NASA received $16.9 billion, which may sound like a lot but, once adjusted for inflation, is roughly what the agency got back in 1986. Just $1.27 billion of that budget goes into funding all robotic exploration in the solar system. And most space policy experts don’t see that number going up anytime in the near future. In 2014, NASA will put many of its robotic missions through what’s known as a senior review. Administrators will have to decide which of its missions will yield the highest scientific return and may recommend canceling some of them.

And that's where some sad calculus comes in.

“We have two very expensive flagship missions, Cassini and Curiosity,” said NASA's planetary science director Jim Green, speaking to one of the agency's advisory councils on Nov. 5. “So, this particular competition we’ll have to do very carefully.”

You wouldn't think the Cassini spacecraft, in orbit around Saturn since 2004, was in trouble. It has lately been beaming back incredible data about the planet's rings and moons. A recent image from the mission (above) showing Earth, Venus, and Mars from Saturn was widely shared on the internet and even landed on the front page of the *New York Times *last week.

But NASA seems to want to focus its dwindling energy on Mars. It is scheduled to launch the MAVEN mission today, which will explore the Red Planet’s atmosphere, and it has at least two more Mars missions in the pipeline but little else in store for the rest of the solar system. The Curiosity rover is the new kid on the planetary exploration block and a media darling. Most in the planetary science community would bet that in a head-to-head competition, Cassini loses.

That would be a shame. Cassini has already been an incredible mission, and scientists estimate it has at least four more years of life left in it. Cassini’s operating budget is about $60 million per year while Curiosity’s runs to roughly $50 million. That’s about what the Department of Defense has budgeted for 3-D printer research and is less than half of what it's estimated to spend maintaining its golf courses. The Cassini mission has already cost $3.26 billion to launch and operate.

Curiosity certainly doesn't deserve to be put down either; both missions could be spared if NASA got a little more money. But Congress is a divided and quarrelsome body these days. If NASA is forced to chose, Cassini could end its mission at Saturn sometime in 2015, two years before its fuel runs out in late-2017. You might be thinking that the mission is going to end anyway, so who cares about those last two or three years? But those years could make a world of scientific difference.

Given the opportunity, Cassini could provide more unprecedented data about Saturn’s many mysterious moons as well as new findings and images of its beautiful rings. Surprising discoveries could appear in the coming years, and the mission offers a front row seat to marvel at the most stunning planet in the solar system. There are campaigns to pressure Congress to increase NASA's planetary science funding, but such efforts might just turn out to be too little, and too late for Cassini.

Here are just a few of the potential research opportunities from the last two years of Cassini's mission. If cancelled, there's no backup spacecraft and not likely to be another Saturn mission for years, if not decades. Losing Cassini means losing a once-in-a-lifetime chance.

Image: NASA/JPL-Caltech/SSI

Cassini arrived at Saturn nearly 10 years ago, in mid-2004. A year on Saturn lasts about 29 Earth-years, so Cassini has only observed the ringed planet for a scant third of its annual trek around the sun and has yet to even witness all the seasons. The ringed planet is tilted on its axis about 26 degrees, quite similar to Earth’s 23.4-degree axial tilt, which causes the seasons to change dramatically on Saturn (Jupiter, in comparison, has an axial tilt of 3 degrees, meaning little seasonal variation).

When Cassini first came on the scene, Saturn was in the waning days of northern winter. Its north pole was obscured by shadow and had a bluish tint, almost like the color of Neptune. Voyager 1 and 2 only flew by Saturn in the early 80s, but Cassini has been up close, watching the Saturnine seasonal change. Rather than provide a brief data point, Cassini has helped scientists see Saturn as a dynamic and complex planet.

One of the most interesting features on Saturn is the north polar hexagon, a perfectly polygonal storm running around the top of the planet. Cassini waited patiently for the tempest to enter sunlight so it could take visible images (previously, it could only see the storm in the infrared). Though they still don’t know how it works, scientists hope to get several more years of prime viewing so they can unravel this mystery.

In 2010 and 2011, Cassini was lucky enough to witness another incredible storm raging through Saturn’s atmosphere. The “Great Springtime Storm” lasted a record-breaking 267 days. It grew so large that the storm-head eventually circled around Saturn and ate its own tail, creating the largest and hottest vortex ever seen in the solar system. Such storms tend to appear roughly once every 30 years, but this event came a bit early.

“They typically form closer to the summer solstice,” said geoscientist Linda Spilker, the mission's project scientist. The question researchers are asking is, “Do we get to see another storm?”

Image: NASA/JPL/Space Science Institute

Saturn's many moons are also excellent targets for new discoveries. Enceladus is among the most interesting. The tiny icy body should be frozen and unexciting but instead has gushing active geysers originating from slashes in the surface called tiger stripes. In just the past year, the Cassini team figured out that the geyser eruptions' power waxes and wanes depending on where Enceladus is in its orbit around Saturn.

Planetary scientists’ working hypothesis is that at times when Enceladus is closest to Saturn, the moon is squeezed, closing the tiger stripes and reducing the jets. But when Enceladus is far from Saturn, the plumes’ power seems to triple, shooting much more material into space. It also just so happens that whenever Cassini has done previous flights through the jets, it was during low eruption times. But in late-2015, Cassini will go zipping through the geysers when Enceladus is far from Saturn, potentially allowing its instruments to detect subtle chemicals that it has so far missed.

“It will be like turning the lights up a bit,” said Spilker.

Because Enceladus could have liquid water beneath its icy crust, it’s a good place to check for life beyond Earth. Using its mass spectrometer to analyze the subterranean chemistry, Cassini could help figure out the conditions below the moon’s surface.

Another interesting moon of Saturn is Titan, a world known to have lakes, rivers, and rain. Hydrocarbons such as methane and ethane, not water, run through this system, making it a familiar and yet alien place. As the seasons change on Saturn and the sun climbs higher in the sky on Titan, the moon’s north pole will be illuminated for longer periods of time. By 2015, scientists may see signs of evaporation at Titan’s northern lakes and perhaps dry lakebeds.

Some global climate models of Titan also suggest that northern summer – which will shift into high gear in 2016 and 2017 – will bring about extreme wind speeds on the moon. These could kick up choppy waves on Titan’s hydrocarbon seas, which Cassini would be able to image with its radar camera. Information about the height and power of Titan’s waves is likely to come in handy for planning future lander missions, which could send a boat to float on and probe these lakes.

Image: NASA/JPL-Caltech/University of Arizona/University of Idaho

Titan is another potential haven for life in the solar system. Along with its hydrocarbon wetness, scientists suspect that it might harbor a liquid water ocean under its frozen crust. If Cassini misses the opportunity to see Titan’s summer close up now, the next chance won’t come for another 30 years, long after Cassini has quit.

Finally, there's the prospect of doing some really incredible new science in 2017 – if NASA doesn't pull the plug before then. Current plans have the spacecraft going through a series of carefully orchestrated gravity assists around Titan to fling itself past Saturn’s rings and enter orbit right up against the planet. NASA engineers have avoided this dangerous maneuver so far for fear that Cassini would hit some debris in the rings, destroying the spacecraft.

“This will really be like a brand new mission,” said Spilker.

By making 22 orbits in the 2,000-kilometer gap between the planet and its rings, Cassini would be able to take data that have been impossible until now. The probe would make extremely precise measurements of Saturn’s magnetic field and help understand the rotation period of the planet. Scientists thought they knew the length of a day on Saturn from Voyager measurements of radio emissions emanating from the planet. Cassini measured those same radio waves and found that the “day” had changed by 10 minutes. So either Saturn somehow slowed down or those emissions came from Saturn’s upper atmosphere. The additional measurements could determine the true day length and help scientists better understand the planet’s interior structure.

Perhaps most importantly, Cassini’s final orbits would allow researchers to determine the mass of Saturn’s rings. In the spacecraft’s current orbits, it is gravitationally tugged by both the planet and rings. Once inside the rings, it would only feel the gravity of Saturn. Subtracting those measurements will give planetary scientists the true mass of the rings for the first time ever (their current models have an uncertainty of 100 percent).

Knowing the rings’ mass could help determine how old they are. If the rings are on the higher end of mass estimates, it would suggest that they formed earlier, perhaps even 4 billion years ago when Saturn itself formed. That’s because the rings have been slowly eroded over time, hit by micrometeorites and other small objects. The more massive the rings, the more material they would have had in the past, and the earlier they would have formed. Thinner rings would suggest a younger formation scenario, perhaps when a large moon wandered too close to Saturn and was torn to shreds by the planet’s gravity.

If Cassini is able to keep operating until 2017, it will be crashed into Saturn to avoid having the probe accidentally impact Titan or Enceladus and contaminate those potential life-bearing worlds with Earth organisms. In September 2017, Cassini will be directed to enter Saturn’s atmosphere, pointing its high-gain antenna at Earth for as long as it can to tell scientists what it encounters before tumbling and burning up.

“It will be really cool to see those final bits of data,” said Spilker.

But none of this will come to pass if Cassini is given the axe. Spilker said that setting up Cassini to be in the right place for the final near-Saturn orbits require extremely careful planning years in advance. It might require a greater amount of fuel than the spacecraft has to do the maneuver earlier if funding cuts end the mission. For now, she says, they are staying the course until directed otherwise and hoping that Cassini will get to complete its full mission.