Cassini sails between Saturn and its rings during its final weeks, courtesy of NASA/JPL-Caltech
The Cassini spacecraft had exhausted 95 percent of its fuel by 2010 after 13 years in space. Then scientists decided to go for seven more years.
Lead propulsion engineer Todd Barber thought the idea was “crazy,” yet navigators worked out a fuel budget that saw the spacecraft sample Enceladus’s geysers and skim Saturn’s rings before burning up this past September. To pull off these decades-long missions, NASA has had to master tightly-engineered propulsion schemes exploiting planetary motion — and the agency continues to explore new horizon-expanding designs.
Need for speed
Here on Earth we tend to frame trips in terms of distance, but in space it’s all about speed. Spacecraft spend their lives in freefall, looping around massive worlds. To reach distant planets, they have to boost their speed and expand those loops, which is why navigators measure fuel budgets in speed change. An extra 2 miles per second puts you on a course for Mars’s orbit, while you’ll need 4 for Jupiter’s.
Cassini launched with enough fuel to get it past Mars, but not all the way to Saturn, forcing planners to ramp up its speed with loops around Venus, Earth, and Jupiter.
The knack lies in letting the spacecraft plummet toward a planet’s surface, but miss the ground. Gravity bends the craft’s path around the back of the planet, which hauls the craft along as it flies through space, much as how Spiderman might get a boost by swinging off of a speeding train.
During the first Venus rendezvous, Cassini received more than twice the speed change contained in its three tons of fuel. Later, the craft would repeatedly harness Saturn’s largest moon, Titan, for more than 30 times the energy stored in the fuel it had launched with.
Fast, light, and furious
Plotting just the right course to harvest planetary speed requires well-tuned propulsion. Cassini could move in two ways: fine twisting or coarse boosts forward. It changed direction with eight thrusters, each capable of nudging the craft with a force similar to the weight of a deck of cards. To move ahead, the two main engines pushed with about 100 pounds of force each.
The thrusters fired only while aligning the craft for a new orbit, or to point a scientific instrument, like a camera to snap a picture of Saturn’s rings. Otherwise, drifting through space costs nothing, which Barber says makes the vehicle’s gas mileage — across nearly 5 billion miles — look fantastic: “If you do the math, it’s ‘move over Prius.’”
Crunching the numbers gives you hundreds of thousands of miles per gallon, but in practice, that’s not how it works. Miles rack up for free during the vast majority of the trip when the craft isn’t actively speeding up or changing direction. Instead of mileage, propulsion engineers gauge a craft’s efficiency with something called “specific impulse” — bang (force) for your buck (fuel). Space has nothing to push off against, so vehicles move by shooting stuff out the back. How much force a craft gets out of its fuel depends largely on how fast it can expel material, so engineers prefer exhaust particles to be as light as possible.
The chemical explosions that power most NASA missions can only push molecules out so fast, and their efficiency hasn’t changed much for decades. Cassini’s big engines share an architecture with the fine-control system of the Apollo missions. Next, NASA is turning to novel designs featuring smaller, faster exhaust.
Toward a galaxy far, far away
Enter ion drives, the technology that puts the I in Star Wars’s T.I.E fighters. These engines use electric fields to knock electrons out of atoms — the single-atom gas xenon specifically — expelling a soup of charged xenon ions and electrons. Xenon is bulky compared to other elements on the periodic table, but the focused electric field gets these ions going 10 times faster than the molecules that would be blown out by chemical alternatives. Available in this galaxy today, ion thrusters got the Dawn craft to the dwarf planet Ceres and the minor planet Vesta.
There’s a tradeoff, however. Ion engines can deliver only a tiny amount of force — less than the weight of a piece of paper — in return for a ten-fold efficiency gain that lets them fire for years straight. That slight but constant push adds up, making the technology ideal for long, continuously powered paths, such as spiraling out to the asteroid belt. But for shorter hops, to the moon for instance, their gentle push would take far too long to get a spacecraft up to speed.
Someday, engineers could get the best of both worlds. NASA recently re-booted Cold War research into an engine featuring both high force and specific impulse — nuclear thermal propulsion. Packing a fission reactor, the craft would heat hydrogen, the lightest element, and spew out the resulting gas.
Nuclear rockets may be distant, but missions like Cassini show that NASA can squeeze impressive performance out of today’s crafts. “It was a photo finish,” says Barber, who estimates that Cassini finished with 1 percent, plus or minus 2 percent of its initial fuel. “None of us could have dreamt it lasted that long.”