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What's next for NASA as Cassini's mission comes to a close

What's next for NASA as Cassini's mission comes to a close
From Engadget - September 15, 2017

Any melancholy felt in the JPL mission control room while Cassini hurtled to its demise at 70,000 MPH, was short-lived. However, as with virtually every planetary mission that NASA conducts, Niebur does wish that some elements had played out differently. "The first thing you realize on every mission you do," he explained, "is that once you get there and get your first measurements taken, is you wish for better instruments."

There's a significant lag between the state of the art when a mission launches and when it finally reaches its celestial destination years later. Instruments used for the Discovery missions -- NASA's bread and butter expeditions throughout the solar system -- are usually between five and eight years old once they start taking readings. And, given the rate of technological advancement these days, five to eight years is an eternity.

Second on Niebur's list: more data. Just as with cowbell and Blue Oyster Cult hits, NASA can never have enough data. "Like on Titan, we still do not have 100 percent coverage of Titan. We are missing half of it, he explained. "We have only done flybys -- now, we have done over a hundred of them -- but a hundred flybys and all you cover each time is a strip of a noodle. Paste all those noodles together you still do not cover the entire globe."

Luckily, it wo not be long before we are back investigating Saturn's largest moon. There's just so much to do there. "The things we have discovered are exciting enough that they definitely justified new missions," Niebur said. "You can send any mission you want to Titan. It's that great a place. Submarines, boats, helicopters, airplanes, rovers -- anything. It all works on Titan." Titan, as well as its smaller lunar sibling Enceladus and Saturn itself, are all on the approved list of targets for the next New Frontiers mission which is scheduled to launch sometime in 2025.

But will we really have to wait until the end of the next decade (assuming it still takes the same amount of time to reach Saturn and its moons that Cassini did)? Unfortunately yes, says Niebur. Solar panels and ionic thruster drives are great but solar only works within a certain proximity of the Sun (hence Cassini's RTGs) and ionic thrusters take forever to get up to speed.

The recently selected Discovery mission, Lucy, will leverage an ionic engine when it inspects "trojan asteroids" circling the Sun. "It actually goes to one side of the sun to visit some trojans and then goes to the other side of the sun to visit others," Niebur explained. "And if we were doing that with wet propulsion, it'd never work but with electric propulsion and one small xenon tank, it can go back and forth. It's not quick but they have the ability to do it."

Gravitational assists, such as the one used by Cassini to slingshot to Saturn, however, are far more effective. "Nothing beats a gravitational assist, Niebur exclaimed. "Getting a free boost, that is the ultimate in efficiency. You know the Voyager spacecraft are the fastest things out there and that's because they have done so many gravitation assists. We could have never got them going that fast."

Indeed, every time these ships spun through Jupiter, Saturn, Uranus and Neptune's gravity wells, they picked up 20km/s of delta V (ie, speed) -- for free. "We can never fit that much gas in a rocket" to accomplish the same, Niebur said. Potentially the only system that wo not have to rely on gravitational assists is the upcoming SLS. When it launches carrying the Europa Clipper around 2022, it should be able to reach Jupiter on a direct trajectory in under three years.

That's a massive improvement over today's propulsion systems and has been a long time coming. "Every mission builds upon the previous mission. That's just the limitation we are operating under," Niebur said.

"A lot of times people come in and say you know let's just skip all those steps and go for the brass ring," he continued. "That sounds very dramatic and it's very tempting. But it's also the wrong way to go about doing this."

That's namely because we are not talking about spending two months coding a mobile app. These missions require decades of design and development, not to mention hundreds of millions if not billions of dollars of investment. "You do not want to make that kind of gamble and get there and realize whoops I brought the wrong tool set," Niebur explained. "You'd feel pretty stupid if you went to Mars with a boat. But you'd feel right at home if you did that on Titan."

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