“Good morning, this is NPR and you’re listening to a special edition of
Science Friday
. I’m Flora Lichtman and today we have a first: a live broadcast from an interplanetary mission. With us is Dr. Rebecca Johansson, who is the senior power engineer aboard the U.S.’s spaceship to Saturn, the
Richard M. Nixon
, which launched two days ago. The
Nixon
is already approaching the moon’s orbit, so there’ll be about a three-second light-speed delay between my questions and Dr. Johansson’s replies.
“First, Becca, thanks for taking the time to talk to our listeners—we know you’ve a busy schedule.”
BECCA:
That’s an understatement, but it’s my pleasure to be here. I’m a big fan of the show.
FLORA:
Becca, you’re a nuclear power plant engineer, not a rocket scientist. Why are you part of this mission?
BECCA:
The propulsion system for the
Nixon
isn’t a conventional nuclear rocket like the Chinese are using—it is, in large part, an electric power plant. That’s where my primary responsibilities lie.
FLORA:
Can you explain the difference in the engines on the two ships?
BECCA:
Sure, the Chinese are using a nuclear thermal rocket. Take a nuclear fission reactor, get it real hot, and pump hydrogen through it. The hydrogen heats up and jets out the back, and there’s your rocket engine. The only difference between their engine and a chemical rocket is that they’re heating the gas in a fission reactor instead of by combustion.
FLORA:
Okay, and the U.S. mission is using . . . ?
BECCA:
An engine that’s never been used on a major space mission before, called a “va-si-meer” . . .
FLORA:
You better spell that.
BECCA:
V-A-S-I-M-R, which stands for “Variable Specific Impulse Magnetoplasma Rocket.”
FLORA:
Oh, well, that clears everything up!
BECCA:
[laughter] Well, then, my work here is done. Okay, before everyone tunes out . . .
FLORA:
It may be too late for that [more laughter] . . .
BECCA:
What we have is a really big, fancy kind of ion engine. We take a gas and ionize it—knock an electron or two off of each atom so instead of being electrically neutral, each atom has a positive charge. We funnel those ions into a big particle gun. It uses magnetic and electric fields to grab those ions and accelerate them to very high velocities. We squirt them out the back, and there’s your rocket exhaust.
FLORA:
So where does the reactor come in?
BECCA:
Reactors, actually. We have two. That way in case something goes wrong with one, the
Nixon
isn’t dead in space. The reactors come in because our rocket engine uses electricity. It takes electric power to ionize the gas and it takes electric power to generate the magnetic and electric fields that accelerate the ions. We use the reactors to power an electric plant, just as we would on Earth.
FLORA:
Then, the ship’s power plant is a lot like the systems you’re used to designing for electric utility companies?
BECCA:
Yes, which is why they want my expertise. But there are some important differences. The ship’s power plant is considerably bigger than any reactor complex in commercial use. It puts out nineteen gigawatts of thermal power. That’s an awful lot of power in a small space, and make no mistake, it is a small space. The reactors themselves wouldn’t fill up this room.
FLORA:
Wow. Why are reactors in regular power plants so much larger?
BECCA:
Partly because they don’t have to be any smaller. It’s a lot harder to manage that kind of power in a small volume than a large one. Also, the reactor on Earth is surrounded by tons and tons of shielding and containment vessels to protect people and the environment from its contents. In our spaceship, we dispense with most of that.
The reactors are all the way at one end of the ship and just need a small shielding cap to provide a radiation shadow for the occupied portions of the ship. We don’t need a big, bulky containment vessel. If something goes wrong and the reactor fails catastrophically, that’s gonna be the end of us, anyway. My job, along with the reactor designers, is to make sure that won’t happen.
FLORA:
Okay, then, what makes a VASIMR better than a tried-and-true nuclear thermal rocket?
BECCA:
Two things—the first is that we can get a much higher exhaust velocity out of the VASIMR. That means we can go a lot faster using the same amount of reaction mass. I think I can explain how that works to your listeners.
FLORA:
I certainly hope so.
BECCA:
Imagine you’re on a pair of roller skates and you’re holding a bowling ball. If you throw the bowling ball away from you, you start rolling backwards. Action balances reaction, thank you, Newton. Now suppose instead of a bowling ball, you have a small pistol that you fire. What happens? The bullet goes forward and the recoil sends you rolling backwards. A very small mass, like that bullet, can push you just as hard as the bowling ball did, if it’s going very fast. The VASIMR gives us a lot more push than a nuclear thermal rocket would, for the same amount of reaction mass.
FLORA:
And the second thing?
BECCA:
This part’s a bit peculiar. The way rocket physics works, it takes a lot more energy to get the ship up to speed with a high-velocity exhaust than with low velocity. Our fast-moving ions are very efficient at using a small amount of reaction mass but very inefficient at using small amounts of energy.
To put it another way, if we want to use as little reaction mass as we can, we want to throw bullets. But if we want to get the most speed from our power plant, we want bowling balls.
In between those extremes is a happy medium. If we adjust the exhaust velocity as we go, we can get by with a ship that’s about half as big as it would be with a fixed exhaust velocity. Rocket scientists
call the exhaust velocity “specific impulse,” which is where the first three words of the engine’s name come from: “variable specific impulse.”
FLORA:
On to my next question. If VASIMRs are better than thermal nuclear rockets, why didn’t the Chinese use them in their ship?
BECCA:
VASIMRs have one big disadvantage—they’re electric. A nuclear thermal rocket puts all its heat energy into the exhaust, it’s essentially one hundred percent efficient. We have to convert the heat of the reactor into electricity and using every trick we know, we can only do that with fifty-five to sixty percent efficiency. The other forty-odd percent? It’s waste heat, and if we don’t get rid of it the ship’ll fry. To give you an idea how much heat I have to deal with, it’s as if you took all the power used by a city the size of, say, Minneapolis, and stuffed it inside our little bitty rocket. My job is to get rid of it!
Believe me, that’s not easy. There’s a lot of specialized plumbing, some humongous heat radiators, the whole thing is very complicated. Our early tests . . . well . . . we had some hiccups.
FLORA:
That radiator test in orbit last January?
BECCA:
Ummm, yeah, that didn’t go so well. But cutting-edge engineering is like that. That’s why we test instead of just flying off. Anyway, a gas core reactor with a hydrogen flow-through, like the Chinese use, is child’s play compared to this. That’s a lot more stable and we have a lot more experience with it. If we didn’t need to be going really, really fast, we’d never be using VASIMRs. The Chinese were originally going to Mars. They didn’t need to be going anywhere that fast. A simple nuclear thermal rocket would get them there in a few months with a very reasonable mass-to-payload ratio. To get that same ship to Saturn, though, they had to hot-rod the whole setup, add some truly monstrous hydrogen tanks for the additional reaction mass they need, and it’s still going to take them a year and a half to get there. They are the tortoise and we’re the hare. This time, though, the hare is going to win the race.
FLORA:
Thank you very much for your time, Becca.
As the
Nixon
passed the moon’s orbit, it officially entered interplanetary space. The ship was finally starting to show what it was made for. By interplanetary standards, its velocity was still modest, at six kilometers per second relative to Earth, but now the
Nixon
was essentially free of Earth’s gravity. The thrust of the VASIMRs produced a small but steady acceleration that piled another four kilometers per second onto the ship’s velocity each and every day. It had taken about sixty hours to pass lunar orbit, but another sixty would see the
Nixon
some two million kilometers from Earth.
The
Nixon
’s nose was pointed directly sunward, so that the VASIMRs’ thrust could push its orbit into a tighter and tighter ellipse, a trajectory that would skim the sun at a scorching thirty million kilometers.
That was well within the orbit of Mercury, and the sun’s light would be twenty-five times as intense as it was on Earth. The small risk was worth it: the sun’s gravity was equally fierce; they’d be whipping past the sun at better than a hundred kilometers per second but the gravity would still bend the spaceship’s heading by forty degrees, putting the
Nixon
on track for a Saturn rendezvous. That was Howardson’s “solar slingshot” trajectory, the one that would let them beat the Chinese to Saturn.
Martinez and the other handymen began prelim work on rigging the solar parasols to protect the ship. The parasols didn’t need to be anything fancy: they weren’t going to be there that long, and once the
Nixon
was past the sun, they’d be jettisoned. They would be large—hundreds of meters on a side, but that was nothing compared to building the
Nixon
in the first place. Deployment wasn’t imminent, as perihelion was a month away. Still, it was something to do.
As the
Nixon
left Earth’s orbit, the crew began self-consciously to develop daily routines.
There was a constant stream of information coming in from Earth, so the Internet and all the related research facilities were there for the academics; and the maintenance people always had work to do.
For most of them, the
Nixon
’s voyage was their first experience living in a low-gravity environment. One-tenth gee wasn’t much in the way of artificial gravity, not enough to prevent loss of bone density, some atypical edema, and other signs of space deterioration.
Too many of the residents were out of shape, and for the first time in their lives, had to maintain a strict regimen of daily physical exercise. If they didn’t, returning to Earth would be a nightmare, with elevated risks of broken bones or heart attacks.
Sandy found himself in demand as a physical trainer, and led two classes a day in core exercises, and spent an hour working out himself under the eye of Marine Captain George Barnes, a physical fitness nut. Sandy enjoyed the workouts, but never, in all his gym experience, had he heard so much bitching and moaning. Fang-Castro ordered monitoring of all physical activity, logging both cardiovascular exercise and resistance training for every crew member.
John Clover taught a weight class, with steel weights fabricated by Martinez from bits and pieces of space junk left behind at the
Nixon
’s former niche in Earth orbit. Clover had been out of shape for years, and now found himself slimming down and adding muscle at the same time.
Roger Ang, the violinist/psychiatrist who’d been worried about the tone of his rare instrument, turned out to have been a college wrestler, and taught a popular wrestling class, which combined both cardio and resistance. The first time they wrestled, he pinned Crow four times in less than two minutes, despite the low gravity, which made it more difficult to keep an opponent pinned to the floor.
As they headed for the showers, Sandy said to Crow, “He kicked your ass. A pencil-necked shrink. A fuckin’ violinist. A snowflake. A delicate little flower . . .”
“In sports, the rules define outcomes,” Crow said. “He won because I wasn’t allowed to bite his nose off, knee him in the balls, or gouge his eyes out.”
“There’s gotta be some rules,” Sandy said.
“Really? I hadn’t heard that.”
—
Seven bands, an orchestra, three string quartets, and two choirs popped up within days. Sandy and Martinez put together a Country/RhythmTech trio, with Imani Stuyvesant, an exobiologist who played drums. Fiorella, as it turned out, having grown up in Bakersfield, California, was an expert two-stepper. She began giving dance lessons. Crow hadn’t brought an instrument along, but admitted to Sandy that he’d once played an upright bass in high school and in the Naval Academy orchestra.
“If you played an upright bass, you could play an electric—it’s all just ears,” Sandy told him. “I’ll start fabricating an electric one in the shop. We need a bass player.”
Crow didn’t say “no.”
Because of the low gravity, activities took on a bit of a slow-motion quality, like living in a special-effects movie. The new crew members learned to act more deliberately, because an unthinking gesture or swipe of the hand could much more easily send objects flying across the room. Tossing a salad without scattering greens all over the table was a delicate art. Get careless reaching for a coffee cup and you’d knock it off your desk. It would take three times longer to hit the floor, but you’d still have to clean up the mess.
Consequently, most crew members found living in one-tenth gee a little surreal. Still, it was more familiar and comfortable for most of them than living in zero-gee, which they would pass through when moving from one habitat module to the other. A few liked it better than Earth gravity
Like Mr. Snuffles. His favorite space was the cafeteria/commons, where there was always an ailurophile or two or three to slip him treats—bits of tofu salmon, tofu beef, tofu chicken, engineered to the point where not even a cat’s taste buds could tell the difference.