A little PSA

You know, I could argue that Iron Man is an allegory for medical devices.

Tony Stark’s heart

However, I really do wish that I could do with my insulin pump what Tony Stark does with his arc reactor at the end of “Iron Man 3.” I also (unfortunately) don’t get super powers from it while it’s still attached to me.

If you agree, then I will point out that I am biking in the Tour de Cure this year in Princeton, NJ. If you like, you can support me with a donation. I only need $206 more to reach my fundraising goal!

A quick reminder for Trekkies

In Star Trek II: The Wrath of Khan, the legendary genetically superior super-bad-guy mastermind genius Khan is defeated by a plot hole.

Allow me to explain: Khan, on board the USS Reliant, is fighting the crew of the USS Enterprise and about to blast them into oblivion when Spock identifies that Khan’s strategic thinking is hampered by his twentieth-century roots. He is treating space like a two-dimensional battlefield. So, the Enterprise sneakily moves vertically relative to Khan’s ship, thus disappearing from Khan’s radar. Moments later, they pop back and obliterate the bad guy.

Okay, first of all, if Khan’s strategy was truly two-dimensional in nature and Starfleet is supposed to be at all effective as a spacefaring organization, then “engage standard battle plan alpha that they teach first-years at the Academy!” should have been sufficient to destroy him. Because any such basic plan will use three-dimensional movement. After all, these plans have been honed by years of war with the Klingons. So, yeah – Kirk ought to have beaten Khan by rote.

Second, the Reliant’s sensors ought to have given some indication that the Enterprise was moving vertically. And they ought to have given some indication of when the Enterprise was coming back into range. The Enterprise, apparently, was able to track Khan’s position while doing its little up-and-over maneuver. Why not the reverse?

Third, the Enterprise crew decides to pop back into the 2D plane before attacking, instead of doing a smarter Princess Leia-style surprise attack from above. Here’s how I think this would have played out in Khan’s mind: “WTF? Where’d they go? Look everywhere in 2D for the Enterp–oh, there they are. Open fire.”

Plus there are all the other weird devices in the story…the Genesis Device is really no better than red matter. We’re supposed to take it that out of all Kirk’s flings in the original mission, somehow he had the most special feelings for this woman we’ve only just met, and we’re only told about that past relationship. And what’s up with his son? My point: I’m not really sure why Wrath of Khan is the sacred cow it’s made out to be. (Personally, I’m more a fan of IV and VI.)

Incidentally, I liked Star Trek Into Darkness.

Spacecraft Research is at it again

It’s been a little while since I checked in with the goings-on back at my Cornell research lab. Totally unsurprisingly, some very cool things are happening there!

One is that the Sprite and KickSat project has gone all the way from a back-of-the-envelope concept when I was at the lab to a flight manifest! Sprites are tiny spacecraft – think the size of a coin – that consist of little more than a solar cell, a little CPU, and a diminutive radio. They are pathfinders for an idea that, rather than relying on a single monolithic (and super-expensive) spacecraft, instead we could just run off a batch of a million tiny satellites and fling them all out into space to cooperatively complete a mission. Some of the applications we talked about included integrating basic lab-on-chip functionality to test for biomarkers, and then rain a bunch of the Sprites down onto Mars or Europa. They wouldn’t return the same wealth of data of a NASA flagship mission, but they would tell us where the interesting things are. Another reason why tiny spacecraft are cool is because they interact differently with Solar System objects than large vehicles do – so they might be able to take advantage of light, magnetism, or planetary atmospheres in different ways.

The KickSat project was the brainchild of grad student Zac Manchester. It’s a simple CubeSat design with a spring-loaded deployer, designed to release a couple hundred Sprites. On the ground, Zac can then track the intermittent radio signals from all these mini-spacecraft, and evaluate how well their unshielded components survive in space. Radiation will eventually kill them, but with many copies of the same spacecraft, we’d expect to see them die out statistically. They’re spacecraft with a half-life, and as long as the half-life is long enough to complete the mission, we don’t care that a huge number of Sprites burned out.

When I left the lab, Zac was applying for grants to build the KickSat hardware. But – despite the cool concept – there weren’t any takers. Eventually, he decided to turn to KickStarter to see if he could crowd-fund some spacecraft research. He ended up raising almost three and a half times as much money as he asked for, and become something of a pioneer for crowdfunded space activities! Zac is now working at Ames Research Center to perfect the Sprite and KickSat designs. They will be launching on the same SpaceX Falcon 9 rocket that will carry supplies to the International Space Station in September. This is actually the first CubeSat from my lab to make it all the way to launch, so I say: Go Zac!

Second, a project that is perhaps a little less flashy but a little closer to my heart has been making some great strides. Ben Reinhardt has been squirreled away in the same basement lab I remember, working on what he calls “eddy-current actuators.” The more fanciful – and very nearly accurate – name for the devices he is working on would be “tractor beams.” He wants to use these to grab onto defunct satellites, the outside of the Space Station, or maybe even some asteroids and comets, all without mechanical contact.

I was still active in the lab when this project got off the ground. In fact, I put together one of our first tabletop demonstrations of the principles involved: a changing magnetic field generates eddy currents in conductive materials; these currents have their own magnetic fields which we can push or pull with magnets. That’s where I left the project, though…a quick video where I waved a magnet around, some rough number-crunching to show that the induced forces were feasible for applications, and then I was out to let other members of the lab hash out the details. (That’s the fifth-year grad student for you!)

The cool news is that Ben has gone from my rough video to a much more carefully controlled demonstration. He’s generated attractive and repulsive forces in a bare piece of aluminum (not unlike the skin of a spacecraft), without touching it, and he’s working on characterizing the design space of his device. This is a critical step in figuring out how to go from proof of concept to a useful technology, and it’s a step I remember quite well. While Ben’s twitching pendulum might not look to you like the tractor beams from Star Trek, it is a clear and measurable experiment illustrating the device. I went from similar experiments in my first two-ish years of grad school to flight demonstrations in my third and fourth; I hope Ben follows a similar trajectory. And who knows – if some companies or space agencies take an interest, we may soon see spacecraft grappling asteroids and assembling components with eddy-current tugs!

Ben and some of the other Cornell Space System Design Studio grad students are keeping a blog about their technology research projects, which you can read here. I think it’s very cool to see what’s going on in the lab!


Way back when I was looking at grad schools, I visited an MIT space propulsion lab where students and faculty were developing something called an electrospray thruster. This is a device consisting of a plate covered in tiny spikes, with a tiny grid layered on top. You feed an ionic liquid onto the plate, where surface tension wicks it up to the tips of the spikes. (Ionic liquids – and this kind of boggled my mind when I first learned about them – are salts that are in their liquid state. They’re just a bunch of sloshing positive and negative ions. Wild!) Then, you apply a voltage to the grid sitting above the spikes. The potential difference between the spikes and the grid yanks ions up and hurls them out through the holes in the grid, and voila – ion thruster.

The MIT Space Propulsion Lab has been developing these as little patch thrusters that they can put on CubeSats. The thrusters are 1×1 cm patches and seem to generate forces in the range of ten or so micronewtons. (That would be, say, 1% of the weight of a postage stamp.) These are very small forces, but we are talking about very small satellites and we can leave the thrusters on for a very long time.

The idea that stuck in my head when I learned about these devices, though, is that they are mechanically very simple: all we have to do is texture a surface appropriately, touch the ionic liquid to it, and energize part of it. We could probably develop a fabrication method to print the thruster “texture” onto a flexible membrane or fabric of some kind.

And then we could deploy it like a sail.

10 micronewtons from a 1×1 cm thruster gives a thrust density of 0.1 N/m^2. So a 1×1 meter sail would produce a thrust force of about a tenth of a newton. On a standard 3U CubeSat, this corresponds to an acceleration of 3.4 milligees – which is actually getting up to the acceleration regime of the chemical thrusters on large spacecraft! With such acceleration, it would take five minutes for the CubeSat to add one meter per second to its velocity. Starting from low Earth orbit, this miniature sailing vessel would need only twenty minutes to hit Earth escape velocity!

3U CubeSat with 1 meter "pusher" sail
3U CubeSat with 1 meter sail

What probably makes the most sense from a propulsion perspective is to deploy the membrane engine behind the spacecraft, maximizing the engine area and minimizing any adverse effects of the ion exhaust. (All those high-energy ions might eat away at the spacecraft’s solar cells or other surfaces!) However, there might be some challenges in running the ionic liquid down to the sail.

A good compromise would be a sail mounted to the back or middle of the CubeSat – think of  the NanoSail-D configuration – where a reservoir of ionic liquid could supply a steady stream of propellant to the membrane and most of the zooming ions will miss the back of the spacecraft. The forward-facing part of the membrane might also be usable area, for things like solar cells. Or CCDs.

Ion engines caused a shift in the way spacecraft engineers thought about propulsion: instead of brief, impulsive maneuvers, they could use a gentle but steady acceleration for a long period of time. The ability to spread an ion engine over a large area might be a way to create a high-efficiency thruster that also produces a large force, and with few moving or complex parts. That’s the kind of device we might use to send a small spacecraft to the outer Solar System. Of course, we’d need a lot of electrical power, but that’s why the DOE is starting plutonium refining again…