Gram-scale StarChip components | Battery

Battery design is one of the most challenging aspects of the mission. Currently under consideration for the energy source onboard are plutonium-238, which is in common use, or Americium-241. 150mg has been allocated for the mass of the battery. This includes the mass of the radioisotope and the ultra-capacitor. As the isotope decays it will charge the ultra-capacitor. Then, at the appropriate time, the StarChip components will be switched on and begin to operate. The power budget is informed by the available energy in the battery. An innovative approach could take advantage of the heating of the frontal surface of the nanocraft through its interaction with the interstellar medium. The heat supply, at a rate of 6mW per cm2, could provide a thermoelectric energy source during the interstellar cruising phase.

It may be possible to coat the lightsail with a thin film of photovoltaic material, which was demonstrated on the IKAROS mission. This could be extremely useful during approach to the host star. The photovoltaics will be able to supply significant energy when they are within 2AU of the target star. Even with just 10% efficient photovoltaics, the energy supplied would be nearly 2kW at 1AU. This is more than 100,000 times the power of the radioactive energy source, and could conceivably allow much higher data rates for laser communication. This is one option that will be explored.

Coating the StarChip itself with PV would allow for high efficiency, and potentially gain several Watts just from the StarChip. These options open up a host of possibilities to greatly enhance functionality at the host star in the crucial data and imaging phases.

Apr 13, 2016 00:54 Posted on: Breakthrough Initiatives

To charge a battery after 20 years of sailing, another idea is to have a reactive metal powder mixture (e.g. Ni+Al) or stacked films, which is reacted (e.g. to NiAl) on arrival, the heat being turned into electricity by the same type of thermoelectric generator needed for a Pu or Am battery. It produces a strong burst of power just when needed. Also, no toxicity/radioactivity which requires a casing because of launch requirements.
See work by Prof. Weihs at JHU

Apr 13, 2016 06:53 Ryan Whitchurch Posted on: Breakthrough Initiatives

Is there hope of using all optical circuits? In interstellar space, I would imagine that electrons will be hard to come by (basically pack your own for the trip), but photons will be plentiful - if they can be harvested and directly used in the spacecraft circuits, wouldn't that be preferred to electron pumps (RTGs and photovoltaics)? Earth can even send photons of a preferred wavelength to the spacecraft using the ground based laser array.

I was intrigued by this:

Apr 13, 2016 18:15 Posted on: Breakthrough Initiatives

Why don't you have considered Strontium-90?

It is cheaper that Pu-238 and Am-241, it can be recovered from exhausted nuclear fuel, and it releases a reasonable amount of heat without affecting too much the surrounding with dangerous emissions.

The 28 years of half-life should cover your need for a nanoship lasting 20 years to reach Apha Centauri.

You can cover one side of your chip with a sheet of Strontium-90 Titanate. 150 mg supplies approximately 38 mW of energy in form of heat, so you can cover up to 6 cm2 of chip.

And, moreover, SrTiO3 is transparent, and I am pretty sure that's a feature you're interested in, especially if you intend to take advantage of a photovoltaic layer. You can conveniently sandwich it between the SrTiO3 glass and the chip: a double energy supply at once!

Raffaele Ricco
University Project Assistant
Institute of Physical and Theoretical Chemistry
Graz University of Technology

Apr 13, 2016 18:51 Karen Pease Posted on: Breakthrough Initiatives

@raffaele: Cost of the radioisotope is irrelevant here - we're dealing with miniscule quantities. Power density over the lifespan of the mission is everything.

Concerning the task at hand: a 150mg ultracapacitor would only power a 1W diode laser for a few seconds according to my calculations. Is that enough to get a statistically significant photon count back on Earth?

Have you considered a nuclear-pumped laser driven by your radiothermal power source?

You'd still have ample decay heat available to you to power everything else. Plus, it opens up a much wider range of possible frequencies to choose from.

As for the photovoltaics-on-the-sail idea, let's just say I have my doubts concerning:

A) Your ability to not ruin your mass budget, and
B) Its ability to withstand the heating loads in the initial acceleration phase.

Apr 13, 2016 20:00 Karen Pease Posted on: Breakthrough Initiatives

Oh hey... I think I've found your fuel source: 232U. It's not normally used in RTGs because it gives off hazardous gamma, making handling difficult... but has been discussed, only tiny amounts are needed for a craft like this, so it's not a major hazard like it would be for a full-sized RTG.

By my calculations 238Pu yields 0,57W/g year 1 and declines from there, down to 0,26W/g after 100 years. 232U is at 0,70W/g year 1, peaks at 4,8W/g year 15, and is down to 2,1W/g after 100 years. It's *far* more energy and power dense. 238Pu basically stops after the first decay down to 234U, but 232U decays to 228Th to 224Ra to 220Rn to 216Po to 212Pb to 212Bi to 212Po to 208Pb. And by far most of that energy is alpha (same as 238Pu), aka quite captureable.

232U is quite produceable - both from 233U(n, 2n) and 230Th(n,gamma). Indeed, it's a common contaminant in the thorium fuel cycle.

You know, this is almost too perfect. Hmm, does anyone have access to this paper?

I'm trying to see what percent of the alpha energy could realistically be converted to laser power output. You know, if you have half a gram of radioisotope putting out over 2W of alpha at the time of the flyby getting a 50% laser efficiency then you've got a 1W laser for communication and maybe 100 mW to run your probe - no battery needed. How perfect is that? :)

Apr 14, 2016 07:56 J Radtke Posted on: Breakthrough Initiatives

Paper regarding the alpha pumped laser is from the Brazilian Journal of Physics, V27, #2, p 129 (1997).
It can be accessed at:

Apr 14, 2016 13:19 Karen Pease Posted on: Breakthrough Initiatives

Thanks for that, J Radtke. Unfortunately they give no power output figures (either calculated or real-world), and the information given lacks several salient characteristics (e.g. they give tube diameter and thickness but not length) to estimate the amount of radioactive material in their design, the total system mass, or the input power.

I can say, looking at their diagrams, that their efficiency isn't going to be spectacular. Given their geometry, most of their alphas aren't going to go into the lasing medium. It looks like an inverted approach would be more efficient, with the emitter source in one or more rods/wires inside the lasing medium.

Their talk about threshold emission levels does raise some good points, however.

Apr 14, 2016 15:24 Vincenzo Romano Posted on: Breakthrough Initiatives

Wouldn't tangled particles (photons?) allow for more energy efficient communication?

There'd be no need for "beaming" them back to (or from) Earth in order to keep the communication up and running.

Apr 15, 2016 21:02 Brian Rauchfuss Posted on: Breakthrough Initiatives

Re: tangled particles - Quantum entangled particles have correlations, but can't be used for communication (assuming you are referring to things like Bell's theorem; obviously you can just send entangled particles back and forth for communication). There is also the practical problem of how you would store entangled particles isolated for 20 years.

Re: U-232 - Gamma radiation is an issue because there is going to be electronic circuitry nearby for 20 years which will be degraded by radiation. Gamma radiation is much harder to shield from than the alpha radiation from P-238. Though this raises the question of how long the circuitry will last under the radiation that always occurs close to stars and in interstellar space. The circuitry will need a lot of redundancy built in anyways to overcome this.

Apr 16, 2016 16:21 Karen Pease Posted on: Breakthrough Initiatives

@Brian: Yes, there will be gamma nearby, but it'll be a tiny flux due to the fraction-of-a-gram amount of fuel. A spacecraft, whether it's 1 gram or half a tonne, doesn't have the mass budget for shielding gamma. So it makes a huge difference whether you're carrying 2kg of gamma emitter onboard or half a gram.

Of course, proximity always matters. Flux drops off proportionally to the square of the distance.

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