The Solar System: a proving ground for interstellar spacecraft

The above picture illustrates a small spacecraft attached to a solar sail, as depicted by By Kevin Gill from Nashua, NH, United States ( Solar Sail, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=42599221 ). Such Solar/Laser sails are the only technology currently proposed for travel to other solar systems in a reasonable amount of time and, as of yet, not practicable except on paper.  Of course, such "starchips," as they are known, would have to be much smaller than the spacecraft pictured above and would travel in larger numbers ("think smaller, think way more legs"). Hugh Podmore and I wondered what the advent of such a technology might mean for planetary science within our own solar system.

Late last year I finally published a fun paper that myself and Hugh Podmore, one of Prof. Regina Lee's PhD students, wrote up in the summer of 2017. It was a pure flight of intellectual fancy for which we sought and received no funding. What was the subject, you may ask: what else could you do with the starchip spacecraft that the breakthrough star-shot initiative is working towards in order to advance our knowledge of our own solar system? While some of the possibilities had been detailed in the original paper by Prof. Lubin, his ideas for how the interstellar laser array could be used to explore the solar system go big whereas we wanted to know what could be bought in terms of time and cost for going small instead.

It seems that there is a quite a bit that one could do. But what surprised us most was how quickly such spacecraft could be of use to planetary science, perhaps as early as the mid 2020s, allowing early starchip prototypes to be tested out while doing valuable science.  They could also be a democratizing force by allowing smaller space actors (such as Canada) to participate in a way that hadn't been previously imagined, due to the  relatively low cost of laser propulsion once the pushing station is set up. Finally, if you stop to think about it, there really are a lot of great uses for small, fast spacecraft. In our solar system there are things that are fiendishly difficult to do with a discovery-class NASA mission or that simply don't justify that classification's $500 million price tag. I'll get into all these interesting possibilities just below the cut.

But first things first, what is a starchip? The short story is that it is a minimalist spacecraft with each of its systems combined together onto a single board or "chip." Right now, cubesats (10 cm x 10cm x 10 cm spacecraft for LEO) approximate this ideal in the 1kg range or so and 300g appears to be easily achievable. Unfortunately, the physics of interstellar flight require a chipsat in the sub-gram range, which is not currently possible, but may be within a few decades.

But do we really need to push that hard to get something useful for our own solar system? Hugh and I found that the answer to that question is a  resounding "No." Even 100g single-instrument spacecraft are incredibly useful. Furthermore, while practical interstellar spacecraft need to be propelled to 20% of the speed of light (about 60,000 km/s), planetary spacecraft need to be accelerated only to a few 10s of km/s in order to be useful. That means that you can bring down the power of your laser array by orders of magnitude from the 100 GW Dr. Evil-esque superweapon/Alien signaling device described by Lubin. Indeed, our calculations show you could get by with only a few MW to a few 10s of MW of beamed power. Making life even easier: current adaptive optics are sufficiently advanced to give the right size of beam for this application, as opposed to the extremely fine control needed for interstellar flight. 

All things considered, the technology is remarkably close to being able to do the job!

And what kinds of jobs could you consider? Well, what about completing our picture of the solar system by visiting every object larger than a few 10s of km in size? How about getting a seasonal picture of how the gas giant planets evolve through multiple flybys? How about dispatching an in-situ sampler to the plumes of Europa or Enceladus only once they are seen to erupt via telescope? Astronomers will love the idea of setting up viewing geometries with baselines of multiple AU on a side. Truly anything that benefits from such "distributed architectures" could be tried, a concept not currently available in large single launches. Or, my favourite as an atmospheric scientist, how about sending small landers to planets with atmospheres, such as Mars or Jupiter - a solar/laser sail makes an excellent ablative entry device!

It turns out that Canada, in particular, is very well placed to participate in this kind of planetary exploration. As a polar nation, we have the right real estate for what Hugh and I call a multi-push architecture (in which you use laser power to raise the orbit of your spacecraft before you send it on its way), not to mention a particularly adept photonics industry, and a robust (if somewhat smaller sized) space agency. Plus, colleagues have already done the calculations and the testing and have found that there are remarkably good seeing conditions on Ellesmere Island (Mauna Kea-equivalent or better), a critical need if the laser beaming station is to be located on the ground.

One aspect of this work which really interests me, as a university prof, is the idea that individual chipsats could be designed by students. Because you could manufacture these in large numbers, the cost of each one should become very small very quickly, allowing lots of organizations that had not previously been involved in space to sponsor such a mission. It's a remarkable thing that university teams now regularly design cubesats to go to low earth orbit. But wouldn't it be something to design and fly a spacecraft to land on Mars or explore the atmosphere of Jupiter as part of an undergraduate project? Let your imagination run wild.