# How could the laser from Earth to a spacecraft be used other than to propel its light sail?

The Starshot Breakthrough Initiative, sponsored by Yuri Milner, now finances the attempt to use a huge laser on Earth to propell a tiny spacecraft to huge velocity. Not having to carry the energy for its own propulsion, the rocket equation is bypassed. But could such an energy beam from Earth also solve some other of the huge challenges of interstellar travel?

• Power. Could a spacecraft transform the energy beam to electric power for use onboard?

• Communication. Could the energy beam be modulated to communicate data to the spacecraft and be reflected in a modulating way by the spacecraft's sail to send data back?

• Impact prevention. Could parts of the energy beam be focused by parts of the sail to first detect impacting dust grains ahead from their reflections, and the refocused to evaporate them?

• Beam reinforcement. Could a convoy of similar sails (without payload) be launched regularly to refocus the energy beam to the next sail ahead?

• Deceleration. Could such a convoy or the spacecraft itself, use the energy beam to decellerate the flyby speed?

• Teleacope. Could the sail be reconfigured to work as a teleacopic mirror to observe the target star system as it approaches?

Could multiple such functions be performed by a single low mass large area thin sail/photovoltaic/reflector/lens component? I understand that the TRL for this is low, but basic physical challenges and opportunities maybe can be identified anyway.

• Funny, I don't see the edit option anymore. I'm on a mobile unit. I wanted to add: -Beam reinforcement. Could a convoy of similar sails (without payload) be launched regularly to refocus the energy beam to the next sail ahead? -Deceleration. Could such a convoy or the spacecraft itself, use the energy beam to decellerate the flyby speed? – LocalFluff Apr 20 '16 at 16:31
• Wouldn't re-purposing part of the laser for some other purpose lower the amount of force being exerted on the sail because you're now absorbing a portion of it? – Ellesedil Apr 20 '16 at 21:26
• To vaporize a small town. – Mark Adler Apr 21 '16 at 5:58

Power. Could a spacecraft transform the energy beam to electric power for use onboard?

Yes, a spacecraft can definitely turn a steady flux of light into electrical energy. A device that does exactly that is pictured below:

Solar are more effective at some wavelengths than others, so the monochromatic light of a laser is especially useful for energy transmission.

Communication. Could the energy beam be modulated to communicate data to the spacecraft and be reflected in a modulating way by the spacecraft's sail to send data back?

At large distances, the targeting of the spacecraft is mostly about hitting it with at least a part of your light cone (yes, lasers diverge too). You then have the opportunity to use single-blinking-light communication, such as Morse code or plain binary bits. Variations in the intensity of the beam can also be used to transfer the information in a analogue manner. There are limitations to this form of communications though. Big lasers are not easy to just turn on and off quickly, and an occultation disc is going to experience the most serious overheating problems ever heard of. A better solution might be to slightly tilt the beam a tiny bit back and forth, but getting it pointed right on target again is perhaps difficult. In short, it can be used for communications, but the bandwidth is surely limited.

Impact prevention. Could parts of the energy beam be focused by parts of the sail to first detect impacting dust grains ahead from their reflections, and the refocused to evaporate them?

I am more in doubt about this one, first of all, looking ahead is simply not enough. This situation, for example, is not going to end well:

As velocities in space are relative, impacting objects may come from every direction. The difficulties with this method is more in the ways of actually being able to discover very small objects at a fairly large distance, and having the ability to illuminate them is not helping that much.

As for combining multiple functions into one, say sail/photovoltaic, I can not see many situations where this is useful. The reason here is the enormous difference in pushing force and energy content of the beam. $1N$ to $150,000,000W$. That means that the area you are going to need for power production is far smaller than the propulsive part of the sail. That is convenient as well, as photovoltaics are much thicker and heavier than the currently used sail materials. Other things, like sail/reflector is much more reasonable, as the sail is a reflector. Throwing optical lensing properties into the mix should certainly possible as well, at the cost of more structural mass for calibration and accuracy.

• Does the sail have to heat up importantly from being pushed by a laser beam? What if a proton beam is used instead, hitting not a physical sail but a magnetic field? Or a beam of several different kinds of particles and wavelengths. And the spread out energy beam would light a cone around and in front of the spacecraft, hitting any incoming particles. – LocalFluff Apr 21 '16 at 0:57
• I've imagined solar sails that have veins and pores that are pressurized by some compressed gas (like argon) on board the payload. The gas slowly covers the sail with atoms of the gas that stick to the sail until energy from the laser cause them to 'pop' off the sail, giving it a tiny bit of thrust. – Howard Miller Apr 21 '16 at 1:04
• This doesn't answer every question the OP had. However, there were quite a few questions... – Steve Apr 21 '16 at 1:58