Crowdfunded spacecraft LightSail 2 prepares to go sailing on sunlight | Hacker News Reader

Crowdfunded spacecraft LightSail 2 prepares to go sailing on sunlight

techcrunch.com

87 pointskouh22 comments

Note that this is an orbital test and not the actual 'launch' to alpha centauri.

Several small spacecraft with solar sails project Breakthrough Starshot. Some solar energy will provide a push on the sails but the main push is from powerful ground-based lasers since solar energy alone isn't enough energy.

At 20% speed of light (c) or about 60,000 kilometres per second it will take 20 years to get there. Then 4 years for the radio signals to get back.

I may see some pictures of Alpha Centauri in my lifetime if they get this project going soon.

Will this thing actually accelerate?

The last project was in such a low orbit that acceleration was never possible. At best it could have managed a measurable decrease in the rate of orbital decay. The light pressure was less than the atmospheric drag. As drag is directly related to sail area, and is constant whereas sunlight isn't, is acceleration possible even in a 700+km orbit?

Drag is not constant, it depends on which way the sail is pointing. One of the things they're testing is constantly swiveling the thing to maximize thrust and minimize drag.

The ISS maneuvers constantly to reduce drag.

Crowdfunded spacecraft are definitely a cool addition to our space exploration program.

Thought it would be fun to receive the downlinks from Lightsail 2 with an SDR. Does anyone have any info on frequencies/modulation modes being used, above and beyond the Morse code beacon [1] at UHF?

[1] http://www.planetary.org/blogs/jason-davis/2016/20160609-lig...

The FCC license registration [0] contains some interesting information.

- Experimental radio service call sign WM9XPA

- Using an Icom IC-821C Transceiver [1], a twenty-something year old VHF/UHF ham radio...wow for a $7M project couldn't they have spent more on a radio? Well, maybe not, I'll get to that in a minute.

- Ground station is in the middle of nowhere? I think something with the Lat-Lon might be off if they mention West Lafayette, IN as the city (Purdue is located here).

- The satellite's datalink is FSK on 437.025 MHz, designation 15K0F1D, which is a standard 15kHz wide FM modulated FSK signal. The signal's parameters are not known from the registration but I googled some more and found their FAQ which links to their protocol description [2]. In a nutshell it's run-of-the-mill AX.25 at 9600baud using a simple encoding for various parameters. You could decode this easily with an SDR and Direwolf [3] or GNURadio. This is why they can use the low-cost ham radio. I'm also willing to bet they'll be leveraging submissions from amateur radio operators and the automated SATNOGS network [4] to harvest more data.

[0] https://fcc.report/ELS/The-Planetary-Society/1029-EX-ST-2019

[1] http://www.rigpix.com/icom/ic821h.htm

[2] https://planetary.s3.amazonaws.com/projects/lightsail/manual...

[3] https://github.com/wb2osz/direwolf/blob/dev/doc/README.md

[4] https://satnogs.org/

Great info, thank you! I'll see if I can rig some of this up in GNU Radio. Thanks again!

PS: Casual trivia. I used to own that Icom model radio ages ago, gave it to my father who used it for SWL.

I'd love to play around with the math, but am extremely ill-equipped to answer my XKCD What If?-Style Question, so I'll just ask it here.

If i'd ignore the time necessary to accomplish my goal, could I use such a craft to steer an asteroid to earth?

I'm imagining to balance the minuscule gravity a craft like the LightSail 2 would effect on an asteroid with the propelling force of the lightsail over a few centuries to slowly but steadily tug the asteroid on a collision course.

When looking at the cost of the LightSail 2 I'd imagine this to be the most cost effective method of putting a deadline on humanity for the enterprising villain on the lookout for a bargain.

That could work if your villain is on a budget, but a time frame of a few centuries is MUCH too optimistic. It would expect diverting even a small asteroid to take several million years, let alone one of the really dangerous ones.

Making the sail (much) bigger would help, but then you have additional problems because people would see it coming and could take countermeasures.

In general, the answer to "If I ignore how physics works, can I XXXX" is "yes", because what you've actually asked is "can I do XXXX with magic".

If your math is good enough, could you just "paint" the surface of a suitable object enough to steer it via differences of light pressure?

Interestingly, this is one of the ways we could deflect an asteroid on a collision course with the Earth, if we detect it early enough.

Yes, some deep space missions need to take solar radiation pressure into account due to effects like this.

Not really. The differences in pressure would create spin, not push the craft in a different direction. You would need something else to push against, to "tack" against the solar wind, like how a sailboat's keel pushed against the water. Magnetic fields are an option, but those get less and less as you move away from the sun.

There are some complicated options involving mirrors that could move a craft in other directions, but they are radically difficult to manage. You send five mirrors with the craft as a formation, one on each side and one ahead. Then you can bounce the lasers off these mirrors to steer the ship. At the destination, the ahead mirror is used to slow the craft. All these mirrors would essentially be separate craft that would need to be themselves accelerated and managed.

If you use a material on the back of the asteroid that had a higher albedo at more oblique angles, you could have a form of feedback.

So we’re living at the beginning of the second Age of Sail?

Would like to learn more about "Age of Sail"

British Empire

I would believe it's a tie to exploration.

Quite literally "Age of Sail"

https://en.wikipedia.org/wiki/Age_of_Sail

Can you rely on sunlight between star systems?

No. In fact, the solar irradiance gets fairly low even by the time you reach Saturn, because the power propogates via an inverse square law. Near Earth, which is 1 Astronomical Unit (AU) away from the Sun, you get about 1000W/m^2. Around Saturn, which is about 10 AU away, irradiance will be down to 1000/10^2 = 10W/m^2. So 1% of what you get near Earth.

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