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Rulers in space! LightSail 2 equipped with boom fiducials

Posted by Jason Davis

14-03-2016 6:01 CDT

Topics: LightSail

Last year, we were both thrilled and surprised when our first and only sails-out picture from LightSail 1 came back from space. Thrilled, because look at that!—a decades-long dream of The Planetary Society was fulfilled, with more excitement yet to come. But we were also surprised, because the sail didn't quite look the way it did during our deployment tests. In space, LightSail 1 wasn't taut like a kite in a summer breeze; rather, it was a little floppy, possibly bending upward in the camera frame. As a result, the obvious question was: Were the sail booms fully extended?

There was no way to tell. We couldn't use image processing to accurately "flatten" the sail and compare it with testing photos because we didn't know how much the sail was flexing when the picture was taken. We also didn't get a chance to download more pictures or try extending the booms farther. A radio problem prevented further communication with the spacecraft, and LightSail 1 reentered the atmosphere about a week later.

LightSail 1 ground test vs. in-space images
LightSail 1 ground test vs. in-space images

The Planetary Society

LightSail 1 ground test vs. in-space images
These two images were captured with LightSail 1's onboard camera. The left image was taken during ground testing, and the right image was taken in space after sail deployment.

For LightSail 2, we want to do better. First of all, the spacecraft's software will now store the number of motor "counts" recorded during deployment. These counts are a dimensionless number that increment as the motor spins, until the they reach a pre-defined value. But that's only one data point. What we really need is an in-space ruler to validate the booms are all the way out. 

That's where fiducials come in. In imaging, fiducials are visual marks used as a point of reference. When you see a photo of a small insect sitting next to a ruler, that's a fiducial in action. Recently, the LightSail team added fiducials to the spacecraft's sail booms. These markings will help us determine how far out the sails are.

And they look pretty sweet! Ecliptic Enterprises Corporation included our logo for good measure. Take a look:

LightSail 2 boom fiducial with Planetary logo

Ecliptic Enterprises Corporation / The Planetary Society

LightSail 2 boom fiducial with Planetary logo
Each of LightSail 2's four booms is marked with a fiducial featuring The Planetary Society's logo. These four fiducials are completely visible only when the booms are fully deployed.

You're looking at the side of a sail boom marked with white epoxy ink to provide extra contrast that will be (relatively) easy to spot with the onboard cameras. I say "relatively" because our cameras have fairly low resolution, and there are a lot of extreme colors in our images: the blackness of space, the silver sail, the gold booms, and possibly, the Earth or sun.

The five tick marks don't signify anything in particular—in total, they're about an inch wide, lengthwise, on the booms. Stephanie Wong of Ecliptic tells me the team chose the tick mark design over a single, inch-long ink block to reduce the likelihood the ink would peel off in one big chunk after being wound so tightly inside the spacecraft. 

The team added six fiducials in all. There are two fish-eye cameras that sit on opposite sides of the spacecraft. When the sail is deployed, each camera sees one boom more or less in the center of the frame, with one or more adjacent booms highly distorted near the edge of the image. That means our best chance of seeing any faraway fiducials comes only on the two booms that align with the cameras. 

So, every boom has a fiducial with a Planetary logo that completely shows when the booms are fully deployed. And on the two booms that align with the cameras, there's a second fiducial at the halfway mark, which looks like this:

LightSail 2 boom fiducial (halfway mark, no Planetary logo)

Ecliptic Enterprises Corporationn / The Planetary Society

LightSail 2 boom fiducial (halfway mark, no Planetary logo)
This LightSail 2 boom fiducial is positioned approximately 2 meters from the spacecraft on the X-axis, at the halfway mark of the boom's total, 4-meter length.

The extra set of tick marks and lack of logo differentiates these two fiducials from the other four, and makes them easier to spot at a distance.

You may be wondering why the team didn't add even more fiducials. The main reason is that the ink adds thickness. The booms are 4 mils thick (a mil is one thousandth of an inch), but the epoxy adds up to 25 mils extra in some spots—a seven-fold increase. And the four, 4-meter-long booms must wrap into a coil less than 4 inches wide, there isn't a lot of room to spare.

Here's Ecliptic's final fiducial schematic, and a corresponding table:

LightSail 2 fiducials schematic

Ecliptic Enterprises Corporation / The Planetary Society

LightSail 2 fiducials schematic
This schematic shows the locations of all six LightSail 2 boom fiducials.

Axis Fiducial Distance to spacecraft Boom thickness Boom thickness with fiducial at thickest point
-X Secondary 75 in. 4 mil (4/1000 of an inch) 25 mil
-X Primary 7 in. 4 mil 24 mil
+Y Single 15 in. 4 mil 29 mil
-Y Single 15 in. 4 mil 26 mil
+X  Primary  7 in.  4 mil 26 mil
+X  Secondary  75 in.  4 mil 19 mil

Here's what all that looks like from LightSail 2's onboard cameras. Can you spot the fiducials?

LightSail 2 boom fiducial test image, +X axis

The Planetary Society

LightSail 2 boom fiducial test image, +X axis
LightSail 2 onboard camera fiducial test, -X axis

The Planetary Society

LightSail 2 onboard camera fiducial test, -X axis

I'd be remiss if I didn't point out that some of the team members can be seen in that minus-X axis view. In the blue lab coats, from left: Stephanie Wong and Riki Munakata, and sitting in the distance, from left: Alex Diaz, John Bellardo (sporting his signature cowboy hat) and Justin Foley. Great job, everyone!

See other posts from March 2016


Or read more blog entries about: LightSail


Tomas OK2PNQ: 03/14/2016 07:44 CDT

You are pointing in the text to the thickness of the boom in milimeters (four milimeters, 25 milimeters), whereas in the table it is written in the mils (1/1000 of inch), which looks more probably.

Bob Ware: 03/19/2016 12:16 CDT

Thanks Jason for a very informative update on LightSail 2. Now if we could have a Light Sail 3 flyby Flight Test in front of Luna to demonstrate flight characteristics that would be a bonus for us too. We could measure (via delta V changes in relation to solar wind to expected gravitational pull) the MASCON effective range on this type of spacecraft. As for a bonus for us with this success we could have a sale of design use during our Patent life and place the sale funds into our current General Fund for that years use. To keep our sales at maximum we would need to have post mission lease of spacecraft restrictions on the owner until the end of the Patent life or new contract time frames with collected funds by a cut off point to meet IRS regulations for non-profits. The business & tax people would have to sort that out. As for a ride, well, another issue for the same day for the customer. We just provide the spacecraft.

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