Delivery and launch dates
LightSail 2 and its Georgia Tech partner spacecraft, Prox-1, are still manifested to hitch a ride on the first operational flight of SpaceX’s Falcon Heavy rocket during the launch of a U.S. Air force payload named STP-2 (STP stands for "Space Test Program”). A demo flight of the rocket is expected before that.
The Falcon Heavy debut date, however, has been in question following the pad explosion of a Falcon 9 on September 1. Because the Falcon Heavy is essentially three Falcon 9 rockets strapped together, SpaceX must first focus on returning the Falcon 9 to flight. Also, the Falcon 9 represents the core of the company’s business; only six of SpaceX’s 71 publicly manifested missions call for the Falcon Heavy.
Last week, at the 67th International Astronautical Congress in Guadalajara, Mexico, CEO Elon Musk said the Falcon Heavy would probably debut "early next year." According to Space News, President and COO Gwynne Shotwell recently amended that date to mid-2017. LightSail’s STP-2 launch, then, would presumably occur sometime after that.
Right now, Prox-1 and LightSail 2 are scheduled to be delivered to the University Nanosat Program before the end of 2016. The two spacecraft will meet up for integration at the Air Force Research Laboratory in Albuquerque, New Mexico, and we’ll be sure to capture the big moment.
Once again, we’re turning to 3D animation guru Josh Spradling to visualize our entire mission. Josh created our LightSail 1 mission trailer. This time, he will add Prox-1 to the mix, and we’ve also tasked him with conceptualizing the complex solar sail tacking maneuvers LightSail 2 will use to raise its orbit.
Never a person to make things easy on himself, Josh is also improving the CubeSat assembly sequence—where the spacecraft’s parts fly together—using actual CAD models. You’ll be able to watch LightSail 2 come together screw by screw, which, as far as I know, is unprecedented. We may also add virtual reality capabilities to the mix, so stay tuned!
Speaking of screw-by-screw assembly, the LightSail 2 flight unit has been assembled for quite some time. It also passed its vibration and thermal vacuum chamber trials last month, which certify it can survive launch and the harsh space environment. One final end-to-end test of the spacecraft’s critical functions, including sail boom deployment (without the actual sails attached) is is still planned before delivery.
In the meantime, the LightSail team, led by engineers at Ecliptic Enterprises Corporation in Pasadena, has been putting the software system through its paces. In particular, there has been a lot of focus on the attitude control subsystem, which is responsible for tacking the spacecraft back and forth through the sun’s photon stream.
This extra testing continues to pay off. As an example, the team recently discovered a glitch on BenchSat, the flight unit’s acrylic-mounted test unit. Because BenchSat does not contain a full suite of flight sensors, some odd behavior was recently discovered when the flight software tried to access a sensor that wasn’t there.
No big deal, right? Actually, the glitch got the team thinking about how the software might behave if one of LightSail’s actual sensors failed during the mission. New code is being added to account for this potential scenario, and more tests will validate the changes.
LightSail in the literature
Lastly, LightSail will be featured in a few papers and presentations at the fourth International Symposium on Solar Sailing this January in Kyoto, Japan. One presentation by systems engineer Barbara Plante focuses entirely on the LightSail 2 attitude control system. Barbara has allowed me to post the abstract below; check it out!
After the successful launch and operation of the LightSail 1 (LS-1) mission in May/June 2015, the LightSail team convened in Dec 2015 at the Planetary Society headquarters in Pasadena, CA to conduct a full mission review and refine objectives for the next mission, LightSail 2. The team identified hardware and software changes to the spacecraft baseline, as well as discussing enhancements to the ADCS subsystem. Unlike LS-1, which flew in an elliptical orbit reaching under 400km at a minimum, LS-2 will fly in a circular orbit above 700 km, a more beneficial sailing environment.
One of the sailing methods considered and modeled was the detection of orbital inclination change due to solar radiation pressure (SRP) as presented in the work by Stolbunov, et al. This option was ultimately rejected since a.) the spacecraft would not be in a truly SRP dominated orbit and b.) modeling results showed that the 32 square meter sail size would not allow an observable inclination change over the anticipated mission duration. An apoapsis raising method using the On/Off switching method presented by McInnes was proposed, which would increase the semi-major axis of the orbit and could be readily measured. “On” has the solar sail normal parallel to the sun vector while “Off“ has the sail normal parallel to the sun vector, in the process gaining orbital energy. Sail orientation would require a 90-degree slew maneuver twice an orbit. Encouraging modeling results for the apoapsis raising method drove the decision to use it for the solar sailing demonstration mission.
Specifics of the hardware and software used to control solar sail deployment, on-orbit imaging, and other aspects of the mission have been the subject of papers by the LightSail team. With respect to ADCS, spacecraft attitude is determined using 4 magnetometers, 5 solar angle sensors, one 3-axis gyro which acts as the primary angular rate sensor, and one 3-axis gyro as the backup. Attitude control is accomplished using 3 variable dipole magnetic torque rods and 1 60mNm-s reaction wheel. Results from the testing and characterization of the ADCS hardware are presented and how these results might affect orbit-raising performance. ADCS flight software architecture and design is described, including operational modes, timing considerations, and fault detection strategies. System and long-duration testing of the integrated LightSail FSW on the flight vehicle, exercising the entire mission CONOPS, serves to mitigate risks and set the stage for mission readiness.
Sail On, LightSail 2!
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