Firming Up the Spacecraft Design

(From the May/June 2010 issue of The Planetary Report)

The LightSail 1 spacecraft development is proceeding well. Our engineering team—led by Jim Cantrell—has completed the preliminary design and made critical decisions to select the hardware and subsystem for the final design—crucial milestones to building the vehicle that will demonstrate the value and potential of using sunlight alone to propel exploratory craft through space.

Thanks to you and your fellow Planetary Society Members, we are well under way with LightSail 1, the first of our planned series of three flights. The three missions will be progressively more ambitious, starting in Earth orbit and moving out into the solar system. One anonymous Member got the program off to a flying start with a donation of $1 million. From around the world, other Members have come through with matching donations, ranging from $5 to $100,000, that have allowed us to begin work on this ambitious project.

Design Reviews

In January, we completed the preliminary design review (PDR). A team of aerospace experts—including former Jet Propulsion Laboratory project managers Harris M. (Bud) Schurmeier, Glenn Cunningham, and Donna Shirley, as well as Aerospace Corporation’s David Bearden—evaluated progress to date. This review panel went over the development of the LightSail program, including mission requirements.

The panel members agreed that our self-imposed requirements might be too ambitious to meet given the available resources and suggested relaxing some capabilities until the second or third LightSail mission. After all, plans for our first spacecraft, LightSail 1, were pushing the limits of what can be accomplished with a four kilogram spacecraft while also introducing new capabilities such as an attitude control system, two radios, onboard imaging, a solar pressure sensor, and, of course, the deployable sail.

As you may remember, LightSail 1 is only the beginning of an innovative program. We will start with a craft launched to an orbit about 800 kilometers (500 miles) above Earth, beyond the first-order effects of the atmosphere, where we can test the sail’s performance in space. LightSail-2 will move farther away from Earth, and with LightSail-3 we hope to reach interplanetary space and test how solar sails could be used to monitor solar storms that may threaten technological civilization on Earth.

The project team has thoroughly reviewed the mission requirements and their implications for the spacecraft. As a result of the review, we've simplified several aspects of the design, such as the onboard cameras, going to a single-channel radio from a two-radio system, and reducing the redundancy among orbital velocity measurements. We can rely solely on ground observations for orbit determination, although I remain devoted to the idea of onboard acceleration data if we can accommodate the instrumentation.

The reviewers also suggested that we build more time into the schedule for testing and evaluation. Our 16-month schedule was built around three key elements: a limited budget, getting ready for a launch late in 2010, and using experience in the Cubesat and ground system. We agree that testing is key to a successful mission, and we’re committed to ensuring sufficient time in the schedule. The team continues to evaluate our existing mission risks, both to balance risks and to identify those we can reduce or work around.

We’ve had a stroke of luck on another front: colleagues have a high-sensitivity, commercially developed accelerometer from Lumedyne Technologies they want to test. With LightSail 1, we can supply a flight test, they get the data they need, and we get an advanced set of microminiaturized, high-performance accelerometers for our spacecraft. We’ve just signed an agreement with Millennium Space Systems to that end; the company will provide the software and special processing algorithms to utilize these accelerometers for the LightSail mission.

Now we're getting ready for the critical design review (CDR). After that, we'll freeze the design and begin building and testing the spacecraft. Following the review team’s recommendations, we decided to hold our CDR in June rather than in April. This will give us more time for the subsystem decisions and enable us to be more specific about our orbit and mission design.

Planning an Uncertain Launch

Our goal was to finish spacecraft development and be ready for launch by the end of 2010. LightSail 1 can ride on a variety of launch vehicles, but we must piggyback as a secondary payload on an existing mission. To fly completely free of atmospheric drag,

LightSail 1 requires a higher orbit than most secondary payload launches can reach. Opportunities exist, but they’re less frequent than other launches to low Earth orbit. It now appears that our earliest opportunity will come no sooner than the second quarter of 2011. The delay offers one positive aspect for planning: it eases our spacecraft development schedule, giving us more time to analyze options and enhance our test program. The bad news is that without a specific launch vehicle agreement, we cannot specify the orbit in which the mission will be conducted. That prevents us from completing the mission design—another reason to stretch out our schedule.

We are working closely with NASA Ames Research Center (through a Space Act Agreement) to identify and arrange launch opportunities. Ames has become a leader in using small satellite launches as secondary payloads. We have identified several good possibilities for launch on U.S. rockets and hope to make launch arrangements soon.

All this means that we’re well on our way to making space exploration history with LightSail. I will continue to update you on our progress here and in The Planetary Report.

You also can keep up with the LightSail program on our website.

Louis D. Friedman is program director for LightSail 1 and executive director of The Planetary Society.