Building a Better Space-weather Buoy: Tests Continue on Sunjammer
WASHINGTON — Days after a successful unfurling of its solar sail, the prime contractor for the experimental Sunjammer spacecraft said the propellant-thrifty vessel could well become a new early warning beacon for potentially harmful space weather.
That is because Sunjammer will carry a pair of sun-gazing instruments to a point on the Earth-sun line about 3 million kilometers away from the planet — twice as far as the United States’ current early warning space-weather buoy, the 16-year-old Advanced Composition Explorer (ACE).
“ACE gives about 40 minutes’ warning time,” Nathan Barnes, president of Sunjammer prime contractor L’Garde Inc., Tustin, Calif., said in an Oct. 7 phone interview. “What we want to do is, near as possible, double that.”
Space weather is the result of coronal mass ejections from the sun, in which waves of ionized particles blast away from the star and hurtle outward through space. These particles can overload electrical circuits, disrupting everything from satellite instruments in orbit to entire power grids on Earth’s surface.
“If you’re a power plant or a grid operator trying to shed load, or an airline operator telling pilots flying over the poles to come land, that extra warning time is extremely valuable,” said Charles Chafer, chief executive of Space Services Inc., L’Garde’s commercial partner on the Sunjammer project.
Barnes and Chafer spoke with SpaceNews after a key Sunjammer test Sept. 29, in which engineers fully extended one quadrant of Sunjammer’s rectangular solar sail on the floor of L’Garde’s manufacturing facility in Tustin. The Sunjammer team is planning a test deployment of the entire sail around June, Barnes said. Launch is tentatively scheduled for January 2015.
Sunjammer will cost about $20 million to build and another $10 million to launch as a secondary payload aboard a Space Exploration Technologies Corp. Falcon 9 rocket, Barnes said. The primary payload on that launch, which the Air Force is paying for as part of Falcon 9’s military launch-certification trials, is the Deep Space Climate Observatory satellite, a refurbished Earth-observing satellite formerly known as Triana.
The budget of Sunjammer, selected in 2011 as a NASA technology demonstration mission, is being covered by the agency’s Space Technology Mission Directorate, which seeks to mature experimental space technologies and spread them throughout the rest of the agency. For NASA, solar sails offer a way to slowly move large, uncrewed payloads through deep space at a fraction of the mass penalty incurred at the launchpad by chemically propelled spacecraft.
Solar sails, which have flown in space before, are analogous to wind sails on Earth, except that solar sails are propelled by photons from the sun rather than air. Like sailboats on Earth, sail-powered spacecraft move slower than similarly massed vessels equipped with engines. Sunjammer’s sail measures about 38 meters on a side and has nearly 1,200 square meters of surface area, but produces only about 0.01 newtons of thrust — about the force a packet of artificial sweetener exerts on an upturned palm, Barnes said.
Because the sun produces a free abundance of photons, Sunjammer can perform station-keeping maneuvers in near-perpetuity. That means the spacecraft need not remain at the gravitationally stable Earth-sun Lagrange point where ACE, dependent on chemical propulsion for station-keeping, is moored. Instead, Sunjammer will creep closer to the sun, where gravitational forces are more turbulent and necessitate more-frequent position adjustments than a chemically propelled vessel could manage for any useful length of time without running out of fuel, Barnes said.
As an early warning system, Sunjammer’s main advantage over ACE would be its proximity to the sun. ACE is a data-rich spacecraft compared with Sunjammer, boasting 10 instruments to the sail-powered craft’s two. ACE also has enough fuel to maintain its current position about 1.5 million kilometers from Earth until 2024, ensuring that the National Oceanic and Atmospheric Administration’s Space Weather Prediction Center in Boulder, Colo., can rely on the venerable craft for years to come assuming its instruments and critical flight systems remain healthy.
Sunjammer’s two solar instruments are the Solar Wind Analyzer, a miniature spectrometer developed by University College London, and the Magnetometer from Imperial College (MAGIC) vector magnetometer, developed by Imperial College London. The U.K. Space Agency is funding both of those instruments.
It is not yet certain that the instruments will function correctly aboard a sail-powered spacecraft. Proving that such sensors can function in conjunction with a solar sail is as much a part of Sunjammer’s mission as demonstrating the sail’s propulsive capability, Barnes said.
“The sail is a great big capacitor,” Barnes said. “And as it charges up, it disrupts the essential measurements that could be made by the instruments. So we ground the sail, we ground the front surface to the back surface,” in the hope of minimizing the effect of accrued electrical energy on Sunjammer’s solar instruments.
Also along for the ride on Sunjammer will be the mortal remains of customers for Chafer’s business, which arranges burials at space on a commercial basis. Space Services has 12 memorial missions under its belt so far.
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