Invention Promises to Change the ,,Small Satellites Are Powered

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Invention Promises to Change the Way
Small Satellites Are Powered
An invention developed by The Aerospace Corporation and recently patented -- the PowerSphere Nanosatellite -- could significantly enhance the way nanosatellites are powered while eliminating some technical challenges that have evolved with the miniaturization of satellite technology.

In September a research team led by Edward Simburger received the patent for the deployable geodesic solar-panel array consisting of connecting pentagon- and hexagon-shaped panels that are solar-energy-collecting cells.

Folded in a flat stack at either end of a strut attached to the payload, the panels of two halves of the PowerSphere unfurl during deployment, creating two hemispheres that interlock and encase the satellite.

Once deployed the PowerSphere becomes a 360-degree solar array that collects "a constant amount of electric power regardless of its attitude relative to the sun," Simburger said. "It eliminates the problem of providing power to nanosatellites with limited surface 'real estate.'"

The invention, co-developed by David Hinkley, Ernest Robinson, Jon Osborn and David Gilmore, also eliminates from small satellites the excess weight and bulk of solar panels and the accompanying attitude-control apparatus required to continually point them in the direction of the sun.

An added advantage, according to Simburger, is that the PowerSphere provides a controlled thermal environment for the nanosatellite electronics and battery it encases, which are subjected to extreme temperatures in space.

"The PowerSphere may find future use in providing power and thermal control for small satellites weighing from under one kilogram (one-half pound) to 60 kilograms (132 pounds)," Simburger added.

Geometry for Power
The development of the PowerSphere began in 1998, when Simburger began investigating novel methods for providing power to picosatellites, the four-by-three-by-one inch, half-pound miniatures developed by The Aerospace Corporation with Defense Advanced Research Projects Agency funding.

"I was exploring various geometries for a solar array for the picosatellites using amorphous silicon solar cells that have extremely low mass and are very flexible," he said.

A rough sketch of a Mars rover inspired Simburger to explore a spherical shape for a solar array because "the rover used three inflatable spheres as tires and a fourth sphere on an antenna mast. The fourth sphere could be a solar array that would provide power to the rover."

That prototype inflatable Mars rover eventually designed for the Jet Propulsion Laboratory ultimately used a solar array that was a deployable parasol, "but I contacted the company that designed that prototype, ILC Dover, and asked them to develop a design for an inflatable sphere for deploying an array consisting of these amorphous silicon solar cells," Simburger said.

Refining the Concept
At that point Simburger imagined the model solar array would be spherical in shape and tethered to the satellite. Testing on thermal control aspects, as well as the manner in which the solar cells of the spherical array were wired together, led the team to refine the concept even further.

"The solution to the difficulties of wiring the solar cells together was to connect them directly to the spacecraft power bus with individual DC-DC converters," Simburger said, a concept that brought the PowerSphere one step closer to encasing its payload.

Simburger received a patent in October 2000 for the method of connecting the solar cells mounted on the spherical array structure.

To verify the operation of his connection scheme, Simburger had the Aerospace machine shop fabricate a two-foot-diameter "buckeyball," which was the size he calculated would be required to produce enough power for a small satellite in low Earth orbit.

At the same time, Hinkley, lead design engineer for the picosatellites, had been working with Gilmore to come up with a thermal design for the tiny picosats.

"David Gilmore told me earlier that the interior space of the PowerSphere would provide a suitable thermal environment for the battery that would power the picosatellites during eclipse," Simburger said.

Energy, Protection
The concept quickly moved from a tethered spherical array to an array that would serve the dual purpose of collecting energy regardless of attitude toward the sun and serving as the protective thermal shell for the nanosatellite. A patent for this configuration was granted last month.

The team was next challenged to devise a deployment scheme for the PowerSphere from a flat stack of hexagon- and pentagon-shaped panels.

Simburger worked with cut-out construction cardboard hexagons and pentagons to devise a workable scheme.

Another patent for the deployment method is pending with the U.S. patent office.

The team has received funding on a proposal in response to NASA's Cross Enterprise Research Announcement, issued in March.

Aerospace, the prime contractor on the project, is working with subcontractors ILC Dover for the design and fabrication of a deployable structure and with Lockheed Martin, which will create the wiring harness for a development model of the PowerSphere.

"The program is on track to complete a preliminary design for the PowerSphere by June 2002," Simburger said.

Contract milestones call for completion of an engineering development model by June 2003 and an engineering development unit by June 2004.

Other References Hinkley et al, "A Mechanical Deployment Structure for the Powersphere Concept," Energy Conversion Conference and Exhibit (IECEC) 35th Intersociety, vol. 1, (2000), pp. 659-669.

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NASA MESSENGER Mission News: Priming Instruments to Map Mercury's Crust

NASA MESSENGER Mission News: Priming Instruments to Map Mercury's Crust

Understanding if ice exists on the surface of Mercury, and if so what types, will mark an important component of the investigations by the MESSENGER spacecraft about the origin and evolution of the solar system's inner planets. This month, instrument engineers at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., turned on the Neutron Spectrometer (NS)-one of several sensors aboard MESSENGER that will be key to sorting out the mystery of Mercury's surface.

The NS will collect data about the composition of the uppermost tens of centimeters of Mercury's crust by measuring the numbers and energies of neutrons that reach the MESSENGER probe as it passes near the planet. The NS together with a gamma ray sensor make up the Gamma-Ray and Neutron Spectrometer (GRNS) instrument.

The NS will map variations in the fast, thermal, and epithermal neutrons Mercury's surface emits when struck by cosmic rays. "Fast" neutrons shoot directly into space; others collide with neighboring atoms in the crust before escaping. If a neutron collides with a small atom (like hydrogen), it will lose energy and be detected as a slow (or thermal) neutron. Scientists can look at the ratio of thermal to epithermal (slightly faster) neutrons across Mercury's surface to estimate the amount of hydrogen-possibly locked up in water molecules-and other elements.

Subtracting the Background Noise

APL's Edgar Rhodes, an instrument scientist on the MESSENGER mission, says the current NS calibration will "tweak the electronic thresholds and voltages for the detectors to assure that the most interesting parts of the neutron spectra are within the energy ranges of the instrument before the upcoming second Venus flyby on June 5."

In space, the spacecraft is bombarded continuously from all directions by galactic cosmic rays, high-energy particles (mostly protons) thought to originate from remnants of supernovae in our galaxy, explains APL's John Goldsten, the GRNS instrument engineer.

"On Earth, we are protected from these penetrating rays by our atmosphere; but in the vacuum of space they collide directly with spacecraft materials, smashing into atomic nuclei, and sending off energetic neutrons," he says. "These energetic neutrons, in turn, collide with other atoms and produce a host of lower-energy neutrons and gamma rays. These signals pose a serious background in the instrument that needs to be 'subtracted out' from the total signal measured near a planet. After all, it's the composition of the planet and not the spacecraft we are trying to measure."

To make matters more difficult, Goldsten adds, some components of this induced spacecraft background build up over time, "and so it is important to make periodic measurements to characterize this background to perform a proper analysis and interpretation of the science data."

Searching for Solar Neutrons

The NS will remain on during most of MESSENGER's cruise phase and return data from Venus flyby 2 in June, three Mercury flybys in 2008 and 2009, and one Earth year in Mercury orbit starting in 2011.

The NS is a low-power instrument that can safely be powered on indefinitely, Goldsten says. "We plan to take special advantage of the unique measurements the NS can produce as it journeys through the inner solar system; a region of space never before studied with this type of instrument."

"Looking for the presence of energetic neutrons streaming away from the Sun during solar flares is of particular scientific interest to physicists trying to model and understand the underlying mechanisms of solar activity," he continues. "Solar neutrons are very difficult to observe from Earth because these sub-atomic particles by themselves-not bound inside an atomic nucleus-are not stable and decay away in about ten minutes, so only the most energetic (fastest moving) neutrons reach Earth before disintegrating, and these are very few. But as the MESSENGER spacecraft journeys closer to the Sun and gets inside the orbit of Venus, the likelihood of observing solar neutrons increases dramatically because we get a chance to capture them before they can decay away."

Another reason to operate the NS during the long cruise to Mercury is to help the Interplanetary Network of satellites detect and locate Gamma Ray Bursts-the most energetic events known in the Universe-which produce monstrous flashes of gamma rays that appear in the sky at random times and from random locations.

"Detecting these gamma ray bursts simultaneously at distant spacecraft helps to triangulate their direction so that observatories can quickly aim powerful telescopes to study the optical counterpart or 'afterglow' of these colossal events, hypothesized to mark the collapse of stars into black holes or the collision of super dense neutron stars," Goldsten says. "While the NS is optimized to detect neutrons, gamma rays appear as a steady background signal, so any sudden changes in this background signal can be used as a gamma-ray burst monitor. Gamma-ray bursts are typically detected about once a day, and no two are exactly alike; some last milliseconds, while others may continue for minutes."

From an engineering point of view, the GRNS is a flexible instrument with many controls and settings that can be adjusted remotely via commands to the spacecraft. "This flexibility is important as we cannot easily simulate the galactic cosmic ray environment on the ground," Goldsten says. "Optimizing the in-flight settings is usually an iterative process where we make small changes and then analyze the results. We then collect long-term data with the final settings to establish the instrument baseline prior to an encounter such as the upcoming Venus flyby."

http://www.windows.ucar.edu/tour/link=/mercury/Interior_Surface/Structure/structure_overview.html

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