April 03, 2010

Spacecraft on a Chip Prototypes Should Be Launched This Year

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A swarm of "smart dust" spacecraft, positioned at a sweet spot between the Earth and the sun, could alert us to the approach of dangerous space storms well before a conventional craft can. The first prototypes are due for launch into low-Earth orbit this year, perhaps as early as May.

Mason Peck, a mechanical engineer at Cornell University in Ithaca, New York, and his colleague Justin Atchison have designed a 1-centimetre-square spacecraft that is 25 micrometres thick and weighs under 7.5 milligrams. The craft is modelled on the dust particles that orbit the sun and are propelled by the photons streaming out from the sun. The chips are essentially small solar panels with a radio antenna, and could act as a solar wind sensor. They can edge closer to the sun than a larger craft monitoring solar activity, buying an extra 13 minutes

At least one prototype should be launched in 2010 as proof of concept and feasibility of a spacecraft on a chip.

A passive, sun-pointing, millimeter-scale solar sail

Taking inspiration from the orbital dynamics of dust, we find that spacecraft length scaling is a means of enabling infinite-impulse orbits that require no feedback control. Our candidate spacecraft is a 25 μm thick, 1 cm square silicon chip equipped with signal transmitting circuitry. This design reduces the total mass to less than 7.5 mg and enables the spacecraft bus itself to serve as a solar sail with characteristic acceleration on the order of 0.1 mm/s2. It is passive in that it maneuvers with no closed-loop actuation of orbital or attitude states. The unforced dynamics that result from an insertion orbit and a launch-vehicle separation determine its subsequent state evolution. We have developed a system architecture that uses solar radiation torques to maintain a sun-pointing heading and can be fabricated with standard microfabrication processes. This architecture has potential applications in heliocentric, geocentric, and three-body orbits.



In collaboration with Sandia National Laboratories, we've developed our first prototype, dubbed "Sprite". Sprite uses a multi-chip module architecture to achieve a form factor of 2cm x 2cm x 2mm. The demo that follows the Sprite will be the system on a chip prototype

Using matched filtering techniques, it can close a communications link from a 500km orbit. A half century later, we expect to duplicate Sputnik’s achievement using less than one ten-millionth of its mass. Our design packages the traditional spacecraft systems (power, propulsion, communications, etc) onto a single silicon microchip smaller than a dime and unconstrained by onboard fuel.

The craft's miniature size would let it hitch a ride into space on the back of another satellite mission headed for the Lagrange point between the Earth and the sun.

The team envisage sending a whole swarm of these "smart dust" chips to the Lagrange point, where they would monitor the strength of the solar wind. They would also warn of any oncoming gusts of charged particles that could disrupt communications and electronics on Earth.

After the tiny craft has been dropped off at the Lagrange point, the effect of solar radiation moves it closer to the sun. Peck estimates that this could give an extra 13 minutes' notice of a storm compared with larger solar monitoring craft such as NASA's Advanced Composition Explorer.

A Passive Microscale Solar Sail (2008)

We consider spacecraft length scaling as a means of enabling achieving passive, feasible infinite-impulse orbits. Taking inspiration from the orbital dynamics of dust, this paper discusses the consequences of length scaling on acceleration due to solar radiation pressure and demonstrates its effectiveness on a candidate microscale spacecraft. We propose to fabricate this dime-sized spacecraft on a single ultra-thin substrate of silicon. This choice reduces the total mass to fewer than 7.5 mg and makes the spacecraft bus itself a solar sail, yielding a lightness number β of 0.0175. This architecture can provide passive solar sail formations and various passive methods of changing orbital energy. We also consider augmenting this architecture with traditional CP1 sail material (β of 0.1095) to reduce transfer times further. The paper surveys and compares passive methods of achieving a marginally stable sun-pointing attitude including the addition of fixed vanes and optical grating of the surface. The microscale infinite impulse (MII) spacecraft design replaces the traditional spacecraft subsystems with a single integrated circuit (IC). Our current fabrication efforts are directed at realizing this spacecraft as a simple sensing and transmitting circuit with standard IC tools.

This paper presents motivation, analytical evaluation, supporting simulation, and sample designs for a microscale infinite-impulse (MII) spacecraft with sufficiently low mass to enable solar sailing. The candidate bus is a 1cm x 1cm x 25 μm silicon IC that conservatively weighs less than 7.56 mg. Each conventional spacecraft subsystem is accounted for and described in the context of a Sputnik inspired temperature sensing mission.

If paired with two other MII spacecraft to form a corner cube, this design can passively reduce the effect of gravity by 1.0% via solar pressure, enabling unique formation opportunities. Using an optical grating scheme to produce stable sun-pointing attitudes, this acceleration can be improved up to 1.75% of gravity. If spin-stabilized and strategically coated with absorptive (η = 0.5, β = 0.0103) and reflective (η = 0.85, β = 0.0175) materials, the spacecraft can passively gain or remove energy and momentum in a simple circular orbit. Equipped with a 10 cm radius ultrathin CP-1 sail, such a spacecraft exhibits a lightness number of 0.1095. If coated, the sail-spacecraft system is shown to passively add or subtract 10% of its orbital energy in a single circular heliocentric orbit at 1 AU.

Table 1 lists this study’s two MII solar-sailing architectures along with two other well known designs. The geometry and performance of these four are compared. Although the lightness numbers are similar, the passive energy-change maneuver using different coatings is much less efficient than a simple logarithmic spiral trajectory.

As a result, a transfer from a circular orbit at 1 AU to the average orbital radius of Mars requires significantly more time with the passive MII design, even when augmented with the disc of thin CP1 material. Nevertheless, we emphasize that the difficult engineering challenges associated with a 22,700 m2 spacecraft introduce risk and cost that the 1 cm2 MII trades for small size.

Table 1. Solar Sail Design Comparison 
           MII Bus               MII – 10cm CP1       DLR/ JPL   Johns Hopkins 
          η1 = 0.50, η2 = 0.85  η1 = 0.50, η2 = 0.85  ODISSEE4   University4 
Design     Square               Disc                  Square     Disc 
Area       1 cm2                31.4 cm2              1600 m2    22,700 m2 
depth      25 μm                25 μm / 5 μm          7.5 μm     7.5 μm 
mass       7.5 mg               43 mg                 77 kg      180 kg 
β          β1 = 0.010           β1 = 0.064
           β2 = 0.018           β2 = 0.110            0.032      0.194 
Time to                                                          logarithmic 
Mars,yr     >100             different coatings,      logarithmic spiral
                                6                     spiral, 3    0.8

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