Microscale Spacecraft for Interstellar Applications
Dust in our solar system experiences a surprising lifecycle. For very small particles, solar pressure and electrostatic forces can compete with gravity to create highly non-traditional orbits. Some dust finds a stable orbit in which to live out its existence; some dust calmly lands on the surface of planets like our own, and some dust is energetically ejected from our solar system altogether, embarking on interstellar trajectories.
Dust particles vary from a few molecules to 100 µm in size and have a mass smaller than a few micrograms. At these mass scales, the acceleration due to what would be considered perturbation forces on larger bodies can no longer be neglected. In fact, they can potentially be harnessed and controlled in order to enable new technologies and missions.
The idea of using tiny spacecraft that capitalize on these kinetics in large numbers and that act synergistically has been pursued in the past due to obvious advantages such as economies of production, reduced launch mass and distributed sensing opportunities. A very good overview in the state of the art and on emerging technologies in the field is given by David Barnhart in his PhD thesis (see references). Mason Peck at Cornell is investigating the dynamics of spacecraft at the microscale in a recent paper. His team at the Space Systems Design Studio (SSDS) has already built and flown a prototype of a “spacecraft-on-a-chip” which they dub Sprite, the dimensions and general design of which can be seen below.
Modeling the Motion of Microscale Spacecraft
In the modeling of the motion of a microscale spacecraft, various forces that act on it must be taken into account. Since the spacecraft has mass it is affected by gravity the same as traditional spacecraft. What separates the study of very small sized spacecraft is that forces that are considered perturbations for bigger spacecraft become very significant as the scale becomes smaller. These forces can be split into three distinct categories:
1. Forces due to Collisions with Particles, such as aerodynamics forces, forces due to collisions with micrometeoroids and the pressure from the energetic particles in the solar wind,
2. Forces due to Radiation, such as the pressure from solar photons and from photons reflected off of planetary surfaces, as well as forces caused by thermal radiation emitted by the spacecraft itself, and
3. Forces due to Magnetic Fields, such as toques on the spacecraft caused by the interaction between the planetary magnetic field and that of the spacecraft, torques that are generated by currents inside the spacecraft that are in turn caused by the external magnetic field and, most of all, the Lorentz force that acts on any charged body with a velocity relative to a magnetic field (see picture)
The above can be taken into account when considering sample mission applications for a microscale spacecraft, including solar sailing, atmospheric reentry, and Lorentz propulsion.
Solar Sailing: A solar sail exploits solar radiation pressure as a means of propellantless propulsion. The above analysis suggests that useful solar pressure can be achieved with small, thin plate-like structures. In fact, solar sail architectures benefit from small size in other ways as well, as typical sail designs are extremely large and challenging to construct, deploy, and actuate
For interplanetary dust, solar radiation pressure can exceed gravity. Highly reflective interplanetary dust particles of this size can escape solar gravity if released from a comet near the sun. This indicates that the candidate microscale spacecraft can capitalize on these mechanics to achieve significant solar pressure acceleration. That is, the spacecraft bus itself, by virtue of its geometry, behaves as a solar sail.
In a typical microscale spacecraft, a magnetic torquer could be used to adjust the attitude of a microscale spacecraft, enabling a form of controlled aerobraking or reentry.
A primary challenge for spacecraft reentry maneuvers is heat management, where heating due to aerodynamic drag can cause catastrophic failure. A spacecraft must be capable of both decelerating and shedding heat rapidly enough to survive reentry. It turns out that dust particles can survive reentry at low temperatures thanks to their small size. Each year, thousands of metric tons of small interplanetary dust particles reach the Earth’s surface unaffected while larger meteoroids energetically ablate as meteorites.
There may be meaningful mission opportunities for a small sensor that can sample many altitudes of the atmosphere continuously throughout the reentry process, and one that furthermore would not experience the plasma-related communications dropout of hotter reentering spacecraft.
Lorentz Force Spacecraft
The Lorentz force is responsible for capturing and ejecting electrostatically charged dust particles in the rings of Jupiter and Saturn. According to the analysis above, if charge can be artificially generated on a spacecraft, it could serve as a means of propulsion.
One promising architecture requires only a power source and two plasma contactors to achieve a net charge. In a plasma environment, if two conductive wires are connected to the terminals of a potential source (e.g. a battery or a solar cell), they acquire dissimilar wire potentials. A spacecraft could be equipped for Lorentz propulsion with such a system, carrying a net negative charge that may be controlled through the power source.
Use in Interstellar Applications
From the above we can see that such tiny spacecraft would be indeed very useful in interstellar exploration. One way to get this type of spacecraft on an interstellar trajectory would be by using the Lorentz Force Spacecraft application above: a charged micro scale spacecraft would use the Jovian magnetic field to accelerate itself in orbit, reaching speeds of thousands of kilometers per second, before turning off its onboard power supply, at which point the Lorentz force would disappear and the spacecraft would be hurled at high velocity out of the Solar System. Continuous, periodic launches of such spacecraft would produce a chain of craft racing out of our solar system and reporting data back to Earth by relaying through each subsequent craft.
Microscale spacecraft could also serve as sub-probes to Icarus, released in large numbers during the interstellar trip and at the target system, taking measurements of the interstellar medium, stellar winds, planetary atmospheres etc. Yet another application for them could be as inspectors of the environment around the parent spacecraft, assessing the state of health of Icarus.
Some very useful work that could be done based on the preceding analysis would be to model this plethora of forces that act on a microscale spacecraft and examine how the trajectory of such a spacecraft would evolve in the various different environments previously mentioned. One could also model all the effects these environments would have on the spacecraft itself and incur the limits of its functionality. Even more useful, considering that this type of spacecraft is meant to be used in swarms, would be to model the evolution of the distribution of large numbers of such spacecraft in various scenarios expected to be encountered during Icarus’ flight.
Overall, this concept deserves to be thoroughly investigated in the context of Project Icarus as it is extremely novel and one of the most applicable to interstellar travel.
Atchison, J. A. and Peck, M. A., “Length Scaling in Spacecraft Dynamics”, Journal of Guidance, Control, and Dynamics, Vol. 34, No. 1, January–February 2011, doi: 10.2514/1.49383
Atchison, J. A. and Peck, M. A., “A passive, sun-pointing, millimeter-scale solar sail”, Acta Astronautica, 67 (2010) 108–121, doi:10.1016/j.actaastro.2009.12.008
Barnhart D. J., “Very Small Satellite Design for Space Sensor Networks”, PhD Thesis, University of Surrey, June 2008
Cornell University Space Systems Design Studio (SSDS) website: http://www.spacecraftresearch.com/MII/MII_overview.html
Feature on the Sprite spacecraft at IEEE Spectrum by Mason Peck, “Exploring Space with Chip-sized Satellites”, August 2011: http://spectrum.ieee.org/aerospace/satellites/exploring-space-
Centauri Dreams article by Paul Gilster, “A Swarm of Probes to the Stars”, August 1st, 2011: http://www.centauri-dreams.org/?p=18968
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