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against gravitational pull is $1 N). A constraint imposed by needed energy density at the debris calls for optics diameter > 50 cm, 1 mJ pulse energy,.

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Orbital Debris Mitigation Using Minimum Uncertainty Optical States By RICHARD FORK, Member IEEE

University of Alabama, Huntsville

Fig. 1. Author’s conception of CW power transmitted from the ISS to a microsatellite as a minimum uncertainty spatial mode (green). The transmitted power is converted to a train of minimum uncertainty space-time pulses (yellow) at a nearby microsatellite and used to push space debris away from the ISS.

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n the Pixar movie, Wall-E, a small robot is left to clean up overwhelming piles of garbage left on Earth by humans who have abandoned their planet. As this paper is being written, we are already accumulating junk in near-Earth space at an alarming rate. We also lack means to clean up this debris other than waiting for it to fall into Earth’s atmosphere. This could take thousands of years for some of the higher orbit material. The recent collision of American and Russian satellites on February 11, 2009, generated a large amount of additional space debris. Some of this recently generated debris is likely to descend to the orbit of the International Space Station (ISS) and could threaten the ISS in a year or two.

Digital Object Identifier: 10.1109/JPROC.2009.2017822

0018-9219/$25.00 Ó 2009 IEEE

We suggest an option for cleaning up this debris (Fig. 1) based on use of minimum uncertainty optical states. These states are formed routinely now as specific spatial and temporal distributions of light generated by optimally adjusted lasers. The minimum uncertainty distribution in space is achieved by stimulating emission of photons into a specific lowest order Gaussian ðT00q Þ mode of the optical field. A continuous-wave (CW) beam of such light (green) is shown delivering power from the ISS to a free-flying microsatellite. Minimum uncertainty states in both space and time are obtained by stimulating emission into ultrashort pulses, e.g. 300 m in spatial length for 1 ps duration (yellow ellipses), propagating in a T00q spatial mode (light yellow trapezoidal area). These minimum uncertainty states are generated and used routinely in terrestrial laser laboratories. Loss and dispersion typically degrade the precisely defined character of these states if they are transmitted over significant distances (tens of meters) in the atmosphere of Earth. In space, however, the loss and dispersion that cause degradation of such minimum uncertainty states are, for all practical purposes, absent. A minimum uncertainty lowest order Gaussian mode of visible light optimally configured will maintain its character while propagating over a hundred kilometers and more in space. Imaging by optics having diameters on the order of a half-meter is necessary, but achievable, in the space environment.

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These ultrashort optical pulses having minimum uncertainty in space and time concentrate energy in intervals of time less than one-trillionth (1012 ) of a second. Such pulses focused on debris at energy densities > 1 J/cm2 cause ablation of a small amount of material from the debris so excited. The ejection at high efficiency of this ablated material exerts a small, but useful, force on debris. Propulsive force can be generated with efficiency comparable to the most efficient form of propulsion, solid-state rocker boosters, although typically at much smaller total force. Optimally applied, forces on the order of a newton per kilowatt of average optical power delivered to the piece of debris appear achievable and useful for precise repositioning, or even deorbiting, of debris. (The force required for you to support a cell phone against gravitational pull is 1 N). A constraint imposed by needed energy density at the debris calls for optics diameter > 50 cm, 1 mJ pulse energy, and an approach to within 10 m of the object. This maximum standoff distance increases linearly with increases in either optics diameter or pulse energy. While the force achieved by ultrashort pulse induced ablation is small, there is negligible friction in space, and the reaction force of light on the emitting and receiving spacecraft, and the debris, is itself negligible (1/ 100 000 of the ablation-induced force). The change in velocity of the debris caused by the ablation-induced force accumulates with time, e.g., a delta-v of 1 m/s is achieved for a force of 1 N exerted for 1 s on a piece of debris of 1 kg mass. This velocity change continues indefinitely to influence the subsequent trajectory of the debris. Care needs to be taken not to worsen the debris problem by unplanned repositioning of debris, or by causing debris to fall to Earth at undesirable locations. This will require carefully planned changes in the orbit of any debris. This approach needs to, and can, have the character of well planned surgical intervention, 952

as opposed, e.g., to the less precise use of explosives. The existing debris problem is illustrated by the diagram in Fig. 2. The dots, which have been scaled to the size of the graphic for purposes of illustration, indicate objects in nearEarth space as of 2007. The amount of debris has grown significantly since then. Ninety-five percent of the indicated material are objects that are no longer used and that are consequently debris. The experiment suggested here and indicated in Fig. 1 calls for installing a state-of-the-art laser on the Kibo (the Japanese word for hope) facility of the Japanese Experimental Module, which is now on the ISS. The Kibo facility can provide adequate power and cooling capabilities as well as a location for the laser, the optics, and the pointing and tracking equipment required to perform simple experiments. Great care must, of course, be taken in performing any experiments on the ISS because of the human presence and the importance

of the ISS to the space program. At the same time, there is also a need to find means of protecting the ISS from space debris that may approach the ISS in the not too distant future. A potential problem with the method outlined here is that the small maneuverable satellite (Fig. 1) will not, itself, support human presence. The final stages of controlling the deflection system will require a remote communication link. In the interest of protecting against any possible third party’s interfering with the communication link between the control center and the microsatellite that exerts the deflection force on a particular piece of debris, some means of secure communication is desirable. Another minimum uncertainty optical state, based on a single photon wave packet, offers arguably unbreakably secure communication. This state differs from the multiphoton minimum uncertainty states in that only a single photon is contained in the space-time wave packet. This method of communication is unbreakable in that any

Fig. 2. Computer-generated image of objects in near Earth space; È95% are debris (dots scaled to graphic). (NASA Orbital Debris Program Office, 2007.]

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attempt by a third party to read a transmitted message will unavoidably perturb the single photon wave packet in a manner that can be detected by those using the communication system. This cyber secure communication system is similar to current systems now available in university laboratories but differs in that the low loss and dispersion of space will allow much longer distances for secure communication. There is, of course, no robot named Wall-E who will clean up the space debris generated by humans. It will be up to engineers and scientists

to keep our path into space open. The good news is that we can access the needed precision, agility, and security. The needed energy can be derived from our Sun, through photovoltaics, or eventually through direct solar pumping of the laser source, and the needed propellant could be provided by the space debris itself. If we use these tools wisely and in a timely manner, we may avoid the fate of the humans in the movie Wall-E, who were forced to abandon Earth because of careless management of their environment. h

Acknowledgment This author wishes to thank students in his optics classes and laboratories at the University of Alabama, Huntsville (UAH), for many ideas and discussions; his wife Donna Fork for many discussions and a critical reading of the manuscript. He thanks Randy Gaillard and Arthur Palosz, UAH, and Debra Shoots, NASA Orbital Debris Program Office, for valuable contributions to the preparation of the figures used in this paper.

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