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As the size and longevity of spacecraft increase, so do the hazards posed by orbital debris. Various models have attempted to predict the future debris population and the effects of impacts with spacecraft. The hazard is taken seriously enough that an international dialogue is taking place on how to limit future production of debris. In addition, several new spacecraft, including the international space station ( ISS) and the Teledesic system, are taking defensive measures to minimize damage from such impacts. In the case of the ISS, this includes shielding the inhabited modules, a measure that is expensive, increases system weight, and adds to station launch costs.
A recent NASA study sought to determine the feasibility of removing the threat to low-altitude spacecraft by deorbiting nearly all debris objects of primary concern. This would be accomplished by irradiating the objects with a ground laser, which would ablate a thin surface layer of the debris and cause plasma blowoff. The resulting dynamic reaction would change the object's orbit, decreasing its perigee and causing its rapid reentry. The study, called Orion after the mythological archer, was cosponsored by the USAF Space Command, directed by the author (then at NASA Headquarters), and managed by John Campbell of NASA-Marshall.
Radar and optical surveillance systems, as well as direct impact measurements, show that there are a great many debris and micro-meteorite objects in orbit. Their numbers increase with decreasing size, and they eventually mingle with the natural micro-meteorite flux. Objects about 1-10 cm across are the most threatening. It is necessary (and relatively inexpensive) to shield against objects smaller than 1 cm, because there are huge numbers of them in LEO, they are extremely difficult to detect, and their impacts could be damaging or catastrophic. On the other hand, objects larger than about 10 cm are relatively few and easy to detect; therefore, it is possible to determine their ephemeris and maneuver to avoid collisions. The remaining objects, measuring about 1-10 cm, are too numerous to avoid, are very difficult or prohibitively expensive to shield against, and could cause catastrophic damage upon collision.
Estimates of the debris population the ISS will likely encounter led NASA experts to incorporate limited shielding to protect it against objects up to about 1-2 cm across. Evasive maneuvers are planned against much larger debris. NASA intends to live with the threat from the remaining 1-10 cm objects, and with the resulting probability of a catastrophic collision.
Among the strategies analyzed for irradiating debris, causing immediate reentry of random debris objects by irradiating continuously during a single pass over a laser was selected as the simplest operationally: Collocate the sensor and laser, point the sensor at a given angle above the horizon, then fire at any debris that enters the sensor's field of view. Firing would, of course, be inhibited when known satellites appear, as per current doctrine.
The study determined that the optimum strategy is to engage the debris from about 30° above the horizon on an ascending pass, and to stop the firing when the object nears its zenith. This will rotate the object's velocity vector and reduce its perigee to 200 km, enough to cause essentially immediate reentry. This strategy also avoids having to track the debris and predict its ephemeris for reengagement on a different pass, a very difficult task because of the uncertain ballistic coefficient of most debris objects.
The statistical characteristics of the debris population show peaks in their altitude distribution at about 800 and 1,500 km. Thus it was decided that a near-term system should be able to remove debris up to an altitude of 800 km (this would protect the ISS as well as systems such as Teledesic and Iridium); a longer term system should be effective up to 1,500-km altitude. A single laser site at sufficiently low latitude would eventually be able to target essentially all such orbital debris.
The velocity change to be imparted to the debris was then calculated to be about 150 m/sec for 800-km-altitude objects and 300 m/sec for 1,500-km-altitude objects, if their orbits are circular. The requirements are closer to 150-200 m/sec for the elliptical orbits typical of most debris. Such a velocity change to its orbit is enough to cause an object's perigee to drop to about 200 km, at which time its orbital lifetime is only a few orbits; it can then be considered to have been deorbited essentially right away.
Laser intensity requirements
The analysis then proceeded to define the laser intensity required at the debris to cause the desired velocity change for various objects; the effects of the atmosphere on the laser beam and how to minimize them; and the required characteristics of the laser and beam director. Four surveillance techniques were also analyzed.
Sid Sridharan of Lincoln Lab, in conjunction with David Spencer of the Air Force Phillips Lab, defined a number of reference target objects spanning the range of observed debris. These included thin sheets, pieces of trusses, metal spheres from a molten Soviet orbital reactor, and tank pieces. The characteristics of these objects were used to determine requirements for designing the Orion systems.
The coupling coefficient between the incident laser energy and the resulting dynamic reaction from the plasma blowoff was determined from calculated and experimentally derived values by Claude Phipps, formerly of Los Alamos Lab. The optimum coupling coefficient was determined for each class of targets. It varied between 4 and 7.5 dyne-sec/Joule and was found to be relatively insensitive to the incident laser intensity after a critical value, one sufficient to cause a plasma to be formed and blown off the object, was reached. This held, provided that the laser pulses were extremely short so as to prevent masking of a pulse by the plasma formed by the previous pulse.
Once the requirements for the incident laser energy at the debris objects were understood, the needed ground laser characteristics were defined by Glenn Zeiders (then with AmDyn), Phipps, and John Rather (then at NASA Headquarters). The atmosphere has two major effects on the laser beam: scintillation, which causes incoherence and spreading of the beam, and nonlinear effects, which spread the beam in wavelength, spatially, or both. The chief nonlinear effects analyzed were turbulence, absorption, dirty air breakdown, stimulated Raman scattering, whole-beam thermal blooming, stimulated thermal Rayleigh scattering, and nonlinear refractive index.
The effects of atmospheric scintillation can be largely removed by creating a point source high in the atmosphere to act as a reference for a rapid adaptive optics system. For the nonlinear effects, a graphical technique developed with the aid of Jim Reilly of Northeast Science and Technology enabled selection of the laser characteristics required for avoiding the deleterious consequences of the various effects. Each of these effects allows a greater intensity of laser energy to pass undisturbed if the laser pulse duration is reduced; thus the solution in general forces the system toward very-short-pulse laser systems to attain the desired intensity at the debris object. In fact, a range of systems could be fielded that allows delivery of the needed energy through the atmosphere, using different combinations of pulse length and intensity.
A further analysis by Reilly varied the laser wavelength and the beam director optics diameter to maintain a 40-cm beam diameter at the debris and to avoid nonlinear atmospheric effects. This analysis determined that while shorter wavelength lasers were desirable from an energy and cost standpoint. they would require multiple sodium guidestars, which are difficult to generate and have not yet been successfully implemented. Increasing the wavelength was beneficial to the laser and guidestar requirements but necessitated a large increase in the beam director optics diameter, rapidly raising the system's difficulty and cost. Largely because of these factors, the selection focused instead on use of the Beamlet Nd:YAG glass lasers that will soon be available from the inertial confinement fusion energy program at Livermore. These lasers operate at a wavelength of 1.06 µm.
Several sensor options for detection, acquisition, tracking, and handoff of debris targets to the laser were also investigated by Sridharan including active radar. passive optical, and active optical lidar using the laser itself. He also analyzed a novel detection technique that uses the many communications spacecraft that are or will soon be in LEO as free illuminators to form a hi static surveillance system.
A number of potential radar systems for detecting and tracking a debris object at sufficient range amid accuracy were studied, such as Haystack, FPS-85, Have Stare, MHR/ TRADEX, and a special-purpose radar for this mission. Of these, Haystack was determined to be the most capable and to meet all needs. A similar analysis was performed for passive optical systems, including the Lincoln ETS, Starfire, AMOS/Maui, and a new optimized Orion system. Starfire was also found to be capable of meeting all system needs. Another study analyzed using the laser itself as an active lidar instrument in a spoiled-beam mode.
In all these cases, the preferred engagement strategy did not require tracking, only detection and handoff to the laser, which simplified the system. This technique also required that the surveillance sensor and the laser be either collocated or within a few hundred kilometers of each other. If use of a sensor far from the laser site was desired, target handoff to the laser then required tracking and accurate ephemeris prediction over many orbits. This is a feat whose uncertainty increases for irregularly shaped, tumbling objects whose ballistic coefficient is low, due to residual atmospheric drag.
A fundamentally different surveillance system proposed by the author was also analyzed. It capitalizes on the large number of existing and planned communications satellites in LEO. These would be used to illuminate the debris, with dedicated receivers on the ground. Past studies had only considered using satellites in GEO as illuminators.
This technique takes advantage of the range from transmitter to receiver, which is roughly 100 times shorter for the LEO than for the GEO system. It also uses the very large cross-section enhancement effect obtained when the transmitter, target, and receiver are nearly in line. This combined gain can be over a million times the signal attained using GEO satellites. The study also determined that, in many communications satellite systems, the uplink transmitters are as attractive for illuminating the debris as the downlinks, even though the cross-section enhancement effect is generally sacrificed, since they usually operate at a much higher effective radiated power.
A summary of the sensor systems analysis shows that Haystack-type radar systems and Starfire-type optical systems are able to meet all the requirements of an Orion system. These two types were identified as low-risk near-term solutions. Using the laser itself for sensing is also feasible and may offer considerable cost savings, but it requires more study before it can be recommended. The bistatic radar technique using communications satellite systems proved very interesting, though it may be limited to detection of debris larger than a few centimeters across until higher power communications satellites are flown. This surveillance system was deemed promising. but it will require more detailed analysis before its ultimate operating parameters and cost can be determined.
Thus the study concluded that, without prejudicing tie other techniques, either a Haystack-type radar or a Starfire-type electro-optical system could function satisfactorily in an Orion system architecture.
Several Orion systems were defined by the team, as were the characteristics and performance of two representative systems. The nearer term system would be able to remove from orbit essentially all of the 30,000 110-cm debris objects at or below about 800-km altitude within three years, for an estimated total cost of $60 million-$80 million, including R&D and operations. The longer term system would be able to remove essentially all of the 125,000 1-10-cm debris objects at or below 1.500-km altitude within two years, for an estimated total cost of $150 million-$180 million.
The nearer term system would consist of a 3.5-m-diameter beam director pointed by a passive collocated optical sensor. The laser would be a Beamlet-type Nd:YAG glass operating at 1.06 µm, a pulse width of 5 nsec, pulse energy of 5 kJ, and a repetition rate of 1-5 pulses/sec. This laser would have an average power of 5-25 kW.
The longer term system would comprise a 6-m-diameter bean director pointed by an active collocated radar sensor. The same Beamlet type laser would operate in the so called hotrod mode at 1.06 pm, with a pulse width of 0.1 nsec, a pulse energy of 20 kJ, and a repetition rate of 1-5 pulses/sec. Its average power would be 20-100 kW. ORION SYSTEM DEFINITIONS
Not an anti-satellite weapon
It is important to note that neither of these systems can even remotely be considered an anti-satellite weapon. In both cases the power is grossly inadequate for this purpose. If pointed at an average satellite , such a system would have to irradiate it continuously for many months before making major reductions in its perigee, and four years before damaging its structure. Optical sensors aboard some spacecraft could be damaged if they looked at the laser and the laser were simultaneously illuminating the spacecraft, but simple avoidance of such pointing by the spacecraft will ensure that this does not occur. An Orion system would also avoid irradiating satellites by simple inhibition of radiation when they are in its field of view. This is current doctrine and practice in laser operation and tests.
Either of these Orion systems could protect the ISS and all other LEO satellites below their operating altitude from debris impacts in the 1-10-cm size range. In fact, if the intent were to protect only the ISS, a considerably cheaper system with a maximum altitude capability of only 500 km would probably suffice. In either case, periodic operation of the system would be needed to clear the debris objects continuing to rain down below Orion's design altitude from debris sources above.
However, even if Orion were developed and operated, the ISS and other vulnerable spacecraft would still have to be designed and shielded against debris smaller than about 1 cm, since such objects are not reliably detected and are too numerous to engage with a ground laser.
Greater benefits at lower cost
But Orion's value to the ISS would become eminently clear if current models of the debris population were shown to be too optimistic, or if damage models for hypervelocity impacts were found to underestimate penetration probabilities for current shield designs. Under these conditions, which are a distinct possibility, the cost and weight required for increasing the station shield protection would likely be considerably greater than for simply fielding an Orion system. Even without such changes in the pertinent modeling, the roughly 10% probability of inhabited module penetration that current protection systems afford over the station lifetime could be viewed as unacceptably high, given that a relatively inexpensive Orion system could reduce it essentially to zero.
If developed, an Orion debris clearing system would be inherently an international capability whoever develops and operates it. Its availability would ensure that all spacecraft are protected from debris impacts larger than about 1 cm and smaller than about 10-20 cm. Thus, since its benefits are international, the development and operation of an Orion system could be a prime candidate for an international undertaking.
This study concluded that a system capable of removing essentially all dangerous debris in the targeted size range from LEO is not currently feasible, but that its costs would be modest relative to those of shielding, repairing, or replacing affected high-value spacecraft. Moreover, the effectiveness of such a system does not depend on which of the current models of debris formation and impact damage ultimately prove correct. The study does not, however, advocate ending international efforts to avoid wanton creation of new debris, as there is no assurance that such debris would fall within the 1-10-cm range against which Orion is likely to be most effective.