ODERACS Deployment, Space Shuttle Cargo Bay A delivery mechanism for synthetic meteors? NASA photo

The fundamental questions surrounding the design of a synthetic meteor involve the mass, velocity, mechanical structure, and chemical composition required to achieve a desired brightness sufficient for observation under a given viewing condition, such as the night lights of a city. The mass requirements are a critical parameter as they drive launch cost and deployment vehicle design. Preliminary estimates were as high as hundreds of kilograms of mass per synthetic meteorite. Recent data from the NASA Orbital Debris Program Office and noted meteor astronomer Dr. Peter Jennniskens have resulted in a significant downward revision of the estimated mass.

The factors of velocity and composition are interrelated with mass in determining brightness. For instance, it is believed that brightness is a function of the square of velocity, in which case it might prove more efficient to accelerate a smaller mass to a higher velocity. Similarly, brightness is a function of composition, using a more massive material which produces a brighter meteor might prove more efficient. In addition, the mechanical structure of the meteoroid is extremely important in insuring desired luminosity characteristics over time. For example, a solid object might require an excessive period of time to heat sufficiently to begin the vaporization which is required for luminous onset, making a less dense, lattice like structure preferable to a solid. To some degree these relationships are unknown or at the forefront of current meteor research, creating the requirement for at least some flight testing of synthetic meteoroids.

Establishing Mass Limits

In an attempt to begin to establish useful ranges for the parameters it was decided to make complete meteoroid reentry demise the determinant for maximum payload mass. The primary objective of the program must be safety for those on the ground. Under no conditions could a payload survive reentry and reach the earth's surface. The NASA Orbital Debris Program Office generously agreed to model various payload masses using their reentry demise modeling software to determine an approximate upper limit for payload mass which would still assure complete reentry burnup. Solid iron spheres were modeled for a variety of reasons. A spherical shape does not require orientation of the payload with respect to its flightpath, simplifying the deployment vehicle. Iron was chosen as a material since many meteorites are primarily composed of iron and this would hopefully provide similar brightness performance by the sacrificial payloads. The Orbital Debris Office performed data runs for .1, 1, 10, and 100 kg. masses. They found that for a zero degree or flat flight path angle, as if reentering from low earth orbit at 78 km at a speed of 8 km/s, .1 and 1 kg solid iron spheres will completely demise before reaching the earth, in times of 88 and 148 seconds respectively. At time of demise they would still be traveling at 6 km/s. The 10 and 100 kg spheres would reach the earth's surface. Solid iron spheres have a diameter of 6.24 cm for a 1 kg sphere and 2.9 cm for a .1 kg sphere, so these are not large objects, effectively pinball to baseball sized assuming a solid structure.

Pegasus Air-Launched Orbital System another Reentry Series launch candidate Laboratory for Atmospheric and Space Physics Univeristy of Colorado, Boulder

Given this object mass range the next question is how bright will a resultant synthetic meteor be and what velocities are required to obtain that brightness? The answer is complicated by the fact that readily achievable velocities from an economic perspective, those of low earth orbit, are significantly slower than meteoroid velocities, 8km/s for LEO versus 20 to 70 km/s for meteoroids. While the luminous performance of naturally occurring meteors has been extensively studied, this data has a floor at 20 km/s which is a velocity which would prove very expensive to obtain. In discussions with Dr. Jenniskens it become quite apparent that a meteoroid with a mass of a kilogram would be an extremely bright object, with a visual magnitude in the range of -8 to -10, brighter than the brightest star and approaching the moon's brightness of -14 (note that the scale is numerically inverted, with brighter objects having increasingly larger negative values for visual magnitude). Naturally occurring meteors with meteoroids of this mass are so bright as to be called "fireballs" in meteor astronomy and would be readily visible in a city night sky.

So, a .1 to 1 kg mass, reentering at meteor velocities would be sufficiently bright. The next question is will this observed behavior on the part of meteors extend to the lower velocities resulting from decay of Low Earth Orbit. The answer is probably yes. The Orbital Debris data indicates the mass will demise, so it will be vaporized and become luminescent. The lower velocity will result in reduced visibility as meteor luminosity is known to be proportional to the square of velocity. This would mean that a meteoroid traveling at 8 km/s would produce only 16% of the instantaneous brightness of one moving at 20km/s. However, the loss in apparent luminosity may not be as great, as the object is slower moving. The integral of the luminosity per subtended viewing angle would be reduced to only 40% of the faster moving meteor and this may be a better measure of the relative apparent brightness.

The Velocity Question

Convergence on a optimal synthetic meteor design solution will require study and recommendations by leading meteor researchers. The issue of reentry velocity has emerged as a major unknown. Preliminary discussions have suggested that the most cost-effective launch technology for payloads in the 10s of kilograms range (nominally 10 synthetic meteoroids and a deployment vehicle) would be to piggy-back on a low earth orbit launch dedicated primarily to another function, such as International Space Station resupply or commercial satellite launch. The reentry velocities available from such a launch profile would be in the 7 to 8 km/s range. Reasonable brightness characteristics would be available from reentry velocities in this range.

However, these LEO reentry velocities are not sufficient to guarantee fidelity to naturally occurring meteors, an issue for some of the parallel science contemplated in association with the mounting of these works. Dr. Jenniskens has indicated that he would like to see a minimum reentry velocity of 15 km/s for certain experiments he would like to perform. The potential problem is that attaining these higher velocities is expensive from an energy expenditure perspective and almost certainly would require a dedicated launch, both factors potentially driving costs higher. On the other hand, these higher velocities would produce brighter meteors by a factor of as much as 4 or reduce the mass requirements for a given visibility by a factor of 4. This is an interesting cost/benefit tradeoff and will be a focus of much of the work in resolving the design of the synthetic meteoroids.

2. Document Similar Reentry Events to Assess Visual Attributes and Experience

Since atmospheric reentry of space vehicles or large fireball size meteors are such infrequent events, few people have had direct encounters with these phenomena. In an effort to better appreciate and convey the visual experience of these events to potential benefactors as well as host cities it is desirable to provide a record of reentry events with similar characteristics to the synthetic meteor artworks. If such similar events could be identified, still and video imagery would be recorded, with backgrounds giving an idea of scale and relative brightness when ever possible.

The Stardust Luanch, 1999 and a sample return capsule after a desert drop test NASA Photo

The NASA Orbital Debris Program Office has been contacted to see if any planned demise events would be suitable for study. Their answer was largely negative, in that most planned demise events are designed to take place over isolated regions of the South Pacific. Since these demises often involve complete vehicles there is substantial chance of material making it to the ground, hence the isolated aiming point. In addition, the reentry visible artifacts of entire vehicles bear little resemblance to the artwork's synthetic meteors. The size and complexity of the reentering vehicles creates large, disorganized visual artifacts. Taking another tack, Orbital Debris was questioned whether smaller artifacts in the size range of the synthetic meteoroids were tracked and if so could a reentry point be determined in order to obtain visual records. The answer is unfortunately no, smaller objects are not being tracked since their reentry does not comprise a significant risk as they invariably will demise.

A very interesting potential for documenting a similar reentry event was suggested by Dr. Jenniskens. He is studying reentering man-made objects for the same reasons as he has expressed interest in studying the Reentry Pieces. A exceptional event of this type is to occur early in 2006 with the return of the Stardust SRC or Sample Return Capsule. Stardust is a mission launched in 1999 which involves the sample and return of cometary matter and interstellar dust, the first return of extraterrestrial material from beyond the moon's orbit. The vehicle made a near pass of comet Wild 2 in January 2004 collecting material and will reenter the earth's atmosphere at 3:00 AM, January 15th 2006, over Utah. The vehicle will mark the fastest vehicle reentry since the Apollo program, of nearly 13 km/s, approaching meteoric velocities. The capsule is only .8 meters in diameter and the expected visual magnitude is -7.8, right in the range of the Reentry Piece synthetic meteors. Dr Jenniskens is studying the event to "probe the delivery of organics for life's origin by measuring the physical conditions during reentry."

At a minimum the artist will mount a concerted effort to obtain as much documentary material as possible of this event as it will mark the only foreseeable opportunity to study an event of such similarity to an individual synthetic meteor in the Reentry Series. The Heaven & Earth Foundation will attempt to obtain grant funding to provide a more in-depth documentation of the event than the artist is able using his own resources. An attempt will also be made to coordinate with Dr. Jennisken's Stardust SRC Observing Campaign to provide and exchange documentary data.


3. Select Launch System

A Japanese TR-IA Sounding Rocket Japan Aerospace Exploration Agency Photo

At present two distinct approaches are available for launching and deploying synthetic meteoroids, a conventional orbital launch and a ballistic launch with stages driving a payload back toward the earth to obtain higher than orbital velocities. Each has its advantages. Conventional orbital launch will provide lower but probably sufficient reentry velocities. Orbital systems have the advantage that the deployment vehicle could be "parked" in low earth orbit, providing a time delay from launch opportunity to eventual realization of the work. Such delays might be advantageous to accommodate unfavorable weather such as clouds or smog. Low earth orbit systems can reach most parts of the earth and can achieve most desired orbital inclinations, which will control the direction of travel of the synthetic meteors. Since orbital launches are relatively commonplace, there is greater opportunity to piggyback on launches serving another primary customer.

Ballistic launches, on the other hand, provide an entirely different set of capabilities. While orbital system reentry paths are restricted to relatively flat flight path angles, essentially parallel to the horizon, ballistic systems could provide very steep flight path angles right up to normal to the earth's surface, or straight down. Ballistic launches also provide the ability to achieve higher reentry velocities. Orbital Sciences Corporation, of Dulles Virginia, USA, have indicated that they may have the capability to achieve reentry velocities as high as 30 km/s in a program designed to test heat shields for the manned Mars mission. A variety of boosters are available for ballistic missions including de-comissioned ICBMs such as the Peacekeeper or Minotaur down to so-called sounding rockets, used for meteorological sampling. A ballistic mission would almost certainly require a dedicated launch, however the lower cost of sounding rockets and potentially simpler deployment vehicle might prove cost-competitive with an orbital launch. This is due to the fact that an orbital system might require a deployment vehicle thruster to provide a de-orbit burn.

The choice of a launch system is critical and is ultimately highly inter-related with the design of the synthetic meteoroids themselves.

4. Design and Construct Deployment System

ODERACS Sphere Delivery System NASA photo

The deployment system is responsible for the release of individual synthetic meteoroids and may also be responsible for providing a guidance system and thruster for de-orbit burn as well as cross-range spacing of the meteoroids. The required functionality of the deployment system as well as its form factor are largely determined by the launch system. The requirement for a guidance and thrust system on the deployment system will significantly increase its cost and development time and every attempt will be made to avoid this requirement, either through employing the capabilities of the launch system or re-purposing existing vehicles. For example, the deployment system may be able to be adapted from existing vehicles such as an ICBM MIRV final stage or "bus".

It has been suggested that the academic community be involved for participation in the design and construction of the deployment vehicle. Graduate programs in aerospace engineering are eager for the rare opportunity to design and construct hardware which will actually be deployed in space and can provide a very cost-effective alternative to commercial aerospace firms. This approach has been used successfully by the NASA Orbital Debris Program Office to develop the deployment vehicle or "sphere delivery system" for the ODERACS project, probably the device most closely resembling the synthetic meteoroid deployment system found thus far. ODERACS, for Orbital DEbris RAdar Calibration Spheres, was a mission flown as a Space Shuttle "Getaway Special" in 1994 to release metallic spheres of a known diameter to accurately calibrate the ground based radars which track orbital debris.


5. Flight Test Payload

A Progress Vehicle atop a Soyuz Booster gantry at Baikonur Cosmodrome, Khazakstan S.P. Korolev Rocket and Space Corporation Photo

Due to the unknowns involved in the design of the synthetic meteoroids, as well as for reassuring the governments and populations of the host cities, it is essential to perform at least one flight test of the complete system, especially the payloads themselves. This will provide empirical validation of the desired optical performance, confirm complete demise in reentry, and validate many aspects of the delivery system including at least a subset of the final deployment system.

A development scenario has been proposed for relatively low-cost and timely flight testing. The International Space Station is routinely re-supplied by Russian Soyuz/Progress robotic vehicles, produced by the S.P.Korolev Rocket and Space Corporation Energia, which deliver two and a half tons of supplies to the station every few months. The vehicles blast off from the massive Baikonur Cosmodrome in Kazakhstan and reenter and burn up in the atmosphere after having been loaded with refuse from the station. It would appear that the Russians are allowing modest piggy-back missions to be flown on Progress flights. It has been suggested to approach the Russian launch program managers via European Space Agency intermediaries about flying a synthetic meteor test as a Progress re-supply piggy-back. After the Progress vehicles have resupplied the Station they are guided to a planned reentry demise over the South Pacific. Prior to the demise the piggy-back mission would be activated, releasing one or more test synthetic meteoroids. The guidance and de-orbit thruster capabilities of the Progress vehicle would be employed for the piggy-back mission, reducing the deployment system requirement to a simple mechanical release system, similar to ODERACS. The deployment over isolated regions is compatible with the test objectives of safely verifying performance, but does somewhat complicate the performance validation aspects. The artist is in the process of investigating this possible test scenario.


6. Debut Reentry Piece

Studies for Synthetic Meteors V1.0 frame from video animation

Having flight tested the hardware one or more times to assure desired performance, it then becomes possible to contemplate the debut of the Reentry Pieces. To meet the artist's objective of producing public art, it would be essential to execute the piece over at least a modestly sized city. Dr. Jenniskens suggested Hawaii as a site for a variety of reasons. Being a group of islands which are isolated in the Pacific there is a very small risk of material reaching inhabited ground should a system malfunction. The work would be mounted in the skies over Oahu, but the trajectories in a typical West to East orbit would take the materials far out to sea to the east of the islands in the unlikely case that any material survived reentry. The Hawaiian Islands also boast a number of superb optical and radar observatories which would be ideal for the scientific study of the Reentry Pieces which the associated scientists would like to conduct.

The potential for a Reentry Piece to alarm the population on the ground is another possible concern for any host city and the relatively small and contained population of the Hawaiian Islands would provide a controlled opportunity to address these concerns. The total population of the islands is only about 1.2 million with nearly 75 percent of that population being in Honolulu City and County. A concerted effort would be made to publicize the event in advance, and then an assessment could then be readily made of any significant increase in 911 or emergency services calls. Unlike the situation on the mainland, where the event might be seen by residents of localities relatively far removed from the host city or recent visitors who are not aware of the artwork, the Island's character allows a more uniform level of notification.

Summary

The above describe the major signposts on the roadmap for ultimate realization of the Reentry Pieces. The Heaven & Earth Foundation will be applying for study grants to allow all parties to begin to address some of the uncertainties as well as develop budgetary requirements and cost analyses of the various trade-offs. Once these analyses are complete basic decisions on candidate technologies can be made and grants for full funding of the first artwork applied for.

1) Results from an Artificial Iron Meteoriod at 10 km/sec, R. E. McCrosky and R. K. Soberman, Smithsonian Contributions to Astrophysics, Vol 7, 1963