Lucas Mancinelli AE 2550

April 16, 2016 Meteoroids and Space Debris

Introduction Since the launch of Sputnik 1 in 1957, humankind has left a notable imprint in Earth’s orbit. According to data from the European Space Agency (ESA) 4800 launches have placed some 6000 satellites into orbit and of those less than a thousand remain operational. In space there is no blanket, like the earth’s atmosphere, that prevents meteoroids and space debris from striking objects. This can impose a serious risk for astronauts and satellites sent outside Earth’s atmosphere. Meteoroids are usually fragments of an asteroid or comet consisting of rock and/or metal. They can be large fragments or small fragments called micrometeoroids which are generally fragments from comets smaller than a grain of sand. (Micrometeoroids and Space Debris) Space debris is “all man-made objects in orbit around the Earth which no longer serve a purpose.” This definition is widely accepted and used after it was adopted by the Inter-Agency Space Debris Coordination Committee (IADC). The IADC is an international government forum for the worldwide coordination of activities related to the issues of man-made and natural debris in space. (NASA Orbital Debris FAQs)

Figure 1: Shows the amount of debris given off by each of the included sources.

Space debris is becoming a growing concern for astronauts and spacecraft engineers. The thousands of space launches carried out have left varying degrees of debris. Derelict spacecraft and upper stages of launch vehicles, carriers for multiple payloads, debris intentionally released during spacecraft separation from its launch vehicle or during mission operations, debris created as a result of spacecraft or upper stage explosions or collisions, solid rocket motor effluents, and tiny flecks of paint released by thermal stress or small particle impacts are all examples of the growing space debris problem. (NASA Orbital Debris FAQs) Although things such as paint chips are small they are still a serious risk to astronauts doing extravehicular activities. They are of such great concern because they travel at speeds of about eight thousand meters per second. Engineers are working towards the advancement in the protection of this space trash. Through various different ways such as thick wall construction and multi-layer shields made of foil and hydrocarbon. Environment and Models Meteoroid models can be a key factor in the safety of an astronaut and a spacecraft. These models can help in the design of exploration missions altering the paths and timing of key elements of the mission to avoid large impact that would be detrimental to the mission. These models range from plotting mass and velocity to determine the severity of the impact to determining when and where an impact will occur.

Graph 1: Meteoroids and space debris flux as a function of diameter. [From Heimerdinger

(2005); copyright 2005 IEEE]

Figure 1: Shows the Hubble Space Telescope (HST) it shows the predicted number of impacts per m2 from meteoroids larger than 10 microns. These results are for the period of 3.62 years

from the start of the HST until the retrieval of the solar array in December 1993. The Grun Meteoroid Model is an isotropic model that uses particle size to estimate meteoroid particle fluxes as shown in graph one. (Grün Meteoroid Model) The model uses an isotropic method to estimate the flux because it is assumed that, relative to the surface of the earth, meteoroids arrive isotropically from all directions and with the same velocity. (Drolshagen) The Grun interplanetary meteoroid flux takes three ranges of particle sizes and sums their integrated fluxes. When meteoroids come in to the vicinity of Earth their density increases due to Earths gravitation.

Graph 2: Grun Meteoroid Model

The Cour-Palais model plots a meteoroids density versus its velocity as depicted in graph two. This is referenced in the Grun model because it correlates with the isotropic environment that meteoroids are in as density increases the closer they are to Earth.

Graph 3: Cour-Palais Model

The NASA orbit debris engineering model (ORDEM) and the NASA meteoroid engineering model (MEM) are important models in viewing the increase of space debris and meteoroids in the atmosphere as well as looking at the number of satellites. MEM is a digital computer model which estimates meteoroid flux in interplanetary space from 0.2 AU to 2 AU from the Sun. ORDEM is a series of models that provides information for spacecraft and debris.

Figure 3: MEM which characterizes the flux, density, speed and direction of meteoroids that a

spacecraft will encounter throughout its trajectory. (Introducing NASA's Meteoroid Engineering Model)

Figure 4 (Left): ORDEM figure is shown from a geosynchronous polar view which is an image

generated from a vantage point above the north pole. The image shows the concentrations of objects in low Earth orbit (LEO) and in the GEO region.

Figure 5 (Right): ORDEM figure is shown is a GEO image generated from a distant oblique vantage point to provide a good view of the object population in the geosynchronous region,

around 35,785 km in altitude. The larger populations in the northern hemisphere is due to Russian objects in high-inclination, high-eccentricity orbits. ()

Figure 6: ORDEM model which graphs LEO, the region less than and up to 2,000 km from the

surface of the Earth. This is the most concentrated area for space debris. (Orbital Debris Graphics)

The ESA Meteoroid and Space Debris Terrestrial Environment Reference Model (MASTER) is a statistical digital computer model of meteoroids and debris originating from comets and asteroids. It can be a good tool in predicting meteoroid flux and the speed of particles in the region up to and above GEO and have a lower size threshold of 1 micrometer. As stated in the above examples the amount of meteoroids and space debris in the atmosphere is continuing to increase. As shown in the picture below the increasing amounts of space debris provide an increased risk for future space missions. When planning future missions more and more analytical factors will need to be implemented to ensure a safe flight occurs in space.

Figure 7: The Menace of Earth-Orbiting Space Debris

Space debris most commonly forms in a debris cloud. A debris cloud is a concentration of debris in a region of space and occurs with the fragmentation of a single object. In the case of a collision two debris clouds will form from each of the objects. The evolution of a debris cloud forms in three phases. The first phase is the initial debris cloud in which the debris stays in a close knit group as it orbits around the earth. The second phase occurs due to differences in

drag and/ or radiation pressure forces, this is when the debris cloud orbit turns into a toroid, or an orbiting ring around the earth. Then, due to slight changes in the inclinations of the debris, the right ascension of the ascending node of the orbit will have a different precession and disperse. These three phases tend to extend a debris cloud both around the orbit and longitudinally. These are all illustrated in figure eight below.

Figure 8: Shows the development of a debris cloud overtime. (Jones)

The Gabbard diagram is used to to plot these debris clouds in a scatter plot. This scatter plot shows the dispersion of the apogee and perigee altitudes as a function of period for each fragment in the debris cloud. This information can be used to be used to infer information on direction and point of impact.

Graph 4: Gabbard diagram displaying a sample disintegration of a Long March 4 booster in

2000. Credit: the NASA Orbital Debris Office

Debris Mitigation Debris mitigation is a plan in an effort to minimize the adverse effects on the debris environments. Four things should be addressed during debris mitigation: debris produced during normal operations, debris produced by explosions and intentional breakups, debris produced by in-orbit collisions, and post-mission disposal. Release of orbital debris during normal operations should be avoided whenever applicable and feasible. If it is not possible to do, it becomes important that debris be minimized in number, area and orbital lifetime. Debris produced by explosions and intentional breakups should also be avoided as much as possible but in order to ensure safety for the astronauts from post-mission explosions all sources of energy such as propellants, batteries high-pressure vessels, self-destructive devices, fly-wheels and momentum wheels, etc. should be depleted and/or saved until the end of the mission, especially the debris that will remain for long periods of time. Debris generated by in-orbit collisions should be minimized as much as possible in review of various debris information to ensure no in-orbit collisions will occur. Post-mission disposal is an

extremely vital element in the minimizing of space debris. Upon completion of the space mission it is recommended that a GEO spacecraft be increased by 235+1000Cr. NASA’s debris mitigation plan has exponentially helped the amount of space debris left in the atmosphere. However, this does not account for the hundreds of other missions that have taken place that have left the atmosphere in the disarray it is in due to the multitude of space debris. The government and private agencies are working harder and harder every day to find solutions to this growing issue. The three most prominent methods for space clean-up are the Phoenix, Cleanspace One, and Earth Based lasers. Phoenix is a satellite launched into the atmosphere that is able to track satellites that are no longer operational. Once the satellite is tracked down, Phoenix attaches to the satellite and extracts any and all usable hardware. This satellite is known as a servicer satellite because it attaches a module that allows a satellite’s salvaged parts to be used on a new mission. The United States Defense Advanced Research Projects Agency is leading the manufacturing of this servicer satellite Cleanspace One is similar to Phoenix, except that Cleanspace One would act more as a single space debris destroyer. This is because Cleanspace One is design to track down a target satellite, grapple it, and then plunge back into the Earth’s atmosphere. This would inevitably cause the destruction of Cleanspace One, along with the destruction of the satellite. This is being designed by the Swiss EPFL. Earth-based lasers are another way that space debris can be decreased. These lasers are ground based and emit a laser beam strong enough to reach through the atmosphere and decrease the momentum of space debris orbiting around earth. Light exerts pressure, the more pressure used the greater the de-orbit. To de-orbit an object 31 inches wide and 11 pounds in mass a laser beam of about five to ten kilowatts would need to be shined upon the object for two hours. This would cause the space debris to fall into the atmosphere and burn up on the decent to earth, essentially removing the space debris from the space environment.

Figure 8: The above image shows a diagram of how an earth based laser works. (Tate)

Graph 5: Number of space debris objects released into the environment by year.

Graph five depicts the amount of space debris that has been added to space by year. As the desire to explore increases the more missions are carried out, this causes more space debris. As the graph depicts the number of payload debris has decreased due to debris mitigation efforts, however the debris has remained consistently higher than it should. This graph shows that even though debris mitigation procedures have been implemented the issue of Space debris is still a growing issue to the space environment. Collision Probabilities Space Debris and Meteoroid collisions are consistent upon a pattern known as the Kessler Syndrome. The Kessler Syndrome, or Kessler effect, is the scenario in which the densities in low earth orbit is large enough that if a collision occurs it could cause a cascade that would increase the likelihood of future collisions. Opposite the Kessler effect is the Poisson Speed Distribution. The Poisson Speed Distribution evaluates data in a “memoryless” fashion. This means that the number of events in any bounded interval at a certain time is independent of the number of events that occurred leading up to the time of the incident.

Hypervelocity Impact Hypervelocity impacts are impacts in which the particle or material involved in the collision reaches speeds greater than the speed of sound, around four to five km/s. Relative velocity plays a key role in hypervelocity impact. The relative velocity will determine the severity of the impact. When a material exceeds a relative velocity of one to two km/s it results in the impacting material to behave like a fluid. Relative velocity is essential to pay attention to in low earth orbits where particles and materials can have relative speeds of up to 14 km/s. There are three main ways to analyze and predict the the potential damage to a spacecraft from a material traveling at speeds that break the sound barrier. The three methods are: hypervelocity impact testing on earth, analytical analyses and digital simulations and in-situ observations on returned space objects. The ability to predict the damage and possibly penetration of a material becomes an ever increasing problem due to complex spacecraft surfaces, such as a honeycomb structure covered with multilayer insulation (MLI).

Figure 9: This image shows just how much damage a material that is ½ an inch wide can do on a

plate of seven-inch thick aluminum block. (Tate) Shields and Bumpers Shielding is just one of the many ways to help lower the risk of severe collisions with micrometeoroid and orbital debris (MMOD) particles. Shields are a necessity when traveling in space because it is not possible to eliminate all impact risk and these shields are the only thing protecting astronauts in a spacecraft or instruments in a satellite to the vacuum of space. Figure

9 illustrates the amount of damage a particle can do to a thick aluminum block. As noted in the picture it did prevent the particle from breaching the wall, however a four-inch-wide block of aluminum surrounding a spacecraft or satellite is heavy. The weight would make the spacecraft bulky and would require an absurd amount of fuel to send out of Earth’s atmosphere. In the 1940s Fred Whipple proposed a shield that would provide the same durability and reliability as a thick, heavy piece of aluminum for a fraction of the weight. The Whipple shield is made up of a bumper, a standoff and a rear wall as seen in figure ten. The bumper, also known as the sacrificial wall is made up of a thin piece of aluminum mounted a distance from the rear wall. The purpose of this wall is to receive the first impact and disperse the projectile into a cloud of materials consisting of the bumper and fragmented particles. The standoff is the vital element in a Whipple shield. The standoff allows for the debris to spread out as it travels through this blank space. This expansion allows for the momentum of the particle to cover a larger surface area of the rear wall, in turn reducing the amount of stress the rear wall feels at a particular point. As the Whipple shield continues to advance with technology it becomes more and more effective. Materials such as Kevlar and Nextel are combined in the inner wall to form a stuffed Whipple shield. Another Whipple shield advancement is a multi-shock shield. This shield consists of staggering multiple layers of Nextel that repeatedly shock the projectile and debris clouds until there is little to no energy for the particles to breach the rear wall. With the further advancements in technology a reduction of up to fifty percent in mass can be achieved. (Micrometeoroid Orbital Debris Shielding)

Figure 10: Shows a Whipple shield as a particle is striking it.

Conclusion As discussed in the above report space debris has an ever-growing impact on spaceflight and travel. If new ways to avoid the creation of space debris or to start removing the amount of debris in the environment are not created, space travel will be faced with a serious obstacle. Although this will not make it impossible to travel, it will increase the cost of future missions. One of the largest costs will come from the amount of extra fuel needed. Extra fuel will be needed to send a spacecraft into space, due to the increased weight of the shields now needed to prevent from the multitude of MMOD impacts. Extra fuel will also need to be stored to allow the spacecraft to move out of the way of space debris. For this reason, astronauts and spacecraft engineers must be careful because even the smallest loose nut or bolt lost on spacewalking repair mission could cause a cascade of debris.

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