OrbitalHub

Credits: NASA

Let us see how the areas mentioned in the previous Sustainability in LEO post are covered at national level in the United States.

The United States has implemented a space traffic management program in the form of the Joint Space Operations Center (JSpOC) of the U.S. Strategic Command at Vandenberg Air Force Base in California.

JSpOC conducts periodic conjunction assessments for all NASA programs and projects that operate maneuverable spacecraft in low Earth orbits (LEO) or in geosynchronous orbits (GEO). Depending on the mission, the conjunction assessments can be performed up to three times daily. If JSpOC identifies an object that is expected to come in the proximity of a NASA spacecraft, and the collision risk is high enough (for manned missions the minimal value accepted is 1 in 10,000, while for robotic missions the threshold is 1 in 1,000), a conjunction assessment alert message is sent to the mission control in order to have collision avoidance maneuver commands sent to the spacecraft. The alert messages contain the predicted time and distance at closest approach, as well as the uncertainty associated with the prediction.

The control of the creation of space debris is addressed by orbital debris mitigation standard practices in four major areas: normal operations, accidental explosions, safe flight profile and operational configuration, and post-mission disposal of space structures. There are also NASA standards and processes that aim at limiting the generation of orbital debris.

The commonly-adopted mitigation methods, which focus on minimization of space debris creation, will not preserve the near-Earth environment for the future generations. As a matter of fact, the debris population increase will be worse than predicted by LEGEND-generated models due to ongoing launch activities and unexpected (but possible) major breakups. Here is where active space debris environment remediation comes into play.

The active space debris environment remediation is mainly concerned with the removal of large objects from orbit. Such large objects are defunct spacecraft (i.e. communication satellites that exceeded their operational life), upper stages of launch vehicles, and other mission-related objects. The removal of large objects from orbit is known as Active Debris Removal (ADR). Several innovative concepts are under study. Among them, tethers used for momentum exchange or electro-dynamic drag force, aerodynamic drag, solar sails, and auxiliary propulsion units. LEGEND studies have revealed that ADR is a viable control method as long as an effective removal selection criterion based on mass and collision probability is used, and there are at least five objects removed from orbit every year. The electrodynamic tethers seem to lead the competition so far, as they have a low mass requirement and can remove spent or dysfunctional spacecraft from low Earth orbit rapidly and safely.

Re-entry in the Earth’s atmosphere of space mission related objects is an important aspect to be considered in this context. Even though no casualties or injuries have been reported so far being caused by components of re-entering spacecraft, fragments from space hardware pose a risk to human life and property on the ground. One big concern is caused by the fact that the point of impact from uncontrolled re-entries cannot be calculated exactly. The uncertainties are due to a large number of parameters that affect the trajectory and the heat of ablation of objects re-entering the atmosphere.

Credits: NASA

Space debris mitigation measures address issues in two major areas: protection from space debris and reduction of the space debris population growth.

Protection methods vary depending on the size of the space debris. Space debris fragments smaller than 0.1 mm in diameter do not have enough energy to penetrate protection panels. Even spacecraft thermal blankets and structural panels can offer protection from such small kinetic impactors.

However, larger debris fragments pose a more serious threat and there are few active measures that can be adopted to minimize the possibility or the consequences of impacts.

First, avoiding space debris is the method that always proves successful, so flying at altitudes and inclinations where space debris density is low should always be considered. Second, orienting sensitive surfaces away from or mounting a bumper on the leading edge can offer more protection for the spacecraft. Multilayered bumpers cause the fragmentation of the space debris and also prevent ejected material from dispersing from the point of impact. Bumpers add mass and volume to the spacecraft and this is why few space missions can exercise this option.

A promising new method for protection from orbital debris impact is shielding with metallic foams. Extensive research and experimental impact campaigns have proven that metallic open-cell foams provide improved protection against hypervelocity impacts with almost no ejecta generated at impact, while offering comparable mechanical and thermal performance to honeycomb structures, which are currently used for shielding. The fact that no ejecta are generated is of great importance, as fragments generated at impact can add to the already existing debris population in Earth orbit.

At this point, it appears that the best protection method is avoiding the creation of space debris.

The long-term projections of the space debris environment generated by models like LEGEND or DELTA have proven that only active methods can maintain a stable environment of artificial objects in Earth orbit. It is quite obvious that unchanged operational practices or even an immediate stop to launch and release activities will not prevent the collisions between already existing space hardware. Scenarios like the one proposed by NASA scientist Donald J. Kessler in 1978 are very likely to occur. In the scenario proposed by Kessler, which is also known as the Kessler Syndrome, space exploration and the use of satellites in proximity of Earth will become unfeasible due to an exponential growth of the debris population caused by collisional cascading. The exponential growth is due to the fact that the material ejected during a hypervelocity impact becomes space debris itself. Laboratory experiments have shown that as a result of such an impact, 1 kg of aluminum can form several hundred thousand 1 mm sized particles.

Failure to address the potential uncontrollable growth of the space debris population will lead to major restrictions on the ability to exploit space. There is a debate in the scientific community over whether or not critical density has already been reached in certain orbital regions and if we are beyond the point where we can address the growth of the debris population.

There are various classifications of the space debris mitigation methods. For example, two broad categories might include, on one hand, measures that restrict the generation of space debris in the near future, such as limiting the production of mission-related objects and the avoidance of breakups, and, on the other hand, measures that restrict their generation for the long term, which include post-mission disposal methods and active measures to remove space debris from protected regions.

There are three major areas that could allow space faring nations to maintain a stable debris environment: space traffic management, control the creation of space debris, and active space debris environment remediation.

Credits: NASA

As mentioned in a previous post, only a small fraction of the existing space debris population is detectable and tracked by ground systems. A smaller fraction is catalogued by special programs and/or departments of national space agencies. This is where statistics comes into play. Numerous models have been created in order to assess present collision risks associated with certain orbits and to predict future evolution of the debris environment around Earth.

The National Aeronautics and Space Administration (NASA) has developed two categories of applications for modeling of space debris environment and risk analysis. The first category, based on evolutionary models such as NASA’s long term debris environment evolutionary model (LEO-to-GEO Environment Debris model or LEGEND), are designed to predict the evolution of the debris environment.

These models cover the near-Earth space between 200 km and 50,000 km, provide space debris characteristics for a debris population consisting of particles as small as 1 mm, and have a typical projection period of 100 years. The second category, which consists of engineering models like ORDEM2000, is used for debris impact risk assessment for spacecraft and satellites, and also as benchmarks for ground-based debris measurements and observations.

The European Space Agency (ESA) has a different set of tools used for modeling the space debris environment and assessing risk associated with collisions in Earth orbit. The DISCOS database (the Database and Information System Characterizing Objects in Space) consolidates the knowledge on all known objects tracked since Sputnik-1, and it is recognized as a reliable and dependable source of information on space objects in Earth orbit. MASTER (Meteoroid and Space Debris Terrestrial Environment Reference) is the agency’s most prominent debris risk assessment tool, which uses statistical methods to determine the impact flux information from all recorded historic debris generation events. ESA also uses DELTA (Debris Environment Long-Term Analysis) to conduct analysis of the effectiveness of debris mitigation measures on the stability of the debris population. Such analysis can cover 100 to 200 year time spans.

Credits: Orbital Sciences Corporation

Orbital Sciences Corporation is proposing a blended lifting body vehicle that will launch atop an expendable launch vehicle in response to NASA’s Commercial Crew Development-2 contract solicitation. The proposed configuration will provide safe and affordable transportation services to and from the International Space Station. The vehicle will carry a crew of four astronauts, and will reenter the Earth’s atmosphere and land on a conventional runway similar to a Space Shuttle.

The launch vehicle proposed for the launch stack is the United Launch Alliance Atlas V rocket. Orbital’s press release mentions that the whole configuration is flexible enough to accommodate other launch vehicles as well.

Orbital is leading a team of world-class space system manufacturers. The pressurized crew compartment will be provided by Thales Alenia Space, the human-rated avionics will be the responsibility of Honeywell and Draper Laboratory, and the United Launch Alliance will supply the launch vehicle. Northrop Grumman will be the airframe structures designer.

Credits: Mark Dowman

Airships are making a big comeback now as the energy consumption for all modes of transportation is being re-analyzed. Missions with special requirements like surveillance and reconnaissance missions and transportation of heavy payloads to remote outposts are the main driver for the reinvention of the airship.

But Earth is not the only place where airships can be deployed. There are a number of destinations in the solar system that would make a perfect environment for deployment and operation of airships, like Mars, Venus, and Titan – Saturn’s largest moon.

The presence of an atmosphere makes possible the use of vehicles that can fly within atmosphere for planetary exploration. Also, planetary exploration with low-powered vehicles like airships really makes sense considering the fact that energy is always at a premium.

So far, the only extraterrestrial deployment of an airship was performed during the Vega mission to Venus, in 1984. Two balloons were released and they floated 54 km above the planet’s surface for nearly two days.

Lighter-Than-Air (LTA) AERial ROBOTS (AEROBOTS) would present some advantages over their Heavier-Than-Air (HTA) siblings and the traditional planetary scouts, the exploration rovers: they would have long-duration mission and long-distance capabilities, they would not have to deal with obstacle avoidance problems, and they have low-power consumption. However, the environment in which the airship will operate will impose some restrictions on the capabilities of the airship (consider things like atmospheric composition and density, temperature, and the amount of solar radiation available). More on the planetary environments in the solar system and airship evaluations for each one of them can be found here.

NASA has funded a number of projects for solar system exploration that make use of aerobots. The Jet Propulsion Laboratory’s Planetary Aerobot Program is developing balloons to support scientific payloads in the atmosphere of other planets in our solar system. You can find more details about JPL’s Planetary Aerobot Program here.

Credits: NASA

In a nutshell, it is really tough! The higher you go, more bad things can happen to you… the increasingly rarefied air, freezing temperatures, ionized atoms, radiation, and space debris make life challenging. So, besides thinking of how to place spacecraft in orbit, engineers must consider all of the factors mentioned above (and much more) when designing a spacecraft.

The space environment (the vacuum, the radiation, the space debris, etc.) definitely poses big challenges to spacecraft design engineers. From 1971 to 1989, more than 2,700 spacecraft anomalies related to interactions with the space environment were recorded. These interactions with the space environment are called space environment effects and the changes in the space environment define what is called the space weather. Believe it or not, there are dedicated programs aimed at developing the ability to predict these changes in the same way the weather forecasting does for terrestrial weather. The Space Weather program was formed in the mid-1990s by the National Science Foundation (NSF). The Europeans developed a similar program under the umbrella of the European Space Agency (ESA).

The space environment effects can be grouped into several categories. Such categories include: vacuum, neutral, plasma, radiation, and micrometeorid/orbital debris. So, basically, we can discuss the effects of the vacuum environment, the neutral environment, etc. Each one of these environments interact with the subsystems that comprise a spacecraft: the propulsion system that provides the means of maintaining a certain orbit or attitude, the electrical power system that provides power to the rest of the subsystems onboard, the thermal control system, the attitude and orbital determination and control system, etc.

The vacuum environment imposes challenges when it comes to designing the structure, choosing the materials, and defining a strategy for thermal control. The pressure differential between the inside and the outside of a manned spacecraft is tremendous (around 350 km above the surface of the Earth, the pressure is ten orders of magnitude less). The lack of atmosphere translates into the fact that the spacecraft will have to deal with solar ultraviolet (UV) radiation (the UV radiation is energetic enough to degrade material properties). Also, the spacecraft can only cool itself by conduction or radiation.

Credits: NASA

Even if very rarefied, the neutral atmosphere in low Earth orbit is dense enough to cause a significant atmospheric drag force. The atoms can physically sputter material from surfaces and even cause erosion. All these mechanical and chemical interactions depend on the atmospheric density.

In low Earth orbit, the solar UV radiation ionizes the oxygen and nitrogen atoms. This environment, known as the plasma environment, can give rise to very interesting effects, like spacecraft charging and arcing between regions of differing potentials.

By far, the most dangerous environment in Earth orbit is the radiation environment. In the regions of charged particles, known as trapped radiation belts, particles with energy levels in the order of MeV pass through the surface layer and interact with the materials inside the spacecraft. Present shielding technology cannot protect living organisms inside a spacecraft in these regions.

Micrometeoroids and orbital debris are a cause of great concern to spacecraft design engineers and spacecraft operators as the kinetic energies associated with impacts at orbital velocities are very high. The main effect on spacecraft in this case is the physical damage upon impact. Other effects include surface erosion, ejecta resulted from impacts, changes in thermal control properties, and generation of electro-magnetic impulses (EMIs).

As most of the characteristics of the space environment were determined by remote observations or during short duration missions, one long duration mission was necessary to verify and validate these measurements.

In April 1984, the Space Shuttle Challenger placed into low Earth orbit (LEO) a spacecraft carrying a number of experiments for the purpose of characterizing the low Earth orbit environment. The spacecraft (known as the Long Duration Exposure Facility, or LDEF for short) was a twelve-sided cylindrical structure three-axis stabilized in order to ensure an accurate environmental exposure. The spacecraft was supposed to spend one year in orbit, but just before the planned retrieval, the Space Shuttle fleet was grounded as a result of the Challenger accident on January 28, 1986.

The spacecraft was returned to Earth by the Space Shuttle Columbia in January 1990. After almost six years in low Earth orbit, the results of the experiments onboard the facility contributed a great deal to the understanding of interactions between artificial objects and the environment in low Earth orbit.

You can find all the above in much more detail in Alan Tribble’s book The Space Environment – Implications for Spacecraft Design. Alan Tribble presents an excellent account of the effects the space environment can have on operational spacecraft. The book offers a unique perspective, as it combines the study of the space environment with spacecraft design engineering. .

Alan Tribble spent over ten years designing spacecraft. He is a technical project manager in the International Software Defined Radios group for Rockwell Collins.