In our everyday lives we take many things for granted. We use car navigation systems, we check the weather forecast, we watch TV without being aware that we are using devices that are located in space.
How would we feel if in the all these amenities would become less reliable or even worse – completely disappear?
The bad news is: This is not a hypothetical scenario. It is certain. The good news is: We can still do something about it if we act now.
But what is happening that threatens our technological status-quo?
Since the beginning of the spaceflight era some 50 years ago there have been close to 5000 launches of satellites and parts that were put together in space to form even bigger satellites, the space stations Mir (controlled re-entry in 2001) and ISS.
All of these carried not only payload but also expendable parts of the launch vehicle to space. Many of these objects have reentered the atmosphere and burned up but to date there are roughly 6500 tons in mass of satellites and rocket upper stages circling the earth. Since their orbits differ speeds relative to each other can be in the order of 40000 km/h. When two such objects collide they break up into a large number of smaller space debris items, increasing the risk of cascading subsequent collisions (Kessler syndrome). This of course not only affects commercial applications. Also, eventually all space based scientific research and space exploration activities would come to a complete halt. Four major collisions have already been observed to date. The number of trackable space debris objects has reached about 17000. This mainly comprises objects that are larger than 10 cm. About 11000 of these objects result from collisions and explosions due to residual fuel. Of the remaining 6000 larger objects only about 1100 are functioning satellites.
The number of space debris fragments between 1 and 10 cm in size is much higher and ranges in the order of 700000 objects. Parts smaller than 1 cm even reach a staggering population of tens of millions. Parallel and independent studies by six space organizations have shown that even with mitigation measures in place, an increase of the number of Low Earth Orbit (LEO) objects larger than 10 cm of approximately 30% is to be expected within a 200 year time span. This indicates that the critical mass today has already been reached.
Experts from all over the world have gathered at ESA’s Euopean Space Operations Center in Darmstadt Germany last week for the 6th European Conference on Space Debris to discuss the way ahead. The urgency is evident. But what can be done to reduce collision risks?
With a lead time of a few days, tactical avoidance maneuvers can be planned and executed for active spacecraft if a conjunction is predicted. Each of these maneuvers reduces the service lifetime of autonomous spacecraft because of the limited amount of fuel available on board. An increasing number of avoidance maneuvers hast been performed in the past few years. ESA executed 11 such maneuvers in the past two years. An extremely critical one was performed by Envisat in 2010 to avoid a rocket upper stage where distance was increased from only 50m to 135 meters. Also ISS has performed 6
maneuvers in the past two years.
Even if the collision risk is reduced, any impact can result in substantial damage or the loss of the spacecraft. Minimization of damage can be achieved by advanced shielding, adding redundancy or improved system design that avoids placing the most critical components near to the surface, if possible.
Shielding can be very effective but is always comes at the cost of mass that has to be transported to orbit. The Columbus laboratory on ISS is shielded by 2 layers of Aluminum with thickness of 2,5 and 4.8mm which are 11 cm apart with inlays of Kevlar and Nextel which weighs 33kg per square meter. Observations and statistics show that ISS has to be protected against about 40 impacts of particles between 1mm and 1 cm per year. Collision of an object as big as ISS with fragments >1cm would currently occur once every ~15 years.
Mitigation actions for future missions comprise a mission design that limits the in-orbit time of a satellite to 25 years which from a statistical point of view does not pose a high risk of collision. Passivation is performed by fuel depletion and battery discharging to minimize explosion risk.
Enough fuel shall be retained to perform a de-orbiting burn and controlled re-entry.
No international law is instated yet to put these measures in place. Compliance is still voluntary.
Unfortunately mankind tends to be rather inert when it comes to problems that don’t concern their immediate future. Experts at the space debris conference stressed the need to put such regulation in place and make compliance mandatory.
Branch 3 of the ESA Clean Space Initiative is currently focusing on mitigation techniques and technologies.
Active Debris Removal (ADR)
In addition to mitigation actions the active removal of debris is necessary to stabilize its population. Simulations show that when starting ADR missions in the next 10 to 20 years, 5 to 10 of the larger objects mus be removed from orbit each year in order to to avoid a further population growth. If we continue with business as usual and begin removing debris at that same rate of 5-10 objects only 50 years from now, the population growth of space debris will be unstoppable. Many more objects would then have to be removed to maintain level.
But which space debris objects should be targeted to reduce the risk of further debris population growth and how can the debris be removed from orbit?
The effectiveness of removal depends on several factors like natural lifetime, spatial density , mass and cross section.
Due to atmospheric drag in the ionosphere, Objects in lower orbits will re-enter the atmosphere quicker than the ones in higher altitudes. The relation between altitude and on-orbit lifetime is exponential. Whereas an Object placed at 600 km altitude will stay in orbit for ~25 years, a comparable object at 800 km altitude will remain in orbit for ~200 years. Active debris removal will therefore focus on orbits higher than 600km unless very large objects are detected for which an uncontrolled re-entry poses a high risk for life on ground. Objects with a mass above 1 ton are likely to not entirely burn up in the atmosphere. However the probability that one of the 7 billion people on earth will be hit is very low. When a large object re-enters the atmoshphere just like the UARS satellite did in 2011, this chance is less than 0.04%.
Calculation of on-orbit collision risk for individual objects remains a statistical exercise. Trajectories of larger debris can well be tracked. Reliable prediction of orbits however is limited to less than a week which makes deterministic analysis impossible.
The highest collision risk is encountered in areas with high spatial density where many spacecraft operate fairly close to each other. The major part of objects is located in low earth orbit. Here we see the highest densities in altitudes between 700 and 1000 km with a peak at 800 km.
This region accumulates many spacecraft with polar orbits. Although fairly well distributed over longitudes, the orbits cross each other over the poles, resulting in a very high collision risk with very high relative speeds.
In LEO one sattellite passes 10 objects in less than 2km distance each week. Although there are only roughly 1300 cataloged objects in geostationary orbit (GEO) the spatial density is quite high since tolerances to remain in coordinated rotation with earth at zero inclination are very low. Although this region is most crucial for communication and broadcasting, first active debris removal missions will surely not be focusing on this region.
Mass and cross section
Mass and cross section of spacecraft usually correlate. However the consequences of an impact would be different. The higher the cross section of a spacecraft, the higher its probability to hit another object. If little mass is spread out by that impact, the cascading effect is less severe. If objects of high mass collide with each other, many smaller objects with still considerable mass are produced which would have higher impact in consecutive collisions. The highest accumulation of mass can be found in altitudes of 800km and 1000km.
All these parameters lead to a list of candidates the removal of which would prove most effective in reducing a debris population growth. However there are even more factors to consider when preparing for a first mission. Legal situation prevents space agencies from targeting any debris they would like to remove. The responsibility for any space object remains with the launching state. Legal conflicts could arise when removing debris that belongs to another state, especially in case a mission fails. Space agencies today are only in the very beginning of cooperation on that matter. The 6th European Space debris conference participants consensually emphasized that the way for cooperation must be paved as quickly as possible.
Taking into consideration all of the above, Envisat and ESR1 would be high on the list of possible priority targets for upcoming ESA ADR missions. However, even if the first demonstration missions (like DEOS of DLR ) which carry the targets they will capture with them to space, have proven successful, the way to the first real ADR mission is sill long. In order to prevent additional collisions in case of a mission failure focus might not necessarily be put on debris targets with high mass in high density regions before more confidence in the ADR technologies has been gained.
To remove debris from its current congested orbit three general concepts are considered.
Uncontrolled re-entry, controlled re-entry, and re-orbitation. The latter does not remove the object from orbit but increases its altitude to orbits with lower collision risk. On-orbit lifetimes are significantly prolonged by that, which might sound like postponing the problem instead of solving it. However for objects at orbits around 1400km this is currently the most feasible solution. It will ensure later reentry well separated from other objects orbits or still leave the option for removal when spaceflight has become more cost efficient.
Mission strategies include launching
- 1 chaser targeting 1 piece of debris
- 1 chaser consecutively targeting several pieces of debris
- multiple deo-rbiting kits, each chasing 1 piece of debris
- mulitiple de-orbiting kits, each consecutively chasing several pieces of debris
Some of the candidate technologies for ADR sound quite adventurous. Scientists are working hard at advancing them and analyzing their feasibility.
Technologies range from robotic arm capture with tentacles to harpoons, net capture, attaching de-orbiting kits, attaching drag sails, electrodynamic and electrostatic tethers, ion beam shepherds and tractor beams.
The upcoming blog posts will take a closer look at each of these technologies and their application envelope. Stay tuned.
I would like to thank Holger Krag from the ESA Space Debris Office at ESOC for taking the time to chat with me and patiently answering all my questions!