Cleaning up space versus cleaning up Everest: which is harder?

Cleaning up places that you can hardly see is something that is not only a rare topic of discussion, but also an unwanted topic when it comes to financial contribution. Yet, it is something that may become a future necessity, and/or something that simply needs to be done, in order to save some of Earth’s most special places from becoming a junk yard.

Let’s take two places that are far away from me but close to my heart: space and Mount Everest.

Few people will have heard about the necessity to clean up space. The junk and litter in space is expanding exponentially, not only due to new trash being left behind but also due to collisions in space (of both functional and non-functional i.e. trash satellites) or sudden breakups of non-functioning satellites by for example an overheating battery exploding. The graph below shows the increase of objects in space, the ones that we can track from Earth, and the trend is clear. It is getting worse and worse. While there are millions of other small pieces in space (that we cannot track from Earth), the objects shown (22300 in February 2020) below are important as a collision with them will release so much energy that whatever is hit by them will break. If this is the centre of a functioning satellite, that satellite will be no more. If it is a habitat on the International Space Station, the astronauts inside will be no more. Protected regions have been identified by the United Nations, such as any orbit below 2000 km, and the world is requested not to leave space behind within the protected regions for more than 25 years. While agencies such as ESA and NASA are indeed now complying to this request, generally there are no laws enforcing this.

On Mount Everest, there is an ever increasing number of climbers each year, and while there is likely to be an increase in ‘responsible climbers’ e.g. climbers leaving no trash behind, or even leaving no trace, one could still expect a fair amount of litter every year from the less responsible climbers or parties that had emergencies etc. In contrast to space, Everest also has several dead bodies lying around. While some bodies have been retrieved, I will focus on removing trash only in this article.

So how do removals of these two places compare?

One similarity is that on both places, trash is located at several important places. At Everest for example, there is trash at basecamp and trash at higher camps. Basecamp trash (at 5.3 km) is relatively easy to remove due to the high amount of people and logistics (I say relatively because it is still a remote place!). Trash at the infamous ‘camp 4’ (at almost 8 km) on the Everest South side, is extremely difficult to remove since it is located at an altitude touching the death zone: only few people visit it every year, and the energy required to load and walk with heavy packs at that altitude is enormous for a person. In space the space debris is also grouped around several locations (a location is often defined in space as the combination of altitude and inclination of the orbit plane with respect to the equator). There is a large cluster of space debris in the altitude band of 600 km to 800 km, with orbit planes close to 90-100 degrees inclined from the equator: (near) polar orbits with satellites covering the entire globe including the poles. These Low Earth Orbits (LEO) are normally used for Earth Observation satellites, and during the Cold War in the past for spy satellites. Not only is it littered with debris of non-functioning satellites (red/green parts in the first graph), there is also an equal amount of final rocket stages (yellow/blue/orange parts); the stage that puts the satellite into orbit is normally also staying in that orbit! At higher altitudes the amount of debris decreases strongly, but at the Geostationary altitude 36000 km there is again a peak, much like camp 4 on Everest. A large amount of energy is required for a rocket to reach that altitude, again a similarity with reaching camp 4 altitude on Everest.

One advantage though in space, is that trash at lower altitudes get cleaned up automatically over time. For trash located at altitudes up to 600 km, the atmospheric drag will eventually lower the orbit of the trash, until it enters the atmosphere and burns up. This can take 25 years for trash at around 600 km, or a few days for trash at 200 km altitude. It would be great if trash at Everest base camp would be removed automatically!

A first difference is that expeditions to clean up Everest have already taken place. Even last year a team of 14 members removed 3000 kg of trash from the high camps of the mountain. This also gives an indication of a amount of trash that can be removed. In space, catching trash is intrinsically complex (as shown in a previous blog) and has never been done before by robot means. It took three astronauts to catch a satellite since from the moment you touch something in space, it floats away. The European Space Agency was studying a satellite to remove a 10-tonne trash using a robot arm. This satellite was weighing 1400 kg itself. This proposal did not get funded however currently ESA did get approval to design a satellite to remove a 100 kg trash by 2025 by means of four tentacles grabbing the trash. This debris is located in the lower altitude band i.e. 700 km.

The financial aspect of these expeditions is incomparable though, and is therefore another large difference. While an expedition for a crew of 14 to remove trash from Everest will cost several hundred thousand of dollars, ESA’s mission to remove 100 kg is costing 100 million euro ($111 million). Since this mission is a first, future trash removals are likely to be at lower cost when one mission removes several pieces of trash, and spacecraft recurring cost will be significantly lower than the first satellite. Even so, removal costs of several million euro per object can be expected.

The expedition/mission itself shows both similarities as differences, with the ascent phase being the phase that shows differences. First one needs to get to base camp, which is climb of several weeks in itself. Making the synergy of Everest base camp with ‘getting into space’, the ascent into space is in the order of minutes and is very direct. Then, a typical climb of Everest would be in rotations e.g. an ascent to camp 1 and back, and ascent to camp 2 and back, and ascent to camp 3 and back, and then finally to camp 4. It is likely though that this can be sped up since it is often the sherpa’s that perform these kind of cleaning expeditions, and they may not require so many rotations to reach camp 4. When it comes to removing several parts of trash in space, another issue occurs. It is easy for a spacecraft to move up and down to catch debris at for example 722 km and 651 km. However the orbit planes of these pieces of trash are likely to be different, and a move from one orbit plane to another is extremely costly in terms of used energy by the satellite’s rocket engines. An alternative is to wait in orbit: orbit planes rotate around the Earth with a rotational speed depending on the altitude. Therefore to go from one piece of trash to the next one, the satellite would 1) wait until its own orbit plane has rotated in such a way that it matches the orbit plane of the trash, and 2) change the orbit altitude to match the altitude of the trash. So while this could be compared to rotations on Everest, the waiting time in orbit could be extensive e.g. several months or even more than a year. Operators will have to optimise the trash removal satellite’s path in space to minimise the total lifetime (and therefore cost).

The descent is in principle similar: Everest expedition members will take the trash down to base camp, from which it is further removed to either trash or storage. From space, trash needs to be lowered to low altitudes, so that atmospheric drag will eventually disintegrate the trash and pieces will burn up. For large objects of trash, some heavy parts like propellant tanks or carbon-fibre antennas may still survive the re-entry; for these parts it is necessary to control the re-entry i.e. the re-entry needs to be steep so that operators can force any surviving part to fall down on uninhabited areas. This is a costly operation. There is however one major difference with Everest: operators may also opt to move the trash from lower altitudes up instead of down, to an altitude of at least 2000 km (and lower than 35800 km). From this altitude onward the amount of satellites is small and trash may be deposited here. For geostationary satellites at 36000 km altitude, moving the trash down to the atmosphere is too costly in terms of energy, and GEO satellites are typically put into a graveyard orbit, 200 km above the 36000 km GEO belt. Unlike Everest, this moving of trash to a different altitude is not exactly cleaning up space, but rather moving it to a different location.

The conclusion is that there are quite a few similarities with these two cleaning projects; the special location that makes it difficult to get to, the general logistics of moving up and bringing down, and the mass of the trash brought down. But cost, duration, and the use of humans or robot arms, is very different between the two. The table below gives a small comparison to summarise.

	Everest	Space
Location of trash	Mainly located at camp 4, some at lower camps	Scattered within 600-800 km lower altitude bands, and at 36000 km altitude
Reason for trash	Litter or unforeseen events	Litter, explosions or collisions
First step	3-week hike to base camp	20-minute launch to space
Core expedition / mission	Rotations	Moving up/down in altitude to match trash altitude, plus orbit plane drift to match orbit plane of trash
Removal	Bring trash down	Bring trash down (for trash at lower altitudes) to burn up into the atmosphere, or moving up to a graveyard orbit
Expedition members	Several crew members	Robotic
Amount to remove	A few thousand kg	A few thousand kg
Cost	Several 100k	Several million
Duration	Several weeks	Days to several months

The space frontier is still more difficult, and more expensive to reach than any location on Earth. While this may change in the future, the removal of trash is currently more complex than the removal of trash on Everest. Within a few years however, a robotic device will remove trash from space. There are strong developments in the fields or robots with locomotion, or rovers, for Earth military applications and nuclear waste cleaning,. It is therefore remarkable that few developments seem to be made in the field of cleaning rovers for mountain terrain on Earth. Any Master of Science student looking for a topic to graduate?

Ciao!
Robin
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