Predicting Orbital Collisions

There are now well over 20,000 objects larger than a softball in orbit around the Earth. We are poised to get even more objects, as several companies are planning on launching mega-constellations of satellites (750-4,000) to provide internet across the world from low-Earth orbiting platforms. While space appears to be quite empty, there are a lot of satellites (and random stuff) out there. The problem is that there is no one “driving” the majority of these objects, and collisions between objects that are moving at 17,000 MPH typically produce even more pieces of debris that could potentially collide with other objects. And the satellites that are being “driven” have only a certain amount of fuel – once that runs out, the satellite is essentially dead. Therefore, satellite operators are not really wanting to move their satellites all of the time.

The United States Air Force (USAF) is in charge of keeping track of the locations and tracks of all objects in orbit around the Earth. There are several websites and apps where you can see which satellites are going to pass over your location at any time. These are pretty cool, because you can often very clearly see orbiting satellites. If you go outside on a clear day just before sunrise or after sunset, while the sun is still shining in the upper atmosphere, but is not shining on the ground (say 10-45 minutes after sunset), and you look up for a long time, you can see little dots of light moving across the sky that are not airplanes. Those are satellites. An app will tell you exactly which satellites those are and how they are moving. You can even see the shape of the International Space Station (ISS) with good binoculars!

These websites and apps get this orbit information from USAF, which tracks them using radars. There are many tracking stations on the ground that are similar to radar guns that police officers use. Simplistically, the radar sends out a pulse that bounces off the satellite and returns back to the radar. The radar can record the time that it took to go from the radar, to the satellite, and back to the radar, which specifies the distance to the satellite. The radar can also record the Doppler shift of the signal, which indicates the speed of the satellite. The strength of the return signal indicates the size of the satellite. This is all a simplification, but in general it is the case. The radar takes a few of these measurements, and then moves on to another object. With a bunch of measurements, the orbital characteristics of the satellite can be determined. The Air Force does this for the more than 20,000 objects on a daily basis. They make the (rough) orbital characteristics available to the public.

(As an aside, you could actually do the same thing with a telescope at home, just by tracking how the objects that you see move across the sky.  It wouldn’t be as accurate as a radar, but just knowing how the object moves across the sky gives you a pretty good estimate of the orbit. Kepler did this with the moons of Jupiter, and you could do it with the ISS!)

The USAF uses an orbit propagator that takes these radar measurements and calculates the position and velocity of the 20,000 objects every second for the next five days (roughly). They then look for any objects that come within a certain distance from each other. Those satellites are flagged to be examined more carefully. There are multiple ways to do this, but I will discuss the easiest to understand here.

First, let’s back up a bit. Let’s say that you want to predict whether two cars that are moving towards the same intersection will collide. One is traveling east and one is traveling south. Consider some scenarios:

  1. The cars are one block apart and both traveling 30 MPH (44 feet/sec) and are exactly 0.1 miles (528 feet) apart. They will meet in the middle of the intersection. A crash is definite, and will take place in about 12 seconds!
  2. Imagine the same scenario (one), but one car is 0.11 miles away. The cars will miss each other by 0.01 miles (roughly 53 feet).
  3. Imagine scenario one, but one car is moving at a slightly slower speeds (say 29 MPH, or 42.5 feet/sec, instead of 30 MPH). In 12 seconds, the car will fall behind its expected distance traveled by about 18 feet, which is about the length of a car, and therefore will end up missing the other car.
  4. Imagine scenario one again, and both cars start off on a perfect collision course, but one of the cars starts to decelerate due to there being a hill in the way. If that deceleration is enough, the speed that the car is traveling decreases enough, and the collision will be avoided.

In predicting whether the cars will crash, there are many things that cause uncertainty: estimating the initial position and velocity as well as figuring out all of the forces on the cars that can cause accelerations. While there are several differences between collisions between cars and satellites, the concepts are the same. Satellites are moving much faster than cars, and there are significantly fewer objects in orbit than cars in, say, downtown New York City. In addition, satellite operators need a couple of days to figure out whether there will be a collision, so they would like to know well ahead of time if they need to move the satellite out of the way.

It seems like it would be relatively straightforward to predict a collision, since the Air Force has the position and velocity of each of the objects, and it uses a really highly accurate orbit propagator to determine the future positions of the satellites. Except, as illustrated above, if there are uncertainties in the initial position or the initial velocity, then it is not clear if there will be a collision or not. In addition, if there are uncertainties in the accelerations that the objects undergo, it will cause uncertainties in whether there will be a collision.

So, what are the sources of those uncertainties? Well, there are a lot of them. Let’s go through them all one by one:

  1. The position could be off due to issues with the radar signals. These signals travel up into the atmosphere, hit the orbiting object, then come back through the atmosphere. As the signal goes through the atmosphere, the speed of the signal actually changes, since the propagation speed is dependent on the medium that it travels through. In fact, the path through the atmosphere is dependent on these atmospheric conditions also. So, moisture in the atmosphere and the ionosphere can both change the path of travel, the propagation speed and therefore the delay. If the atmosphere is not modeled correctly, then this will cause the exact position of the orbiting object to have some uncertainty. And remember that the uncertainty only needs to be about as big as the satellite (a few feet) to cause issues in determining whether there will be a collision.
  2. The velocity of the orbiting object needs to be known really, really, really well. As an example, let’s say that there may be a collision 24 hours into the future, and the object that is moving is 10 feet across. That means that the speed needs to be known to within 0.000116 feet/sec, or 0.000079MPH. That is crazy small. Considering that the objects are orbiting at about 17,000 MPH, this is an impossible task. This is a source of very large uncertainty!
  3. The forces that act on the orbiting objects are quite complex and challenging to model. For example, some of the forces include:
    1. Gravity: While most people think of gravity as 9.8 m/s2, it is more complicated than this. The Earth is to first order a flattened sphere, so that the satellites feel more gravity near the equator than near the poles. To higher orders (literally), the orbit propagators need to take into account many of the features of the Earth, such as mountain ranges and oceans. The equation that is used to (accurately) describe the gravity has several hundred terms in it. If you discount these terms, the positions of the objects can be systematically miscalculated, which is bad. The European Space Agency has launched several satellite missions to accurately model the gravity of the Earth, such as CHAMP, GRACE, and GOCE.  Neglecting higher order gravity terms can cause the satellite orbits to be off by several hundred meters after a day.
    2. Sun and moon Gravity: The sun and the moon both exert forces on objects in orbit around the Earth, so they need to be taken into account. Luckily, the orbits of the Earth around the sun and the moon around Earth are pretty well known, so these are easy forces to account for.
    3. Sunlight: Sunlight bouncing off the orbiting object actually imparts momentum to it. While this force is quite small, it actually is very important to model correctly. It is dependent on the materials that the orbiting object is made out of (if the material absorbs sunlight it feels a different force than if it reflects the sunlight), and the orientation of the object (if it has a large area pointed at the sun, then there is a lot of force, but if there is a small area, the force is smaller).  Neglecting this force can cause an error of 10s of meters after a day.
    4. Earth shine: Sunlight reflecting off of the Earth adds pressure to the orbiting object, similar to sunlight described above. This only happens on the dayside, and is pretty weak compared to the direct sunlight, but it still needs to be accounted for. Further, the Earth radiates infrared energy (i.e., the Earth glows). Satellites feel this glow, just like they feel the sunlight, but instead of being directed from the sun, it is directed from the Earth. It is complicated to accurately take into account the Earth shine, but luckily it is a pretty weak force, so for collision avoidance, it can be approximated and not modeled exactly.  Neglecting this results in errors of a couple of meters at most after a day.
    5. Drag: Just like a biker with a headwind feels wind resistance, a satellite in orbit feels a tiny bit of atmospheric drag force that causes it to lose energy all of the time. (See a post on drag here!) The drag force is directly dependent on density of the atmosphere and is dependent on the difference between the velocity of the object and the winds in the atmosphere squared. There is significant uncertainty in both the winds and the density of the atmosphere. As described below, it can be one of the main sources of error in the probability of collision, and can cause the positions to be uncertain to hundreds of meters after a day.

It is impossible to definitively say “there will be a collision between these two objects in 24 hours from now”. What is done instead is determine the probability of collision. This information is then passed on to satellite operators, so they can choose whether they want to move their satellite or not. If the decide to move their satellite, the operators typically speed it up or slow it down to definitively avoid the collision.

A simple method of calculating the probability of collision is to do what is called a “Monte Carlo” simulation of the interaction. By this, the modelers create about a few million versions of object 1 and a few million versions of object 2. They give these objects slightly different initial positions, velocities, and satellite characteristic (using random numbers to perturb these quantities) and see how many of them collide. This number, divided by the few million scenarios, gives the probability of collision.

The satellite characteristics that are perturbed have to do with the drag. Satellites have different shapes and sizes and masses. While the satellites’ shapes are typically well described, the orbital debris is not well described at all. For example, several years ago, the Chinese blew up one of their own weather satellites, resulting in several thousand pieces of debris of unknown size and mass. There are estimates of the size and mass of each of these objects, but there is significant uncertainty. Therefore, the few million objects in the Monte Carlo simulation are given slightly different sizes and masses (technically ballistic coefficients – the ballistic coefficient also depends on the shape of the object and what the object is made out of, but that is a detail.  Well, this whole post is a detail, so it is a detail on a detail!)

The USAF propagates these millions of Monte Carlo satellites through a single atmosphere. While the majority of the time this is fine (since the upper atmosphere is calm most of the time), at times this can give huge systematic errors. For example, when the northern (and southern) lights become active, they add a bunch of energy to the atmosphere, causing it to heat up and expand. This expansion causes the density to increase, which drives a stronger drag force. If this isn’t accounted for, then the probability of collision will be incorrect.

Recently, a paper was published that showed that the behavior of the atmosphere has a large effect on the probability of collision. An event was explored, where the probability of collision was determined to be above the threshold where something should be done. The million objects were then simulated over and over and over again, propagating them through different atmosphere, depending on what was predicted. It was shown that if the sun and the aurora was a tiny bit more active, the probability of collision would be increased, while if the aurora and sun were either a lot more active or any less active, the probability of collision would decrease. It was suggested that this uncertainty in the sun’s brightness and the auroral activity should be taken into account when calculating the probability of collision. (This paper was published by Charles Bussy-Virat, and there is a youtube video of him explaining all of this in a seminar here.)

Finally, the probability of collision that is typically taken as a threshold to do something with the satellites is typically 0.0001%. This is incredibly low! But, considering that a satellite may cost several hundred million dollars and many tens of millions of dollars to launch into space, the operators want to be as cautious as possible.

In summary, calculating the probability of collision between objects in orbit is really hard. When the atmosphere is really calm, the hardest part is figuring out the velocity of the objects – a tiny error in this can cause a large error in the position of the objects at the time of closest approach. When the atmosphere goes a bit crazy, due to the aurora or the sun having more activity than expected, the satellite’s drag force can change pretty dramatically, acting to change the acceleration, velocity, and ultimately the position of the objects at the time of closest approach. While the distribution of the velocity errors is really well understood, so the Air Force can very accurately account for this in determining the probability of collision, the uncertainty in how the atmosphere is behaving is very hard to account for. This lack of knowledge in how to treat the future state of the atmosphere is one of the largest challenges in accurately determining the probability of collision of objects in orbit around the Earth.

A Side Note:

There are a lot of issues in determining the density of the atmosphere. The most accurate models of the upper atmosphere, at this time, are empirical models, meaning that the models were created fitting a ton of data. These models get the mean state of the atmosphere (i.e., the climate) (mostly) correct, but have a hard time with the “weather”. Indeed, the Air Force uses a very old model of the upper atmosphere that is no longer the state of the art, but they have a way of compensating for this. There are over 50 perfect spheres orbiting the Earth at different altitudes. The Air Force can get very good orbital characteristics from these spheres. In addition, they know the shape, mass and what the sphere is made out of, so they can use these to figure out what the drag force actually is, given the change of orbit from one time to another. They can compare the “actual” drag force and the drag force predicted by the model and adjust the model until it matches. Then they use that corrected model to predict the density into the future.

The science community is attempting to improve models of the upper atmosphere all of time.  These are like weather models for the troposphere in that they use fluid dynamic equations to simulate the atmosphere.  Ultimately, the USAF will have to use these types of models if they really want to improve the forecasts when there are large storms in space that can dramatically alter the trajectory of the satellites.  They will make predictions like hurricane forecasters make predictions of landfall – using many different models to better understand the uncertainty.

Satellites, Space Stations

It Fell From the Sky

Recently, a news article talked about China’s “Failed” Space Station (Tiangong-1) re-entering Earth’s atmosphere.  There are several aspects of this that are interesting:

  1. China launched a Space Station!  That is sort of cool and crazy.  The article calls it a “failed” space station, but really, it was just about as successful as any country’s first space station. Really, only three countries have launched space stations, besides the International Space Station: Russia, USA, and now China.  All of them had problems with their first stations. But, this post isn’t about space stations.  I will write about those some other time!
  2. The space station is going to re-enter Earth’s atmosphere, as most objects in low Earth orbit do.
  3. Researchers don’t know exactly when it will re-enter the atmosphere.  They gave an estimate of around April 2nd, with a two-week window size.  It seems like this is a really large window size.
  4. The article discusses that the space station could land in the United States! Should you run for the hills and hide for the entire two-week period that it could re-enter the atmosphere?
Tiangong-1 Space Station

First, let’s talk about how many objects there are in low Earth orbit.  There are a LOT – about 22,000 that are larger than a softball.  There are even more that are smaller than this size, but we don’t really have the ability to track those objects.  Very few of these objects are operational satellites, like less than 1,000.  You can find the most objects at an altitude around 700 km.  We put a lot of satellites in this range for pretty much the same reason that you find a lot of stuff there – the atmosphere above about 700 km is super weak, so that the drag is extremely low.  Objects at 700 km altitude will take well over 50 years to be pulled back down into the atmosphere.  This means that anything put up there will stay for a really long time.


Objects below about 500 km altitude will re-enter the atmosphere within about 10 years.  Anything put there or lower is just sort of swept into the atmosphere relatively quickly. This is why there is not much stuff at these altitudes – it all re-enters the atmosphere.

Altitude distribution of objects in low Earth orbit around the Earth. NASA Orbital Debris Program Office (ODPO).

About one object re-enters the atmosphere every day.  Most of these objects are really small and burn up completely.  Others are very big and make it through the re-entry process and land on Earth.  Things like first and second stages of rockets are examples of relatively large objects that don’t always burn up in the atmosphere.

The Department of Defense (DoD) tracks all of these objects, and attempts to predict exactly when and where they will enter the atmosphere.  This is really difficult to do.  First, the orientation of the objects are not really well known, and they could be tumbling. Therefore, it is really hard to predict the area that they are presenting to the incoming air, so the drag is difficult to calculate.  Also, the objects may have lots of protrusions, like antennas and solar panels.  If the satellite is tumbling, the area could change dramatically.  Or, if there is enough force to rip the panels off, then the area could change quite suddenly and stay at a lower value.  Think about driving a minivan down the road with a mattress strapped onto the top.  The minivan feels a lot of drag while the mattress is attached, but suddenly feels quite unburdened when the mattress flies off onto the cars behind it.  The same happens when satellites or space stations are unburdened of their solar panels as they enter the atmosphere.  Just like a minivan with a mattress strapped onto the top, it is difficult to predict if and when this unburdening event may happen, so determining the exact drag for the last few weeks of the object’s life is quite difficult.

Another thing that adds to the difficulty of predicting the drag is that there are a lot of aspects of the atmosphere between about 100 and 150 km that we don’t really understand that well.  For example, in this region, the temperature goes from being the coldest part of the atmosphere (about -75°C at 100 km) to the hottest part of the atmosphere (about +700°C at about 200 km).  That exact transition is not well understood.  Part of the reason is that it is impossible for a satellite to survive there and take measurements.  It is also impossible for an airplane to fly there, or a balloon to ascend to there.  Really, the only way to take measurements is either with rockets (so about 10-20 measurements a year at most), or through remote sensing, which has a lot of assumptions.

One of the great ironies in NASA is that when the Upper Atmosphere Research Satellite (UARS) was going to re-enter the atmosphere, NASA researchers also had a window of a couple of weeks.  One would think that given the name of the satellite, we would be able to specify the atmosphere well enough to predict when it was going to de-orbit!  But, not so much.  It is a hard problem!  Another interesting thing about UARS is that it actually re-entered the atmosphere over the United States! UARS was about the size of a school bus, so a lot of pieces may not have burned up in the atmosphere and may have made it all the way to the ground. No one was hurt.

NASA has come computers codes that you can run that will predict what will make it to the ground.  You actually have to runs these codes before you can get permission to launch a satellite. Most stuff like aluminum and plastics burn up,  but things like tungsten and other really dense metals may not.  You can then predict how fast the objects will hit the ground by estimating the surface area and mass, and predicting the terminal velocity.

Given how much stuff has re-entered the atmosphere, why has no one ever died due to getting hit my something? Well, the surface of the Earth is really, really large.  One website estimated that the percentage of area that we humans have covered with artificial surfaces is about 0.6%.  That is not much.  If the objects were falling randomly over the Earth, then one might expect about 2 objects a year to re-enter the atmosphere over a populated area.  Since the vast majority of objects burn up in the atmosphere, there is not too much to worry about.

A rocket stage that made it back to Earth. And not the way Space-X get’s their stages back.

There have been objects like meteors that have recently entered the atmosphere over populated areas.  For example, on January 16, 2018, a meteor landed just north of the University of Michigan.  This meteor was large enough to cause a huge boom and could be seen by thousands of people.  Still, no one was hurt. Even in a relatively dense population center, there is a lot of empty land.


So, there is not very much chance that when the Chinese Space Station re-enters the atmosphere and some of the bits make it back to Earth, they will land on anyone or cause much damage.  Most likely the pieces will land in the ocean or over land where there are not too many humans.  But, I guess there is always a first time.


Two Reasons Why the Humanity Star is Not a Complete Waste of Time

If you have not seen this, you should look at this web page that describes The Humanity Star.  It is basically a nearly spherical object that was launched into space in January of 2018.  It has 65 reflective surfaces that will reflect sunlight while it is in orbit. The general idea is that whenever it is in the sun, it will be so bright in the sky that you can see it.

Normal satellites can be seen in orbit around the Earth from the ground.  What happens is that when it is dark on the ground, but still light at orbital altitudes (around 250 miles high and above), sunlight can reflect off the satellite and it can look like a star in the sky.  This happens just after sunset or just before sunrise.  If you are very patient and look up at the sky during these times (preferably from an outdoor hot tub), you can sometimes see objects that look like stars that are moving from south to north or north to south. To give you an idea, it should take them about 10 minutes to go from horizon to horizon.

The Humanity Star is so bright that it should be be visible during the day.  The web page talks about how this will be a beacon to draw humanity back together and to make them look to the stars.  I personally don’t think that a tiny star-like object in the sky will bring humanity back together unless the star-like object is getting bigger and bigger every day and has the potential to wipe out humanity.  Other people that I have talked to have a similar feeling, and so it seems like The Humanity Star really has no real value. Except it does. There are a few good reasons, some intended, and others maybe not.

The Humanity Star. It is not in orbit in this picture. It is sitting on the ground. (From the website).

The true purpose of The Humanity Star was really to test the Electron rocket by Rocket Lab. This was the first flight of the Electron.  While Space-X just launched the Falcon Heavy, Rocket Lab launched a small rocket that can take only 150-225 kilograms to orbit for an estimated price of $5M.  This is a huge deal because constellation missions would like to spread out satellites.  It is incredibly difficult to truly distribute a constellation of satellites from a single launch vehicle (rocket).  If you could buy 8 tiny rockets that could take one or two satellites to orbit for the price of one medium sized rocket that could take 8 satellites, it would allow you to distribute the satellites immediately.

When you test a rocket for the first time, the probability of failure is quite high (like, explosively high).  Some companies give a special deal to satellite companies to launch their satellite on a very risky rocket launch. If it blows up, then everyone loses, but they are not out a huge amount of money.  If it doesn’t blow up, everyone wins – the rocket is proven to work, and the satellite gets to orbit for cheap. Other companies just launch dummy payloads in order to prove that the rocket works.  If it works, then they have a proven rocket.  If it doesn’t, no one is harmed.  This path doesn’t make the company any money (if the rocket works), but also doesn’t make people really angry (if it doesn’t work).

The Humanity Star was a dummy payload for the first test launch of the Electron rocket.  This is similar to Space-X launching a Tesla on the Falcon Heavy (another dummy load with an actual dummy in the driver’s seat). Instead of just saying that it was a test load, Rocket Lab made a big deal about The Humanity Star instead of talking about their super cool and super small rocket.

The second interesting thing about The Humanity Star is that it can actually be used to do science, even though it has no power or sensors or anything. The Air Force has many dummy spheres like this in orbit. The reason for this is that all objects in low Earth orbit feel atmospheric drag.  Since the projected area of a nearly spherical object is known exactly and basically never changes (since it looks exactly the same from every angle) the only change in the drag force that the object feels is due to changes in the atmospheric density. Normal satellites are strange shapes and have lots of protrusions, like antennas and such.  If the orientation of the satellite changes, the drag changes. It is often extremely difficult to model this behavior accurately.  So, simple spheres are used and are tracked with radars from the ground.

The Humanity Star will allow us to more accurately track the thermospheric density since it is really big (about 1 meter across) and pretty light (about 8 kg).  Its area to mass ratio means that the drag that it feels will be pretty big, so it will reenter the atmosphere pretty quickly (less than a year). Because it feels such a large drag, the drag force will be easy to determine and any changes will be caused by only by changes in the thermospheric density.  This is the type of research that I do!

Another really minor thing about The Humanity Star is that because it can be visible from just before sunrise to just after sunset, including the whole day, it could be used for educational purposes.  You see, a satellite’s orbit can be determined just by tracking how it moves across the sky.  If you point a telescope at the satellite in the sky and mark down the direction that the telescope is pointed, and do this over and over again as the satellite moves across the sky, the math is relatively easy to do to determine the orbit (well, students do this in Junior-level Aerospace Engineering classes). This is a great real life example that students could use to put their education to use! In the daylight!

Hopefully this has convinced you that The Humanity Star is not a complete waste of time and money!



News: QB50 and Space Debris Conference

Yesterday was a very long and a very busy day for me – I traveled to Europe to attend the 7th European Conference on Space Debris and we had two satellites launch into space as part of the QB50 mission.

QB50 is a European led mission that has about 35 CubeSats that have been launched to the international space station (ISS).  Each of the satellites, which are about 4 inches by 4 inches by 8 inches (like, really small), carries one of three different sensors that measure the space environment.  The Europeans provided the instrument and the launch, while each group provided the satellite.  University of Michigan build two satellites, called Atlantis and Columbia, that carry the FIPEX instrument.  FIPEX measures the atomic and molecular oxygen density in the thermosphere.  Oxygen is the main gas in the thermosphere, so, in effect, these satellites will measure the air density.  This is important for satellite orbit prediction and collision avoidance. Below is a picture of these two satellites with a bunch of the students, faculty, staff, and engineers that worked on them.


On April 18, 2017, the satellites were taken up to the ISS on a normal resupply mission. They are in deployers called NanoRacks, which push the satellites out into space from the ISS. According to QB50 officials, this should happen in the first few weeks of May. The satellites will then turn on, deploy some drag panels, and start to communicate with the ground station at UM.  We will then command the FIPEX instrument to turn on and start to take data.

While the launch was happening, I was participating in the 7th European Conference on Space Debris.  This conference has about 350 people who are investigating all sorts of aspects of space debris: new techniques for discovering it, quantifying how much there is, and looking at ways of removing it, just to name a few.

A quick refresher on space debris: There are over 20,000 objects orbiting Earth that are about the size of a softball or larger. Since we have hundreds of active satellites, this debris cloud is a problem, since if a piece of debris hits an active satellite (or another piece of debris), it will destroy it and create even more debris. People talk about a Kessler Syndrome, which is basically where low Earth orbit becomes crowded with debris which leads to collisions, which leads to more debris, which leads to more collisions, etc.  This has the potential for running away and basically making low Earth orbit unusable.

I got to watch a talk by Kessler yesterday.  He is a retired NASA employee. Sort of cool to see such a talk.

So far, I have watched a bunch of talks on how to measure debris and some missions that are trying to raise money to remove debris.  Measuring the debris is very interesting, since you can sort of do this with a relatively inexpensive camera.  Just before sunrise and just after sunset, the ground is in darkness, but the sun is still shining on satellites. If you look up in the sky during these times, you can see this reflected light and observe the satellites. Which is pretty awesome.  If you take pictures with a camera, you can figure out the speed of the debris, which gives you its orbital characteristics and roughly how big the object is (from its brightness).  The better your camera, the smaller the debris you can see. I may try to do this with some students. It seems like a great project.

For the debris removal missions, there are a bunch of hurtles: (1) getting to space is very expensive, so it may cost so much to get the junk down, that it is not worth it; (2) rendezvousing with the debris is really hard, since it is quite difficult to automatically track and maneuver into position; (3) capturing the debris is hard, since it may be spinning and oddly shaped; and (4) deorbiting the debris is a challenge, since you have to rigidly attached the debris to some sort of thruster and then use a bunch of fuel to deorbit it.  This means that the missions are pretty expensive and have a LOT of technical hurtles to get over in order to be feasible.  But, they are pretty interesting to learn about!