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!