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.

Engines, Rockets

Why the Falcon Heavy Makes Good Business Sense

This week, Space-X had a test launch of the Falcon Heavy, the largest rocket ever launched besides the Saturn-5. It was a great success, putting a Tesla Roadster into trans-Martian orbit. Space-X has taken a fantastic first step. If you have not seen the launch, you should definitely watch it – it is fantastic.

Falcon Heavy at liftoff (Elon Musk via SpaceNews).

While the Falcon Heavy is very cool and a great achievement, it is also a good business investment.  There are two reasons for this.

The first reason is that the Falcon Heavy is a good business decision is that the Falcon Heavy is simply three Falcon-9s strapped together. Other rocket companies have different lines for different weight classes.  For example, Orbital/ATK has the Pegasus rocket, which last launched the eight CYGNSS satellites.  The next mission that will use the Pegasus is ICON, which will launch later this year.  That is about 1.5 years between launches.  The Pegasus launches off of an airplane, which is super cool.  The next size up from the Pegasus that Orbital makes is the Minotaur-C, which is capable of of carrying about four times as much mass to orbit as the Pegasus.  The Minotaur-C uses some of the same components as the Pegasus, but they are pretty different.  This means that Orbital needs to keep up two lines of production, which is quite difficult and costly.

The Falcon-9 uses 9 Merlin 1D engines in the first stage and 1 Merlin 1D (Vacuum) engine in the second stage. So, it uses essentially 10 of the same engines.  The Falcon Heavy uses 3 Falcon-9 first stages for a total of 27 Merlin engines in the first stage, with a second stage that is identical to the Falcon-9 second stage, for a total of 28 Merlin engines.  The engines and first stages can be used as Falcon-9s or Falcon Heavies.  This makes the production costs much lower, since they don’t have to maintain different manufacturing lines for different rockets.

What would be super awesome is if they could have a rocket that used one Merlin engine (a Falcon-1 rocket, which used to exist) that could compete with the super small rockets, like the Pegasus or the Electron (as discussed in a previous post).  But, Space-X made a choice that their smallest rocket would be the Falcon-9, which was a good decision, since the Falcon-1 could not really be expanded, like the Falcon-9 to the Falcon Heavy.

(Side Note: The Falcon Heavy is a relatively small rocket compared to the BFR that Space-X is planning for going to Mars.  There are plans for more, extremely large, rockets. The BFR is going to be a fundamentally different design than the Falcon Heavy, which is somewhat sad, since I just wrote a bunch of words above about how awesome it is that Space-X are combining the same rockets to get bigger rockets. I have no real idea how much development has gone into the BFR yet.  I will find out and get back to you!)

The future of rockets compared to the Saturn V. The Falcon Heavy is the biggest rocket available today, but it is very small compared to what is needed to go to the moon or Mars with humans. (

The other thing that Space-X has done is to make the Falcon-9s reusable. The fact that they can fly back down and land makes them very valuable.  While the reusability of the rocket engines, and the number of times that they can be reused, is still questionable, it is quite certain that they will get there and the engines will be able to be used many times.

The other reusable space vehicle has been the Space Shuttle.  The problem with the shuttle was that it was extremely costly to reuse it – about $500,000,000 to launch the shuttle. (I will write a post on this soon!) The Space-X rockets are fundamentally different things.  They do not have a super complicated heat shield, or relatively complicated solid rocket boosters.

Interestingly, the primary reason that the Falcon-9 lands the way that it does, standing up using thrust, is that this is the way that it would have to land on Mars. Because Mars has such a weak atmosphere, it is very hard to land with parachutes or with wings, like the shuttle. Space-X therefore designed the Falcon-9 so that it could land vertically.  Not that there will be any Falcon-9s on Mars, but the vertical landings are great tests of the technology so that when rockets do land on Mars, they will have undergone significant real-world tests. That is in addition to making the rockets reusable, which drives the price down significantly.

In summary, Space-X is making really good business decisions in its Falcon-9 and Falcon Heavy lines: using the same engines and the same structures is really smart, and making the rockets reusable is genius.


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!


Star Wars

Why Does the Millennium Falcon Fly the Way it Does?

Since we are all humans (well, maybe), who have been raised on planet Earth (again, maybe), we pretty much understand how things work in an atmosphere. We end up thinking about a wide variety of things with an atmosphere in mind. Take airplanes for instance.

Airplanes fly by generating lift with their wings. There is an air pressure difference between the top of the wing and the bottom of the wing that causes an upward pressure difference, giving the plane lift.  When this lift is larger than gravity, the airplane goes up.  When it is less than gravity, it goes down. When it exactly balances gravity, it flies level.

Since an airplane typically has two engines, one on the left and one on the right, you may think that an airplane can turn by adjusting the engine speeds.  Like, in order to turn right, the left engine thrusts a bit harder and the airplane is rotated to the right. Any who has flown on an airplane knows that this is not the case.  As you probably are well aware, an airplane turns by tilting one way or the other: it banks to make a turn. This banking, or tilting, points the lift vector from being perfectly vertical to being directed in one direction or the other. The airplane literally uses air pressure to change directions!  This force then causes an acceleration that is perpendicular to the direction of motion (centripetal acceleration). Interestingly, tilting the airplane reduces the effective lift, so the plane will start to descend if it doesn’t speed up to compensate for this. Take a look at the diagram below.

(Left) When an airplane is flying in a straight line, the lift balances the gravity.  (Right) When the airplane tilts, some of the lift force is directed perpendicularly to the velocity, so that the airplane starts to turn.  This is called “banking”.

If an airplane wants to turn a 90° turn, it tilts in the direction that it wants to go, causing a force (and acceleration) that points towards the center of the turn (centripetal acceleration).  It continues like this until it is done with the turn, then stops tilting.


It would be incredibly difficult for airplanes to not fly like this.  Pretty much by definition, they use the atmosphere to help with flight: Air-Plane.

In space, this is not the case at all, since there is little to no atmosphere.  This means that spaceships (in space) can’t fly like airplanes. They don’t use air pressure to give them lift, since there is no air. They use thrusters to move from one place to another.

This means that spaceships don’t have the atmosphere to allow them to make banking maneuvers. So, how do they make turns if they can’t bank? Well, ships that fly in space have thrusters that cause an acceleration in the direction that they want to move. If a spaceship wants to move to the right, it has to thrust towards the left, which will accelerate it to the right. If the ship wants to reverse course, it doesn’t make a big circle, it just turns around and starts accelerating in the opposite direction that it is moving. This would look like it is slowing down, eventually stopping completely, then moving in the other direction, getting faster and faster all of the time.

Spaceships don’t have to turn in giant circles like airplanes have to. Airplanes can’t simply turn around like a spaceship, since the air puts a huge amount of pressure on the forward facing side of the airplane.  It would be very bad if you simply rotated a 747 by 180° while it was still moving at 600 miles per hour.  Very bad. Like, wings being ripped off and fuselage breaking in half type of bad. Seriously, don’t do it.

But, in space, there is (essentially) no atmospheric pressure to rip the wings off of the ship or break the fuselage in half. So, space ships can orient themselves in whatever direction they want when they are not thrusting.

The Millennium Falcon, on the other hand, banks all of the time.  Take a look at this youtube video of just the Falcon flying. It is constantly banking.  In an atmosphere, this way of flying totally makes sense (except it is not shaped like a wing at all, so I don’t understand the lift, but we will set that aside.) And, the Millennium Falcon does fly in the atmosphere, so maybe that is why it is portrayed as constantly banking.

Since ships like the Millennium Falcon have one large thruster, and not thrusters on each side of the ship, they have to rotate so it is facing the way that it wants to go, then thrust, thrust, thrust.  No banking!  Imagine, in reality, what it would look like for the Millennium Falcon to turn to the right.  It would be going in one direction (not even having to use thrust, since you don’t have to thrust to move in space – just to change velocity!). Then it would have to rotate around facing perpendicularly to the velocity, then thrust, continuously rotating, so it always thrusts towards the center of the turn.  When it is done turning, it can turn in whichever direction it wants to, but it would probably face the direction that it is moving. Take a look at the figure.



If the Millennium Falcon wanted to reverse directions, it would simply flip over so that it was facing in the opposite direction, and would thrust, slowing down, and then accelerating in the opposite direction.  There would be no back flips or anything like that, since backflips are another form of maneuvering in an atmosphere.

One very cool thing about spaceships is that they don’t have to face in the direction that they are moving (if they are moving at a constant speed and want to continue moving in that speed).  This is very convenient if you want to shoot another ship – you can simply rotate your ship around and shoot in whatever direction you want. Battlestar Galactica did a really fantastic job of portraying this.  If you watch this video at around 7:30, you can see the ships moving in one direction while shooting in a different direction, then rotating around to thrust in another direction to change their velocity.  Pretty nice!  This is how ships should really move in space!

Ok.  I love the Millennium Falcon.  Who doesn’t?  But ships in the Star Wars universe really don’t follow the laws of physics.  I have often wondered why.  I think that back in 1975, George Lucas didn’t really understand the physics, and movies that were made back then didn’t really care as much about the physics.  Now though, people do care more, and they have started to take the pains to get it closer to correct. But, I have to imagine that George Lucas (or Disney) had to make a decision about whether he (they) should change the physics to be more realistic, thereby changing some of the fundamental things about the universe.  It was most likely a hard choice.  I probably would have given in to the dark side and changed it.  I am weak.



Lagrange Points

In orbital dynamics, there are very strange things that can happen.  One of those is the idea of Lagrange Points.  When you have two bodies that are orbiting each other (like the Earth orbiting the Sun or the Moon orbiting the Earth), you can get points in which the forces cancel such that you can put satellites there and they will basically stay in the same spot (mostly).  There are five Lagrange points, as illustrated below. These are very strange and sort of unexpected!  I thought that I would explain how L1 and L2 are created and wave my hands for the others, since the physics is exactly the same.

The five Lagrange points between the Sun and Earth (from

To illustrate, we will consider the Earth-moon system, since that is a bit closer to home. Let’s start with the most basic thing: Earth’s gravity.  Earth has gravity that essentially goes outwards forever . Even Jupiter “feels” Earth’s gravity – it is just really weak.  Just to make sure that we are all on the same page, Earth’s gravity is towards Earth. 🙂


Let’s switch to the moon. When you are standing on the moon, you are always being pulled towards the moon, but if you look at this from the Earth’s perspective, this “towards the moon” can be either “towards the Earth” or “away from the Earth”, depending on which side of the moon you are on.  If you are on the side facing the Earth, the moon is pulling you away from the Earth.  If you are on the “Dark Side of the Moon” (i.e., the side of the moon facing away from the Earth), then the moon is actually pulling towards Earth.  Weird, right? Take a look at the plot:


So, if you next consider Earth’s gravity and the moon’s gravity together (plot below), you can see three things: (1) when you are close to the Earth, Earth’s gravity “wins”, and you are pulled towards Earth; (2) on the far side of the moon, Earth’s gravity and the moon’s gravity point in the same direction (towards the Earth), so they add together and you would be a little heavier on this side of the moon; and (3) when you are close to the moon, but on the Earth side of the moon, the moon pulls you away from the Earth and the Earth pulls you away from the moon, so you would weigh a bit less on this Earth side of the moon.  At some point between the Earth and the moon, there is a location in which Earth’s gravity (pulling you towards Earth) and the moon’s gravity (pulling you towards the moon) cancel each other and you have a gravitation null.


Solving for this gravitational null (see below) is a pretty standard high school physics problem.


Up until this point, we have only considered gravity.  This would be fine if Earth and the moon were fixed in place, instead of the moon orbiting around the Earth. But since this is not the case, we have to consider the orbital motion.  The moon goes around the Earth once every 28 days (roughly).  You can then ask the question about how much force is needed to keep the moon moving around in a circle instead of the moon flying off in a straight line.  This is the centripetal force.  You can experience this force if you go on a merry-go-round that is spinning fast. You have to hold on to something to keep from being flung off of it.  If you are standing at the exact center, then you don’t feel any force at all, but as you move further and further towards the edge, you end up feeling more and more force.  You should totally go and try this.

Well, the same force works in space.  If you have an object going around another object, it feels an outward force (well, the object wants to continue to move in a straight line, which we interpret as an outwardly directed force).  If we pretend that the moon is sitting on a gigantic merry-go-round at the position of the moon, and the merry-go-round is spinning exactly the same speed as the moon is going around the Earth (one revolution every ~28 days), the acceleration that you would feel at any point along the merry-go-round would be this:


Notice that the acceleration is always away from Earth, just like on the merry-go-round, you always feel a force pushing you away from the center.  Now, the Earth and the moon are not sitting on a gigantic merry-go-round, so this is really a thought experiment, but you get the idea.

This ends up being a third force/acceleration that is felt in the Earth-moon system, and so it needs to be included in all of our accelerations that we talked about earlier:


Notice that the centripetal acceleration is pretty much nothing compared to the Earth’s gravity until really close to the moon.  Is that a coincidence?  No!  What is the force that makes the moon orbit the Earth? Gravity balanced with centripetal acceleration!  If you trace the Earth’s gravity line (which turns into a dotted red line) and the centripetal acceleration line, they cross at the moon’s orbit (~60 Re)!  Physics!

But, we are really looking at the sum of the forces.  So, the centripetal acceleration becomes larger than all of the accelerations in the gravitational null point (since the gravity of the Earth and the gravity of the moon cancel so there is almost no acceleration there), and when you go on the other side of the moon and are far enough away, the centripetal acceleration becomes larger than the sum of the Earth and moon’s gravity.  Notice that the sum of the gravity is red (towards Earth), and the Centripetal acceleration is blue (away from Earth), which means that where they cross, the sum will be zero:


The above plot shows the sum of all three of the accelerations: gravity from the Earth, gravity from the moon, and centripetal acceleration. Now you can see that there are two areas where the three accelerations cancel each other out: around 52 Re away from the Earth and about 71 Re away from the Earth, both along the Earth-moon line.  These are the first two Lagrange points (L1 and L2).

These Lagrange points exist in any system where you have one body orbiting another.  You can look at this plot and think of the sun as being the main body, and the Earth as being the second body, so that L1 is between the sun and Earth, and L2 is away from the sun on the dark side of the Earth. These Lagrange points are very useful, since we can place satellites near them so that they can look at the sun all of the time (this is useful for solar physics missions), or look back at the dayside of the Earth all of the time (for climate and weather missions), or be in the shade of the Earth all of the time (well, no missions really want this, since they need solar power to run, but they do want to be far away from the Earth and not have the sun be too large in their view).

There are three other Lagrange points that I have not discussed (as shown in the top picture as L3, L4, and L5).  All of these points result from the balance between the three accelerations, just like L1 and L2. L3 is very cool because it is on the opposite side of the sun, and we can’t really observe it.  Some people would argue that there could be a mirror Earth at L3, which is pretty funny.  We would be able to tell though, since the hidden planet would change the orbits of Venus and Mercury.  But, can we really trust science? 🙂

L4 and L5 are strange because they are in Earth’s orbital path around the sun, but about 60 degree behind and in front of the Earth.  The math is a tiny bot more complicated, since you have to consider two dimensions, but the concept is exactly the same. These two Lagrange points are really interesting, since you can observe the sun from unique vantages.



Mars vs Moon

Recently, the Trump administration discussed sending astronauts back to the moon, instead of on the Mars. It is well known that Space-X would like to send people to Mars. This has brought up a debate about whether the moon or Mars would be better to colonize first. Mars should obviously be the winner here, since, you know, Mars is awesome, but I thought that it would be interesting to look at both sides from a number of different points of view.

First let us look at advantages for Mars:

1. Mars spins at a pretty reasonable rate. Its day is just over 24 hours, so a day on Mars will be roughly a day on Earth. A day on the moon is 28 Earth days, which will take a fair bit of adjusting too. There is no need to readjust on Mars. Also, because Mars’s day is relatively short, the day-to-night temperature difference is not too dramatic, while on the moon, the day is very hot and the night is very cold. There is an almost 300C degree temperature difference between the day and night temperatures on the moon. Yikes.

2. Mars has an atmosphere. Not really a great atmosphere, but some atmosphere. It is all Carbon Dioxide (well, not all, but a hefty bit), which is great for plants, but really sucks for us humans. The atmosphere allows wind to blow, which helps to equalize the day-to-night temperature differences, but causes a lot of dust to move around too. We can pressurize domes and structures using air from outside. Having some atmosphere is better than not having any, which is roughly what the moon has. The side of the moon that is facing away from the Sun (the literal Dark Side of the Moon, not the metaphorical one), is one of the more vacuous places in the solar system, since the solar wind of the sun just plows into the moon, which leaves a giant wake of nothing behind it. We think of space as being pretty empty, but there are something like 12 orders of magnitude more particles per square meter where the International Space Station flies than on the dark side of the moon. That is pretty empty. (And dark – remember that the night lasts for 14 days on the moon!)

3. Mars may have a lot more water than the moon. There would only be certain places we could colonize on the moon and have easy access to water, while water may exist in the subsurface all over Mars. This is not really definitive, but is a relatively strong possibility. Of course, Mars doesn’t have much Oxygen to breathe in the atmosphere, but, hey, neither does the moon. We can make Oxygen out of water, though. So, it may be easier to sustain life on Mars, simply because of the (possibly easy) access to water. On the moon, we would have to live near the poles, which is probably ok, since you might not have as much of a day-to-night temperature difference.

4. Mars’s atmosphere could stop a bunch of harmful EUV and energetic particles from the sun. This would stop you from frying if you went outside. The moon doesn’t really have anything to stop you from getting zapped (technical term). So, really, you would have to definitely live in caves on the moon, but on Mars it might be possible to live above ground. You would still get a lot of radiation exposure though.

5. Gravity on Mars is much larger than the gravity on the moon. This keeps the atmosphere on the planet, and would help us keep things down on the ground and keep our bones a bit more robust.

6. Mars is really cool. It is red, and there are gods named after it (or it is named after a god? Not sure which…) Mars is capitalized, while the moon is not. Clearly, Mars is better. Heck, life could have formed there, then been transported to Earth and flourished here. So, if you are interested in science, Mars wins.

Advantages for the moon:

1. The moon is really close to Earth. In fact, it is orbiting Earth. So, we don’t actually have to leave the Earth system to get to the moon. This saves a HUGE amount of fuel (money). We have rockets right now that can land a lot of stuff on the moon. Like, we could start dumping supplies on the moon in months if we really wanted to. We would have to do a lot of research to really dump an equal amount of stuff on Mars (i.e., years away). Don’t get me wrong, we do have rockets that can land tiny amounts of stuff on Mars, but that is tiny. We had been talking about a sample-return mission to Mars, but NASA has basically dropped it, since it would be about $20 billion dollars, and we find Mars rocks quite often. It is really hard to get big things to Mars, while it is much (much) easier to get big things to the moon.

2. It only takes 3 days to get to the moon, while it takes 6 months to get to Mars, and you can only do that every 2 years. If you miss the two-week launch window to get to Mars, you have to wait another 2 years. In the two-week launch window to go to Mars, we could literally go to the moon and back. If we need to rescue people from the moon, it would only be a few days, while on Mars it could be years. Remember the movie “The Martian”??? Also, if you want to have a conversation with someone on the moon, it would only be a few seconds of time delay, while the delay to Mars is between 4 and 24 minutes, depending on whether Mars is on “our side” of the sun or the opposite side. Imagine trying to have a conversation with someone with a 24 minute delay. Ugh.

3. Because the moon is much smaller than Mars, it is easier to get stuff down to the surface and back up into orbit (i.e., it takes less fuel and therefore less money). In fact, shipping stuff from the moon to Earth is very, very cheap (read the Robert Heinlein book, “The Moon is a Harsh Mistress”) . So, if we mine the moon, we could get stuff back to the Earth for almost no cost at all. If we mined Mars, it would be incredibly expensive to get it back to Earth (harder to get off the planet, and it is much further away). So, from a purely commerce oriented stand point, a moon colony is a much better economical venture.

4. Remember the 6 month trip to Mars? That is on a tiny ship out in the vastness of space. If there were a large solar event that caused the radiation to increase significantly (happens a lot), then you would die unless there was significant (very heavy) radiation shielding on the ship. The space between here and Mars is not friendly at all. This is true of the space between here and the moon too, but it is only 3 days to get there, so it is easier to avoid times in which there may be bad solar events.

5. Earth-rise. That would be awesome. Unless Earth was a nuclear wasteland. Then it would just be a reminder of what used to be.

So, in summary, Mars is a nice idea, and could probably sustain life better and provide a much better scientific exploration opportunity, while the moon is a much more economically feasible location for a colony. Because of that, Mars will continue to be far off in the future unless the funding and political winds shift. If something horrific happens on the Earth, and it can no long sustain life, then Mars is probably a much better place to live because of the possibility of finding water, its higher gravity, and its atmosphere.  The moon is a bit too harsh to be anything but a temporary stepping stone to the future.


Liquid Fueled Rockets

The opposite of solid is liquid.

Back before the 1900s, the only type of rocket fuel used was solid. Then, in 1926, Robert Goddard flew a very small rocket named Nell, after his daughter. This rocket had the distinction as being the first rocket to use liquid propellant. It reached the amazing height of about 100 meters. Not super high, but it was really a “Wright Brothers” moment. Nothing would be the same.

Robert Goddard with his rocket Nell.

An aside on Robert Goddard: Goddard was basically the father of modern rocketry. When the press found out what he had done, they interviewed him and actually made fun of him, because they didn’t understand how rockets work. They thought that the rocket exhaust pushed on the atmosphere, enabling the rockets to go. They did not understand that you don’t need an atmosphere, or the ground, to lift something into space. In fact, the New York Times printed an editorial about Goddard that stated that he “does not know the relation of action and reaction, and of the need to have something better than a vacuum against which to react—to say that would be absurd. Of course he only seems to lack the knowledge ladled out daily in high schools.” Oops. That journalist didn’t read this blog, where I explain that rockets go because of Newton’s laws of motion. Goddard became very withdrawn and basically did his research in private. But, people still knew about him. In fact, German rocket scientists knew about him. They would call him up and ask him about different aspects of rockets. Being a scientist, he would share what he knew. Being an American, he told the government about the German’s interest in rocketry, which the government ignored. Robert Goddard died in August 1945. Just north of Washington DC, there is the NASA Goddard Research Facility, where scientists and engineers build many of the satellites that are in orbit today. The New York Times published an apology to Goddard the day after the launch of Apollo 11, which stated “Further investigation and experimentation have confirmed the findings of Isaac Newton in the 17th Century and it is now definitely established that a rocket can function in a vacuum as well as in an atmosphere. The Times regrets the error.”

An example of a liquid fuel for rocketry is hydrogen with oxygen used as an oxidizer. When these two molecules are mixed together, they release a very large amount of energy, and water results. I am sure that you all know that oxygen is a gas at room temperature, and I am sure that many of you know that hydrogen is also a gas at room temperature. Why, therefore, is this post about liquids and not gasses? Well, a gas takes up a huge amount of space compared to a liquid. Oh, sure, you could store gasses in pressure tanks, but have you ever tried to lift a gas tank? They are extremely heavy, since they have to hold the massive pressures of the gasses. So, the rockets would have to be very, very heavy in order to hold all of the gas. It is much easier to cool the gas down to extremely low temperatures and cause them to liquify. Then, you can store the liquids in relatively light weight tanks.

A liquid propellent engine typically uses two different liquids, a fuel and an oxidizer to create a large amount of energetic (hot) gas that is expelled at extremely fast speeds. The liquids need to be kept in separate tanks to keep them apart until they are pumped into the combustion chamber. The word “pumped” is important here, since, the flow rate of a pump can be controlled. This means that a liquid rocket engine can be throttled, such that the burn-rate of the propellent can be controlled, unlike a solid rocket booster. In fact, the liquid can be cut off completely, stopping all thrust. The flow can then be resumed at a later time, allowing the engine to restart. These are huge advantages over solid rocket boosters.

Liquid propellents are typically more efficient that solid propellents. This means that less fuel is needed, and since the amount of fuel exponentially increases as more is needed (see the rocket equation post!), any increase in fuel efficiency can dramatically decrease the size of the rocket.

Why would anyone ever use a solid propellent rocket, then? Well, there are a few disadvantages to liquid fueled rockets.

The first is that the liquids typically have to be stored at an extremely cold temperature. For example, oxygen turns into a liquid at -183 C (-297F). It is not super easy to store, nor is it super easy to fuel the rocket. Also, the rocket sits on the ground for a while before being launched into space, so there sometimes is insulation on the storage tank. For example, the large orange thing on the space shuttle is not rocket, but is a storage tank for the liquid oxygen and hydrogen. It is orange because that is the color of the foam insulation that wraps the entire tank. This insulation was the direct cause of the Columbia accident in 2003. I will talk about this in a separate post.

The Space Shuttle Columbia during its final launch. The orange thing that the shuttle is attached to is the storage tank for the liquid hydrogen and oxygen.  It is orange because the tank is completely covered in several inches of (orange) foam insulation.

There is one advantage of having super-cooled liquids on the rocket, though. The combustion chamber and the rocket’s nozzle can get extremely hot, since the whole point of the rocket engine is to make extremely hot gasses and direct them out of the nozzle. The combustion chamber and nozzle can actually melt because of this. But, because there is very cold liquid around, the rocket’s plumbing goes around the nozzle and combustion chamber before leading into the chamber. This cools down the extremely hot metal, and warms up the liquids before they are combined together. All of this plumbing and pumping makes liquid engines much more complicated than solid engines.

Another disadvantage of liquid propellents is the storage tanks. Imagine trying to ride a bike with a half-empty five-gallon bucket of water balanced on your handlebars. As the water slushes around, it moves the center of gravity around. If you are trying to go in a (relatively) straight line, and stay balanced, this can significantly complicate things. The same is true with a liquid propellent rocket. The tanks move from being full, to being empty, with every stage in between. All the while, the rocket has to stay on course. The tanks, therefore, need to have baffles and be specially designed so that the liquids won’t slosh. Yet another complication.

If you drove a car before about 1995, you probably remember having a carburetor. These were devices that combined the fuel with oxygen. Now, pretty much all cars have fuel injectors that automatically adjust the amount of fuel and oxygen mixture. So, imagine 50 years before the first automobile fuel injectors were built, building them for rockets. The fuel and the oxidizer had to be mixed just right in order for the burn to be as efficient as possible. On the Saturn-V, the fuel was being used at such a huge rate, that they couldn’t get the mixture to be very good, so they ended up having pockets of fuel and oxidizer, which would then cause explosions. The engineers didn’t have time to develop new fuel injectors, so they made the walls of the combustion chamber thicker in order to withstand the explosions. Engineering! Fuel injectors are complicated when you are using hundreds of gallons of fuel a second. Today, fuel injector technology is much better, but we don’t have many rockets that use fuel as quickly as a Saturn-V did.

In summary, liquid fueled rockets have some big advantages over solid rockets, namely the engines can be throttled and even turned off, allowing more precise orbital insertion; and the liquid fuels tend to be more efficient than solid fuels. On the other hand, rockets that use liquids as fuels tend to be more complicated, with lots of pumps, massive plumbing systems, and fuel injectors. Most rockets that launch satellites into space tend to have a main stack that is liquid with strap-on solid boosters that provide a bit of extra lift in the first minutes of the launch. That way the rockets compromise between the simplicity, but reduced performance of the solid rockets, while allowing the complex, but precise orbital insertion and raw power of the liquid fueled rockets.