When most people think of space, they think of astronauts, rockets, and space ships. Since the late 1950s, people have been sending people and probes into space aboard rockets to explore, allowing us to see the Earth and the Universe in ways we haven’t seen before. From space stations to space telescopes to interplanetary probes, our exploration of space has changed our understanding of ourselves, our solar system, and all of astronomy.

Don’t see your question in the archives below?

Earth Orbit

How do rockets break out of the atmosphere? (Beginner)

You don’t ever really need to “break” the atmosphere since there’s no solid boundary where the atmosphere ends. As you go higher and higher, the air pressure/density just gets lower and lower until it’s effectively nothing. This is why people climbing up Mount Everest have to carry oxygen tanks, because otherwise the air would be too thin for them to breathe. “Space” above Earth is really just the altitude where the air is so thin that it doesn’t really matter to the spaceships and satellites that are there.

Why do satellites fall out of their orbits after a few years or decades, but the Earth has stayed stable in its orbit for billions of years? (Beginner)

The reason for why satellites fall out of orbit forces us to talk in more detail about what we actually mean by “space”. Most people would agree that satellites orbit Earth in space, but in reality, most satellites are actually in the very upper part of the Earth’s atmosphere. The Earth’s atmosphere never really stops, it just gets thinner and thinner, and at a point around 100 km high, it gets so thin that people have agreed that it’s effectively a vacuum, so they call it “space”. However, if you were an astronaut on the International Space Station and you measured the barometric pressure outside, you would find that it is actually not zero, but rather about 1 trillionth the atmospheric pressure of the Earth’s surface. This is tiny enough that it doesn’t make a big difference most of the time, but for satellites that are moving through this very thin atmosphere at thousands of miles per hour with giant solar panels acting as sails, this thin atmosphere can lead to real drag, and over time, that drag adds up and slows the satellites down enough that they fall.

So low satellites will fall out of orbit after a few years or decades due to the drag from the Earth’s atmosphere, unless they actively boost themselves back up (like the International Space Station does). The Sun doesn’t have nearly as extensive of an atmosphere though, so the Earth and the other planets don’t experience drag in the same way, meaning that nothing slows them down as they orbit and they can stay in the same position for billions of years.

How many miles you would have to be in space to be to see Earth about the size of a basketball with your naked eyes? (Beginner)

This works out to just a simple problem of proportions. The Earth is approximately 53 million times larger than a basketball, so it would have to be about 53 million times further away to appear the same size. If your arm is 1 meter long, then this corresponds to about 53,000 km or 33,000 mi, which is about 14% the distance to the Moon.

How do you keep time in space if you're not able to measure the Earth's rotation anymore? (Beginner)

Funnily enough, even though the day was originally defined by the rotation of the Earth, it actually isn’t anymore, and we have clocks that can keep time more consistently than the rotation of the Earth now. The Earth actually changes its rotation speed by several milliseconds randomly over time, which is enough that every few years we have to adjust the clocks by one second to keep our time aligned with the Earth’s rotation, meaning that the Earth loses one second of time every few years (this is called a leap second). By contrast, our most accurate atomic clocks only lose one second ever 300 million years, so if we want to keep our time accurate then we should look to the lab, not the skies! In fact, the real problem with timekeeping in space is that Einstein’s theory of relativity tells us that time moves at different speeds on objects moving at different speeds (like satellites or other planets), so things like GPS satellites have to adjust their clocks to stay consistent with us on the surface of the Earth.

Is it possible to build a building so tall it reaches into space? (Beginner)

The Earth’s atmosphere doesn’t really have an “edge” like you might be thinking (it’s not like there’s an “inside” and an “outside”). Instead, the atmosphere slowly gets thinner and thinner as you go up until there’s so little air you can’t breathe anymore. The line that scientists usually say marks the start of “space” is about 60 miles or 100 km up (about the distance from Santa Cruz to San Francisco), and beyond that point the atmosphere is so thin that it’s basically a vacuum.

So what would you need to build a building in space? It would have to be airtight so the air for the people on the inside wouldn’t all leak out, which means it would have to be built like a submarine or a space station. The walls would have to be thick and the doors would have to be hatches that close tightly so the air doesn’t get out. However, there isn’t a way to actually build a building that tall because we don’t have materials that are strong enough. The supports would have to be so big that they would crush themselves under their own weight, making the building fall over. And even if you did build a building that tall, it would be so heavy that it would sink into the ground because the dirt wouldn’t be strong enough to support it.

So unfortunately we can’t build a building tall enough to reach out of the atmosphere, which is why astronauts have to use rockets and space stations to get there instead.

Are the sky and space the same thing? What's the difference? (Intermediate)

Fundamentally, the assertion that the sky is space isn’t wrong, you just need to define how far up in the sky you’re talking about. Most people will define space as the area between astronomical bodies (planets, stars, etc.) that is free of almost all material. The problem in this definition though is the word “almost”. Even if you’re in intergalactic space, thousands of light years away from the nearest star, space is still not totally empty. There’s always a tiny bit of gas floating around, so you’re never technically in a complete vacuum. Even on Earth, the atmosphere doesn’t stop abruptly. Instead, it slowly gets thinner and thinner as you get further from the Earth, with no clear point where you would say there is no air anymore. So at some point, you need to decide exactly how much air qualifies you as being “in space” and how much means you’re still in the atmosphere.

The most widely accepted approach to answering this question was first articulated by a physicist named Theodore von Kármán, who in 1957 decided to calculate the altitude at which an airplane stops being an airplane and starts being a spaceship. Airplanes use wings to keep themselves up, but spacecraft just use their own orbital velocity and inertia to continue around the Earth, not requiring any wings. At some point in the Earth’s atmosphere, the amount of lift generated by a plane’s wings using the thin atmosphere gets overcome by the centrifugal force of the plane’s velocity, meaning it ceases to support itself with air and instead continues on like any other orbiting spacecraft. This happens at an altitude of around 100 km, which has come to be known as the Kármán line.

Most definitions of space are based on this definition. The specifics vary from agency to agency (NASA says that space starts at 80 km but the UN says it starts at 100 km), but fundamentally the answer to your question is that the sky is space, but only if you go up about 100 km into the sky (by most definitions). Sky that is any closer to that is atmosphere.

How can we use expired satellites in a better way? (Intermediate)

Currently, nothing is really being done with old satellites and I am not aware of any plans to use them at all, so all of my answers will be entirely hypothetical, but I’ll give my opinions based off of what I know about orbital dynamics.

Satellites are incredibly hard to get to because they are very small and are moving very fast in seemingly random directions. If you sent a spaceship of some kind up into Earth orbit to capture satellites, it would have to expend a lot of time, effort, and fuel to go rendezvous with each individual satellite it wants to capture. For instance, going from the ground to docking with the International Space Station takes days for spacecraft to do because it is very hard to create the appropriate orbit to match yourself up. And if you were to capture one satellite, you would have to expend a ton of energy to change your orbit to capture another one because they are so far apart (there are about 4000 satellites in orbit, which works out to about one every 50,000 square miles / 130,000 square kilometers, and there’s also the height dimension to worry about). You would probably end up expending more weight in fuel to get to the satellite than the amount of material you would gain from the satellite.

Beyond all of the difficulty you would face in getting to the satellite, the things you would find on a satellite would probably not be too useful. Many satellites die because their electronics short circuit because of the radiation of space, so their old broken electronics would probably not be useful for modern purposes. Their solar panels may be useful, but for old satellites, their solar panels are not going to be as energy efficient as newer ones. Raw materials like sheet metal may be the most useful components, but that hardly seems worth the journey.

The best thing to do with old satellites is just to return them to the Earth. Despite the fact that space is so large and satellites are so sparse, popular orbits are actually becoming cluttered enough that people are worried about satellites colliding (it has already happened at least once). Newer satellites have equipment for removing themselves from their orbits once they are decommissioned, but many older satellites are just sitting in space going around in circles forever. The most productive use for old satellites is to somehow destroy and remove their debris from orbit to make way for new ones, but that’s just as hard of a problem to solve.

Why haven't astronauts been back to the Moon since the Apollo program? (Intermediate)

This is a complicated question that has scientific and political answers to it but no real conclusive answer. Though no people have been to the Moon since 1972, there have been plenty of missions to the Moon before then and since then (here is a list of all of them). The main motivation for sending people to the Moon was of course the Space Race, and when the US first put people on the Moon in 1969, most of that political motivation was taken away. The lunar programs for the US and USSR were hugely expensive (the NASA budget was about 9x larger in the 60s than it is now), so once the prize was claimed, the USSR stopped pursuing Moon landings, and the US followed suit a few years afterwards. Since then, computers have become much more capable, so scientific missions to the Moon don’t require actual people to perform their experiments and they instead send robotic landers. It has been more cost effective to have a lot of cheap robotic missions than a few expensive manned missions, so that is what has happened. There is currently a push to return people to the Moon with bigger and better landers in the mid 2020s for political and economic reasons (the Trump administration wants to land by 2024 so they can have a potential political win), so we’ll see how that turns out in the next few years.

Astronauts

How does radiation affect an astronauts bones? How do astronauts keep their bodies healthy while in space? What happens if an astronaut gets sick in space? (Beginner)

The radiation in space doesn’t cause any specific problems to bones. Bones are mostly just made of solid material, so the small radiation particles in space are not that harmful to it (other than elevating your risk for bone cancer along with every other type of cancer since you’re receiving at least 10 times the normal amount of radiation). Astronauts have to worry about their bones a lot though because they have a tendency to deteriorate while in zero gravity. Since the bones aren’t being used to support any weight when astronauts are just floating around, they get lighter and weaker over time (like the bones of old people but 10 times as fast). This condition is called spaceflight osteopenia and is a serious risk for long term space travel.
The main thing astronauts do to stay healthy is exercise. As I mentioned above, floating around in weightlessness puts much less stress on the body than being in gravity, so bones and muscles deteriorate quickly. On the International Space Station, there is a stationary bike and a treadmill to keep the astronauts in shape (the treadmill has tethers to keep the astronauts pulled down, otherwise they would float away while running). However, these treatments still don’t work very well, which is another serious risk for future space travel.
Astronauts don’t get sick in space because they’re completely isolated from all of the germs on space. Nothing can get up to them while they are up there, and they are basically put into quarantine for 10 days before launch to ensure that they don’t carry any diseases up to space with them. There are big problems with motion sickness though since being in space really messes with your inner ear. Astronauts have to take anti nausea medicine while they are in space suits so they don’t throw up in zero gravity and have it float everywhere, which can be dangerous or fatal.

If an astronaut exercises and loses weight while in space, where does the weight go? (Beginner)

This is a complex question that relies more on biology than on physics and astronomy. So what actually happens when we burn fat while exercising? The molecules that make up fat store energy in their chemical structure using the bonds between atoms, so when cells break down those molecules, the energy is released and the cells can do things (like exercise). The atoms that used to be in the fat molecule are then reformed into waste products, which are mostly carbon dioxide and water vapor. So the stuff you breathe out is actually the waste products from the process of cells using energy!

So what happens to the carbon dioxide and water in space? It’s almost always recycled for future reuse. Spacecraft like the International Space Station have fancy atmospheric purifiers that condense out the water vapor so the astronauts can drink it later and convert the carbon dioxide back to oxygen so the astronauts can breathe it again later. This allows rockets to only carry a small amount of water and oxygen up to space and then reuse it endlessly instead of having to bring weeks or months worth of water and oxygen. So the matter doesn’t actually go anywhere, it just gets converted to another form where it can get reused again later.

Should humans be sent to space despite all the risks of space exploration being preventable by sending robots? (Beginner)

This is certainly a complicated topic and I don’t necessarily have expertise in the right areas to answer it fully (this is probably better posed to someone at NASA or something) but here’s my opinion:

Robots can do a lot of the things that humans can do in space like run experiments, take observations, and send back data. They can move around, travel great distances, and go for decades. But what they can’t do is solve problems. A Mars rover that costs hundreds of millions of dollars can get stuck forever because of a small rock (Spirit), and a probe’s orbit can be forced to change because its operators are too afraid to reactivate its engine because they don’t know if it is faulty (Juno). Essentially, a robot can do exactly what its job is reasonably well, but once it is faced with an unexpected challenge, it frequently can’t recover.

There have obviously been huge developments in robotics over the years that have led to very capable and and adaptable (see Boston Dynamics robots for a good example), but you will notice that these aren’t the types of robots that go into space. The technology that is included in these missions is usually older and well-tested because if you spend hundreds of millions of dollars putting something on a rocket and sending it across the solar system, you really don’t want it to fail because of a bug in new software or hardware. So it’ll probably be a while before the robots going to space are less conservative and more advanced than the ones today.

It’s worth talking about a few of the disadvantages of sending people to space though. Mostly these are health benefits that result from low gravity and radiation, as I’m sure you know from your research. There are already ideas for countering both of these, with rotating spaceships creating artificial gravity (see 2001: A Space Odyssey) and water/fuel storage tanks lining the walls to block radiation (see SpaceX). Of course, there are still risks and there is ongoing research in other effects, but science has overcome many challenges in the past so I personally think we’ll get there.

Beyond all of these immediate scientific considerations, I believe the most important function of people in space is to expand the horizons of humanity and inspire technological development. There were several computerized landers that went to the Moon before the Apollo program, but it was still a pivotal moment in human history when people first walked on the Moon. Ultimately I believe that humans can and should travel to other planets to expand its reach, and that is something that a robot could never adequately do. Sometimes these scientific pursuits are not entirely objective and there has to be some human emotion involved on occasion.

So that’s my own individual opinion.

Why do we need to go to other planets? I like Earth and I would like to stay here... (Beginner)

Your question is a very interesting and difficult one that is hard to answer in a rigorous way. There are some concrete reasons why people want to go to other planets, but many of them are difficult to articulate in a logical way. Here are some ideas though:

Science: We can learn a lot of things about how the solar system was formed by visiting other planets. Landers on the Moon, Mars, and Venus have discovered interesting things about their structure that has told us how they formed and what their conditions have been like over their lifetimes, giving us a better understanding about what makes Earth the way it is. By looking for planets around other stars, we can see other kinds of solar systems that can form, telling us even more about how we came to be.
Resources: Other places in the solar system likely have different resources than the Earth does, making them useful for different purposes. Because of the Moon’s low gravity and lack of atmosphere, it has been proposed as a good place to build telescopes and rockets so they don’t have to deal with the complications of the Earth. Asteroids contain lots of valuable metals that could be used in manufacturing on Earth or in space to significantly reduce the prices of some goods.
Safety: If something bad happens to the Earth, be it a giant meteor, climate change, or some other unforeseen threat, humanity currently has no backup plan. If we could set up a settlement on another planet or something, then we would know that at least some humans could be safe in the event of an Earth-ending disaster. If we explore other planets in our solar system and look for planets in other solar systems, we can find good places to live
Exploration: This one is a little more abstract, but humans are naturally curious about their environment and they have always done things that may not have been the safest or the most logical in pursuit of exploration. Brazil would not be the way it is today if European explorers didn’t set off across an ocean with no idea what they would find. Nobody would know about Antarctica if some very reckless explorers hadn’t sailed south just because they wanted to. The same applies to going to other planets. Humans just want to go look at things.

There are definitely a lot of arguments for staying on Earth, like safety, cost, and resources, and I’m sure nobody is ever going to force you to go to space if you don’t want to. But sometimes people have to do things that don’t make sense in the moment because they will lead to big advances later down the line. Hopefully this answers your question.

Why isn't the International Space Station rotating to create artificial gravity, which would be healthier for the astronauts? (Beginner)

This idea has been floating around for a while but it would be hard to actually implement. In order to build a space station that was actually capable of supporting its own weight (as a rotating space station would have to), you would have to do a lot more structural engineering, which would mean larger and heavier materials being sent up to space. The International Space Station is really not that strong since is held together by relatively small docking adapters and don’t have any large exterior structure. The solar panels and radiators don’t have anything holding them up either, so they’re pretty weak. Building a circular rotating space station would be kind of like building a bridge in space, and right now it would be too expensive to launch that many steel beams (or whatever) would be super expensive, and we barely have the money to maintain the current space station as it is.

Deep Space Exploration

Could we build a telescope in the Oort cloud and use the gravitational lensing of the Sun and planets to make a giant telescope? (Intermediate)

There has been a fair amount of consideration given to using the gravity of planets or the Sun as the lens of a telescope and would allow unprecedented views of extrasolar planets, but there are a lot of technical factors that make it wildly impractical. General relativity allows us to use anything with mass as a lens, but the distance you have to go for the light to actually come to a focus is absolutely insane. The Sun’s focal point is at about 550 AU, which is almost 4 times further away than the Voyager spacecrafts which have been travelling outwards since the 1970s (and other planets have focal points that are even further), so it would take decades (at least) to get there, meaning whatever you send out there would likely be outdated by the time you got to use it. Even if you get there, you have to be able to block out the Sun’s light extremely well to actually see what you are looking for, which would push the limits of modern telescope design. Furthermore, if you wanted to look at more than one thing, you would have to stop the spacecraft and make it travel another huge distance to get in the right alignment to look at something else. With the amount of fuel, effort, and time that would take, it would likely be easier to just launch a different telescope for every observation, which would again be wildly expensive and impractical. Even getting the data back would be really hard, since it takes huge radio telescopes to pick up the signals from the Voyager spacecrafts, which adds more expense to deep space missions. So overall, cool idea, probably won’t happen any time soon.

Could we figure out where the Wow Signal came from and send a probe there? (Intermediate)

The Wow signal is a topic that has drawn a lot of interest in the decades since its detection, and like you, many people have thought about trying to determine the source of the signal. The field of view of the radio telescope that detected it is very small (two patches of about 0.13 by 0.66 degrees, about the size of a crescent moon), so the source of the signal is known to pretty good precision, lying within the constellation Sagittarius. The frequency of the signal was very close to the 21cm line of a hydrogen, a very specific frequency that astronomers use to observe clouds of gas in the Universe, and by looking at how much its frequency was shifted by the Doppler effect, we can reasonably assume that it must be from some source moving towards Earth at about 10 km/s. This is very slow by cosmological standards (1/3 the orbital velocity of the Earth around the Sun), so it must have been from some source within the Milky Way.

Despite the fact that the Earth is constantly moving through the solar system and the galaxy, we don’t really need to take that into account when looking for the sources of signals in the sky because in the grand scheme of things, our motion is very slow. It takes over 200 million years for the Earth to complete one orbit around the Milky Way, so even the 44 years since 1977 only represent about 0.00002% of an orbit. This tiny fraction of an orbit still represents a fair amount of distance (about 300 billion km), but because it is such a short amount of time in the life of the galaxy, the stars within it have hardly moved at all. All of this is to say that unless the source of the Wow signal was something very close (like something passing through our solar system) it is likely still in the exact place we observed it.

However, things would get more complicated if we ever wanted to send a probe in that direction. Even if we were to launch a probe going as fast as the fastest probe we have ever launched out of the solar system (the New Horizons probe going 31000 mph), it would take huge amounts of time to get to wherever it was going. This probe would take over 20,000 years to go 1 light year, so it could only go about 10,000 light years (or 10% of the galaxy’s diameter) in one orbital period of the Milky Way. Over timescales this long, stars move around quite a bit, drifting around and slingshotting off of each other’s gravity, so by the time this probe got to where it was going, the stars that were formerly in that area would likely have been scattered to the winds. We can try to predict where these stars will go, but that’s kind of like trying to predict where one bee within a swarm of bees will be. In all likelihood, this probe would end up just floating through interstellar space forever and never getting close to its target.

So instead we are left with the option most astronomers have taken since the signal was observed: try to see it again so we can figure out what it is. Unfortunately, this has been fruitless so far, and astronomers haven’t seen anything that looks similar either. Instead, all we can do is wonder and prepare for the next one.

Could we put a mirror really far away and use the fact that light takes time to travel to look back in time? (Intermediate)

Fundamentally, there’s no reason why an idea like this couldn’t work. Any light that you see in a mirror is delayed by the amount of time it took to travel to and from the mirror, so a mirror that is sufficiently far away would be able to delay time significantly like you describe. However, a few practical concerns make this not a super viable idea in reality.

First, you would have to actually put the mirror very far away, meaning you would have to put it on a spaceship and fly it out to wherever you wanted it to be. Since we can never travel faster than light, that means that by the time you got to wherever you were putting the mirror, all of the “old” light would have already passed you and you would only be able to reflect “new” light that was created after the mirror itself was created. So you couldn’t actually learn any unknown information from this mirror, only events that had happened in the time since you launched the mirror.

Second, the mirror would have to be ridiculously huge and precise. In order to see the reflection of an entire object in a mirror, the mirror needs to be at least half its size, so the mirror would need to be about the size of Mars in order to show the whole Earth at the same time. It would also have to be ridiculously flat so that the image wasn’t distorted over the massive distances involved, and it would have to be oriented exactly perpendicularly to the Earth’s line of sight so we could actually see ourselves in the mirror.

In addition, another practical concern to worry about is the size of the telescope you would need to see anything useful. In order to see small objects, your telescope needs to be very large, so in order to see detail on the surface of the Earth from very far away, your telescope would need to be a significant fraction of the size of the Solar System (or larger), depending on how far away your mirror was.

So there’s nothing fundamentally wrong with your idea. A faraway mirror would indeed allow us to see back into the Earth’s past, but you wouldn’t get any new information and it would be prohibitively difficult to set up the system so it works.

Why does Mars have to be 44 degrees ahead of the Earth in its orbit during a launch window? (Advanced)

When talking about launch windows for interplanetary missions, astronomers are usually referring to when is the best time to perform a Hohman Transfer orbit, which is an elliptical orbit that has its near point at the inner planet and far point at the outer planet, timing the arrivals perfectly so no adjustment is needed. Here is a schematic:

The alignments of this system must be precise, because if the spacecraft reaches the orbit of Mars but Mars is somewhere else in its orbit, the spacecraft would either have to expend a lot of energy to meet up with Mars or wait until things align properly. The appropriate arrangement turns out to be when Mars is 44 degrees ahead of the Earth at launch, which we can show using a back of the envelope calculation of the system:

The most important thing to understand in this system is how long it takes for the spacecraft to go from Earth’s orbit to Mars’s orbit, which is just half of its orbital period. Conveniently, the orbital period of an elliptical orbit is easy to figure out as long as we know the length of the long side of the ellipse, which in the picture above is just the orbital radius of the Earth (R_E) plus the orbital radius of Mars (R_M). What we actually care about is called the semimajor axis, which is half the length of the long side: a = 1/2(R_E + R_M). This is just the average of the two planets’ radii.

From there, we can use Kepler’s Third Law to calculate the orbital period T (how long it will take a spacecraft to go all the way around its elliptical orbit). Plugging this in, we find that T = 518 days, which is between the length of Earth’s year and Mars’s year. With this information, we are almost done. We know that the spacecraft and Mars must both arrive at the same point after the spacecraft travels half of its orbit (259 days) and that the spacecraft will get there faster because it’s on a smaller orbit, so we need to figure out how much of a head start Mars needs to get there at the same time. Mars’s year is 687 days, so in order to get around half of its orbit, it needs about 343 days, which is 84 more days than the spacecraft needs to travel the same way around. 84 days is about 12% of a Mars year, and 12% of a circle is 44 degrees. So Mars needs a 44 degree head start to make it to the other side of its orbit in time to meet the spacecraft.

Now that we know the arrangement that Earth and Mars need to be in for this to work (or any other pair of planets if you want to generalize), we can figure out when is the right time to launch. Using a website like this one we can find the positions of the planets at any given point in time and then see when they’re in the right position. Of course, the calculations above are just approximate and there are a lot of other factors that need to be taken into account for an actual spacecraft launch, like the ellipticity of the Earth and Mars orbits, orbital inclination, gravity assists, and many other things, so the launch windows you find may not be 100% correct. If you are going for accuracy, I recommend you use a table that somebody else has already calculated.

Extraterrestrial Life & Astrobiology

Do you think intelligent life exists elsewhere in the Universe? (Intermediate)

These are difficult questions to answer directly, and I’m not an astrobiologist so I don’t necessarily have all of the most current information, but I can tell you what I know. The conditions to create life likely occur all over the Universe. Any rocky planet with liquid water will probably get the necessary chemicals dissolved in the water somehow, either via erosion, lightning, or hydrothermal vents. From there, it’s just random chance whether a self replicating molecule (the precursors of life) will occur, and we have no idea how often that happens. As far as we can tell, it only took a few hundred million years for the Earth to go from a molten ball of lava to having simple life, so it doesn’t seem too difficult, but as far as we can tell it only happened once on Earth so it isn’t happening all the time. So we know that life has some chance of spontaneously occurring on a watery rocky planet. There are about 100 billion stars in the Milky Way, and if we say (conservatively) that only 1% have a watery rocky planet, then that’s still 1 billion chances for it to happen in our galaxy. Multiply that by the ~1 trillion galaxies in the observable universe and we have 10^21 chances to get it right.

Of course, there’s a long way from life to intelligent life. It only took a few hundred million years for life to evolve from lava, but it took another ~2.5 billion years for multicellular life to evolve. So that suggests that it’s way harder for life to become complex than to exist in the first place, meaning that most occurences of life we find are probably going to be single-celled things like bacteria. Once life became multicellular, it didn’t take long for plants and animals to exist, but it took a another billion years for human-level intelligence to happen, so that suggests that intelligence isn’t easy to make either. Many many organisms have existed/continue to do very well for themselves and are not very intelligent, so there must be very specific circumstances that must happen that makes it the right evolutionary choice to waste a giant amount of energy on thinking, much less building a civilization that is interested in contacting other ones.

Beyond these evolutionary barriers, there are also astronomical ones. The Earth has been safe from any major disasters for the past 4 billion years or so, allowing life to evolve relatively uninterrupted. We could have been vaporized by a nearby supernova, had our atmosphere stripped away by a gamma ray burst, had the planet flung out to interstellar space by a passing star, been crushed by a collision between planets, burned up by the Sun expanding, or been subject to any number of other astrophysical calamities that can happen at any time. But we have been safe. This is helped by the fact that we are in a relatively calm galaxy that hasn’t gone through any major star formation events during the Earth’s lifetime (meaning fewer supernovas/gamma ray bursts) and we are in a relatively calm part of the galaxy (not near the center where stars are denser so they are more likely to disturb the Earth’s orbit or explode nearby). If we had been in a merging galaxy that is making a bunch of new stars that then explode, or if we had been closer to the center of the galaxy and gotten stripped from the Sun, or if we had just gotten less lucky where we are, then any life on Earth could have been wiped out.

Regardless of all of those barriers, it’s still hard to believe that out of the 10^21 chances, we were the only success, and the Drake equation is a way of putting that thought into numbers. The Drake equation is a true statement of probability (ie if you had all the information, then you would get the right answer) but the problem is that nobody knows what any of the probabilities in the equation are. Estimates vary, but 10^21 is a huge number, so I personally think it would be way creepier if we were alone than if we weren’t alone. I don’t know what that means for the Fermi paradox, though. Maybe other intelligent life isn’t nearby enough to see (we can only see a relatively small part of our galaxy well), or it’s not as ambitious as we are (maybe they have a hard time getting off their planet or don’t use radio communication), or it’s taking some form we don’t recognize, or maybe they don’t want us to see them, or any number of other possibilities. That’s where I run out of answers.

How quickly would we be able to spot an alien spaceship coming to visit us with modern telescopes? (Intermediate)

This is obviously a very sci-fi question that is impossible to give a real answer to, but I can actually give a pretty reasonable scenario based on some recent events. In 2017, an object called ‘Oumuamua entered the inner Solar System from interplanetary space. It was a thin sliver (likely a fragmented comet or something like that) that was several hundred meters long and around 100 meters wide, and it dove closer to the Sun than Mercury before being slingshotted back out of the Solar System forever. And it did all of this before we even noticed it at all. It was only randomly discovered by a telescope survey about 40 days after its closest approach, when it was already on an escape trajectory. So while we obviously have never seen an alien spaceship coming into the Solar System, we have seen this thing, which is similar in size, shape, and trajectory to a hypothetical spaceship (so much so that it prompted some conspiracy theories). If we believe this example, then it is reasonable that we wouldn’t see a spaceship of similar scale until it had already arrived (unless they wanted to be found). New telescopes coming online in the next few years will significantly improve our ability to see dim, fast-moving objects like this though, so our space defenses should soon improve dramatically.

Will the James Webb Space Telescope be able to see things on the surface of exoplanets? (Intermediate)

The James Webb Space Telescope is a highly anticipated telescope within the astronomy community that will enable many new and amazing observations when it is operational, but it is important not to get too carried away with what is physically possible. The laws of optics say that the diameter of a telescope dictates the size of the smallest thing it can resolve, even when the telescope is in space and there is no atmosphere in the way. So when JWST launches in December (not October) and unfolds its mirror to 6.5 meters (over 21 feet) across, this is the factor that will limit the smallest objects it can see.

With the wavelength of light it will be using (infrared), JWST will have an angular resolution of 0.07 arcseconds, which is equivalent to about 400 feet when you look at the Moon from the Earth. So even if we were using it to look for things. It will still be able to collect light from things that are smaller than this, but all of the light will be blurred into one blob that it won’t be able to see details in. So even on the Moon, JWST wouldn’t be able to see actual creatures. Kepler 186f is very far from the Earth (almost 500 light years), so the angular resolution works out to about 10 times the size of the Earth’s orbit, so not even close to being able to pick out features on the surface of the planet.

Angular resolution isn’t the real reason exoplanet scientists are excited about JWST. By putting such a large infrared telescope into space, they instead hope to use its onboard instruments to look at the light shining through the atmospheres of the other planets (something that doesn’t require actually resolving the surface of the planet). If they can find the signatures of molecules that could only come from living things, then they could confirm that there is life on those planets, something that wouldn’t be possible from the surface of the Earth because we have our own atmosphere to worry about.

So it’s really the molecules in the atmosphere that astronomers are trying to get past when they launch a space telescope, not necessarily the blurriness of the atmosphere. Astronomers have gotten pretty good at correcting for the blurriness of the atmosphere using systems called adaptive optics (where you bend the mirror of the telescope to undo the blurriness from the atmosphere), so ground-based observatories can now get to their theoretically best angular resolution too. However, even for the next generation of telescopes that will have mirrors 100 feet wide, they will still only be able to resolve objects that are about 60 times the size of the Earth. In order to really see the surface of other planets, you’ll need to try out some truly crazy ideas.

Can we do spectrometry on transiting exoplanets to see whether there are signatures of life in their atmospheres? (Intermediate)

Atmospheric spectroscopy of transiting exoplanet atmospheres is still in its infancy, but the field will mature significantly in the next few years as more nearby exoplanets are discovered and we gain the ability to look at their atmospheres more easily. Taking the spectrum of a transiting exoplanet is very difficult for a number of reasons. First, the window of time to perform the work is small, so conditions must be appropriate and resources must be available exactly when they are needed. Second, the signal itself will be extremely small. Seeing the signature of a transiting exoplanet is often compared to looking for a fly in front of a searchlight, and looking for the atmosphere would be like looking for a thin haze around that fly. The atmospheres of small rocky planets like Earth are also much smaller than those of large gas giants too, so it’s even more difficult. Also, the limited amount of exposure time (due to the short transit) means that you have a strict time limit too. Third, stars themselves are weird in a lot of ways, so you need to make sure your “planetary atmosphere” isn’t just a temporary feature of the star. Finally, a lot of this spectroscopy is incredibly difficult to do from the ground because our own planetary atmosphere is in the way. If you claim to see a molecule on another planet, you need to be very very sure you’re not just looking at the same molecule on Earth by accident. If you’re interested in a deeper dive, here is a relatively approachable paper on the subject of transiting exoplanet atmospheric spectroscopy and its prospects in the near future from a few years ago.

The Kepler space telescope discovered several thousand transiting exoplanets, but they are comparatively far away from Earth, so all of the problems above make measurements very difficult to overcome. A newer satellite called TESS has been focused on finding nearby transiting exoplanets in the past few years, which should be much easier to study. When the James Webb Space Telescopes launches in (hopefully) a few months, it will be able to take infrared spectra of nearby transiting Earthlike planets from TESS, which will give the best chance of detecting biosignature molecules in the near future. I think it’s safe to say that we’re many decades away from doing similar work across our galaxy or in other galaxies since doing so would require significantly larger space telescopes (the only planets we even know about in other galaxies are one-offs in extreme environments).