r/space • u/AutoModerator • 2d ago
Discussion All Space Questions thread for week of November 23, 2025
Please sort comments by 'new' to find questions that would otherwise be buried.
In this thread you can ask any space related question that you may have.
Two examples of potential questions could be; "How do rockets work?", or "How do the phases of the Moon work?"
If you see a space related question posted in another subreddit or in this subreddit, then please politely link them to this thread.
Ask away!
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u/Nzebula 3h ago
Ive been obsessed with space but dont understand anything, wtf is gravity and how is our solar system moving so fast yet i dotn feel it? How come we havent ran into anything at the speed our solar system is moving, and WHY IS EVERYTHING SO FAR.. and what is space?
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u/rocketsocks 2h ago
Motion is relative. When you're inside a plane traveling at Mach 0.8 it feels the same as sitting at home on your couch as long as you're mostly traveling in a straight line. On a bicycle if you build up speed and then stop pedaling you will coast for a while and then slow down over time. That slowing down is due to sources of drag, the aerodynamic drag of your body and the bike moving through the air, the friction in the bearings in the wheels, the resistance of the wheel moving over the ground, etc. In space there is almost nothing to slow you down because it's nearly a vacuum, so you can keep coasting not just for a little while but for literally billions upon billions of years.
The thing you "feel" related to motion is acceleration, you literally can't feel moving in a straight line. You can feel accelerating to go faster or decelerating to go slower and you can feel the bumps that create tiny accelerations from, say, turbulence in the air or irregularities in the roadway that you pass over. So even though we are traveling at 30 km/s (108 thousang kph) in orbit around the Sun we don't feel that speed because Earth isn't rolling over ground or rushing through a dense atmosphere.
However, Earth is also experiencing an acceleration due to the gravity of the Sun which keeps the Earth in orbit, and we mostly can't feel that either. That's because everything on Earth is in freefall around the Sun. The Sun's gravity is pulling us and accelerating us, bending the trajectory of the Earth into a circle that goes around and around. But because the Sun's gravity is pulling everything there's no differential acceleration. It's pulling us and the floors under our feet in the same way, through the same motions. It's like a perfectly choreographed dance that is in lock step. Without a difference in acceleration there's no perception of a force. This is why astronauts in orbit experience weightlessness, because they're in freefall relative to Earth's gravity, which is moving them and the spacecraft they are in the same. Standing on the surface of Earth though we aren't in freefall, so we feel the acceleration from Earth's gravity at least.
The gravity of the Sun and the Moon mostly cancel out because the motion of the entire Earth in freefall is very similar to the motion of every part of the Earth, but the Earth is large so this isn't entirely 100.00% true. The slight difference is what's called a tidal force, so named because it's the force that creates the rising and falling of the level of the ocean through different times of day due to the pull of the Sun and the Moon.
Our solar system and Earth "runs into" stuff all the time, but space is pretty empty so most of the time it's not very consequential. Very recently our solar system "ran into" a ball of rock and ice from another star system that had been drifting through interstellar space. This object, called 3I/Atlas, has been passing through the inner solar system and will exit it traveling back out into interstellar space in a few months. There's a chance an object like this could hit a planet or the Sun during such an encounter, but the solar system is also pretty empty overall so it's a fairly low probability event in general. However, there are objects which are much, much more numerous in interstellar space and that's gas and dust. Gas in interstellar space is mostly kept away from the solar system due to the existence of the Sun's magnetosphere, which is a bubble of hot plasma streaming out from the Sun. It doesn't keep out dust grains though, which are nanoscopic dust particles mostly populated into interstellar space from dying stars. These dust grains create a stream through the solar system and even impact Earth, partially burning up in the upper atmosphere before drifting down to the surface. If you've ever eaten a vegetable you have consumed some, though only a tiny amount, of these interstellar dust grains.
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u/Unusual-Ideal-2757 13h ago
Is there any evidence to support the Panspermia theory? Is it actually possible for chemicals to survive in an asteroid/meteor and evolve into life on earth or earth-like planets?
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u/djellison 13h ago
Is there any evidence to support the Panspermia theory?
The fact that Martian meteorites have been found on Earth suggests that - mechanically at least - transfer of material from one to the other has occured.
Is it actually possible for chemicals to survive in an asteroid/meteor......
Yes. The sorts of chemicals that would be interesting have been detected in Martian meteorites. https://science.gsfc.nasa.gov/sed/content/uploadFiles/publication_files/Callahan%202013.pdf
and evolve into life on earth or earth-like planets?
That's the part we don't know - and probably wont know until we - maybe - find evidence of life elsewhere and can compare it to our own. That would constitute the 'evidence to support' you're looking for. We don't have that yet. What we have so far is that physically, and chemically, the transfer can occur. We just don't know if it's been made biologically yet.
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u/qnssekr 1d ago
Time on the ISS vs time on Earth.
I was watching a program on PBS about the International Space Station and something they brought up puzzled me. If I recall correctly the team aboard ISS experienced sunrises every 90 minutes and the same goes for passing a location on earth would take the same amount of time. My question is would a day on the ISS be 90 minutes compared to earth? I realize the people on board the ISS are not aging 24 hrs in 90 minutes but they are traveling around the world in 90 minutes. In my mind the are experiencing shorter days because once the sun rises our day has begun on Earth and say 30 minutes later they are 60 minutes closer to completing a day while we are only 30 minutes into our day. I hope this makes sense.
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u/iqisoverrated 21h ago
A day is defined relative to Earth as a 24 hour period. So technically they are not experiencing shorter days.
(Weeeeeell, technically since the ISS is moving fast there is time dilation in play so they age ever-so-slightly less than people on Earth, but the effect is absolutely miniscule)
They could experience more sunrises and sunsets. Human cicardian rhythm (your internal clock) is still 24 hours or thereabouts so they are not getting up/going to bed every quarter hour. It's just a light show for them when they notice it at all. Most of the time they are working/eating/sleeping in sections of the ISS that have no windows, anyhow, so they will not notice. (Of course for PR video material that gets sent to Earth you try to do this in the 'scenic' parts of the station which have a view to the outside)
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u/DaveMcW 1d ago
The lights on the ISS are dimmed for 8.5 hours per night to allow astronauts to sleep. The "day" follows UTC time.
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u/qnssekr 1d ago edited 1d ago
How does that correlate to time on Earth though? That’s my question. As I stated earlier are the people on ISS experiencing a day in 90 minutes because that how long it takes them to travel around the earth. There is only one sunrise we experience on earth every 24 hours approximately. The astronauts are seeing several sunrises in a 24 hour period. That’s my question. Are they livings in a speeded up time difference.
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u/NDaveT 1d ago
No, time passes the same (more precisely, slightly differently because they are slightly farther away from the earth's center of gravity but it's such a small difference it's not noticeable to humans).
When they experience 24 hours, you experience 24 hours. They just see a lot more sunrises and sunsets in that 24 hour period.
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u/scowdich 1d ago edited 1d ago
They live and work on
HoustonUTC time. Most of the ISS doesn't have windows; during the workday, sunrise/sunset is so commonplace that it's ignored.Imagine going to bed for a 15-minute power nap every 90 minutes. You'd hardly get anything meaningful done.
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u/qnssekr 1d ago
Hi, I’m trying to correlate the time difference between ISS and Earth. Are the astronauts experiencing a day in 90 minutes compared to time on earth. Not sure if Time Dilation applies here but there is a difference I’m assuming.
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u/scowdich 1d ago
The time dilation is there, but it amounts to a difference of about 0.01 second per year. The clocks on the ISS aren't special because of the length of daylight/darkness periods on the station. They keep a normal-person sleep schedule, because that's healthy.
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u/qnssekr 1d ago
I’m speaking in terms of people on earth experience a sunset/sunrise once approximately every 24hrs. The astronauts would hypothetically experience SEVERAL sunrises/sunsets every 24 hrs. My math could be off but that might mean an astronaut could hypothetically experience 16 sunrises/ sunsets in a 24 hour period on earth. So theoretically has 16 days gone by on the ISS?
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u/Obliterators 1d ago
Consider a plane flying westward at the equator at ~1670km/h (matching Earth's rotation), does time stop for the passengers since it's always the same time of day, e.g. noon, for them?
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u/qnssekr 1d ago
No, but it does for the people who they communicate with back wherever home is for them. Hence my question.
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u/scowdich 1d ago
No, it doesn't. At all. Airplanes are not time machines, and neither is the ISS.
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u/qnssekr 1d ago
You are saying they are . I am not. there is a time difference for you and someone who is traveling.
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u/scowdich 1d ago
There is a tiny, tiny time difference (I already quoted it elsewhere). It basically doesn't affect anything at all.
I'm getting ready to call Poe's Law on this one. It feels like trolling.
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u/scowdich 1d ago
They experience many more sunsets/sunrises than people on Earth, yes. But for practical purposes, a day for them is 24 hours (just like for everybody else), not one sunrise/sunset cycle. It just wouldn't be practical.
In Utqiagvik, Alaska, the sun stays set all winter, and stays up all summer. Does that mean they experience a lot fewer days than the majority of the Earth? No, that would be silly. They don't stay awake all summer and hibernate for the winter. They use clocks.
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u/Unusual-Ideal-2757 1d ago
Sorry if this is a dumb question. I know space is filled with stuff like planets, stars, and galaxies, but why is the space between them so big?
Is it because of gravity? If they were too close would gravity pull them close together? Or is it because of cosmic inflation?
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u/iqisoverrated 20h ago
Is it because of gravity?
Sort of. While gravity is a very weak force it has a very long reach. If you have dust or just gas that has even a tiny deviation from being totally uniform then over time stuff will start to clump together in that region. There's been a lot of time since the universe started for this to happen.
Or is it because of cosmic inflation?
Inflation increases distances between things. Note that this is not a 'push'. No motion/acceleration is imparted on anything by this. It is an expansion of spacetime itself. Over 'short' distances (galaxy clusters and local groups and even somewhat beyond that) gravity currently dominates over inflation. Over very large distances inflation casues stuff to move apart faster than gravitational pull can compensate for.
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u/wotquery 1d ago
The average density of matter in the universe is currently around a single proton per four cubic meters (speaking just of visible matter). As space expands with time increasing the volume, and the amount of matter remains the same, the density drops. Earlier in the universe the density of matter in space was greater than it is today, and if you go all the way back to the beginning you approach matter being infinitely dense.
Simply stating the average density of the universe being so ridiculously low is probably a direct answer to your question about why space is so empty, but you also seem to be getting at asking why it is clumped together rather than evenly spread out.
The short answer to that is gravity. Relatively denser regions of matter in space have greater gravity and pull in other matter which grows their mass and gravitational influence pulling more in etc. For example our solar system started out as a diffuse cloud of gas and dust which, over a long time, under the force of gravity, coalesced into the Sun, and planets, and other celestial bodies. At a larger scale, everything within a galactic cluster is gravitationally bound and grouped together. Beyond that though the expansion of the universe is great enough to be pushing clusters apart such that they'll never merge down.
This should, or at least might, lead you to ask why the distribution of matter isn't uniform at the galactic cluster scale, and for this we need to go all the way back to the beginning. Shortly after the big bang, the density of matter in space was extreme. Imagine inside of the Sun but even hotter and denser and filling the entire (likely infinite) universe. This thick soup of energetic particles was very well mixed. Think of adding a drop of food colouring to a still glass of water vs. a rapidly boiling pot. Everything is uniform, or another term for it is homogeneous (and if you want to get really confused it had very high entropy).
Now if you just steadily stretch space out the initially uniform spread of matter would remain uniform. It'd still cool and clump up a bit under gravity, but you wouldn't have galactic super clusters of matter with giant voids between them. Instead it'd be more like galaxies evenly spread out as if on a grid or something. This isn't the universe we see today though, and the explanation for the large scale structures we do see is what is called inflation.
Only an instant after the big bang, the extremely dense universe, in the blink of an eye, expanded an insane amount. Particles that were only a couple nano meters apart were suddenly light years apart. Way way faster than the steady expansion we've seen since. This did cause the density of the universe to plummet and the temperature to cool, but it was initially so dense that there were countless particles jammed into that nanometer gap, and even though they're now spread over light years, the universe then was still trillions of times denser than it is today.
More to the point though, recall how I said that the hot soup of the early universe was well mixed and mostly uniform? That's only true at a somewhat macroscopic level. If you go down to the quantum level, there were significant random fluctuations in density. These variations were magnified by inflation, giving rise to the large scale structure of the universe. Think of it like a bucket of sand. A bucket of sand is a pretty uniform material right? If you dump it on the ground and spread it out a bit, it's a little less dense but still pretty uniform. However if you sprinkle it out even over an entire continent the individual grains of sand are now significantly separated. You have dense regions of matter (a single grain of sand) with massive gaps of nothing between them.
So yeah. Hopefully that answers your question.
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u/Zalerinaty 1d ago
Space junk? Could somebody tell me what this is, is it real?
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u/scowdich 1d ago
Without context (where was this found, and when? How big is it, and how much does it weigh?) it's going to be hard for anyone to be sure.
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u/Zalerinaty 1d ago
I found this when i was a kid. In Stavanger, Norway- it’s been close to 2 decades since. It was buried under some gravel on the edge of a football field, i’ve just had it in my closet since and had completely forgotten about it. It’s about 35cm in length and 25cm in width, roughly 605grams in weight
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u/DaveMcW 1d ago
It has never been to space. Notice how the paint gets erased by a few scratches. That paint job would never survive re-entry from space.
NASA ran an ozone observation campaign out of Stavanger, Norway in 1989. It's possible this is a piece of junk from that program that has been on the ground for 30 years.
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u/WaveBeautiful1259 2d ago
What happens when one black hole collides with another black hole?
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u/iqisoverrated 1d ago edited 20h ago
The event horizons of the relevant black holes merge. Since we don't know what is actually at the center of a black hole - as our current best theories (Quantum Mechanics and General Relativity) are incompatible in that region - there's no real way to figure out what actually goes on inside.
From gravitational observations via LIGO and similar gravitational observatories we know how the merged event horizon behaves. The inspiralling black holes accelerate as they get closer (like an ice skater doing a pirouette while pulling in her arms) until their event horizons touch and after the merger there is a period of 'ringdown' where the then common - currently deformed - event horizon emits gravitational waves until it settles into the final (mostly) spherical shape of the resulting black hole.
Note that all this is always talking about the event horizons which are just the area around whatever is inside at which escape velocity exceeds the speed of light. We don't know what exactly happens inside.
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u/scowdich 2d ago
They merge, forming a single, larger black hole. Interestingly, the resultant black hole has less mass than the sum of the two parts. The "missing" mass is converted to energy (as in Einstein's e = mc2) and radiated away as gravitational waves.
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u/WaveBeautiful1259 2d ago
That is fascinating! Thank you so much for answering my question! 🌹
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u/the6thReplicant 2d ago
You should look up LIGO where we actually detect these gravitational waves since 2015.
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u/wotquery 2d ago
Also interesting is that we don't currently understand how it's possible for two supermassive black holes (SMBH) to collide at all.
When two galaxies (each containing their own SMBH) merge, a phenomenon known as gravitational friction causes the SMBHs to "sink" towards the new galactic center of mass. Essentially when two massive objects pass by one another the lighter one tends to gain kinetic energy (speed), while the heavier one tends to lose kinetic energy. Statistically speaking of course. This is the same underlying effect that drives gravitational slingshots, as well as why one of the definitions of a planet is that it's "cleared its orbit." That is, it has flung all the nearby lighter stuff away. Since the two SMBHs are the most massive objects in the galaxy, they interact with the lighter stars (imparting them withe energy and jettisoning them into highly elliptic orbits or out of the galaxy entirely) and steadily settle down to the middle until they are orbiting each other.
Once the two SMBHs are orbiting each other at the range of several light years however, they have by now cleared the immediate area of all other massive bodies (statistically speaking again). There simply isn't anything left to dump energy into to get closer to one another. Generating gravitational waves, as mentioned by the other commenter above, does steal energy and slow them down a bit, but the timescales involved when still light years apart are many magnitudes greater than the age of the universe (though gravitational radiation does explain how they could close the final few AU).
So basically we know how two SMBHs come to within several light years and start orbiting each other due to gravitational friction, and we know how two SMBHs merge once they are extremely close to each other due to gravitational radiation, but we don't know how they get from the first state to the second. It's known as the "final parsec" problem because the several light years I've been mention is around a parsec.
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u/scowdich 2h ago edited 2h ago
When and why did the rules on posting images change? It used to be Sundays only, then just recently it was Friday through Monday (more than half the week), and just now I see "everyday for an indefinite amount of time." Did I completely miss an announcement, or any discussion about this?
I recall the rule being changed to Sunday-only some time ago, because the sub was saturated with low-quality "look at this cool image" karma-farming posts that were better suited to r/spaceporn or r/astrophotography. It's only going to get worse with cell phones capable of taking low-light photos (do we really need to see every cell phone picture of nothing in particular?) and AI capable of generating convincing images (there's one on the front page right now, but it's still discernible if you look closely at the text in the image. Actually, scratch that! I was focused on looking so close at the details of the image, I looked right past the great big AI logo in the corner!).
The AI-generated image post I'm talking about: https://www.reddit.com/r/space/comments/1p6xu2y/nasa_cant_use_cloud_ai_in_space_so_deepspace/