Kurzgesagt – In a Nutshell
Kurzgesagt – In a Nutshell
Embedded Files
Sources – The Barrier
Sources – The Barrier
We thank the following experts for their feedback:
Blakesley Burkhart
Associate Professor, Physics & Astronomy Dept., Rutgers University
Abel Mendez
Associate Research Scientist, Center for Computational Astrophysics, Simons Foundation
—The fastest a human ever traveled in space were Astronauts at 40,000 km/h, returning from the Moon in 1969.
#NASA (2019): “50 Years Ago: Apollo 10 Clears the Way for the first Moon Landing”
https://www.nasa.gov/history/50-years-ago-apollo-10-clears-the-way-for-the-first-moon-landing/
Quote: “By the time it made first contact with the Earth’s atmosphere 16 minutes later at an altitude of 400,000 feet, the point called Entry Interface, Apollo 10 was traveling at 24,791 miles per hour [or 39,897 kilometers per hour], the fastest reentry for any crewed space mission. To this day, Stafford, Cernan, and Young hold the record as the fastest humans.”
—The fastest thing humans have ever built by far is the Parker Solar Probe, which at this moment uses a physics cheatcode, hurling around the gravity well of Venus, accelerating to get closer to the sun. After seven years of doing this, the probe is shooting through space at a breathtaking 635,000 km/h.
#Deeks, Russell (2025): “Fastest human-made objects ever built”, BBC Sky at Night Magazine
https://www.skyatnightmagazine.com/space-missions/fastest-object
Quote: “The current space speed record holder for fastest human-made object is NASA’s uncrewed Parker Solar Probe.
On 21 September 2023 – assisted by several fly-bys of Venus that allowed it to slingshot off the planet’s gravity – Parker Solar Probe clocked up a speed of 635,266 km/h (394,736mph).”
#NASA’s Parker Solar Probe: “The Mission: Timeline” (retrieved 2026)
https://parkersolarprobe.jhuapl.edu/The-Mission/index.php#Timeline
—Fast enough to go around the world in 4 minutes or to cover the distance from Earth to the moon in 36.
The speed of the probe is 635,266 km/h:
#Deeks, Russell (2025): “Fastest human-made objects ever built”, BBC Sky at Night Magazine
https://www.skyatnightmagazine.com/space-missions/fastest-object
Quote: “The current space speed record holder for fastest human-made object is NASA’s uncrewed Parker Solar Probe.
On 21 September 2023 – assisted by several fly-bys of Venus that allowed it to slingshot off the planet’s gravity – Parker Solar Probe clocked up a speed of 635,266 km/h (394,736mph).”
The mean radius of the Earth is 6,371.0 km:
#Fasullo, John: ”Earth Fact Sheet” (retrieved 2026)
https://atoc.colorado.edu/~fasullo/1060/resources/earthfact.html
Then, the total distance to cover to go once around the Earth is:
2 × 𝜋 × 6,371.0 km= 40,030 km
And covering that distance would take the probe:
t=d/v= (40,030 km) / (635,266 km/h) ~ 0.063 hours ~ 4 min.
The average distance between the Earth and the Moon is 384,400 km
#UK Royal Museums Greenwich: “How far away is the Moon?” (retrieved 2026)
https://www.rmg.co.uk/stories/space-astronomy/how-far-away-moon
Quote: “The average distance between the Earth and the Moon is 384 400 km (238 855 miles).”
By the same type of calculation:
t=d/v= (384,400 km) / (635,266 km/h) ~ 0.61 hours ~ 36 min.
—Current rockets would need 7-10 months to reach Mars, but our new spaceship covers the distance in just two weeks! Pretty good!!
#NASA (2025): “How Long Does it Take to Get to the Moon… Mars… Jupiter? We Asked a NASA Expert: Episode 51”
Quote: “NASA’s Mars Reconnaissance Orbiter mission took about seven and a half months to reach Mars. And NASA’s MAVEN mission took about ten months to reach Mars.”
The speed of the probe is 635,266 km/h:
#Deeks, Russell (2025): “Fastest human-made objects ever built”, BBC Sky at Night Magazine
https://www.skyatnightmagazine.com/space-missions/fastest-object
Quote: “The current space speed record holder for fastest human-made object is NASA’s uncrewed Parker Solar Probe.
On 21 September 2023 – assisted by several fly-bys of Venus that allowed it to slingshot off the planet’s gravity – Parker Solar Probe clocked up a speed of 635,266 km/h (394,736mph).”
The mean distance to Mars is 140 million miles, or 230 million km:
#NASA: “Hazard: Distance from Earth” (retrieved 2025)
https://www.nasa.gov/hrp/hazard-distance-from-earth/
Quote: “Mars is, on average, 140 million miles from Earth [230 million km].”
And covering that distance would take the probe:
t=d/v= (230 × 106 km) / (635,266 km/h) ~360 hours ~15 days.
—The next stop is Pluto which we will reach after about a year – ok much longer, but still ok! It only takes two more years to pass the heliopause and one more to pass Voyager 2, which traveled over 50 years to get this far. Great!
The speed of the probe is 635,266 km/h:
#Deeks, Russell (2025): “Fastest human-made objects ever built”, BBC Sky at Night Magazine
https://www.skyatnightmagazine.com/space-missions/fastest-object
Quote: “The current space speed record holder for fastest human-made object is NASA’s uncrewed Parker Solar Probe.
On 21 September 2023 – assisted by several fly-bys of Venus that allowed it to slingshot off the planet’s gravity – Parker Solar Probe clocked up a speed of 635,266 km/h (394,736mph).”
#NASA: “Dwarf Planets: Pluto Facts: Orbit and Rotation” (retrieved 2026)
https://science.nasa.gov/dwarf-planets/pluto/facts/#h-orbit-and-rotation
Quote: “Pluto's orbit around the Sun is unusual compared to the planets: it's both elliptical and tilted. Pluto's 248-year-long, oval-shaped orbit can take it as far as 49.3 astronomical units (AU) from the Sun, and as close as 30 AU. (One AU is the mean distance between Earth and the Sun: about 93 million miles or 150 million kilometers.) But on average, Pluto is 3.7 billion miles (5.9 billion kilometers) away from the Sun, or 39 AU [5.8 billion km].”
For simplicity, we assume that the distance between the Earth and Pluto is around 39 AU or 5.8 billion kilometers. Then, covering that distance takes the probe:
t=d/v= (5.8 × 109 km) / (635,266 km/h) ~ 9100 hours ~ 380 days.
The heliopause is at around 18 billion km:
#Encyclopedia Britannica: “Heliopause” (retrieved 2026)
https://www.britannica.com/science/heliopause
Quote: “The heliopause is about 123 astronomical units (AU; 18 billion km [11 billion miles]) from the Sun.”
And covering that distance would take the probe:
t=d/v= (18 × 109 km) / (635,266 km/h) ~28,000 hours ~3.2 years.
The Voyager 2 was at the Heliosphere, at a distance xV0 of 18 billion km, in 2018:
#PhysicsWorld (2019): “Voyager 2 spacecraft goes interstellar as it leaves the solar bubble” https://physicsworld.com/a/voyager-2-spacecraft-goes-interstellar-as-it-leaves-the-solar-bubble/
Quote: “The spacecraft Voyager 2 left the heliosphere and travelled into interstellar space over the course of a day in November 2018, according to a suite of papers published today by scientists working on the mission.
The spacecraft was launched in 1977 along with its twin Voyager 1, which crossed-over into interstellar space seven years ago. Scientists analysing data from Voyager 2 have found both similarities and differences to the crossing of Voyager 1.”
It travels at an approximate speed vV0 of 15.4 km/s or 54,000 km/h.
#NASA: “Voyager 2” (retrieved 2026)
https://science.nasa.gov/mission/voyager/voyager-2/
Quote: “At 9.6 miles per second (15.4 kilometers per second [54,000 km/h]) relative to the Sun, it will take about 19,390 years for Voyager 2 to traverse a single light year.”
Meanwhile, the Parker Solar Probe travels at vP= 635,266 km/h.
#Deeks, Russell (2025): “Fastest human-made objects ever built”, BBC Sky at Night Magazine
https://www.skyatnightmagazine.com/space-missions/fastest-object
Quote: “The current space speed record holder for fastest human-made object is NASA’s uncrewed Parker Solar Probe.
On 21 September 2023 – assisted by several fly-bys of Venus that allowed it to slingshot off the planet’s gravity – Parker Solar Probe clocked up a speed of 635,266 km/h (394,736mph).”
If we sent a probe today at the same speed in the direction of Voyager 2, its position could be described by:
xP = vPt
While the position of Voyager 2 would be:
xv = xV0 + :vV0(t + 8 years)
Equating the positions and isolating t, we find that they would meet in:
t= (xV0 + (vV0 × 8 years) ) / (vP - vV0) =
(18 × 109 km + 54,000 km/h × (8 years × 365 days/year × 24 hours/day) ) / (635,266 km/h - 54,000 km/h)
~ 37000 hours ~ 4.3 years
By that time it would be 2030, so 53 (rounded to 50) would have passed since the launch of Voyager 2 in 1977.
—The next stop is the end of the Oort cloud, the true edge of the solar system where open space begins! We reach it in… 2,500 years.
The speed of the probe is 635,266 km/h:
#Deeks, Russell (2025): “Fastest human-made objects ever built”, BBC Sky at Night Magazine
https://www.skyatnightmagazine.com/space-missions/fastest-object
Quote: “The current space speed record holder for fastest human-made object is NASA’s uncrewed Parker Solar Probe.
On 21 September 2023 – assisted by several fly-bys of Venus that allowed it to slingshot off the planet’s gravity – Parker Solar Probe clocked up a speed of 635,266 km/h (394,736mph).”
The end of the Oort cloud is at 100,000 astronomical units, or 1.5 × 1013 km:
#NASA: “Oort Cloud Facts” (retrieved 2026)
https://science.nasa.gov/solar-system/oort-cloud/facts/
Quote: “In 1950, astronomer Jan Oort proposed that some comets come from a vast, extremely distant spherical shell of icy bodies surrounding the solar system. This giant swarm of objects, now named the Oort Cloud, occupies space at a distance between 5,000 and 100,000 astronomical units [1.5 × 1013 km].”
So reaching the end of the Oort cloud would take the probe:
t=d/v= (1.5 × 1013 km) / (635,266 km/h) ~ 2.3 × 107 ~ 2,700 years
—But things like a propulsion system that uses nuclear fusion or antimatter, powerful enough to reach a substantial fraction of the speed of light, are in the realm of the physically possible.
Right now we do not have rockets that use any of these two methods of propulsion, but designs that use each of them have been considered:
#US National Air and Space Museum: “Propulsion Test Vehicle, Project Orion” (retrieved 2026)
#NASA Marshall Space Flight Center: “Antimatter Propulsion” (retrieved 2026)
https://ntrs.nasa.gov/api/citations/20200001904/downloads/20200001904.pdf
Depending on the size of the mission, other propulsion systems could be favoured. We thank the expert Blakesley Burkhart for the following comment:
Quote: “Light sails [may be a better system], if the mass of the probe is small enough.”
—we can get a spaceship to fly at around 20% the speed of light!
Confirmed as a reasonable horizon by expert feedback, though it depends on the payload and could be too difficult for a crewed mission like the one we describe.
—Which is extremely fast – 60,000 kilometers per SECOND, 216 million kilometers per hour. Fast enough to reach the edge of the solar system in just 3.5 days and the outer edge of the Oort cloud in eight years. Alpha Centauri, the closest star system to our own in 20 years! We can work with that!
We can express 20% of the speed of light using different units:
0.2 c = 60,000 kilometers per second = 2.16 ×108 kilometers per hour
We consider the heliopause to be the edge of the Solar System. It is 123 AU or 0.001945 light years from the Sun.
#Encyclopedia Britannica: “Heliopause” (retrieved 2026)
https://www.britannica.com/science/heliopause
Quote: “The heliopause is about 123 astronomical units (AU; 18 billion km [11 billion miles]) from the Sun.”
Then, getting there at 0.2c would take:
#E = mc2 Explained: “Time dilation calculator” (used 2026)
0.0095 years ~ 3.5 days
Notice that at these scales the results for an outside observer and from the moving probe are approximately the same. For simplicity, we will round them to agreement.
By the same calculation:
#NASA: “Oort Cloud Facts” (retrieved 2026)
https://science.nasa.gov/solar-system/oort-cloud/facts/
Quote: “The inner edge of the Oort Cloud, however, is thought to be located between 2,000 and 5,000 AU from the Sun, with the outer edge being located somewhere between 10,000 and 100,000 AU from the Sun.”
#E = mc2 Explained: “Time dilation calculator” (used 2026)
#Encyclopedia Britannica: “Alpha Centauri” (retrieved 2026)
https://www.britannica.com/place/Alpha-Centauri
Quote: “Alpha Centauri, triple star system, the faintest component of which, Proxima Centauri, is the closest star to the Sun, about 4.2 light-years distant.”
#E = mc2 Explained: “Time dilation calculator” (used 2026)
— Because while space is very empty, it is not actually empty. There are energetic particles, gas, dust and things bigger than dust.
#The Center for Astrophysics | Harvard & Smithsonian: “Interstellar Medium and Molecular Clouds” (retrieved 2026)
https://www.cfa.harvard.edu/research/topic/interstellar-medium-and-molecular-clouds
Quote: “Most of the space between stars is much emptier than the best vacuum we can make in a lab on Earth. However, it’s not truly empty: the interstellar medium (ISM) is complex, with a variety of physical processes governing its behavior. This is the place where stars are born, and where stars recycle their atoms when they die.
Hydrogen and helium make up about 98% of the mass of the ISM, with the remaining 2% being heavier elements like carbon, oxygen, and other elements astronomers call “metals”. Most of these atoms are in the form of gas, but about half of the heavier elements form dust: relatively large grains composed of carbon-containing molecules.”
#Encyclopedia Britannica. “Interstellar medium” (retrieved 2026)
https://www.britannica.com/science/interstellar-medium
Although they are much rarer, bigger objects than dust exist in the interstellar medium.
#The Planetary Society (2025): “Where do 3I/ATLAS and other interstellar visitors come from?”
https://www.planetary.org/articles/where-do-3i-atlas-and-other-interstellar-visitors-come-from
Quote: “An average star system in the Milky Way throws off roughly 10 quadrillion objects into interstellar space throughout its lifetime. Most of these bits of dust, rock, and ice get ejected soon after a star is born, when planets are still forming and settling into stable orbits. Some debris gets slung out as these orbits rearrange, while other bits get pulled away by the gravitational influence of passing stars.”
—An individual iron atom that hits your spaceship at this speed has enough energy to carve a damage track tenths of a millimetre deep. Over time its hull would be riddled with microscopic bullet holes, which is not great.
#Hoang, Thiem et al. (2017): “The Interaction of Relativistic Spacecrafts with the Interstellar Medium”, The Astrophysical Journal, vol. 837, 1
—Of course our engineers knew this and created a shield for our ship. A sort of protective sail that absorbs or at least slows down most molecules we smash into.
The shield in this video is loosely based on the type of shield described here:
#Drobny, Jon et al. (2020): “Survivability of Metallic Shields for Relativistic Spacecraft”, Journal of the British Interplanetary Society, vol. 73, 12, 446-456.
Other shield designs are possible. We thank expert Blakesley Burkhart for the following comment:
Quote: “Possible to build a magnetic field shield that travels ahead of the spacecraft. This is the often imagined work around since the dust grains are charged, one could deflect them.”
—Running into a single grain of dust at 20% the speed of light is like running into a grenade, causing a tiny detonation as the dust explosively evaporates, grinding away our protective shield.
The impact of a typical grain of interstellar dust at 0.2c is much less energetic than an actual grenade, but it causes the dust to explode and create damage in all directions, like a grenade.
#Hoang, Thiem et al. (2017): “The Interaction of Relativistic Spacecrafts with the Interstellar Medium”, The Astrophysical Journal, vol. 837, 1
https://iopscience.iop.org/article/10.3847/1538-4357/aa5da6
https://arxiv.org/pdf/1608.05284
Quote: “The effect of dust bombardment erodes the spacecraft surface and produces numerous craters due to explosive evaporation of surface atoms. For a spacecraft speed v=0.2c, we find that dust bombardment can erode a surface layer of ~0.5 mm thickness after the spacecraft has swept a column density of 𝑁𝐻∼3×1017 cm−2, assuming the standard gas-to-dust ratio of the ISM.”
Each of the impacts creates a micro-crater on the surface of the spacecraft. The repeated impacts end up eroding the surface away.
—Like a small rock, the size of a golf ball. Hitting that at 20% the speed of light releases more than double the energy of the nuclear bomb that was dropped over Hiroshima. We’ll not even talk about anything larger.
From the point of view of the spacecraft, a rock that sits still in space is going at 0.2c towards it. The kinetic energy of a, say, 100 g rock, measured from the spacecraft going at 0.2c is:
#Szyk, Bogna (2025): “Relativistic Kinetic Energy Calculator”
Which is more than twice the energy released in the bombing of Hiroshima:
#Encyclopedia Britannica:”The First Atomic Bombs Tested and Used During World War II” (retrieved 2026)
Quote: “The first atomic bomb detonated over a populated area occurred on August 6, 1945 at 8:15 AM over the Japanese city of Hiroshima. The bomb name was Little Boy. The bomb type was a gun-assembly bomb. It was deployed by a B-29 bomber named the Enola Gay. It was airburst at 580 m (1,900 ft) above the city with a TNT equivalent of 15,000 tons (estimated). An estimated 140,000 people were killed by year’s end.”
—The first proper target outside the solar system is the Alpha Centauri System. Which is roughly 10,000 years with our original slow, more realistic ship but only 20 years with our super fast fictional ship!
#Encyclopedia Britannica: “Alpha Centauri” (retrieved 2026)
https://www.britannica.com/place/Alpha-Centauri
Quote:“Alpha Centauri, triple star system, the faintest component of which, Proxima Centauri, is the closest star to the Sun, about 4.2 light-years distant [4.0 ×1013 kilometers].”
#Deeks, Russell (2025): “Fastest human-made objects ever built”, BBC Sky at Night Magazine
https://www.skyatnightmagazine.com/space-missions/fastest-object
Quote:“The current space speed record holder for fastest human-made object is NASA’s uncrewed Parker Solar Probe.
On 21 September 2023 – assisted by several fly-bys of Venus that allowed it to slingshot off the planet’s gravity – Parker Solar Probe clocked up a speed of 635,266 km/h (394,736mph).”
(4.0 ×1013 km)/(635,266 km/h ) ~ 6.3 ×107 hours ~7,000 years
#E = mc2 Explained: “Time dilation calculator” (used 2026)
—Four years after their arrival, their message reaches earth.
From the perspective of Earth, the spaceship takes 21 years to arrive at Alpha Centauri, and it takes a bit more than 4 years to receive the message after it is sent.
#Encyclopedia Britannica: “Alpha Centauri” (retrieved 2026)
https://www.britannica.com/place/Alpha-Centauri
Quote: “Alpha Centauri, triple star system, the faintest component of which, Proxima Centauri, is the closest star to the Sun, about 4.2 light-years distant.”
#E = mc2 Explained: “Time dilation calculator” (used 2026)
—Alpha Centauri and its planets are very pretty up close! Unfortunately they are all super deadly and uninhabitable.
We thank our expert Abel Méndez for the following comment:
Quote: “I presume that, long before we have the capability to travel to the stars, we would be constructing large networks of ground- and space-based telescopes that would provide us with much more detailed information about any exoplanet, including the presence of cities from their light or heat signatures. Still, if we need to travel to them, it's because something is unknown or unexpected.”
And our expert Blakesley Burkhart for the following comment:
Quote: “It would be enormously exciting to visit the nearest star system, a triple star system. We have no idea what we might find but it is possible to find life in a form we don't expect it.”
In this scenario, we sail to Alpha Centauri with incomplete information about the star system.
Our representation of the star system in this video reflects what we know of it today. Proxima Centauri, the star in the Alpha Centauri system that is closest to Earth, has three known planets. At least one, Proxima Centauri b, is in the habitable zone. It is slightly larger than Earth, and generally considered the one with the most habitability potential:
#Encyclopedia Britannica: “Alpha Centauri” (retrieved 2026)
https://www.britannica.com/place/Alpha-Centauri
Quote:“Proxima has three planets, Proxima b, c, and d, the closest extrasolar planets, the first of which, Proxima Centauri b, was discovered in 2016. Its mass is at least 1.3 times that of Earth and thus is deemed a rocky planet like Earth. Its orbital period of 11.2 days puts it within Proxima’s habitable zone, at the distance from the star where water can exist in liquid form on a planet’s surface. Where there is liquid water, there may be the conditions to support life. Proxima c has a mass about 5.8 times of Earth and has an orbital period of 5.2 years. Proxima d orbits with a 5.1-day orbital period between Proxima b and the star. It is not within Proxima’s habitable zone and has a mass at least 0.26 times that of Earth, making it one of the lightest known extrasolar planets.”
However, with what we currently know, there is no way to tell if it is actually habitable for humans or how difficult it would be to make it habitable. For example, if the CO2 content of its atmosphere is high enough, it could be so hot as to make it uninhabitable.
#Meadows, Victoria S. et al. (2018): “The Habitability of Proxima Centauri b: Environmental States and Observational Discriminants”, Astrobiology, vol. 18, 2,133-189.
https://pmc.ncbi.nlm.nih.gov/articles/PMC5820795/
Quote: “Proxima Centauri b provides an unprecedented opportunity to understand the evolution and nature of terrestrial planets orbiting M dwarfs. Although Proxima Cen b orbits within its star's habitable zone, multiple plausible evolutionary paths could have generated different environments that may or may not be habitable. Here, we use 1-D coupled climate-photochemical models to generate self-consistent atmospheres for several evolutionary scenarios, including high-O2, high-CO2, and more Earth-like atmospheres, with both oxic and anoxic compositions. We show that these modeled environments can be habitable or uninhabitable at Proxima Cen b's position in the habitable zone.[...]
We find several post-runaway states that are uninhabitable either due to extreme water loss or inclement surface temperatures. In particular, a dense Venus-like CO2 atmosphere will result in extremely high surface temperatures at Proxima Cen's current semimajor axis.”
There may be other planets in the Alpha Centauri system, in particular around its other two stars:
#Open Exoplanet Catalogue: “Alpha Centauri” (consulted 2026)
https://www.openexoplanetcatalogue.com/planet/Alpha%20Centauri%20B%20c/
—But if 20% of the speed of light might be our speed limit, and we give humans a realistic travel time of say, 40 years, we could reach things that are up to 8 light years away.
We consider that becoming an astronaut may take some training and that the total space travel time should not exceed the working life of a person, as their capabilities may deteriorate with age. Provided that a human being is capable and willing to travel for roughly 40 continuous years to visit other stars, they can reach a neighborhood of 8 years around the Sun.
#E = mc2 Explained: “Time dilation calculator” (used 2026)
—Our solar system is a bit unlucky in that regard, we are in a relatively empty pocket of the galaxy.
The shape of the local bubble was described here:
#Zucker, Catherine et al. (2022): “Star formation near the Sun is driven by expansion of the Local Bubble”, Nature, vol. 601, 334–337
https://www.nature.com/articles/s41586-021-04286-5
https://arxiv.org/abs/2201.05124
And interactive visualizations can be found here:
#The Local Bubble (2022): “Visuals”
https://sites.google.com/cfa.harvard.edu/local-bubble-star-formation/visuals
The existence of the local bubble had already been known:
#The Center for Astrophysics | Harvard & Smithsonian (2021): “Gigantic Cavity in Space Sheds New Light on How Stars Form”
https://www.cfa.harvard.edu/news/gigantic-cavity-space-sheds-new-light-how-stars-form
Quote: “Astronomers have discovered a humongous cavity in space while mapping interstellar dust. The sphere-shaped phenomenon may explain how supernovae lead to star formation. [...]
Astronomers analyzing 3D maps of the shapes and sizes of nearby molecular clouds have discovered a gigantic cavity in space.
The sphere-shaped void, described today in The Astrophysical Journal Letters, spans about 150 parsecs — nearly 500 light years — and is located on the sky among the constellations Perseus and Taurus. The research team, which is based at the Center for Astrophysics | Harvard & Smithsonian, believes the cavity was formed by ancient supernovae that went off some 10 million years ago.”
—This might change in a few million years as we move through the galaxy, and there might be small, localized star spanning civilizations – we discussed this in another video.
The Sun is currently moving through the Local Bubble we described in the section of this document just above this one, it won't stay in it forever.
#Center for Astrophysics (2022): “A Bubbly Origin for Stars Around the Sun”
https://www.youtube.com/watch?v=08UlpJBt5Ic&t=35s
In fact, the Sun orbits around the center of the Galaxy with a period of 225 million years, so the stars in the solar neighborhood change in timescales of millions of years:
#OpenSpace: “Sun: The Motion of the Sun in the Galaxy” (retrieved 2026)
https://docs.openspaceproject.com/latest/content/solar-system/sun/index.html
Quote:“The Sun is moving through the Galaxy as it revolves around its center. It has a relatively stable orbit around the Galaxy, moving at 828,000 kilometers per hour (514,000 miles per hour). Its bevy of planets are along for ride, tipped 62 degrees. At this speed, it takes about 225 million years to complete one orbit around the Galaxy. Given the Sun has been around for about 4.6 billion years, we’ve only made about 22 revolutions around the Galaxy. Compare that to Earth, which has made 4.6 billion revolutions around the Sun in that time.”
We talked about how interstellar civilizations could form in the Milky Way here:
#Kurzgesagt – In a Nutshell (2024): “There Are Thousands of Alien Empires in The Milky Way”
—Let’s say that we could go much further, to any of the stars within our stellar neighborhood, a sphere 25 lightyears in diameter, with the sun at its center. 0.02% of the milky way by the way. This tiny bubble is currently inhabited by 34 stars, and 6 brown dwarfs. Only three stars in our neighbourhood are similar to the sun.
#European Southern Observatory (2006): “Stars in the solar neighbourhood”
That sphere’s diameter is 0.02% of the diameter of the Milky Way.
#Encyclopedia Britannica: “Milky Way Galaxy” (retrieved 2026)
https://www.britannica.com/summary/Milky-Way-Galaxy
Quote:“Milky Way Galaxy, Large spiral galaxy (roughly 150,000 light-years in diameter) that contains Earth’s solar system.”
25 light-years / 150,000 light-years ~ 0.0002
There are 40 stellar-type objects within 12.5 light years, including the Sun and 6 brown dwarfs, as per:
#Stellar Catalog: “List of nearest stars” (consulted 2026)
https://www.stellarcatalog.com/stars.php
The 6 brown dwarfs we count are: Luhman 16 A, Luhman 16 B, WISE 0855-0714, Star 2MASS J1812-2608, Epsilon Indi Ba, Epsilon Indi Bb.
The Sun-type stars within this radius are the Sun, Alpha Centauri A and Tau Ceti.
—The other stars close to us are mostly red dwarfs that have no planets or planets that vary from deadly to super deadly. There are a few planets in the habitable zones of their stars, where water can be liquid.
22 out of the 40 stellar-type objects within 12.5 light years are red dwarfs.
#Stellar Catalog: “List of nearest stars” (consulted 2026
https://www.stellarcatalog.com/stars.php
Our reference for habitability will be the planets in the TRAPPIST-1 system, situated around 40 light-years away. They will set the benchmark of what we consider a “good” planet.
#NASA (2025): “Largest Batch of Earth-size Habitable Zone Planets Found Orbiting TRAPPIST-1”
https://science.nasa.gov/exoplanets/trappist1/
Quote:“The most studied planetary system, aside from our own solar system, lies about 40 light-years away. We've looked at the seven rocky exoplanets orbiting the TRAPPIST-1 star with ground and space telescopes like Spitzer, Kepler, Hubble, and, now, the James Webb Space Telescope.”
We will consider that for an exoplanet to be “good”, it must be in the habitable zone of its star and be at least as Earth-like as the second most Earth-like planet in TRAPPIST-1. We measure likeness to Earth using the Earth Similarity Index (ESI).
#Planetary Habitability Laboratory (2026): ”Habitable Worlds Catalog”
Of the 22 red dwarfs systems within human-travel distance around the sun, only 6 have exoplanets reasonably similar to Earth (Teegarden’s b, Proxima Centauri b, GJ1061 d and c, Ross 128 b, and GJ 273 b) which means that the majority have no planets or only deadly planets. Of these, only Proxima Centauri b and GJ1061 d are in the habitable zone.
#NASA: “Eyes on Exoplanets: Teegarden’s Star” (retrieved 2026)
https://eyes.nasa.gov/apps/exo/#/system/Teegarden's_Star
#NASA: “Eyes on Exoplanets: Proxima Centauri” (retrieved 2026)
https://eyes.nasa.gov/apps/exo/#/system/Proxima_Cen
#NASA: “Eyes on Exoplanets: GJ 1061” (retrieved 2026)
https://eyes.nasa.gov/apps/exo/#/system/GJ_1061
#NASA: “Eyes on Exoplanets: Ross 128” (retrieved 2026)
https://eyes.nasa.gov/apps/exo/#/system/Ross_128
#NASA: “Eyes on Exoplanets: GJ 273” (retrieved 2026)
https://eyes.nasa.gov/apps/exo/#/system/GJ_273
Note that we are considering habitability for humans, other life forms may prefer very different conditions on a planet.
—But, Mars is also in the habitable zone of the Sun and it is really bad.
#Fischer, Debra et al.: “Cosmic Origins:The Science of Life in the Universe” , Ch. 16: “The Habitable Zone” (retrieved 2026) https://wisconsin.pressbooks.pub/astrobiology/chapter/the-habitable-zone/
Quote: “Two ranges for the habitable zone are often quoted: one is a conservative estimate, with a more narrow range of distances, and the second is the optimistic estimate, which is wider. The conservative estimate accounts for changes due to water in a planet's upper atmosphere that can lead to a "moist" greenhouse effect, which sets in earlier than the "runaway" greenhouse effect. For the Sun, the conservative range is 0.95 - 1.37 AU, while the optimistic range extends the borders to 0.85 - 1.7 AU. Figure 2 shows the optimistic habitable zone around the Sun; the conservative estimate would not include Mars, which is 1.5 AU from the Sun.”
Despite being in the extended habitable zone, Mars is a very hostile environment to humans:
Martian soil contains perchlorates in toxic levels to humans. But it is probably only one of the many others that are dangerous for us.
#M. H. Hecht et al. (2009): “Detection of Perchlorate and the Soluble Chemistry of Martian Soil at the Phoenix Lander Site”, Science, vol.325, 64-67
https://www.science.org/doi/10.1126/science.1172466
Quote: “Several microorganisms on Earth are known to harvest energy by anaerobic reduction of the perchlorate molecule (36, 37). Under martian conditions, perchlorate does not readily oxidize organics (although the presence of perchlorate salts may indicate a vigorous oxidant-forming chemistry in the martian atmosphere or on the surface), but the low water activity associated with such a strongly desiccating substance may inhibit many forms of life. The high-temperature oxidizing properties of perchlorate will, however, promote combustion of organics in pyrolytic experiments, compromising the ability of Phoenix’s TEGA experiment to readily detect organics (38) and possibly affecting the Viking mass spectrometer experiments (39).”
Since Mars has no magnetosphere, it can not protect us from radiation coming from the sun or cosmic rays.
#NASA’s Goddard Space Flight Center (2015): “Real Martians: How to Protect Astronauts from Space Radiation on Mars”
Quote: “A human mission to Mars means sending astronauts into interplanetary space for a minimum of a year, even with a very short stay on the Red Planet. Nearly all of that time, they will be outside the magnetosphere, exposed to the harsh radiation environment of space. Mars has no global magnetic field to deflect energetic particles, and its atmosphere is much thinner than Earth’s, so they’ll get only minimal protection even on the surface of Mars.”
Mars has around a third of Earth’s gravity, which can make you sick beyond merely nauseated.
#Powley, Kathryn(2017): “Getting Sick In Space On The Way To Mars”, Pursuit
https://pursuit.unimelb.edu.au/articles/getting-sick-in-space-on-the-way-to-mars
Quote: “There is no gravity on the International Space Station, and Mars only has about a third of Earth’s gravity. This instantly plays havoc with the human body.
Astronaut’s faces grow puffy and round, and they constantly feel like they have the flu with blocked sinuses.
“Your body has developed to push fluid up to your brain against gravity. In space, too much fluid gets pushed up to the top half of your body so it then tries to get rid of fluid by making you urinate more, and you end up dehydrated,” says Dr Jurblum.”
—Even the planets that we think look most like Earth, like GJ1061 d, come with no guarantees.
We will consider that for an exoplanet to be “good”, it must be in the habitable zone of its star and be at least as Earth-like as the second most Earth-like planet in TRAPPIST-1. We measure likeness to Earth using the Earth Similarity Index (ESI).
#Planetary Habitability Laboratory (2026): ”Habitable Worlds Catalog”
GJ1061 d is also in the habitable zone of its star:
#NASA: “Eyes on Exoplanets: GJ 1061” (retrieved 2026)
https://eyes.nasa.gov/apps/exo/#/system/GJ_1061
But much like Proxima Centauri b, we can't say for sure it will be habitable for humans.
#NASA (2020): “Discovery Alert: New Planet — a Heavyweight, but Habitable?”
https://science.nasa.gov/universe/exoplanets/discovery-alert-new-planet-a-heavyweight-but-habitable/
Quote: “We don’t know some of the basic properties of GJ 1061d, such as how big around it is, its true composition or whether it has any atmosphere at all. All this will require future investigation.”
—They could be dozens, if not thousands of light years away.
The clear example of “good” planets dozens of light years away are the planets in the TRAPPIST-1 system.
#NASA (2025): “Largest Batch of Earth-size Habitable Zone Planets Found Orbiting TRAPPIST-1”
https://science.nasa.gov/exoplanets/trappist1/
Quote:“The most studied planetary system, aside from our own solar system, lies about 40 light-years away. We've looked at the seven rocky exoplanets orbiting the TRAPPIST-1 star with ground and space telescopes like Spitzer, Kepler, Hubble, and, now, the James Webb Space Telescope.”
We consider that for an exoplanet to be “good”, it must be in the habitable zone of its star and be at least as earth-like as the second most Earth-like planet in TRAPPIST-1. We measure likeness to Earth using the Earth Similarity Index (ESI).
As explained above in this document, Proxima Centauri b and GJ1061 d are the only exoplanets within 12.5 light-years that comply with both requirements.
Still, it is possible that none of the planets in TRAPPIST-1 is habitable, and that the actually habitable planets are thousands of light-years away.
—In 1903 the New York Times published an editorial, "Flying Machines Which Do Not Fly” that confidently predicted it would take humans one to ten million years to achieve powered flight. 69 days later the first successful flight happened. 66 years later humans landed on the moon.
#Anonymous (1903): “Flying Machines Which Do Not Fly”, The New York Times
https://www.nytimes.com/1903/10/09/archives/flying-machines-which-do-not-fly.html
https://www.newspapers.com/article/the-new-york-times/105009177/
Quote: “Hence, if it requires, say, a thousand years to fit for easy flight a bird which started with rudimentary wings, or ten thousand for one which started with no wings at all and had to sprout them ab initio, it might be assumed that the flying machine which will really fly might be evolved by the combined and continuous efforts of mathematicians and mechanicians in from one million to ten million years-provided, of course, we can meanwhile eliminate such little drawbacks and embarrassments as the existing relation between weight and strength in inorganic materials. No doubt the problem has attractions for those it interests, but to the ordinary man it would seem as if effort might be employed more profitably.”
#US National Air and Space Museum: “1903 Wright Flyer” (retrieved 2026)
https://airandspace.si.edu/collection-objects/1903-wright-flyer/nasm_A19610048000
Quote: “The Wright brothers inaugurated the aerial age with the world's first successful flights of a powered heavier-than-air flying machine. The Wright Flyer was the product of a sophisticated four-year program of research and development conducted by Wilbur and Orville Wright beginning in 1899. After building and testing three full-sized gliders, the Wrights' first powered airplane flew at Kitty Hawk, North Carolina, on December 17, 1903, making a 12-second flight, traveling 36 m (120 ft), with Orville piloting. The best flight of the day, with Wilbur at the controls, covered 255.6 m (852 ft) in 59 seconds.”
#US National Air and Space Museum: “Apollo 11” (retrieved 2026)
https://airandspace.si.edu/explore/stories/apollo-11-moon-landing
Quote: “On July 20, 1969, humans walked on the Moon for the first time.”
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