life in the universe Flashcards
3 recent developments indicate the high possibility of life to exist elsewhere as well:
Life arose quite early in Earth’s history It may form quickly elsewhere as well, provided it has the right conditions
Organic molecules could be formed even with laboratory chemical reactions
strong indicator that Life may appear by naturally occuring chemistry wherever it has the right conditions Microscopic living organisms were discovered to survive in conditions similar to those on at least a few of other planets in our Solar system.
Earth formed
~4.5 b years ago (b.y.a).
Initial period (4.5…3.8 b y.a.), especially in the 1st several m y.: Heavy bombardment
intense flux of many asteroids & comets, including large ones (even planetesimals) Moon formation (30…50 m y. after Solar syst. formed )
At the end of this period: the Late Heavy Bombardment (3.9…3.8 b y.a., for a period of about 20 to 200 m y.)
→ nr. of impacts in the Solar syst. may have increased tremendously.
Most probably because of shifts in the orbits of the giant planets.
Mineral evidence suggests oceans had formed after the first 200 m y.!
Natural chemistry laboratories that could lead to life.
Some impacts possibly vaporised early oceans → no life possible (on the surface; but theoretically possible below it, or in hot hydrothermal vents!)
Living organisms quickly arose after impacts had subsided.
Evidence suggests Life was already thriving prior to 3.85 b y.a !
History of Life on Earth
deduced from study of fossils buried in layers of sedimentary rocks.
The key issue: determine the correct dates at which fossil organisms lived Radiometric dating → used both in geology & for fossil age estimation (but with different isotopes for each purpose)
Old rocks are much rarer! Earth constantly renews itself!
Those old rocks which can be found have usually been transformed
Geological evidence of Life shows more detail in the last ~500 m years: All prior life were microscopic → Finding fossil evidence of microscopic life organisms is extremely difficult
Stromatolites (colonies of microbes) found in very old rocks.
Date back to 3.5 b years ago Ancient organisms became advanced enough to build stromatolites
There is evidence that Life already thrived even 3.5 b years ago!
Life arose as soon as conditions first allowed it
What is Life ?
There is no simple & clear-cut definition of Life! A more practical alternative is to identify the basic unit of Life
CELL = the smallest unit alive. Cells have 8 basic properties → by extension, these properties can also be considered to be inherent resultant qualities of Life.
Life on Earth:
Is based on carbon (C) → the best ability to readily form bonds and many types of molecules/structures with other C atoms & other atoms, giving flexibility to the form and function that biomolecules can take.
Other chemical elements essential for life are: H, O, N (the most abundant, but others may also play crucial roles, even if their amount is not large). CANNOT SURVIVE without water. Oxygen is also essential for the metabolism of most organisms, especially the most complex ones.
key macromolecules necessary/used in living cells & organisms:
Nucleic acids alongside proteins , carbohydrates and lipids are the major types of key macromolecules essential for all known forms of Life on Earth.
DNA & RNA are nucleic acids
Other important macromolecules are ATP, vitamins & enzymes, hormones, and neurotransmitters.
DNA stores all the essential genetic information necessary for a cell, and even for the entire body the key component involved in evolution
Theories for the origin of Life → 2 categories:
De Novo [from nothing] &
Panspermia [from already existing ‘seeds’]
1) De Novo theories
1.1- Organic Soup Theory:
Abiotic chemical interactions within a ‘primordial soup’
Special polymeric molecules, such as RNA, were created, with the ability to reproduce: the beginning of life.
The Miller-Urey experiment (1953): Organic compounds (including a small percentage of amino acids – the building elements of proteins) were obtained from gases (believed to imitate early Earth’s atmosphere) in a closed system which subjected to heat and electrical discharges.
Subsequent experiments showed that RNA & DNA bases could be obtained through simulated prebiotic chemistry with a reducing atmosphere.
Problems with the Miller-Urey experiment:
Early Earth’s atmosphere was different
Lightning was not continuous
Amino acids and other organic compounds may not have been formed in the large amounts which the Miller/Urey experiment produced
1.2- Surface Metabolists Theory (Günter Wächtershäuser, 1988):
Primitive microorganisms called ‘surface metabolists ’ synthesised and polymerised from inorganic compounds on the surface of minerals with a positive surface charge via an autocatalytic process
1.3- Through iron monosulphide bubbles at hydrothermal vents on the seafloor of the primordial Ocean (Russel & Hall, 1997) :
Problems: Relies on very specific conditions which cannot be ascertained =unclear whether it can be verified and proven experimentally
2) Panspermia theories
Life was already present elsewhere in the Universe, and was transported to Earth, most likely via meteorites or by comets (e.g. during the Heavy Bombardment period)
Its feasibility is well supported by some experimental facts
Delicate bio-molecules are known to have been carried and brought on Earth by meteorites.
ISM clouds and even protoplanetary disks contain not only many simple hydrocarbon molecules, but also many types of complex organic molecules and even important biomolecules Key problem: Panspermia does not answer directly how Life itself arose
HOW and WHEN did RNA form? Before or after the DNA?
Since nucleic acids (DNA & RNA) are necessary to build proteins, and proteins are necessary to build nucleic acids, until the early 1980s it was unclear which came first, the nucleic acid or the protein?
This problem was solved when it was discovered that RNA can both store genetic information and cause or catalyze the chemical reactions necessary to copy itself. Nucleic acids (specifically, RNA) came first — and later on, life switched to DNA-based inheritance
HOW and WHEN did ATP form? Before or after the DNA?
ATP = Primary energy source at cellular level for ALL Earth life.
Studying ATP and other ‘molecular fossils’ revealed that they are closely related to nucleic acids ATP was soon synthesized after nucleic acids
In an RNA world, how did DNA appear? ?
RNA plays a central role in the mechanism of protein synthesis.
DNA synthesis and replication actually requires many proteins.
DNA can be considered as a modified form of RNA
Why was DNA selected to replace RNA?
It is more stable and can be repaired more faithfully.
Why was DNA selected to replace RNA?
→ It is more stable and can
be repaired more faithfully.
One hypothesis: viral replication systems achieved the transition from RNA to DNA : DNA & DNA replication proteins originated in viruses, and DNA replication mechanisms have been transferred subsequently from viruses to cells.
The ‘invention’ of DNA required a complex multi-step process, much more complex than previously thought. Interestingly, this complex process was discovered independently more than once.
The origin of homochirality
how only certain orientations of chiral biomolecules have been selected and used in living cells?
Homochirality is essential to the correct functioning of many of contemporary life-forms
Such highly evolved molecules would not have been available from the outset and it is NOT clear how they could have appeared.
An extraterrestrial source has been suggested for homochiral molecules & their discrimination
Life has appeared only ONCE on Earth! WHY?
Unique conditions, which could not be repeated again There is only one basic life-form on Earth All living species did NOT arise from multiple independent first ancestors. .
How/where did the 1st organisms come from?
We may never know for sure.
Living organisms quickly arose after impacts had subsided
Probably the 1st to appear may have had just a very simple inorganic membrane inside which a large RNA molecule performed all (most?) functions/chemical reactions
Very simple micro-organisms appeared initially
The first to appear: prokaryotes → single-celled organisms (bacteria) with a lipid bilayer membrane but do NOT have a nucleus.
The membrane’s role is critical because it provides the barrier that marks the boundaries of a cell (Isolates the interior of the cell & protects it from the outside, also allowing control over the type & flow rate of molecules in/out of the cell)
Next: Archea → differ from bacteria in that they have a different cell membrane material (NOT a bilayer).
Evolution then led to eukaryotes = single-celled organisms with bilipid layer membranes, nuclei and membrane-bound organelles.
Appearance of eukaryotes
subsequent emergence of multicellular organisms
This ultimately led to the evolution of complex, i.e. highly structured and organized organisms, with specialized organs
Micro-organisms can be further categorised based on their metabolism:
Autotrophs → gain energy & synthesize their own organic materials for growth from inorganic compounds (e.g. CO 2 or CH4) and hence may be thought of as ‘producers’.
Chemotrophs (Chemo-autotrophs) → use inorganic chemical reactions.
Phototrophs (Photo-autotrophs) → use sunlight.
Heterotrophs → require organic material for growth (e.g. dead animals, etc.) and may be thought of as ‘consumers’
Life is divided into 3 main branches
(domains = Archaea, Bacteria, and Eucaria) but all share a common ancestor. Cyanobacteria produced O 2 through photosynthesis :
May have started 3.5 b y.a
Chemical reactions with rocks removed O 2
From 2.4…2 b y.a. O 2 started to accumulate in atmosphere
Oxygen reached higher
% (breathable levels) by 700…600 m y.a. → first complex organisms!
Life on land became possible when atmospheric
O 2 accumulated enough to form an ozone (O 3) layer.
O2 =
initially toxic to most organisms living before ~2 b y.a.
Dramatic change in fossil record ~540 m y.a.
Cambrian explosion → sudden increase in animal diversity!
This sudden burst of new life is also called “Darwin’s dilemma” because it appears to contradict the gradual evolution by natural selection
Age of Reptiles
Life forms developed, complexified and continuously changed WHY? → Evolution Early dinosaurs & mammals arose ~225…250 m y.a.
Dinosaurs dominated for over 100 m years.
Died out ~65 m y.a., most probably due to asteroid/comet impact
Paved the way for large mammals Earliest humans appeared only a few m y.a.
Our industry/technology only existed over the last few centuries
Life, as we know it, is carbon-based. Its requirements are:
Liquid water
Source of energy
Abundant oxygen
Atmosphere that is not poisonous
Abundant & varied food sources
Suitable & stable environmental conditions
Fairly narrow temperature range (–40…60 oC) It may seem that these conditions for Life are narrow & specific. But Life does exist in more hostile environments!
Life has also been found in all sort of extreme conditions, from very cold (Antarctic lakes) to very hot (volcanic vents) very high salinity, acidity, etc.
Organisms (mainly bacteria) which can survive in such extreme conditions are called extremophiles
Examples of extremophiles:
acidophiles thermophiles lithotrophs halophiles methanogen methane ice worms tardigrades
Acidophiles or Alkaliphiles
live in very acidic or basic environments.
Thermophiles & Hyperthermophiles
creatures that live in hot (e.g. between 45 and 80°C ), and VERY hot (>80°C ) environments.
Lithotrophs
organisms using inorganic materials to use in biosynthesis (e.g., CO 2 fixation) or energy (i.e., ATP) production via aerobic or anaerobic respiration → (Crypto)endoliths = live in microscopic spaces within rocks (sometimes deep within the crust!).
Halophiles
creatures or Archean bacteria that thrive in very salty environments.
• Halophilic bacteria (halobacteria) may be the oldest life form on earth!
• Large family of bacteria, with great variations in in nutritional demands, which are very dfferent from normal bacteria. • VERY relevant as Mars & Europa may have subterranean salt(y) formations/seas
Methanogen
A microorganism that produces CH4 as a byproduct of its metabolism
- All known methanogens are both Archaeans and obligate anaerobes, that is, they cannot live in the presence of O 2 !
- More than 50 species have been identified, including some extremophiles which can thrive in hot springs, submarine hydrothermal vents, and hot dry deserts.
• Some scientists speculate that methanogens may be responsible for the CH4 detected in the atmosphere of Mars.
Methane ice worms
A kind of worm discovered in the cold, dark depths of the sea-floor in the Gulf of Mexico at lakes of ‘methane clathrate’ = CH4-trapping ice that forms at high p and low T. It is believed that these worms graze on bacteria that feed on the clathrate.
The existence of a higher order organism in this habitat suggests the possibility of life in similar extraterrestrial habitats such as the huge ocean of Europa
Tardigrades (“water bears”)
Invertebrate animals which can withstand a large range of environmental extremes.
They survive by going into cryptobiosis, a state of suspended animation, in which body functions like metabolism temporarily shut down → unlocking the secrets of this process is important for future long interstellar flights
Life (as we know it) as a whole has only 3 basic requirements:
Nutrients source(s) from which to build living cells Energy to fuel activities of life Liquid water
Liquid water is the limiting factor! →
search for liquid water is
the currently the key criterion in the search of Life on other planets
The water requirement rules out most worlds in our Solar system
There are 2 major possibilities beside Earth: Mars
& Europa
Mars: possibly the best candidate to host life?
It has been warm & wet for some periods in its distant past, similar to early Earth.
IF water existed on Mars → Life was possible!
Subsurface ice was discovered recently, and apparently even presently liquid water still flows sometimes on the Martian surface.
IF Life may still exist near such sources of liquid water
need to sterilize future probes/rovers sent to Mars to prevent contamination
Huge plumes of CH 4 in the northern hemisphere of Mars were discovered in 2003, but has also been removed from the atmosphere 600 × faster than expected!
whats fucked about mars
However, Mars water and/or regolith are heavily laced in salts (perchlorates) that ↓ the freezing point of water, but also are a strong oxidizing agent –and hence highly toxic for Life as we know it.
Mars’ tilt cyclically changes every 120,000 years as it undergoes precession, strongly changing the amount of sunlight at the poles.
Best candidates for extraterrestrial Life: 2) Ganymede
Ganymede boasts a lot of water, perhaps 25 × the volume of Earth’s oceans. Its sub-surface oceans are estimated to be up to 800 km deep !
Ganymede: the largest moon in the Solar system
Ganymede and Callisto are clearly deficient in rocky materials
The only moon in the Solar System known to have magnetosphere hence it has a source of internal heat (hot liquid Fe core)
Galileo mission confirmed it may have an ocean extending to depths of hundreds of miles. It also found evidence of salty seas, which may contain magnesium sulfate ( MgSO 4)
New research suggests Ganymede may have alternate ice & liquid oceans stacked up like a multi-layered sandwich
Ganymede is NOT
but no -or limited- energy source for Life in these oceans CONCLUSION: As yet, Ganymede is NOT considered a prime contender in the search of Life.
Europa is considered one of the most biologically interesting worlds in the solar system due to several key factors:
a) Likely presence of a salty sub-surface ocean of liquid water (perhaps as much as 150 km deep), which may contain 2 × the volume of Earth’s oceans, and could provide a medium and solvent for life
b) Intense radiation from Jupiter’s magnetosphere striking ice on Europa’s surface and releasing oxygen, which, IF it finds its way into the ocean could provide a fuel for life; and
c) Possible presence of undersea volcanic vents, which could furnish energy (heat) and nutrients for organisms, possibly with volcanic vents on ocean floor
Occasional geysers were observed to
eject water in the South pole region
Chemicals from Europa’s sub-surface ocean
(or maybe just from liquid
water “lakes” in near-surface pockets?
) are leaking to the surface, and
chemicals from the surface are cycling back into the ocean, too
Some scientists consider Europa to be the best place for finding life in the Solar system, but… There are important hurdles for on-site studying Europa in detail:
Many, large and expensive missions would be needed to provide a clear answer to the compelling question “Is there Life on Europa?”
Far from Earth ↑ time & cost of the total mission
Thick shielding against Jovian intense radiation & a source of independent energy (nuclear power) are necessary ↑ ↑ weight ↑↑↑ cost is necessary
A manned mission is 10 × to 100 × more challenging than a robotic probe
Enceladus (6th moon of Saturn) is another good candidate, and might be more easily explored (than Europa ) because:
Signs of active geology indicative of internal heat & the right ingredients for life.
Plumes of water ice and other materials on Enceladus are known to erupt regularly from cryovolcanic vents in the S polar region
Enceladus’ interior may be warm, contain a subsurface ocean and that its surface is presently tectonically active (with probable volcanic/hydrothermal vents on the seafloor).
It may host a large, possibly regional, ocean about 10 km deep, beneath an ice shell about 30 to 40 km thick.
In contrast to Europa, the exposed crevasses on Enceladus that may hold liquid water are thought to be only about a half-km deep.
Enceladus continually produces astounding amounts of heat. Tidal heating cannot explain the release of so much energy.
Some consider Titan an even more interesting & promising place even than Europa! Possibility of surface Life?
Complex organic molecules are known to be present in Titan’s atmosphere: could they lead to formation of precursors of Life?
Problem: lack of O 2 (traces of CO 2 & CO are present in the atmosphere) & of liquid water.
Could life-forms exist based on a hydrocarbon instead of water? (less risk of biomolecules being hydrolysed).
Could Life exist in lakes of liquid CH
4 on Titan, just as organisms on
Earth live in water?
Such creatures would inhale H2 in place of O 2, metabolize it with C2H2 instead of glucose, and exhale CH4 instead of CO 2 → This hypothesis was supported by findings that concentrations of both H2 & C2H2 ↓ from the upper atmosphere towards the surface, where they apparently disappear. However, inorganic reactions are also possible using a catalyst in the surface ice.
Titan’s simple organic molecules in its atmosphere can form larger molecules with carbon-nitrogen-hydrogen bonds, called
“tholins”. In laboratory, tholins exposed to (sometimes NH3-rich) liquid water over time developed into biologically significant macromolecules, such as amino acids and the nucleotide bases that form RNA (oxygen from H2O was incorporated in the macromolecules).
However, IF Life hypothesis will be confirmed on Titan (e.g. in lakes)
it would have revolutionary implications: it would mean a second, independent origin of Life within the Solar System, implying that Life has a high probability of emerging on habitable worlds throughout the Cosmos.
Scientists have developed a Planetary Habitability Index based on factors including
characteristics of the surface and atmosphere, availability of energy, solvents and organic compounds. Using this index, based on data available in late 2011, Titan has the highest current habitability rating of any known world other than Earth!
Triton
-The 7th & largest of Neptune’s 14 moons:
Its icy surface has one of the higest albedo in the Solar system: 0.7…0.8!
it reflects most of what little sunlight reaches it that it is one of the coldest objects in the Solar system, at about –240 o C. At this temperature, CH4, N2, and CO 2 all freeze solid.
Lies inside the magnetosphere of Neptune, which is harmful to life.
Regions on its surface show evidence of past cryovolcanic and cryotectonic activity. The internal heat source for Triton’s geologic activity is not known, but it may involve tidal heating. Unknown processes pump unusual plumes of gas (probably a mixture of liquid N2, CH4 and dust) and particles into the atmosphere
Streaks of dark deposits (probably tholins?) form downwind of geysers.
May still have a thin layer of liquid NH3-rich water today which could form a sub-surface ocean – but only if it started as a moon with a highly eccentric orbit that slowly circularized over time (orbit now is a perfect circle) in order to benefit from possible tidal heating. Radioactive decay in rocky core (2/3 of mass!) may also contribute
The watery ocean on triton
most probably contains a lot of ammonia (up to 15% NH3) which keeps the liquid from freezing although its temperature is ~ –97 °C. o Such low temperatures would slow down biochemical reactions significantly, and impede evolution. However, terrestrial enzymes have been found to speed up biochemical reactions down to temperatures of –103 °C ! o So, while it may be the outermost & coldest watery ocean in the Solar system, it is not as cold as the –180 °C hydrocarbon lakes on Titan !
Pluto:
very difficult to estimate if its tidal heating (resonance with Charon!) is sufficient for a liquid ocean (at what depth under the crust, how deep?, etc.) and it also is very small (smaller than Triton!) and extremely cold
has the least chance of harboring Life.
Only certain stars may have habitable planets
Habitability Conditions: Last long enough so that Life can arise in a few 100s of m y.
Allow for stable planetary orbits
cannot be (most of) binary systems or around young and massive/hot stars.
To be a relatively quiet star, with few & mild activities (flares; CMEs)
. Water can exist as liquid on surface
The Habitable Zone (H.Z.) of a star
=
the range inside which a terrestrial planet’s orbit can be found to maintain liquid water on its surface.
The Habitable Zone (H.Z.) of a star
= p2
Very low mass stars/red dwarfs are present in huge numbers! But they have… Much narrower H.Z.
Extremely high sensitivity to variations in location (planet must be exactly in the right place). Conversely, the HZ around hotter stars (e.g. F or A type) is much wider and further away from the star.
H.Z. is very close to star Planet is tidally locked (always faces the star possible large temperature extremes between its 2 sides)
Prone to (sometimes HUGE) X-ray flares!
Possibly even luminous flux & spectrum could be insufficient for plant life
An H.Z. can also be formulated at the level of a galaxy, too
Galactic Habitability Conditions: Outer regions of a galaxy have little abundance of heavy elements.
Even within the galactic disk, the abundance of heavy elements ↓ with distance from the center of the galaxy
Inner regions are unfavourable due to:
High density of stars & of supernova rates High levels of high-energy radiation
Current planet-finding methods favour
gas giants & “superEarths”. To unequivocally find Earth-like planets, direct visual observation is preferrable, but…. direct visual observation is extremely difficult (quasi-impossible) at current technologygical level, even with Hubble & Kepler
possible future solutions: an interferometric array of satellites and/or a large telescope on the Moon’s dark side.
Images and spectra of exoplanets are necessary to deduce:
The planet’s atmosphere composition (search for O 2, or…. pollution?).
The presence of Life – either from the atmosphere (if it has O 2, or CH4) or from possible surface spectra (presence of plant foliage!).
E.T. Life may be very different → useless to look for same biomolecules or probe for Earth-similar signs of Life ? Can we recognize it even if we saw it??
ow many civilizations exist in our galaxy with whom we
could make contact?
A formula was first expressed in 1961 by “Drake’s equation” A simplified variation of that first equation is: Number of civilizations NC = NHP × flife × fciv × fnow
NHP = number of habitable planets in the galaxy. flife = fraction of habitable planets which actually contain life. civ = fraction of life-bearing planets where civilization at some time has arisen. fnow = fraction of communicating civilizations which still exist NOW
Unfortunately, we do not know the value of ANY of the factors, hence cannot calculate NC accurately A lot of speculations for the factors values can lead to “rare-Earth” or “abundant-Earth” conclusions which are hotly debated.
Fermi’s paradox:
If so many alien civilization could exist, why we have not detected yet any one or being visited by one?
Another argument: We are very rare!
At least 10 m species on Earth → we are the only one with our level of intelligence & technical capability
this slow progress of life towards intelligence might suggest that the appearance of a civilization is very rare even on planets with complex life
We could slowly populate the galaxy with our current technology Civilizations should already exist in the galaxy if we are typical
Possible Solutions to Fermi’s Paradox
We are alone.
Life is common, and there are many Civilizations, but none has colonized the galaxy.
There is a galactic meta-civilization (“club”) which is deliberately concealed from us.
SETI: Search for ExtraTerrestrial Intelligence
If we are typical & there are other intelligent species, then some should also be interested in making contact….
Radio telescopes used to listen for encoded radio signals
SETI now privately funded due to lack of significant results in the last decades, estimated low chance of success & large amount of time required
Can we actually visit worlds in other star systems?
Pioneer 10 & 11, Voyager 1 & 2 will take >10,000 years to travel 1 l.y.
Current technology (chemical propulsion) is impractical for
interstellar distances (VERY long journey time) and also not applicable/up-scaleable for large life-sustaining systems.
o make interstellar journeys within a human lifetime ,
starships must travel close to the speed of light ( c).
]
This may ↓ the journey time and probably could make feasible interstellar journeys in our ‘close’ galactic vicinity (10s of l.y. only).
…but close to the speed of light relativistic effects appear:
Velocity: limited to ~0.5c → beyond this value, mass ↑↑↑ very rapidly
Relativistic time dilation: time slows down for interstellar traveler (the “twins paradox”). The space travelers become ‘ageless’ as they move fast forward into the future
for long journeys, returning back to Earth becomes useless! … length also changes (length contraction)
Key rocket engine parameters:
Thrust = the reaction force according to Newton’s 3rd law: Mass expelled/ejected in one direction will cause a force of equal magnitude but opposite direction onto the rocket [Newtons] rocket acceleration = thrust/(payload)
Specific impulse = change in momentum obtained per unit of propellant mass [seconds]
Solar sail
Very low thrust (9 μN/m 2!) & specific impulse small payload NO fuel needed! Also, it is very economic; can be easily fabricated
A ‘nanosail’ was already tested successfully in low-Earth orbit in 2011, and a bigger version, “Sunjammer”, was supposed to be launched in Jan.2015 Can be used within the Solar system, but may require power lasers to be placed on intermediary “stepping stones” for usage in areas where sunlight is too faint
Small payload + may gradually achieve high velocities but cannot decelerate
Less attractive for interstellar travel send a “fleet” of small but many probes of a few types for quick feasibility study of target
Ionic rocket engine (Ion thruster)
Very feasible & in advanced stages of R&D for usage in near future (e.g. trips to Mars) Still limited in terms of capability (useful for ‘small’ distances; rather within the Solar system; delivers only small accelerations, ~g/1000)
Nuclear pulse propulsion
Uses nuclear explosions (behind a ‘pushing’ plate) for thrust.
In principle can be implemented even today; USA had even done prototype experiments in the 50s.
Provides BOTH high thrust and high specific impulse simultaneously!
Present international treaties that ban usage of nuclear weapons in space prevent considering such an alternative.
Shielding the crew against the intense emission of energetic radiation is one key problem
Nuclear Thermal Rocket (NTR) propulsion
A working fluid, usually liquid hydrogen, is heated to a high temperature in a conventional nuclear (fission!) reactor, and then expands through a rocket nozzle to create thrust.
It produces a superior effective exhaust velocity and can roughly double or triple the payload carried to orbit.
Although extensively tested between 1950s-1970s it has not been used practically, but it may be considered for future interstellar missions
Interstellar ramjet engine
An astronomically-sized magnetic scoop collects interstellar hydrogen and uses it for fusion-based acceleration
It has to get up to a significant percentage of lightspeed before lighting its fusion reaction!
Requires huge amounts of energy for the magnetic scoop
Fusion reactors are NOT available yet!
Space warp (Alcubierre) engine
Miguel Alcubierre proposed a device that causes the space-time ‘fabric’ in front of the spacecraft to contract, while that behind it expands. This creates a warp bubble that carries the spacecraft through space-time at 10 times the speed of light !
Purely theoretical, no one knows how to build or fuel one!
It may require incredibly large amounts of energy, which were hypothesized that could result from matter-antimatter annihilation → again, nobody knows how to obtain antimatter, and how to store it in complete isolation!
Wormhole
→ a hypothetical topological feature of spacetime: a shortcut through spacetime by uniting 2 otherwise separate & distant points after bending the spacetime fabric
Unfeasible as it raises numerous technologic and scientific fundamental unsolvable problems: nobody knows how to produce and use one!
Interstellar travel is technologically feasible with some available technologies even today,
but travel time is still prohibitive Other aspects may also need to be first studied and advanced technologically, e.g. life-support systems; improving the capability of humans to withstand very long periods (decades) in space without biological problems; closed space ecologies; cryo-suspension & reanimation of humans
Theory of relativity will complicate life for space travelers.
Long-distance targets will probably require only one-ticket journeys! more new demands (as yet unstudied) on implanting/tranplanting the human species in a totally new habitat
Enormous obstacles to interstellar travel!
Possible if we do not destroy ourselves first
