Module 26: Space & the Solar System

Venus is hellish in ways Mars is not. Venus's surface temperature is ~465°C -- hot enough to melt lead -- due to an extreme greenhouse effect. The atmosphere is 90× denser than Earth's (equivalent to 900m underwater) and composed of carbon dioxide with sulphuric acid clouds. Every spacecraft that has landed on Venus has been destroyed within 2 hours by the pressure and heat. Mars, while cold (avg −63°C) and with a very thin atmosphere (1% of Earth's), is survivable for robotic missions. Rovers (Curiosity, Perseverance) have operated for years. Mars also has evidence of ancient water (dried riverbeds, polar ice caps) making it interesting for life-detection. There are also serious proposals for human missions to Mars. A human mission to Venus would require floating in the upper atmosphere -- the surface is inaccessible with current technology.

Tidal locking: Earth's gravity created tidal bulges in the Moon's solid crust. The gravitational pull on these bulges created a torque that gradually slowed the Moon's rotation over billions of years until its rotation period exactly equalled its orbital period (27.3 days). At that point, the same face perpetually points toward Earth -- the system is in its lowest energy state, and there's no longer a torque to change it. From the far side of the Moon, you would never see Earth -- it would always be below your horizon. You would see the stars and Sun in a completely black sky (no atmosphere means no scattered light). The far side has a much more heavily cratered surface -- the near side's mare (dark volcanic plains) are largely absent. China's Chang'e 4 became the first mission to land on the far side in January 2019. Communication requires a relay satellite positioned at the Earth-Moon L2 Lagrange point because the Moon itself blocks direct radio contact.

No -- this is one of the most common misconceptions in space science. At 400 km altitude, Earth's gravity is still about 88–89% of its surface value. You feel weightless on the ISS not because gravity is weak, but because you are in continuous freefall. The ISS (and everything in it) is falling toward Earth at the same rate -- but moving horizontally fast enough (~27,600 km/h) that Earth's curved surface keeps falling away beneath it at the same rate it falls toward Earth. This is what an orbit is: perpetual freefall in which you miss the ground because you're moving sideways so fast. Astronauts experience microgravity because every object in the ISS -- the station itself, their bodies, their breakfast -- is all falling at exactly the same rate. There is no relative gravitational force between them. True weightlessness (absence of gravity) doesn't exist in the solar system -- gravity has infinite range, just diminishing with distance.

All four gas giants actually have rings -- but Saturn's are uniquely prominent and bright. Jupiter, Uranus, and Neptune all have ring systems, but they are faint and dark (composed of dust and dark rocky material). Saturn's rings are made primarily of water ice particles ranging from tiny grains to chunks several metres across -- and ice is highly reflective (albedo ~0.9 vs dark rocky material's ~0.05). This is why Saturn's rings are brilliant white and visible from Earth with a basic telescope while the other planets' rings are nearly invisible. The rings likely formed from the tidal disruption of an icy moon (or captured comet) that came within Saturn's Roche limit -- the distance at which tidal forces overcome an object's self-gravity. Saturn's rings are only ~10–100 metres thick despite being 282,000 km wide -- if scaled to the thickness of a sheet of paper, the rings would be several kilometres across. Saturn's rings are also surprisingly young -- estimated at only 10–100 million years old, meaning dinosaurs existed before Saturn had its current ring system.

Proxima Centauri is 4.24 light-years away -- ~40 trillion km, or ~268,000 AU. To put this in perspective: if you shrank the Sun to the size of a grapefruit (13 cm), Earth would be a grain of sand 14 metres away, Jupiter a marble 73 metres away, and Neptune a pea 440 metres away. The Oort Cloud's outer edge would be about 50 km away. Proxima Centauri, at the same scale, would be 3,800 km away -- roughly the distance from London to New York. The nearest star is as far from our solar system as a different continent is from another, while our entire solar system (planets) fits on a football pitch. The Voyager 1 probe, launched in 1977 and travelling at ~17 km/s (the fastest human-made object at the time), is currently ~24 billion km from Earth -- which sounds enormous but is only ~0.000025% of the way to Proxima Centauri. At its current speed, it would take ~73,000 years to reach there (it's not heading that direction).

The Fermi Paradox is physicist Enrico Fermi's famous question: "Where is everybody?" Given the universe contains ~2 trillion galaxies, each with ~100–400 billion stars, many with planetary systems, and the universe is 13.8 billion years old -- there has been ample time and space for intelligent civilisations to arise, spread, and make themselves detectable. Even at 1% of the speed of light (far slower than theoretical limits), a civilisation could colonise the entire Milky Way in ~10 million years -- a blink of cosmic time. Yet we detect nothing: no signals, no megastructures, no visitors. Proposed resolutions include: (1) The Great Filter -- some evolutionary step is extraordinarily rare, and either we have already passed it (life, eukaryotes, multicellular life, intelligence -- take your pick), or it lies ahead of us; (2) The civilisation lifetimes hypothesis -- intelligent civilisations self-destruct before going interstellar; (3) The "Dark Forest" hypothesis -- civilisations deliberately hide; (4) We simply haven't looked hard or long enough. The silence of the cosmos is one of the deepest unsolved questions in all of science.

For a sufficiently large black hole, you would feel nothing special at the moment of crossing the event horizon. There is no physical surface, no wall, no flash of light -- just empty space. The event horizon is a mathematical boundary defined by the escape velocity equalling the speed of light. A local observer falling freely through it experiences no discontinuity. This is a consequence of the equivalence principle: freefall locally feels the same as being in empty space. However, two things would eventually kill you. First, tidal forces: as you approach the singularity, the difference in gravitational pull between your head and feet grows without limit, stretching you lengthwise and compressing you sideways -- physicists call this "spaghettification." For a stellar-mass black hole (~10 solar masses, Schwarzschild radius ~30 km), tidal forces at the event horizon are already fatal. For a supermassive black hole like M87* (6.5 billion solar masses, horizon radius ~19 billion km), tidal forces at the horizon are negligible -- you would cross it comfortably and only be torn apart much closer to the singularity. Second, time: once inside, every possible future path in spacetime leads to the singularity. You cannot escape, turn around, or send a message out. From a distant observer's perspective, you would appear to slow down asymptotically, growing redder and dimmer, never quite reaching the horizon -- frozen in time. The two perspectives are both physically valid and irreconcilable from the outside.

There are several compounding obstacles, each of which alone would likely be fatal. First, exotic matter: keeping a wormhole throat open requires material with negative energy density -- something that pushes outward against gravity rather than inward. The Casimir effect (a quantum vacuum phenomenon) demonstrates that negative energy densities are physically real in tiny amounts, but the quantities needed to stabilise a wormhole of traversable size would be astronomically large, possibly exceeding the total mass-energy of the observable universe for any useful wormhole. Second, stability: even if exotic matter were available, wormholes are dynamically unstable. Any small perturbation -- a single photon passing through -- causes the throat to collapse on a timescale far shorter than the crossing time. Third, Hawking's chronology protection conjecture: if a wormhole were stabilised, moving its two mouths to different gravitational potentials or at relativistic speeds relative to each other would create a time machine. Hawking argued that quantum vacuum fluctuations would build up in a feedback loop at the moment a closed timelike curve (time loop) became possible, delivering enough energy to destroy the wormhole before it could be used. This has not been proven rigorously but is consistent with the absence of time travellers. Fourth, the ER=EPR conjecture suggests that the wormholes that do exist in nature (connecting entangled black holes) are non-traversable by construction -- the quantum information link and the spacetime tunnel are real, but sending a signal through requires a process that is equivalent to faster-than-light communication, which quantum mechanics independently forbids. The mathematical existence of wormholes in general relativity is therefore considered a feature of the theory's geometry rather than a blueprint for construction.