Space exploration

What you will learn

distinguish optical, radio and space-based telescopes and what each reveals,
explain how rockets achieve orbit using Newton’s third law and the rocket equation,
outline key crewed missions (Apollo, Skylab, Mir, ISS, Artemis, planned Mars missions),
describe applications of satellites: GPS, weather, communications, Earth observation,
identify the major challenges for humans in space: microgravity, radiation, isolation, distance.

Worked example 0 Real-world example: GPS and relativity

Your phone uses timing signals from four GPS satellites to triangulate your position. GPS satellites orbit at $\sim 20\,200$ km and travel at $\sim 14\,000$ km/h. Why must the satellite clocks be corrected relative to Earth’s clocks for GPS to work?

Each satellite broadcasts the time at which its signal left.
The phone calculates its distance from each satellite: $d = c \times \Delta t$ , where $c$ is the speed of light.
A timing error of just $1$ microsecond would produce a position error of $c \times 10^{-6} \approx 300$ m.
Relativity predicts the satellite clocks tick at a slightly different rate (by about $38$ microseconds per day) compared with ground clocks, due to velocity and weaker gravity.
Without correcting this, positions would drift $\sim 10$ km per day — useless for navigation.

Key idea: modern space applications rely on extraordinary precision in physics. GPS is a daily demonstration of Einstein’s relativity.

1. Telescopes and discovery

Different wavelengths reveal different physics:

Optical telescopes (visible light): stars, galaxies, nebulae. Limited by Earth’s turbulent atmosphere, so the largest are on mountain tops (Hawaii, Chile) or in space (Hubble).
Radio telescopes: long wavelengths pass through dust clouds that block visible light. CSIRO’s Parkes dish helped relay the Apollo 11 Moon-landing broadcast and discovered the first pulsar’s periodic signals.
Infrared telescopes: detect heat from cool stars and young, dust-shrouded protoplanetary disks. The James Webb Space Telescope (launched 2021) works in the infrared at L2, $1.5$ million km from Earth.
X-ray and gamma-ray telescopes: high-energy events like black-hole accretion and supernova remnants. Must be in space, because Earth’s atmosphere blocks these wavelengths.

Worked example 1 Why is the James Webb Telescope in space?

Give two reasons the James Webb Space Telescope operates at L2 rather than on Earth.

Earth’s atmosphere blocks most infrared light (absorbed by water vapour and CO $_2$ ), so ground-based IR telescopes work only at narrow wavelengths.
L2 is far enough from Earth and the Sun that the telescope sun-shield keeps the detectors at $-233\,^\circ\text{C}$ , essential because a warm telescope would emit its own IR and drown out faint signals from distant galaxies.

(Additionally, L2 is a gravitationally stable point where minimal fuel is needed to stay in position.)

2. Rockets and orbits

Rocket propulsion: a rocket pushes hot exhaust gases backward. By Newton’s third law, the gases push the rocket forward. The faster and heavier the exhaust, the greater the thrust.

Getting to orbit: a rocket must reach a horizontal speed of $\sim 7.8$ km/s ( $\sim 28\,000$ km/h) at low-Earth-orbit altitude. At that speed, the vehicle falls toward Earth at the same rate the Earth’s surface curves away beneath it — continuous free-fall, which is what “being in orbit” means.

Orbit types:

Low Earth Orbit (LEO) $\sim 300$ - $2\,000$ km: ISS, Hubble, Starlink. Fast orbit period ( $\sim 90$ min).
Medium Earth Orbit (MEO) $\sim 20\,000$ km: GPS satellites. $12$ -hour period.
Geostationary Orbit (GEO) $\sim 35\,786$ km: period exactly $24$ hours, so the satellite appears fixed over one spot. Used for weather, TV broadcast.

Worked example 2 Apparent weightlessness on the ISS

Astronauts on the International Space Station float, even though gravity there is still $\sim 90\%$ of Earth’s surface gravity. Explain.

The ISS orbits at $\sim 400$ km altitude, moving at $\sim 7.7$ km/s horizontally.
Both the station and the astronauts are in continuous free-fall toward Earth.
Since everything in the station falls at the same rate, no support force is felt — the astronauts appear weightless.
Gravity is not absent — it is what keeps them in orbit. “Microgravity” is a more accurate term.

Key idea: orbit is free-fall that never hits the ground because the surface curves away as fast as the object falls.

3. Crewed missions and milestones

1957: Sputnik 1 — first artificial satellite (USSR).
1961: Yuri Gagarin — first human in space.
1969 - 1972: Apollo 11 - 17 — six crewed Moon landings.
1971 - 1986: Salyut, Skylab, Mir — first space stations.
1998 - present: International Space Station — continuous habitation since 2000.
2020s: SpaceX crewed Dragon flights; Artemis lunar programme; Chinese Tiangong station.
Future: Mars landing missions planned for 2030s, with robotic precursors ongoing.

Worked example 3 Why Mars missions are hard

A one-way trip to Mars takes $\sim 7$ - $9$ months. List three physical challenges crews will face that Moon astronauts did not.

Radiation: no protective magnetic field on the way; cosmic rays and solar flares accumulate a dangerous dose over many months.
Microgravity: Apollo missions were short ( $<2$ weeks); Mars crews will spend $\sim 18$ months in microgravity or Martian $0.38g$ . Bone and muscle loss are severe.
Resupply and rescue: communication with Earth has a $\sim 20$ -minute one-way delay; a sick astronaut cannot be evacuated home.

(Also: psychological isolation, food and water recycling, dust toxicity.)

4. Satellites and applications

Application	Orbit	How it works
GPS	MEO	four satellites’ timing signals triangulate position
Weather imaging	GEO	same satellite continuously over one hemisphere
Earth observation	LEO	polar orbit images each region daily
Communications	GEO	TV, phone, internet relayed across continents
Scientific research	varies	Hubble (LEO), Webb (L2), ISS

Worked example 4 Why is a weather satellite geostationary?

Why are weather satellites usually placed in geostationary orbit rather than low Earth orbit?

GEO satellites stay above the same point on Earth ( $24$ -hour period matches Earth’s rotation).
They can produce continuous video-like imaging of developing storm systems.
An LEO weather satellite passes overhead for only a few minutes twice a day; GEO gives $24/7$ coverage of an entire hemisphere.

Practice: Year 10

Fluency

Telescopes

Name one advantage of space-based telescopes over ground-based ones.
Why must X-ray telescopes be placed in space?
What wavelength does the James Webb Space Telescope use?
What kinds of objects do radio telescopes detect that optical ones cannot?
Name one significant astronomical discovery made with a space telescope.

Fluency

Rockets and orbits

State Newton’s third law and explain how it applies to a rocket.
Roughly what orbital speed is needed at low Earth orbit?
What is a geostationary orbit, and why is its altitude fixed at $\sim 36\,000$ km?
Why does an astronaut on the ISS appear weightless even though gravity is strong there?
What is “escape velocity” and how does it differ from orbital velocity?

Fluency

Missions and applications

Name the first human to orbit Earth and the year.
In which decade did the Apollo crewed lunar missions take place?
When did the ISS begin continuous crewed occupation?
Name one weather satellite system and one GPS-style navigation system.
Name two countries or agencies currently running crewed space programmes.

Reasoning

Explain and justify

Explain why a rocket’s fuel is $\sim 85\%$ of its launch mass.
Why do most rockets carry both fuel and oxidiser, even though there is plenty of oxygen in the lower atmosphere?
Explain why a single GPS satellite is not enough to give your location — the phone needs at least four.
Why is radio astronomy often done in remote locations (outback Australia, Atacama desert)?
Compare the dangers of a Moon mission with a Mars mission. Which mission poses greater risks, and why?

Problem solving

Apply

A GPS satellite orbit altitude is $20\,200$ km. Earth’s radius is $\sim 6\,400$ km. What is the satellite’s distance from Earth’s centre? If the satellite moves at $14\,000$ km/h, how long does a signal take to reach you (use $c = 3.0 \times 10^5$ km/s)?
The Apollo missions sent crews a distance of $\sim 384\,000$ km to the Moon. At a travel speed of $\sim 5\,500$ km/h (average), estimate the travel time in days.
A geostationary satellite broadcasts a TV signal. The signal reaches the ground after $\sim 0.12$ s. Estimate the satellite altitude using $d = ct$ . Does this match $36\,000$ km?
Explain why Mars landing missions must launch during specific “windows” every $\sim 26$ months.

Challenge

Reasoning

Harder reasoning

The rocket equation is $\Delta v = v_e \ln(m_0 / m_f)$ . Explain what each symbol represents and why small increases in $\Delta v$ requirement (e.g. from LEO to the Moon) dramatically increase the initial mass $m_0$ for a fixed payload.
A Mars-surface sample return mission has multiple spacecraft elements — orbiter, lander, ascent vehicle, sample capsule. Explain why this staged approach is preferred over a single monolithic design, referencing the rocket equation.
Argue for or against sending humans (rather than robots) to Mars. Give one scientific, one economic and one ethical reason for your position.
A near-Earth asteroid is discovered on a collision course with Earth, impact due in $20$ years. Describe, in general terms, how current space technology could deflect it, and why early warning is crucial.