Choosing the mission

Every mission is a trade-off between cost, risk and science return.

• Flyby — the probe sweeps past once. Cheapest and quickest, needs no fuel to slow down, and can visit several targets (Voyager 2 flew past Jupiter, Saturn, Uranus and Neptune). But you get one look and cannot follow changes over time.
• Orbiter — the probe must brake into orbit, which costs fuel and mass. In return it maps the whole body, monitors it for years, and can act as a communications relay for landers.
• Lander — touches down and analyses rock, soil and atmosphere directly. It must survive entry, descent and landing, and it can only study one spot.
• Rover — a mobile lander: it can drive to interesting rocks, drill, and analyse many sites (Curiosity, Perseverance).
• Sample return — brings material back to Earth laboratories, where instruments are far better than anything that can be flown (OSIRIS-REx from asteroid Bennu; Hayabusa2 from Ryugu).

Why bother going at all? Ground telescopes can never match a close-up. Probes measure magnetic fields, sample the atmosphere, look at surfaces at centimetre resolution and land instruments in the dirt.

Pick the mission

Gravity assists and transfer orbits

Rockets can only carry so much fuel, so mission planners let gravity do the work.

• A transfer orbit is an efficient elliptical path from the Earth's orbit to the target's. Because the target must be in the right place when the probe arrives, missions have a launch window — for Mars, roughly every 26 months (its synodic period).
• A gravity assist (slingshot) swings the probe close past a planet. Seen from the planet the probe leaves with the same speed it arrived — but the planet is moving, so seen from the Sun the probe gains (or loses) speed and changes direction, free of fuel. Voyager 2's grand tour and the Cassini mission to Saturn both depended on them.

Conservation is not broken: the probe's gain in energy is exactly balanced by an unimaginably tiny loss to the planet's orbital motion — Jupiter is slowed by a truly negligible amount.

Powering and talking to a probe

Power. Sunlight obeys the inverse square law, so it fades fast with distance:

• Near the Sun (Mercury, Venus, Earth, Mars): solar panels work well.
• Beyond Jupiter, panels would have to be absurdly large, so probes carry a radioisotope thermoelectric generator (RTG) — heat from decaying plutonium-238 is converted to electricity. Voyager, Cassini, New Horizons and Curiosity all use RTGs.

Communication. Radio signals travel at 3 × 10⁵ km/s, but the distances are colossal, so there is a real time delay — minutes to Mars, hours to Pluto. You cannot "drive" a Mars rover live, which is why rovers must be partly autonomous. Signals also grow very weak, so probes use high-gain dish antennas and NASA's giant Deep Space Network dishes.

Other hazards: intense radiation (especially around Jupiter), extreme cold, micrometeorites, and journeys lasting years — every component has to survive.

Your turn — talking to Mars

Your turn — sunlight at Jupiter

Your turn — a signal from Pluto

Why an RTG?

A free speed boost

The search for life

Life as we know it needs liquid water, an energy source and the right chemical elements. That focuses the search:

• Mars — dry now, but orbiters and rovers have found dried-up river channels, clays and mineral salts that only form in water. Rovers hunt for organic molecules and ancient microbial traces.
• Europa (Jupiter) — an icy crust with a probable salty liquid ocean beneath, kept warm by tidal heating.
• Enceladus (Saturn) — Cassini flew through plumes of water vapour jetting from its south pole and found organic molecules.
• Titan (Saturn) — a thick atmosphere and lakes of liquid methane; rich organic chemistry, but very cold.

Planetary protection: probes are sterilised before launch, and orbiters are sometimes deliberately crashed at end of mission (as Cassini was, into Saturn) so that they cannot contaminate a possibly habitable moon with Earth microbes.

Which kind of mission?

Tap a statement, then tap the mission type it describes.

Match the description to the term

Tap a description on the left, then its matching term on the right.

Why not drive it live?

What the probes discovered

A quick tour of the headline results — good material for a "why is space exploration worth it?" question:

• Venus — landers survived barely an hour: a crushing 90 atmospheres of carbon dioxide and a surface at about 460 °C, the result of a runaway greenhouse effect. A stark warning about atmospheric CO₂.
• Mars — orbiters and rovers found dried river channels, deltas, clays and salts: Mars was once warm and wet. It has the largest volcano in the solar system (Olympus Mons) and a thin CO₂ atmosphere.
• Jupiter — the Great Red Spot, a storm bigger than the Earth, and a fierce radiation belt. Its moon Europa hides a subsurface ocean.
• Saturn — Cassini showed the rings are countless chunks of water ice, and dropped the Huygens probe onto Titan, which has a thick atmosphere and lakes of liquid methane.
• Comets and asteroids — Rosetta orbited and landed on a comet; sample-return missions have brought asteroid material home. These bodies are pristine leftovers from the formation of the solar system.

Spin-offs and cost: exam questions often ask you to evaluate. For: pure science, planetary defence, understanding the Earth's climate by comparison, and technological spin-offs. Against: billions of pounds that could be spent on other things, and the risk of contaminating other worlds.

A lesson from Venus

The big ideas to know

That is Exploring the Solar System covered for Edexcel GCSE Astronomy. Press Finish to see your score.