Mars rovers and what they have found on the Red Planet
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Send a rover on Mars the command "turn left" and, depending on where the two planets are in their orbits, the message takes somewhere between about 3 and 22 minutes to arrive. The rover cannot wait for a human to react to every rock. That single fact shapes how every Mars rover is designed, landed and driven.
Why rovers?
Orbiters map a whole planet from above, and fixed landers can study one spot in detail. A rover can do what neither can: drive to the most interesting rock in sight and examine it up close. Behind most Mars missions sits one question: did Mars once have liquid water, and could anything have lived there?
Six rovers
| Rover | Agency | Landed | Size | Known for |
|---|---|---|---|---|
| Sojourner | NASA | July 1997 | About the size of a microwave oven | First wheeled rover on Mars |
| Spirit | NASA | January 2004 | About the size of a golf buggy | Minerals that formed in water |
| Opportunity | NASA | January 2004 | Twin of Spirit | Planned for 90 sols, worked for over 14 years |
| Curiosity | NASA | August 2012 | About the size of a small car | Evidence of an ancient lake in Gale Crater |
| Perseverance | NASA | February 2021 | Similar to Curiosity | Collecting rock samples; carried a helicopter |
| Zhurong | CNSA (China) | May 2021 | Smaller than Curiosity | China's first Mars rover |
A sol is a Martian day, explained below.
Spirit and Opportunity
The twin rovers landed on opposite sides of Mars in January 2004, each expected to last about 90 sols. Spirit became stuck in soft soil in 2009 and fell silent in 2010. Opportunity kept going until a planet-wide dust storm in 2018 smothered its solar panels, having driven about 45 km. Among its finds were tiny round grains of the mineral haematite, nicknamed "blueberries", of a kind that on Earth often forms in water.
Curiosity
Curiosity was too heavy to land on airbags like the earlier rovers. Instead, a hovering, rocket-powered "sky crane" lowered it to the ground on cables, then flew off to crash at a safe distance. Curiosity has no solar panels: a nuclear power source turns heat from decaying radioactive material into electricity, so dust storms cannot starve it of power.
It landed in Gale Crater and has spent years climbing the layered mountain in the middle. Its results show that the crater once held a lake, calm enough for fine mud to settle, with conditions in which microbes could have survived, had any existed.
Perseverance and Ingenuity
Perseverance landed in Jezero Crater, chosen because it contains what looks like an ancient river delta, where a river once flowed into a lake. The rover drills cores of rock and seals them in tubes so that a future mission might bring some back to Earth. Plans for that return trip have changed several times and remain uncertain.
It also carried a small helicopter, Ingenuity. On 19 April 2021 Ingenuity made the first powered, controlled flight on another planet. It was planned for five test flights and made 72, until damage to its rotor blades ended the mission in January 2024.
At the time of writing, Curiosity and Perseverance are both still working. Zhurong went into hibernation for the Martian winter in 2022 and has not woken since.
Why there is no joystick
Radio signals travel at the speed of light, about 300,000 km per second. Mars is never close, and its distance from Earth changes all the time.
Worked example: signal delay
Suppose Mars is 225 million km away.
- Time = distance ÷ speed = 225,000,000 ÷ 300,000 = 750 seconds.
- 750 ÷ 60 = 12.5 minutes.
A command takes 12.5 minutes to arrive, and the picture showing the result takes another 12.5 minutes to come back. So engineers send each day's instructions as a plan, and newer rovers can pick their own way around obstacles.
Landing has to be fully automatic too. The descent from the top of the atmosphere to the ground takes about seven minutes, less than the time a signal takes to reach Earth. By the time engineers hear that the descent has started, the rover is already down, safely or not.
Your turn: Mars is 100 million km away. What is the one-way delay? (100,000,000 ÷ 300,000 ≈ 333 seconds, about 5.6 minutes.)
Worked example: Martian days
A sol lasts about 24 hours 40 minutes. How many Earth days is 90 sols?
- 40 minutes is two thirds of an hour, so a sol ≈ 24.67 hours.
- 90 × 24.67 ≈ 2,220 hours.
- 2,220 ÷ 24 ≈ 92.5 Earth days.
At the start of a mission, some teams live on Mars time, starting work about 40 minutes later each day.
A hard place to work
- Cold. The average temperature is around −60 °C, and nights are much colder.
- Thin air. The atmosphere is mostly carbon dioxide, with surface pressure less than 1% of the Earth's. Parachutes have little air to grip, which makes landing harder.
- Dust. Fine dust settles on solar panels and can rise in storms that last weeks.
- Rough ground. Sharp rocks have punched holes in Curiosity's aluminium wheels.
The red colour, incidentally, comes from iron oxide, a form of rust, in the dust. It says nothing about temperature; Mars is far colder than Earth. For the full comparison with other worlds, see the planets of the Solar System.
What the rovers have not found
No rover has found life. What they have found is strong evidence that Mars once had lakes, rivers and groundwater, and places where microbes could have lived. Whether anything actually did is still an open question, and the sealed samples in Perseverance's tubes are meant to help answer it.
Activity: design a rover mission
- Choose a landing site: an old lake bed, a polar ice cap or a volcano. Give one scientific reason.
- Choose solar or nuclear power, with one advantage and one problem.
- List three instruments the rover should carry and what each is for.
- Rovers travel to Mars in about seven months, and good launch opportunities come round roughly every 26 months. If a window opens in January of one year, when is the next one? (26 months later: March, two years on.)
Every rover starts its journey on top of a rocket; how rockets work explains the physics of getting it off the ground.
Sources and further reading
- Mars 2020: Perseverance rover – NASA Science – the mission, Jezero Crater and the sample tubes.
- Curiosity rover – NASA Science – Curiosity's findings in Gale Crater.
- Opportunity – NASA Science – the rover that worked for nearly 15 years.
- Ingenuity Mars helicopter – NASA Science – the first powered flights on another planet.
