August 24, 2026

Target Pillar: Key Milestones in the History of Space Exploration

Last Updated: August 19, 2026 Author: Sarah Mitchell

Introduction

Five NASA rovers have driven across the Martian surface since 1997, and the story of why three of them eventually stopped working — while two are still operating in 2026, years or even decades past their original mission plans — comes down to a single design decision: what powers them. It’s a detail that says a lot about how NASA’s approach to Mars exploration has evolved across nearly three decades of missions.

Sojourner: Proving the Concept (1997)

Sojourner, part of the Mars Pathfinder mission, became the first successful Mars rover when it landed at Ares Vallis on July 4, 1997, cushioned by an airbag landing system rather than a controlled rocket-powered touchdown.[^1] It was a genuinely modest vehicle by later standards — equipped with an X-ray spectrometer and a handful of cameras, designed for a mission lasting just 90 sols (Martian days, each about 24 hours and 40 minutes, slightly longer than an Earth day).[^2]

Sojourner actually operated for roughly three months, modestly exceeding its planned lifespan, and its real significance wasn’t scientific spectacle but proof of concept: it demonstrated that mobile robotic exploration of the Martian surface was genuinely feasible, opening the door to every more ambitious rover mission that followed.[^1]

Spirit and Opportunity: The Twins That Outlasted Everyone’s Expectations (2004)

NASA’s Mars Exploration Rover mission sent two identical rovers, Spirit and Opportunity, launched in 2003 and landing on opposite sides of the planet in January 2004 — Spirit on January 4, Opportunity on January 25.[^3] Both were built to the same conservative 90-sol design lifespan as Sojourner. Both dramatically exceeded it.

Spirit operated until 2010, eventually becoming stuck in soft sand it couldn’t drive out of — its wheels essentially trapped, ending its ability to reposition itself for adequate sunlight during the Martian winter.[^4] Opportunity fared even better, continuing to operate for nearly 15 years before a massive, planet-wide Martian dust storm in 2018 blocked enough sunlight to drain its batteries beyond recovery, ending communication for good.[^5][^1] Between them, Spirit and Opportunity provided critical, direct evidence of Mars’s watery past, identifying geological formations and mineral deposits that could only have formed in the presence of liquid water.[^3]

Curiosity: A Fundamentally Different Kind of Rover (2012)

Curiosity, part of the Mars Science Laboratory mission, landed in Gale Crater on August 5, 2012, representing a genuine step-change in scale and capability rather than an incremental upgrade.[^6] Car-sized and equipped with 17 cameras along with a full suite of spectrometers, radiation detectors, and environmental sensors, Curiosity was built to assess Mars’s past and present habitability directly, not just search for water-related geology.[^7][^6]

Its most significant scientific finding came from the Sample Analysis at Mars (SAM) instrument, which discovered organic carbon in ancient rocks at Mount Sharp, the central peak inside Gale Crater — direct evidence of the carbon-based chemical building blocks associated with life, though not evidence of life itself.[^7] Curiosity’s radiation detectors separately measured the radiation levels Mars’s thin atmosphere allows through from galactic cosmic rays and solar particles, generating data NASA considers directly relevant to assessing health risks for future human Mars missions.[^7] As of 2026, Curiosity remains active, continuing to explore Mount Sharp’s transition from clay-rich to sulfate-rich terrain — geology that records a specific chapter of how the region’s climate and water availability changed over deep time.[^7][^8]

Perseverance: Searching for Life, Not Just Water (2021)

Perseverance launched July 30, 2020, and landed in Jezero Crater on February 18, 2021 — a site scientists identified as an ancient lake bed fed by a river delta, making it one of the strongest candidate locations on Mars for preserved evidence of ancient microbial life, if any ever existed.[^9] Unlike its predecessors, Perseverance’s core mission isn’t simply to study Mars in place — it’s actively collecting and sealing rock and soil samples for eventual physical return to Earth, where laboratory instruments far more sensitive than anything that can be flown to Mars could analyze them directly.[^5] As of early 2025, the rover had sealed 30 of its 38 planned sample tubes while exploring the rim of Jezero Crater.[^5]

Perseverance also carried Ingenuity, a small robotic helicopter that achieved the first powered, controlled flight by any aircraft on another planet — a genuine historical first, comparable in spirit to the Wright brothers’ original flight on Earth.[^9] Ingenuity vastly outperformed its own expectations, completing 72 flights before a hard landing on January 18, 2024 ended its mission, after its navigation system lost track of surface features over a sloped, sand-rippled area during its final descent.[^5] Perseverance also carries MOXIE, an experimental instrument working to produce oxygen directly from Mars’s carbon dioxide-heavy atmosphere — technology with direct, obvious relevance to any future human mission that would need to manufacture breathable air or rocket propellant on-site rather than hauling it from Earth.[^2]

Why Three Rovers Died and Two Didn’t: The Power Source

The single biggest factor separating Mars rover longevity isn’t ruggedness or luck — it’s the power source each rover was built around. Sojourner, Spirit, and Opportunity all relied on solar panels, which face a specific, unavoidable problem on Mars: fine dust settles on panel surfaces over time, gradually reducing their power output, and the planet periodically produces storms severe enough to blot out sunlight for weeks at a stretch.[^5] All three of these earlier rovers ultimately lost power for exactly this reason — whether through gradual dust accumulation, a sand trap preventing repositioning toward the sun, or a storm severe enough to drain their batteries past recovery.[^5]

Curiosity and Perseverance both use radioisotope thermoelectric generators instead — power sources that generate electricity from the natural radioactive decay of onboard material, entirely independent of sunlight, weather, or surface dust.[^5] This single design choice is the primary reason both rovers have operated for a decade (Curiosity) or are on track to (Perseverance) well past their originally planned mission durations, without the slow, dust-driven power decline that eventually ended each of their solar-powered predecessors.

Frequently Asked Questions

How many rovers has NASA sent to Mars?

Five: Sojourner (1997), Spirit and Opportunity (2004), Curiosity (2012), and Perseverance (2021). A sixth rover, China’s Zhurong, landed in 2021 as well, making it the only non-NASA rover to successfully operate on the Martian surface, though it entered hibernation in 2022 and hasn’t resumed operations since.

Are any Mars rovers still working today?

Yes — Curiosity and Perseverance are both still active and operating on Mars as of 2026, exploring Gale Crater and Jezero Crater respectively.

Why did Opportunity stop working after so many years?

A massive, planet-wide Martian dust storm in 2018 blocked enough sunlight to drain Opportunity’s solar-powered batteries beyond recovery, ending communication with the rover after nearly 15 years of operation — far beyond its original 90-sol design lifespan.

Why don’t all Mars rovers use nuclear power if it lasts longer?

Cost and mission scope are the main factors — radioisotope power systems are more complex and expensive than solar panels, and NASA’s earlier rovers (Sojourner, Spirit, Opportunity) were designed as shorter, lower-cost missions where solar power was considered an acceptable tradeoff at the time.

What happens to the Mars samples Perseverance is collecting?

They’re being sealed in dedicated sample tubes and left in a planned cache for eventual physical retrieval and return to Earth via a future mission, so scientists can study them using laboratory instruments far more capable than anything that can currently be transported to and operated on Mars.

Sources

Note on methodology: mission details and current rover status above reflect NASA/JPL published data and recent (2025-2026) reporting as of mid-2026. Curiosity and Perseverance remain active missions with evolving status — consult NASA/JPL’s Mars missions pages directly for the latest updates.

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Sarah Mitchell