“Our nuclear program, SR-1, is not about going and lobbying for billions of dollars to undertake a brand-new mission,” Isaacman said. “Honestly, we haven’t won the right to be able to do that after $20 billion worth of failed programs over time. This is why we’re taking hardware that we already have, a reactor that’s mostly built, fuel that’s mostly paid for over time.”

NASA officials did not disclose an estimated cost for the SR-1 mission.

After proving nuclear propulsion works, “then you can come back and maybe ask for more [funding] in the future when you show that it can be done,” Isaacman said.

“SR-1 Freedom primarily has that one new system, the reactor, on a spacecraft bus that already exists,” Sinacore said. “The timeline will match the need with the next Mars launch window in December 2028. Orbital mechanics does not negotiate, and the scope must bend around this deadline.”

There are still some hurdles that won’t be easy to jump. Readying any large space mission, especially one as novel as a nuclear propulsion demo, for launch in less than three years will require sharp focus, resistance to mission creep, and near-perfect execution. Sinacore laid out an ambitious timeline for SR-1, with mission design complete by June and large-scale assembly beginning at the start of 2028. If the mission misses a launch opportunity in late 2028, the next Earth-Mars alignment won’t happen until early 2031.

“We are not trying to do everything,” Sinacore said. “We are trying to do the hard thing, which is operate a coupled nuclear reactor, power conversion, and electric propulsion thruster system beyond Earth orbit for the first time ever.”

Although NASA will be the “prime integrator” for SR-1, actually launching radioactive fuel into space requires input from multiple federal agencies, including the Department of Energy. Any rocket selected to launch a nuclear-powered mission must undergo a special certification. SpaceX’s Falcon Heavy, which NASA originally booked to launch the Gateway core module, is undergoing a nuclear certification to launch NASA’s Dragonfly mission to Saturn’s moon Titan.

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When NASA’s Perseverance rover landed in Jezero Crater in 2021, its primary mission was to scour the remnants of a dried-up Martian lakebed for signs of ancient life. Scientists have been focused on the crater’s spectacular Western Delta, a fan-shaped geologic feature deposited by a river flowing into the basin billions of years ago. But now Perseverance’s ground-penetrating radar (called RIMFAX) detected what is likely another, even older river delta buried tens of meters beneath it.

“I think it’s a promising place to look for signs of biosignatures at depth,” says Emily L. Cardarelli. “Microbial life could have potentially developed in those types of environments.” Cardarelli, an astrobiologist at the University of California Los Angeles, led the team interpreting RIMFAX imagery.

Peeking underground

Perseverance’s RIMFAX, the Radar Imager for Mars Subsurface Experiment, continuously fires radar waves into the ground, acquiring soundings each time the rover traveled 10 centimeters. When these radio waves hit boundaries between different types of rock, ice, or sediment layers, some of the signal bounces back. The timing and intensity of these reflections allow scientists to construct a two-dimensional, vertical slice of the subsurface, much like a sonogram of the Martian crust.

During a campaign spanning from September 2023 to February 2024, or over 250 Martian sols, Perseverance drove across a geological zone known as the Margin unit. The Margin unit is an expansive deposit flanking the inner rim of Jezero’s inlet valley, occupying the space between the western fan deposits and the crater rim. It is rich in magnesium carbonates, which was one of the main reasons Jezero Crater has been chosen as the Perseverance’s landing site: on Earth, carbonates are exceptionally good at preserving the chemical fingerprint of life. “You can think of the Cliffs of Dover, for example, that are all carbonate—they have tons of fossils in them,” Cardarelli says.

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This is not science fiction. In remote corners of the world, geologists have found tiny relics of Earth’s very ancient surface.

I have been part of this scientific endeavour, looking at the treasure trove of information in the bedrock of the Makhonjwa Mountains in South Africa and the adjacent small kingdom of Eswatini.

These rocks reach back more than three quarters of the way through our planet’s long history of nearly 4.6 billion years. In my new book, The Oldest Rocks on Earth, I describe the graphic images “beamed back” by this geological time machine.

World of oceans

The ancient rocks reveal a world with extensive oceans and intense volcanic activity on the sea floor.

Deep beneath the crust, Earth was much hotter than today, giving rise to an unusual white-hot magma, rich in elements from its interior. Huge volumes of super-heated water continually gushed out of underwater cracks, building up chimneys of valuable metals. And life was thriving around these undersea vents.

Volcanic islands rose up from the ocean depths. These were dangerous places. Pools of hot bubbling mud dotted their shores, and clouds of volcanic ash periodically exploded from volcanic craters.

Life was already there, forming microbial mats in the sheltered nearshore waters.

Periodically, large earthquakes violently shook the bedrock, triggering submarine avalanches that cascaded down into the deep ocean, creating vast jumbles of rock on the sea floor. Giant asteroid impacts disturbed this world, but crucially, did not extinguish it.

Deep-seated forces were pushing up new land, creating the early continents.

Ocean waves moved back and forth on sandy beaches along coastlines with bays, lagoons, inlets and estuaries, with tides similar to those today.

During floods, large rivers brought muddy water from the continental interior. Farther in the distance, their headwaters drained a mountainous terrain, often enveloped in thick cloud.

It was a blue planet because, like today, the oceans scattered light in the blue part of the colour spectrum.

But the atmosphere contained a lethal cocktail of gases, including high concentrations of methane and carbon dioxide. These greenhouse gases kept the surface at the right temperature for liquid water, at a time when astrophysicists calculate the Sun was much weaker. But there was no oxygen.

The earliest life forms were anaerobic microbes, although brightly coloured – pink or purple have been proposed.

Oceania today

Oceania, in the southwestern Pacific, may illustrate best what this early world was like. Here, the ocean is peppered with volcanic islands and small continents, rocked by great earthquakes where tectonic plates rub against each other. There are even clues to how life began.

The 2022 eruption of the Hunga volcano, near Tonga, created a mushroom cloud of ash that burst out of the ocean and reached up into space with an estimated energy of a 60-megaton atomic bomb. It generated more than 200,000 lightning strikes and left behind a deep underwater crater filled with a chemical soup derived from numerous underwater hot vents.

Experiments show that lightning strikes can trigger the synthesis of basic organic molecules needed by living organisms. Millions of Hunga-like eruptions on early Earth would have created myriad opportunities to kick start the chemistry of life in underwater volcanic craters – life was born out of extreme geological violence.

Staying blue

Going back in time beyond the Makhonjwa Mountains, we still find evidence for oceans, life and, I argue, plate tectonics. Earth became blue within the first tenth of its history.

Mars and Venus may have started this way, too. But our planet uniquely lies in the so-called Goldilocks Zone, receiving just the right amount of solar energy to avoid becoming a boiling Venusian hell or freezing Martian world.

It is also big enough to have a magnetic field and pull of gravity sufficient to retain its atmosphere. And right at the start, a dramatic collision with a Mars-sized asteroid spalled off our Moon, stabilising Earth’s spin axis so that day and night were less extreme.

Finally, the biochemistry of living organisms may have played a key role in keeping Earth this way by helping the bedrock absorb greenhouse gases in the face of a steadily warming Sun.

We must not be the first to let Earth lose its distinctive life-giving blue, a colour so wonderfully referred to in the Siswati language of Eswatini as luhlata lwesibhakabhaka, literally “green like the sky”.

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