Patrick M. Shober, NASA Much of what scientists know about the early solar system comes from meteorites – ancient rocks that travel through space and survive a fiery plunge through Earth’s atmosphere. Among meteorites, one type – called carbonaceous chondrites – stands out as the most primitive and provides a unique glimpse into the solar system’s infancy. The carbonaceous chondrites are rich in water, carbon and organic compounds. They’re “hydrated,” which means they contain water bound within minerals in the rock. The components of the water are locked into crystal structures. Many researchers believe these ancient rocks played a crucial role in delivering water to early Earth. Before hitting the Earth, rocks traveling through space are generally referred to as asteroids, meteoroids or comets, depending on their size and composition. If a piece of one of these objects makes it all the way to Earth, it becomes a “meteorite.” From observing asteroids with telescopes, scientists know that most asteroids have water-rich, carbonaceous compositions. Models predict that most meteorites – over half – should also be carbonaceous. But less than 4% of all the meteorites found on Earth are carbonaceous. So why is there such a mismatch? In a study published in the journal Nature Astronomy on April 14, 2025, my planetary scientist colleagues and I tried to answer an age-old question: Where are all the carbonaceous chondrites?

Sample-return missions

Scientists’ desire to study these ancient rocks has driven recent sample-return space missions. NASA’s OSIRIS‑REx and JAXA’s Hayabusa2 missions have transformed what researchers know about primitive, carbon‑rich asteroids. Meteorites found sitting on the ground are exposed to rain, snow and plants, which can significantly change them and make analysis more difficult. So, the OSIRIS‑REx mission ventured to the asteroid Bennu to retrieve an unaltered sample. Retrieving this sample allowed scientists to examine the asteroid’s composition in detail. Similarly, Hayabusa2’s journey to the asteroid Ryugu provided pristine samples of another, similarly water-rich asteroid. Together these missions have let planetary scientists like me study pristine, fragile carbonaceous material from asteroids. These asteroids are a direct window into the building blocks of our solar system and the origins of life.

The carbonaceous chondrite puzzle

For a long time, scientists assumed that the Earth’s atmosphere filtered out carbonaceous debris. When an object hits Earth’s atmosphere, it has to survive significant pressures and high temperatures. Carbonaceous chondrites tend to be weaker and more crumbly than other meteorites, so these objects just don’t stand as much of a chance. Meteorites usually start their journey when two asteroids collide. These collisions create a bunch of centimeter- to meter-size rock fragments. These cosmic crumbs streak through the solar system and can, eventually, fall to Earth. When they’re smaller than a meter, scientists call them meteoroids. Meteoroids are far too small for researchers to see with a telescope, unless they’re about to hit the Earth, and astronomers get lucky. But there is another way scientists can study this population, and, in turn, understand why meteorites have such different compositions.

Meteor and fireball observation networks

Our research team used the Earth’s atmosphere as our detector. Most of the meteoroids that reach Earth are tiny, sand-sized particles, but occasionally, bodies up to a couple of meters in diameter hit. Researchers estimate that about 5,000 metric tons of micrometeorites land on Earth annually. And, each year, between 4,000 and 10,000 large meteorites – golf ball-sized or larger – land on Earth. That’s more than 20 each day.

Today, digital cameras have rendered round-the-clock observations of the night sky both practical and affordable. Low-cost, high-sensitivity sensors and automated detection software allow researchers to monitor large sections of the night sky for bright flashes, which signal a meteoroid hitting the atmosphere. Research teams can sift through these real-time observations using automated analysis techniques – or a very dedicated Ph.D. student – to find invaluable information. Our team manages two global systems: FRIPON, a French-led network with stations in 15 countries; and the Global Fireball Observatory, a collaboration started by the team behind the Desert Fireball Network in Australia. Together with other open-access datasets, my colleagues and I used the trajectories of nearly 8,000 impacts observed by 19 observation networks spread across 39 countries.

By comparing all meteoroid impacts recorded in Earth’s atmosphere with those that successfully reach the surface as meteorites, we can pinpoint which asteroids produce fragments that are strong enough to survive the journey. Or, conversely, we can also pinpoint which asteroids produce weak material that do not show up as often on Earth as meteorites.

The Sun is baking the rocks too much

Surprisingly, we found that many asteroid pieces don’t even make it to Earth. Something starts removing the weak stuff while the fragment is still in space. The carbonaceous material, which isn’t very durable, likely gets broken down through heat stress when its orbit takes it close to the Sun. As carbonaceous chondrites orbit close, and then away from the Sun, the temperature swings form cracks in their material. This process effectively fragments and removes weak, hydrated boulders from the population of objects near the Earth. Anything left over after this thermal cracking then has to survive the atmosphere. Only 30%-50% of the remaining objects survive the atmospheric passage and become meteorites. The debris pieces whose orbits bring them closer to the Sun tend to be significantly more durable, making them far more likely to survive the difficult passage through Earth’s atmosphere. We call this a survival bias. For decades, scientists have presumed that Earth’s atmosphere alone explains the scarcity of carbonaceous meteorites, but our work indicates that much of the removal occurs beforehand in space. Going forward, new scientific advances can help confirm these findings and better identify meteoroid compositions. Scientists need to get better at using telescopes to detect objects right before they hit the Earth. More detailed modeling of how these objects break up in the atmosphere can also help researchers study them. Lastly, future studies can come up with better methods to identify what these fireballs are made of using the colors of the meteors. Patrick M. Shober, Postdoctoral Fellow in Planetary Sciences, NASA This article is republished from The Conversation under a Creative Commons license. Read the original article.

Benjamin Roulston, Clarkson University Powering spacecraft with solar energy may not seem like a challenge, given how intense the Sun’s light can feel on Earth. Spacecraft near the Earth use large solar panels to harness the Sun for the electricity needed to run their communications systems and science instruments. However, the farther into space you go, the weaker the Sun’s light becomes and the less useful it is for powering systems with solar panels. Even in the inner solar system, spacecraft such as lunar or Mars rovers need alternative power sources. As an astrophysicist and professor of physics, I teach a senior-level aerospace engineering course on the space environment. One of the key lessons I emphasize to my students is just how unforgiving space can be. In this extreme environment where spacecraft must withstand intense solar flares, radiation and temperature swings from hundreds of degrees below zero to hundreds of degrees above zero, engineers have developed innovative solutions to power some of the most remote and isolated space missions. So how do engineers power missions in the outer reaches of our solar system and beyond? The solution is technology developed in the 1960s based on scientific principles discovered two centuries ago: radioisotope thermoelectric generators, or RTGs. RTGs are essentially nuclear-powered batteries. But unlike the AAA batteries in your TV remote, RTGs can provide power for decades while hundreds of millions to billions of miles from Earth.

Nuclear power

Radioisotope thermoelectric generators do not rely on chemical reactions like the batteries in your phone. Instead, they rely on the radioactive decay of elements to produce heat and eventually electricity. While this concept sounds similar to that of a nuclear power plant, RTGs work on a different principle. Most RTGs are built using plutonium-238 as their source of energy, which is not usable for nuclear power plants since it does not sustain fission reactions. Instead, plutonium-238 is an unstable element that will undergo radioactive decay. Radioactive decay, or nuclear decay, happens when an unstable atomic nucleus spontaneously and randomly emits particles and energy to reach a more stable configuration. This process often causes the element to change into another element, since the nucleus can lose protons.

When plutonium-238 decays, it emits alpha particles, which consist of two protons and two neutrons. When the plutonium-238, which starts with 94 protons, releases an alpha particle, it loses two protons and turns into uranium-234, which has 92 protons. These alpha particles interact with and transfer energy into the material surrounding the plutonium, which heats up that material. The radioactive decay of plutonium-238 releases enough energy that it can glow red from its own heat, and it is this powerful heat that is the energy source to power an RTG.

Heat as power

Radioisotope thermoelectric generators can turn heat into electricity using a principle called the Seebeck effect, discovered by German scientist Thomas Seebeck in 1821. As an added benefit, the heat from some types of RTGs can help keep electronics and the other components of a deep-space mission warm and working well. In its basic form, the Seebeck effect describes how two wires of different conducting materials joined in a loop produce a current in that loop when exposed to a temperature difference.

Devices that use this principle are called thermoelectric couples, or thermocouples. These thermocouples allow RTGs to produce electricity from the difference in temperature created by the heat of plutonium-238 decay and the frigid cold of space.

Radioisotope thermoelectric generator design

In a basic radioisotope thermoelectric generator, you have a container of plutonium-238, stored in the form of plutonium-dioxide, often in a solid ceramic state that provides extra safety in the event of an accident. The plutonium material is surrounded by a protective layer of foil insulation to which a large array of thermocouples is attached. The whole assembly is inside a protective aluminum casing.

The interior of the RTG and one side of the thermocouples is kept hot – close to 1,000 degrees Fahrenheit (538 degrees Celsius) – while the outside of the RTG and the other side of the thermocouples are exposed to space. This outside, space-facing layer can be as cold as a few hundred degrees Fahrenheit below zero. This strong temperature difference allows an RTG to turn the heat from radioactive decay into electricity. That electricity powers all kinds of spacecraft, from communications systems to science instruments to rovers on Mars, including five current NASA missions. But don’t get too excited about buying an RTG for your house. With the current technology, they can produce only a few hundred watts of power. That may be enough to power a standard laptop, but not enough to play video games with a powerful GPU. For deep-space missions, however, those couple hundred watts are more than enough. The real benefit of RTGs is their ability to provide predictable, consistent power. The radioactive decay of plutonium is constant – every second of every day for decades. Over the course of about 90 years, only half the plutonium in an RTG will have decayed away. An RTG requires no moving parts to generate electricity, which makes them much less likely to break down or stop working. Additionally, they have an excellent safety record, and they’re designed to survive their normal use and also be safe in the event of an accident.

RTGs in action

RTGs have been key to the success of many of NASA’s solar system and deep-space missions. The Mars Curiosity and Perseverance rovers and the New Horizons spacecraft that visited Pluto in 2015 have all used RTGs. New Horizons is traveling out of the solar system, where its RTGs will provide power where solar panels could not. However, no missions capture the power of RTGs quite like the Voyager missions. NASA launched the twin spacecraft Voyager 1 and Voyager 2 in 1977 to take a tour of the outer solar system and then journey beyond it.

Each craft was equipped with three RTGs, providing a total of 470 watts of power at launch. It has been almost 50 years since the launch of the Voyager probes, and both are still active science missions, collecting and sending data back to Earth. Voyager 1 and Voyager 2 are about 15.5 billion miles and 13 billion miles (nearly 25 billion kilometers and 21 billion kilometers) from the Earth, respectively, making them the most distant human-made objects ever. Even at these extreme distances, their RTGs are still providing them consistent power. These spacecraft are a testament to the ingenuity of the engineers who first designed RTGs in the early 1960s. Benjamin Roulston, Assistant Professor of Physics, Clarkson University This article is republished from The Conversation under a Creative Commons license. Read the original article.