Small Worlds

Big Questions

Since the beginning of time, humans have looked up at the night sky with a mix of wonder and awe. Flashes of bright light across the darkened heavens inspired fear and dread in ancient cultures. What were these brilliant streaks? Something similar to lightning? Or harbingers of doom? With the invention of telescopes and better observation tools, scientists came to understand these magnificent bursts of light as comets, asteroids and meteoroids, leftover bits of rock and ice from the formation of our solar system.

Woodcut showing destructive influence of a fourth century comet from Stanilaus Lubienietski's 1668 Theatrum Cometicum.

(From “Comets: A Chronological History of Observation, Science, Myth and Folklore” by Don Yeomans.  Used with permission.)



About 4.6 billion years ago, at the very beginning of our solar system, a vast disk of dust, gas and ice circled the young Sun. Over thousands of years, this "solar nebula" was pulled together by gravity to form the celestial bodies as we know them today. Some of the small rocky and icy bodies in this disk would come together to build the planets, but billions of these ancient objects remained after the rocky planets and gas giants began to take shape. These small worlds, the remnants of that time long ago, are the asteroids, comets, dwarf planets, and meteoroids of today. They still circle the Sun, reminders of that early, fast and furious time of planet building.

Many of these worlds have been altered very little since they first formed. Their relatively pristine state makes the comets, asteroids and dwarf planets wonderful storytellers with much to share about what conditions were like in the early solar system. It is the tale of our own origins, chronicling the processes and events that led to the birth of our world and the delivery of the water and raw materials that made life possible – from the iron in our blood to the air we breathe.

The purpose of this section is to spotlight these Small Worlds and provide a wide-ranging yet concise view of where they are, how we explore them, and why scientists are so interested in what we can learn from them. After you experience Small Worlds, we hope you will also be intrigued and have many questions of your own.


Getting to Know the Neighborhood

As the technology of telescopes and the capabilities of spacecraft improve, new observations change our perceptions of the bodies that orbit our Sun. In recent years many distant icy worlds have been discovered in the vast cold region beyond Neptune. Some of these discoveries have prompted a passionate and very public discussion about how these objects should be classified. New revelations that challenge our assumptions are an exciting part of the process of exploration.


As we learn more about these primordial building blocks of our solar system, the distinction between asteroids, comets and dwarf planets has become increasingly difficult to make. But even with the shuffling of names and categories in the past several years, we can divide the small worlds into three main categories and share some of their general characteristics.



Ceres is a great example of how scientists’ thinking evolves regarding classification of objects in the solar

system.  Ceres was discovered in 1801 by Giuseppe Piazzi.  He initially reported it as a comet, but with more observations noticed its movement was slow and uniform.  It was ultimately classified as the eighth planet.  However, half a century later, as other objects were discovered in the area, scientists realized Ceres represented the first of a class of many similar objects that were called “asteroids,” meaning “star-like.” Ceres retained its comfortable position as asteroid #1, largest and first discovered, for about 150 years. 

Why We Explore


Several missions in NASA's Discovery Program focus on the small, primitive bodies we know as comets, asteroids and dwarf planets. But why all the effort? Why should we devote time, effort and money to exploring them and what does it have to do with you?


Keys to the Past

The small worlds of our solar system conceal answers to some of the most pressing questions about our own origins. How did our Sun's family of planets, moons and small bodies originally form? How did our solar system evolve into the diverse collection of worlds we see today? How did life begin on Earth, and could the same processes have allowed life to get a foothold on other worlds?


At the very beginning of our solar system, before there was an Earth, Jupiter or Pluto, a massive swirling cloud of dust and gas circled the young Sun. The dust particles in this disk collided with each other and formed into larger bits of rock. This process continued until they reached the size of boulders. Eventually this process of accretion formed the planets of our solar system.


Billions of small space rocks never evolved. Amazingly, many of these mysterious worlds have been altered very little in the 4.6 billion years since they first formed. Their relatively pristine state makes the comets, asteroids, and dwarf planets wonderful storytellers with much to share about what conditions were like in the early solar system. They can reveal secrets about our origins, chronicling the processes and events that led to the birth of our world. They might offer clues about where the water and raw materials that made life possible on Earth came from. Comets and asteroids probably delivered some of the water and other ingredients that allowed the complex chemistry of life to begin on Earth. The amino acid glycine was discovered in the comet dust returned to Earth by the Stardust mission. Glycine is used by living organisms to make proteins. The discovery supports the theory that some of life's ingredients formed in space and were delivered to Earth long ago by meteorite and comet impacts.


Like forensic detectives, scientists follow clues about what happened when the solar system was young to piece together the story of our origins. What we learn will also teach us about systems of planets around other stars, and how life might develop there as well.


Resources for the Future

We also explore small worlds to understand the hazards and resources in the solar system that will affect human expansion in space. As we venture outward from our home planet, what kinds of challenges will we face?


Might we find new sources of raw materials and natural resources that we could use on Earth? Could humans use asteroids or comets as refueling stations someday? Might we find new, cleaner energy sources in space to help protect our environment?


What is the future habitability of Earth? Studying the small worlds will give us a better understanding of the solar system forces and processes that affect our future at home and enhance our ability to assess suitable locales for future human exploration. Examples of global climate change in the solar system can help us to model Earth's long-term climate.


Protection from Potential Impacts

Impacts are a process in the solar system that are capable of ending life as well as advancing it. These cosmic collisions are as natural as rain, although they happened a lot more often when the solar system was young. Scientists believe stray objects or fragments from earlier collisions slammed into Earth in the past, playing a major role in the evolution of our planet.

With increasing regularity, scientists are discovering asteroids and comets with unusual orbits, ones that take them close to Earth and the Sun. Very few of these bodies are potential hazards to Earth, but the more we know and understand about them, the better prepared we will be to take appropriate measures if one is heading our way. Knowing the size, shape, mass, composition and structure of these objects will help determine the best way to divert a space rock found to be on an Earth-threatening path. Missions to comets and asteroids provide valuable information about their composition and structure, helping scientists assess the best methods to deal with those in potentially hazardous orbits.



By immediately tracking potentially hazardous near-Earth objects, we have more time to study potentially threatening situations. NASA's Near Earth Object Program was established in 1998 to coordinate NASA-sponsored efforts to detect, track and characterize potentially hazardous asteroids and comets that could approach the Earth.

How We Explore


NASA's robotic spacecraft allow us to visit comets, asteroids, and dwarf planets up close, and even bring back samples to study. We are just beginning to figure out what these places are like, what they are made of, and how they formed. There are so many places to explore and so much to learn.


While actually going there is perhaps the most exciting and direct way to learn about these small worlds, it's not the only way we can experience what they have to teach us. There are lots of ways in which we learn about the small worlds of our solar system.


What Our Eyes See


For most of human history our best tool for observing the sky has been our eyes. With our natural senses, we witness comets, meteor showers, and shooting stars. In ancient times, before we knew much about them, these phenomena were often sources of awe and fear. Learning about what causes these apparitions has removed our fear of them, but done nothing to lessen our awestruck gaze. We are simply fascinated by these messengers in the heavens.

Orionid Meteor Shower

The Orionids meteors streaking across this image started as sand-sized bits expelled from Comet Halley during one of its trips to the inner solar system.

Credit & Copyright: Tunc Tezel 


We see comets when they enter the inner part of the solar system where the Sun's light and heat begins to warm their surfaces, causing them to emit jets of gas and dust. When Earth encounters the trail of small particles left behind in the orbital path of a comet, we witness these particles burning up in Earth's atmosphere as a meteor shower.


The magnificent tail of Comet McNaught was visible to Southern Hemisphere observers in 2007.

Magnificent tail of Comet McNaught


Shooting or falling stars are another name for meteors. These are bits of space debris that create lots of friction as they fall through our atmosphere, causing them to glow. Most meteors are no bigger than a pea, but larger ones can create brilliant colorful fireballs that break apart as they fall, dropping meteorites over large areas.

Observing Through Telescopes
Kid looking through Telescope


From the ground, both scientists and amateur observers use telescopes to search the skies. When an object is discovered, telescopes carefully check its path on multiple nights to determine its orbit. An object that is small and orbits close to the Sun might appear just as bright as an object that is much larger and orbits very far from the Sun. How fast an object moves and the path it takes across the sky helps to determine where an object is in the solar system and what the shape of its path around the Sun looks like.

Looking through Telescope





Scientists also use telescopes to watch for changes in the brightness of comets and asteroids. Comets generally brighten as they get closer to the inner solar system and the Sun begins to evaporate ices on and near their surfaces. This material streams out into space and catches sunlight, causing a comet to brighten substantially. Telescopes can observe comets as they first begin to brighten, sometimes as far out as the orbit of Saturn. Outbursts of dust and gas can also cause comets to temporarily flare in brightness.

Most asteroids appear as mere points of light moving against the background stars, but we can use telescopes to watch them change in the brightness as they rotate. Researchers use this information to learn about the shapes of the space rocks and even their structure and composition. For example, from measuring how fast asteroids rotate, scientists realized that many are probably loose rubble piles. If a pile of rubble in space rotates too quickly it will fly apart, but many asteroids spin slowly enough to keep this from happening.


Telescopes come in many shapes and sizes, all helping to advance the frontiers of astronomy. The W. M. Keck Observatory's twin telescopes on the summit of Hawaii's dormant Mauna Kea volcano are the world's largest optical and infrared telescopes.

Keck telescopes

Keck Telescopes


Also, although it may seem incredible, the light that comes from objects in space contains information about what those things are made of, like a chemical fingerprint. Telescopes can be fitted with special instruments called spectrographs that break apart light like a prism to analyze the composition of comets and asteroids. Comparing this chemical fingerprint, called a spectrum, with the composition of meteorites analyzed in the laboratory is how scientists determined that meteorites are pieces of asteroids. Still another way we use telescopes to explore small worlds is in the search for space rocks that could hit our planet, causing large-scale damage. Telescopes survey the skies, trying to find any objects big enough to cause global catastrophe or even significant regional damage.

Radar astronomy uses the world's most massive dish-shaped antennas to beam directed microwave signals at their targets, which can be as close as our moon and as far away as the moons of Saturn.


An asymmetric, irregularly-shaped object. Caption


These pulses bounce off the target, and the resulting "echo" is collected and examined. Radar antennas can beam powerful blasts of radio waves at an asteroid or comet and use the reflected signal to measure an object's size, how fast it is spinning, and some properties of materials on its surface.

Radar observations can also help precisely determine an object's location and even create pictures to show what it looks like.


Image: Radar observation of asteroid 1999 JM8 taken August 3, 1999 with the Arecibo radar. The image reveals an asymmetric, irregularly-shaped object.


Space-based Telescopes Capture History

The Hubble Space Telescope floats above the Earth in this 2002 photo taken from the International Space Station during a servicing mission. Launched in 1990, the Earth-orbiting Hubble has beamed back hundreds of thousands of images, shedding light on many of the great mysteries of astronomy and transforming the way scientists look at the universe.

Hubble over ISS


Telescopes in space can provide us with observations of comets and asteroids that are unobscured by Earth's atmosphere. The Hubble Space Telescope has obtained incredible images of the two largest asteroids, Vesta and Ceres, giving scientists a chance to examine the shapes, composition and large-scale surface features of these small worlds in preparation for the visit by the Dawn spacecraft in 2011.

Montage of Hubble Space Telescope images showing scars on Jupiter caused by the impact of comet Shoemaker-Levy 9 in 1994, from 5 minutes (top) to 5 days (bottom) after impact.



In addition, space observatories like the Hubble and the Spitzer Space Telescopes can tell us about the disks of rocky and icy debris that surround many young stars, indicating many infant star systems are full of comet- and asteroid-like bodies, very much as we think our solar system was early in its history.


Getting Up Close With Spacecraft

Many spacecraft have visited comets and asteroids, and our capabilities to explore these bodies continue to increase. Some missions fly past small bodies on their way to other destinations, allowing us an exciting, quick glimpse of a comet or asteroid. From simple flybys we have progressed to orbiting these objects over a period of time, touching down on their surfaces, collecting samples to return to Earth, and even punching craters into them to examine what's inside.


Spacecraft allow us to carefully choose a set of tools with which to examine the small bodies of our solar system. As technology continues to improve, we can fly missions to answer specific questions about these intriguing little worlds. The Stardust mission collected tiny particles from the halo of dust and ice that surrounds comet Wild 2 and returned them to Earth where scientists are analyzing them and making amazing new discoveries. It's now on its way to peer into the crater made by the Deep Impact spacecraft on the nucleus of comet Tempel 1. Future missions will likely touch down on the surfaces of comets and asteroids and gather samples for us to study back on Earth.



Artist's concept of Stardust capturing dust from the comet Wild 2.



Examining Samples on Earth


The solar system's most primitive materials, those modified the least by processes over the ages, are a time capsule of our origin and evolution. These materials include dust, rocks, solar wind, and meteorites.


Examining these mysterious materials on Earth in futuristic laboratories with the most advanced instruments can yield answers to many of humanities oldest questions about where we come from.


The Great Benefits of Sample Return Missions


Humans who traveled to the Moon during the Apollo missions from 1969 to 1972 brought back to Earth 842 pounds of awe-inspiring rocks, pebbles, sand, and dust. The lunar surface geology preserves the record of nearly the entire 4.6 billion years of solar system history. Analysis of these rock and soil samples combined with observations from orbit have played a huge role in reconstructing planetary evolution and continues to generate new knowledge about the early history of the Moon, the Earth, and the inner solar system. The lunar sample studies have inspired the development of new analysis methods while honing the skills of two generations of scientists.

Eugene Cernan operating the Lunar rover during Apollo 17 in December 1972. Cernan covered 22 miles during 22 hours of exploration and sample collecting. In that final lunar landing mission, Cernan became the last man to walk on the Moon.

NASA Apollo 17


The Discovery Program, which began in 1992, gave scientists the opportunity to pursue more sample return opportunities. Samples of solar wind returned to Earth by the Genesis mission and comet dust and interstellar dust from Stardust are yielding amazing results. From just a pinch of comet dust, the Stardust mission proved that comets contain a fantastic record of the birth of the solar system. With a larger amount of material and even some ices to study on Earth, scientists could learn so much more.

A comet particle collected by the Stardust spacecraft, found to be made up of the silicate mineral forsterite, also known as peridot in its gem form. The particle is about 2 micrometers across.

Stardust Mineral



While spacecraft instruments tell us a great deal, there are limitations on mass, power, reliability, data rate, and the ability to work autonomously. On Earth, the best instruments that exist, regardless of size, can be used to analyze samples. As new analytical techniques are developed, they can be used to make even more discoveries from older samples. Larger numbers of scientists can participate, requesting samples to study in their own labs worldwide. This facilitates the ability to replicate important results, an essential element of scientific research.

A researcher examining a Stardust aerogel tile under a stereo microscope.

Stardust Examining Samples


Even citizen scientists can participate in sample return missions. Stardust@home has about 30,000 volunteers using their own computers to search for the first pristine interstellar dust particles ever brought to Earth.


What Meteorites Tell Us

Most meteorites are chunks of asteroids that fall to Earth. In fact, several thousand tons of these space rocks and dust fall to our planet every day. Most burn up harmlessly in the atmosphere, but some make it to the ground without being vaporized. Scientists collect meteorites to study their structure and chemical composition. From such studies we have learned a great deal about the different kinds of asteroids – for instance, there are three main types. Some asteroids are mostly metal, consisting of nickel and iron, like Earth's core. Others are a combination of these metals and rocky minerals, like magnesium and silicon. A third type of asteroid, the most common by far, is very dark and rich in carbon and has about the same composition as the Sun, minus hydrogen, helium and other easily evaporated chemicals.

This composition of this meteorite suggests it is a fragment of the asteroid Vesta, blasted off that small world by an impact long ago. 
Credit: R. Kempton (New England Meteoritical Services) 



The only asteroid that has been identified as a meteorite source is Vesta, one of the targets of the Dawn mission. By determining the composition of Vesta from the way it reflects sunlight, scientist know it is the only large asteroid whose 'light signature' matches the basaltic rock of HED meteorites, those composed of the howardites, eucrites and diogenites.

These basaltic meteorites from Mars were found in California's Mojave Desert. A 1-cm cube is shown for size comparison.

Credit: Ron Baalke

Mars Meteorite LA001Mars Meteorite LA002


Not all meteorites come from asteroids, however. Some are actually pieces of the Moon and Mars that were blasted off those worlds by powerful impacts. Wherever they are from, meteorites are amazing – they give us actual samples of other worlds to study. And the best part is that they come to us!

This iron meteorite was found in Antarctica. This sample is made of mainly iron and nickel and is probably a small piece from the core of a large asteroid that broke apart.

Iron Meteorite


It is easiest to spot meteorites in sandy desert regions, like Namibia in Africa, or permanently snow-covered places in Antarctica. In these areas, dark rocks stand out against the light-colored sand or white snow. You can find pieces of meteorites for sale in most rock & gem shops.

Scientists look for meteorites in Antarctica, where the dark rocks from space are easy to see against the icy ground.

Scientists In Antarctica


Cameras on NASA's Spirit and Opportunity rovers have both spotted iron-rich meteorites lying on the Martian surface during their travels on the Red Planet.


In the Future - Human Exploration

People may someday visit asteroids and comets themselves, to perform detailed scientific studies or to look for natural resources. In April 2010, President Barack Obama announced a plan for U.S. astronauts to embark on a mission to an asteroid by 2025. By then, he said, new spacecraft designed for long journeys could allow us to begin the first ever crewed missions beyond the Moon into deep space.

Human in Space


When humans finally travel to these primitive bodies, they will rely on the large body of knowledge collected by all the other types of exploration that paved the way.


Hubble Over ISSShoemaker-LevyStardustNASA Apollo 17Stardust MineralStardust Examining SamplesVestaMars Meteorites LA001 And 002Iron MeteoriteScientists In Antarctica




What We're Learning

Small Worlds - What We're Learning

At the core of NASA's Solar System Exploration is the quest to understand the origins of planets, asteroids, comets and objects in the Kuiper belt. Exciting missions to explore these places begin with our desire to know about how the solar system formed, how it is still evolving today and how the story of life on Earth fits into that picture.


The small worlds tell us about the conditions that prevailed when the Sun and planets were still forming. Call them "original members" of the solar system. Comets are the leftover building blocks of the outer planets, while asteroids are the remnants of the process that built the inner planets. Both likely played important roles in the development of life on Earth as well. So if we are to understand the story of our origins, we must explore these primitive small bodies. And since a comet or asteroid impact could still pose a threat to our civilization, it is prudent for us to learn more about them.


Cosmic Puzzle

MESSENGER will unlock many of the mysteries of Mercury, the smallest planet in our solar system.


The groundbreaking missions of NASA's Discovery Program provide an excellent way for us to reach out to the small worlds, getting up close and getting to know them as real places. Each innovative new exploration has a unique focus, doing something that's never been done before. Every mission adds more pieces of knowledge to help solve the expanding cosmic puzzle, while raising exciting new questions that beckon future explorers to venture out into the solar system. These missions teach us there are always surprises, changes to long-held beliefs, and new wonders to behold.


Where are these missions going and what are we learning from them?


Listen to scientists on the Stardust and Deep Impact comet missions as they react to some amazing images from their investigations in Image Impact!


Near Earth Asteroid Rendezvous (NEAR)

The NEAR spacecraft was the first to perform a comprehensive study of an asteroid, spending one year in orbit around asteroid Eros. The mission had three main scientific goals: determine the physical and geological properties of a near-Earth asteroid; clarify relationships between asteroids, comets and meteorites; and further our understanding of how and under what conditions the planets formed and evolved.


NEAR produced our first detailed, global map of an asteroid. The mission revealed bizarre and surprising


These four images show the boulder-strewn surface of Eros at increasing resolution, just days before landing. The top images are from 8.4 miles (left) and 6.9 miles (right). The top scenes are about 1,815 feet across. The bottom images were taken from 3 miles above the surface, showing 760 feet across.

aspects of surface structures on Eros. Scientists were puzzled by the lack of small craters and the profound number of boulders they saw. Later analysis showed that most of the boulders were produced by the impact that created the asteroid's largest impact crater.


NEAR also showed that the composition of Eros is very similar to the meteorites called chondrites, but there was a discrepancy. The abundance of the element sulfur was less than in chondrites. However, NEAR's data tell us only about the thin, uppermost layer of the surface. A future mission to sample the asteroid could tell if the sulfur is depleted from only a thin surface layer or throughout the asteroid.


The NEAR team devised a spectacular finish to the yearlong orbit at Eros – the first-ever spacecraft landing on an asteroid. On February 12, 2001, NEAR made a gentle landing on the tips of two solar panels and the bottom edge of its body. Then, to much amazement, the craft continued to operate and send signals back to Earth. For two weeks the team gathered the first scientific readings from an asteroid's surface, adding to the legacy of a mission that collected 10 times more data than planned and advanced the field of asteroid studies tremendously.


Dawn Location

Dawn’s location in December 2010, about 7.1 million miles and 6 months to go before arrival at Vesta. 


Dawn will orbit the large asteroid Vesta and the dwarf planet Ceres, two very different objects. Vesta is dry, differentiated, and shows signs of resurfacing. It resembles the rocky bodies of the inner solar system. Ceres has a primitive surface containing water-bearing minerals and has many similarities to the large icy moons of the outer solar system. By observing them both with the same set of instruments, Dawn can compare their different evolutionary paths. Dawn will arrive at Vesta in 2011 and Ceres in 2015. Dawn is the first spacecraft to orbit an object, study it, and then re-embark under powered flight to a second target. All previous multi-target missions were planetary flybys. Dawn's ion propulsion engines make its unique journey possible.


Dawn has the potential for making many paradigm-shifting discoveries. Ceres could have active processes leading to seasonal polar caps of water frost. Vesta may have rocks more strongly magnetized than on Mars, altering our ideas of how and when magnetic fields arise on planets, and with important lessons for Mars, Earth and Mercury. Ceres might have a thin, permanent atmosphere distinguishing it from the other minor planets.


The three big scientific drivers for the mission are first that it captures the earliest time in the origin of the solar system, enabling us to understand the conditions under which these objects formed. Second, Dawn determines the nature of the building blocks from which the inner, terrestrial planets formed, improving our understanding of this formation. Finally, it contrasts the formation and evolution of two small worlds that followed very different evolutionary paths – helping us understand what controls that evolution.


What we learn from Dawn will tell us volumes about not just these small worlds, but also about our own planet and its origins.



Comet particle tracks in aerogel.


The Stardust spacecraft flew through the cloud of dust that surrounds the nucleus of comet Wild 2 in January 2004. The particles of cometary material and interstellar dust gathered were returned to Earth aboard a sample return capsule which landed in the Utah desert in January 2006. In 2007 the still-healthy spacecraft was retargeted to head for comet Tempel 1 on a new mission called Stardust-NExT.


One of the most exciting findings from Stardust was the presence of organic materials – chemicals that are important to living things on Earth – in the comet dust. Scientists found simple hydrocarbons, as well as the amino acid glycine, in samples from Wild 2. Hydrocarbons are used for energy by life on Earth, and amino acids are used to build proteins. These discoveries support the idea that the fundamental building blocks of life are common in space, meaning life may not be a rarity.


A surprising finding from the Stardust samples was the presence of minerals that came from the inner part of the young solar system, which has temperatures much warmer than the icy region where comets form. Many scientists expected that a lot of the dust in comets would be of interstellar origin, meaning it formed around other stars. But most of the dust Stardust collected from Wild 2 contains minerals that formed in the presence of high heat, meaning they had to have condensed in the inner solar system close to the Sun and been blown out to the cold comet forming region beyond Pluto to be incorporated in the icy matrix of the comet. So it turns out that, rather than containing a nearly pristine record of the primitive interstellar material that originally formed our Sun and its planets, Wild 2 is a time capsule containing a diverse mixture of materials from locations all over the young solar system.


One of the big mysteries of Wild 2 is why it looks so dramatically different than the four other comets that have been imaged by a spacecraft. Most of the others are relatively smooth, but this one is dramatic with its really deep impressions, vertical cliffs, and spires sticking up into space. It has no impact craters which indicates its original surface has been lost and replaced by an incredibly rugged, intriguing surface. It has depressions that look like the material has just collapsed, as a result of a very weak structure. It has no surface ices. Wild 2 showed us that the cometary nuclei are quite different from one another.


deep impact

This sequence of images depicts the development of the ejecta plume after impact. The red arrows highlight shadows due to opacity of the ejecta. The yellow arrow indicates the “zone of avoidance.” The 8 images were taken 0.84 seconds apart.

Deep Impact

Deep Impact provided the first look at what is inside a comet. In July of 2005, the spacecraft released a small, 820 pound copper impactor directly into the path of comet Tempel 1. With a closing speed of about 22,800 miles per hour, the resulting collision produced an impact crater on the surface of the comet's nucleus. The impact and the 10,000 tons of material thrown out of the forming crater were observed in detail by the flyby spacecraft. The bright plume contained far more dust than scientists expected – one of the many surprises that makes science so exciting.


Scientists found that 96% of sunlight hitting the comet's surface is absorbed, meaning the nucleus is darker than coal. Analysis of the comet's outbursts of gas and dust revealed that the outbursts are not random events; instead, they are caused by active regions rotating into sunlight.


Scientists analyzed the material ejected from the impact, revealing an array of mineral components that formed at both very high temperatures (>1,000K) as well as low ones (<200K), indicating that the early solar nebula has been stirred up.


Deep Impact showed that there is very little compositional variation with depth in the nucleus. Scientists found there is a tremendous amount of layering, some of which is thought to be primordial. Integrating the results from the mission, Principal Investigator Mike A'Hearn developed a new theory of cometary formation whereby low velocity impacts early in the history of the solar system produced "cometessimals" in the Kuiper belt that grew into comets by building up layer upon layer. At low velocities, the material "splats" upon impact. A'Hearn calls the layers talps ("splat" spelled backwards).



This photo of Hartley 2 shows the carbon dioxide, dust, and ice apparently coming from the same area on the nucleus. The water vapor has a different distribution, implying a different source region and process.



The EPOXI mission recycles the already in flight Deep Impact spacecraft for a new mission comprised of two projects with different scientific objectives. EPOCh, or Extrasolar Planet Observation and Characterization, observed stars with known transiting giant planets to characterize those planets and to search for others. DIXI, the Deep Impact Extended Investigation, continues the original Deep Impact theme of studying comets by flying past comet Hartley 2 in November 2010. One goal of the mission is to compare a small active comet with the larger inactive comets previous seen up close by spacecraft to get a better understanding of what features are due to evolution and which are a result of recent processing.


In 2008, EPOCh observed seven stars with "transiting extrasolar planets" and also searched for evidence of rings and moons associated with the known giant planets of the targeted stars. In addition, the spacecraft paved the way for future observations of Earth-size extrasolar planets by taking a special set of observations of our own planet. Researchers used data from EPOCh to create a simulated view of Earth as seen from light years away. The results of this work reveal how an image in which a planet fills only a single pixel could be used to infer the presence of oceans and continents like those on our home world.


The DIXI flyby of comet Hartley 2 captured spectacular images of this small, peanut-shaped active comet. The images show bright plumes of material spewing from the surface and are clear enough for scientists to link jets of dust and gas with specific surface features. The photos reveal a cometary snow storm created by carbon dioxide jets spewing out tons of golf-ball to basketball-sized fluffy ice particles from the comet's rocky ends. Stereo images reveal snowballs in front of and behind the nucleus, making it look like a scene from a snow globe. At the same time, a different process caused water vapor to escape from the comet's smooth mid-section. The smooth area of comet Hartley 2 looks and behaves like most of the surface of comet Tempel 1, with water evaporating below the surface and percolating out through the dust.


Carbon dioxide appears to be a key to understanding Hartley 2. This information sheds new light on the nature of comets and even planets, showing that Hartley 2 acts differently than the four other comet nuclei that have been imaged by spacecraft. The finding that carbon dioxide is powering the many jets could only have been made by traveling to a comet, because ground based telescopes can't detect CO2 and current space telescopes aren't tuned to look for this gas.



This image from the Deep Impact mission shows layering on Tempel 1, which provides clues to accretion during its formation or subsequent processing. The Stardust-NExT spacecraft will obtain new measurements of the comets features.


The Stardust-NExT (New Exploration of Tempel 1) mission recycles the already in flight Stardust spacecraft for a very cool new opportunity to flyby and investigate comet Tempel 1, the same comet that took a hit from the Deep Impact impactor. In February 2011 the Stardust spacecraft will take a new look at Tempel 1, adding much more to the wealth of discoveries scientists made from the Deep Impact experiment.


Tempel 1 has completed a trip around the Sun since Deep Impact visited in 2005, and this close pass through the inner solar system could have caused changes on the comet's surface. The Stardust-NExT mission will capture up-to-date images of the comet's surface, revealing what's new since 2005. Many hope it will finally give us a view of the crater that was created by Deep Impact but not visible at the time because of all the dust that was stirred up. This will be the first time a comet has been visited by two different spacecraft, giving us a much deeper understanding of processes that affect the formation and evolution of these ancient building blocks of the solar system.


Image Impact

Deep Impact

The Deep Impact mission sent a probe into the path of comet Tempel 1 with spectacular results, revealing clues about the comet’s internal composition and structure.


Deep Impact

67 seconds after impact


Deep impact

Ice Patches on Tempel 1
First ice found on a comet’s surface

photo of Mike A’Hearn


Mike A’Hearn Principal Investigator of the Deep Impact and EPOXI missions

Mike is a professor of astronomy at the University of Maryland. His distinguished career
includes many contributions to the field of cometary science, including developing observational techniques to study their structure and composition.

+ Reflections on “Bulls-Eye” and more about the Deep Impact Mission (MP3, 3:22 min.)




photo of Lucy McFadden

Lucy McFadden Co-Investigator on the Deep Impact, EPOXI, and Dawn missions

Lucy is a planetary scientist and the chief of Higher Education and University Programs at NASA’s Goddard Space Flight Center and Co-Investigator on the Dawn mission to asteroids Vesta and Ceres. Previously, she was a research professor at the University of Maryland. Lucy also directed the Education and Public Outreach programs for Discovery’s Deep Impact, EPOXI and Dawn missions.

+ Reflections on “Bulls-Eye” (MP3, 2:25 min.)
+ Reflections on “Ice Patches on Comet Tempel 1”(MP3, 1:22 min.)




Tempel 1

Quiet Before the Storm
Tempel 1 Five Minutes before Impact


Comet Wild 2

Incredible Features on the Nucleus
Comet Wild 2 as viewed by Stardust

photo of Don Yeomans

Don Yeomans Co-investigator on the Deep Impact and EPOXI missions

Don is a Senior Research Scientist at the Jet Propulsion Laboratory and manages NASA's
Near-Earth Object Program, coordinating efforts to detect, track and characterize potentially
hazardous asteroids and comets that could approach the Earth.

+ Reflections on “Quiet Before the Storm” (MP3, 2:15 min.)
+ Reflections on “Incredible Features on the Nucleus” (MP3, 1:16 min.)