Small Worlds

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