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.
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
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 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.
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 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).
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.
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.