JPL BLOGS
Inside Views on Outer Space
Hear from scientists, engineers and other experts as they offer a fascinating look at JPL missions, science and history.
Dawn Journal | August 22, 2018
Dear Sinceres Readers,
People have been gazing in wonder and appreciation at the beauty of the night sky throughout the history of our species. The gleaming jewels in the seemingly infinite black of space ignite passions and stir myriad thoughts and feelings, from the trivial to the profound. Many people have been inspired to learn more, sometimes even devoting their lives to the pursuit of new knowledge. Since Galileo pointed his telescope up four centuries ago and beheld astonishing new sights, more and more celestial gems have been discovered, making us ever richer.
In a practical sense, Dawn brought two of those jewels down to Earth, or at least brought them more securely within the scope of Earthlings' knowledge. Science and technology together have uncloaked and explained aspects of the universe that would otherwise have seemed entirely inscrutable. Vesta and Ceres revealed little of themselves as they were observed with telescopes for more than two centuries. Throughout that time, they beckoned, waiting for a visitor from distant Earth. Finally their cosmic invitations were answered when Dawn arrived to introduce each of them to Earth, whereupon the two planet-like worlds gave up many of their secrets.
Even now, Ceres continues to do so, as it holds Dawn in its firm but gentle gravitational embrace. Every 27 hours, almost once a day, the orbiting explorer plunges from 2,500 miles (4,000 kilometers) high to as low as about 22 miles (35 kilometers) and then shoots back up again. Each time Dawn races over the alien landscapes, it gathers information to add to the detailed story it has been compiling on the dwarf planet.
Dawn began its ambitious mission in 2007. (And on Aug. 17, 2018, it passed a milestone: three Vestan years of being in space.) But the mission is rapidly approaching its conclusion. In the previous Dawn Journal, we began an in-depth discussion of the end, and we continue it here.
We described how the spacecraft will lose the ability to control its orientation, perhaps as soon as September. It will struggle for a short time, but it will be impotent. Unable to point its electricity-generating solar panels at the Sun or its radio antenna to Earth, the seasoned explorer will go silent and will explore no more. Its expedition will be over.
We also took a short look at the long-term fate of the spacecraft. To ensure the integrity of possible future exploration that may focus on the chemistry related to life, planetary protection protocols dictate that Dawn not contact Ceres for at least 20 years. Despite being in an orbit that regularly dips so low, the spaceship will continue to revolve around its gravitational master for at least that long and, with very high confidence, for more than 50 years. The terrestrial materials that compose the probe will not contaminate the alien world before another Earth ship could arrive.
Like its human colleagues, Dawn started out on Earth, but now its permanent residence in the solar system, Ceres, is far, far away. Let's bring this cosmic landscape into perspective.
Imagine Earth reduced to the size of a soccer ball. On this scale, the International Space Station would orbit at an altitude of a bit more than one-quarter of an inch (7 millimeters). The moon would be a billiard ball almost 21 feet (6.4 meters) away. The Sun, the conductor of the solar system orchestra, would be 79 feet (24 meters) across at a distance of 1.6 miles (2.6 kilometers). More remote even than that, when Dawn ceases operating, it would be more than 5.5 miles (9.0 kilometers) from the soccer ball. The ship will stay locked in orbit around Ceres, the only dwarf planet in the inner solar system. The largest object between Mars and Jupiter, that distant orb would be five-eighths of an inch (1.6 centimeters) across, about the size of a grape. Of course, a grape has a higher water content than Ceres, but exploring this fascinating world of ice, rock and salt has been so much sweeter!
Now let's take a less terrestrial viewpoint and shift our reference to Ceres. Suppose it were the size of a soccer ball. In Dawn's final, elliptical orbit, which it entered in June, the spacecraft would travel only 37 inches (94 centimeters) away at its farthest point. Then it would go in to skim a mere one-third of an inch (8 millimeters) from the ball.
Dawn is one mission among many to explore the solar system, dating back almost 60 years and (we hope) continuing and even accelerating for much longer into the future. Learning about the cosmos is not a competition but rather a collective effort of humankind to advance our understanding. And to clarify one of the many popular mistaken notions about the solar system, let's take advantage of reducing Ceres to the size of a soccer ball to put some other bodies in perspective.
Because it is in the main asteroid belt, there is a common misconception that Ceres is just another asteroid, somehow like the ones visited by other spacecraft. It is not. The dwarf planet is distinctly unlike the small chunks of rock that are more typical asteroids. We have discussed various aspects of Ceres' complex geology, and much more remains to be gleaned from Dawn's data. Vesta too has a rich and complicated geology, and it is more akin to the terrestrial planets (including Earth) than to asteroids. But for now, let's focus simply on the size in order to make for an easy comparison. Of course, size is not a measure of interest or importance, but it will illustrate how dramatically different these objects are.
With a soccer-ball-sized Ceres, Vesta would be nearly five inches (more than 12 centimeters) in diameter. (This writer's comprehensive knowledge of sports inspires him to describe this as a ball nearly five inches, or more than 12 centimeters, in diameter.)
What about some of the asteroids being explored as Dawn's mission winds to an end? There are two wonderfully exciting missions with major events at asteroids (albeit ones much closer to Earth than the main asteroid belt) in the second half of 2018. Your correspondent, a lifelong space enthusiast, is as hopeful for success as anyone! Hayabusa2 is revealing Ryugu and OSIRIS-REx is on the verge of unveiling Bennu.
Ryugu and Bennu are more irregular in shape than Ceres and Vesta, but they would both be so small compared to the soccer ball that their specific shapes wouldn't matter. Ryugu would be less than a hundredth of an inch (a quarter of a millimeter) across. Bennu would be about half that size. They would be like two grains of sand compared to the soccer ball. In the first picture of the June Dawn Journal, we remarked on the detail visible in a feature photographed on one of Dawn's low streaks over the alien terrain. It is also visible in the first two pictures above. That one structure on Ceres is only a part of Cerealia Facula, which is the bright center of the much larger Occator Crater. Occator is a good-sized crater, but not even among the 10 largest on Ceres. Yet that one bright feature in the high-resolution photo is larger than either of these small asteroids. In many of Dawn's pictures that show the entire disk of the dwarf planet (like the rotation movie and the color picture here), Ryugu and Bennu would be less than a pixel, undetectably small, just as invisible specks of dust on a soccer ball.
The tremendous difference in size between Ceres (and Vesta) and small asteroids illustrates a widely unappreciated diversity in the solar system. Of course, that is part of the motivation for continuing to explore. There is a great deal yet to be learned!
Although Ryugu and Bennu aren't in the main asteroid belt, the belt contains many more Lilliputian asteroids closer in size to them than to the Brobdingnagian Ceres and Vesta. In fact, of the millions of objects in the main asteroid belt, Ceres by itself contains 35 percent of the total mass. Vesta has 10 percent of the total.
Readers with perfect memories may note that we used slightly smaller fractions in earlier Dawn Journals. Science advances! More recent estimates of the mass of the asteroid belt are slightly lower, so these percentages are now correspondingly higher. The difference is not significant, but the small increase only emphasizes how different Vesta and Ceres are from typical residents of the asteroid belt. It's also noteworthy -- or, at least, pretty cool -- that Dawn has single-handedly explored 45 percent of the mass between Mars and Jupiter.
Dawn will end its mission in the same orbit it is in now, looping around from a fraction of an inch (fraction of a centimeter) to a yard (a meter) from the soccer-ball-sized Ceres. In the previous Dawn Journal, we described what will happen onboard the spacecraft. We also saw that the most likely indication controllers will have that Dawn has run out of hydrazine will be its radio silence. They will take some carefully considered steps to verify that that is the correct conclusion.
But it is certain that emotions will be ahead of rationality. Even as team members are narrowing down the causes for the disappearance of the radio signal, many strong feelings about the end of the mission will arise. And they will be as varied as the people on the Dawn team, every one of whom has worked long and hard to make the mission so successful. Your correspondent can make reasonable guesses about their feelings but won't be so presumptuous as to do so.
As for my own feelings, well, I won't know until it happens, but I'm not too presumptuous to guess now. Long-time readers may recognize that your correspondent has avoided writing anything about himself (with a few rare exceptions), or even using first person, in his Dawn Journals. They are meant to be a record of a mission undertaken by humankind, for everyone who longs for knowledge and for adventures in the cosmos. But now I will devote a few words to my own perspective.
My love affair with the universe began when I was four, and my passion has burned brighter and brighter ever since. I knew when I was a starry-eyed nine-year-old that I wanted to get a Ph.D. in physics and work for NASA, although it was a few more years before I did. I had my own Galileo moment of discovery and awe when I first turned a telescope to the sky. Science and space exploration are part of me. They make me who I am. (My friend Mat Kaplan at The Planetary Society described me in the beginning of this video as "the ultimate space nerd." He's too kind!) Adding to my own understanding and contributing to humankind's knowledge are among my greatest rewards.
Passion and dedication are not the whole story. I recognize how incredibly lucky I am to be doing what I have loved for so long. I am lucky to have had access to the resources I have needed. I am lucky that I was able to do well in my formal education and in my own informal (but extensive) studies. I am lucky I could find the discipline and motivation within myself. For that matter, I am lucky to be able to communicate in terms that appeal to you, dear readers (or, at least, to some of you). My innate abilities and capabilities, and even many acquired ones, are, to a large extent, the product of factors out of my control, like my cognitive and psychological constitution.
That luck has paid off throughout my time at JPL. Working there has been a dream come true for me. It is so cool! I often have what amount to out-of-body experiences. When I am discussing a scientific or engineering point, or when I am explaining a conclusion or decision, sometimes a part of me pulls back and looks at the whole scene. Gosh! Listen to the cool things I get to say! Look at the cool things I get to do! Look at the cool things I know and understand! Imagine the cool spacecraft I'm working with and the cool world it is orbiting! I am still that starry-eyed kid, yet somehow, through luck and coincidence, I am doing the kind of things I love and once could only have dreamed of.
Dawn will continue to be exciting to the very end, performing new and valuable observations as it skims incredibly low over the dwarf planet on every orbital revolution. The spacecraft has almost always either been collecting new data or, thanks to the amazing ion propulsion, flying on a blue beam of xenon ions to somewhere else to gain a new perspective, see new sights and make more discoveries. Whether in orbit around Vesta or Ceres or traveling through the solar system between worlds, the mission was rarely anything like routine.
I love working on Dawn (although it was not my first space love). You won't be surprised that I think it is really cool. I could not be happier with its successes. I am not sad it is ending. I am thrilled beyond belief that it achieved so much!
I was very saddened in graduate school when my grandfather died. When I said something about it in my lab to a scientist from Shanghai I was working with, he asked how old my grandfather was. When I said he was 85, the wiser gentleman's smile lit up and he said, "Oh, you should be happy." And immediately I was! Of course I should be happy -- my grandfather had lived a long (and happy) life.
And so has Dawn. It has overcome problems not even imagined when we were designing and building it. It not only exceeded all of its original goals, but it has accomplished ambitious objectives not even conceived of until after it had experienced what could have been mission-ending failures. It has carried me, and uncounted others (including, I hope, you), on a truly amazing and exciting deep-space adventure with spectacular discoveries. Dawn is an extraordinary success by any measure.
It did not come easily. Dawn has consumed a tremendous amount of my life energy, many times at the expense of other desires and interests. (Perhaps ironically, it even comes at the expense of my many other deep interests in space exploration and in science, such as cosmology and particle physics, interests shared by my cats Quark and Lepton. Also, writing these Dawn Journals and doing my other outreach activities take up a very large fraction of what would otherwise be my personal time. As a result, I always write these in haste, and I'm never satisfied with them. That applies to this one as well. But I must rush ahead.) The challenges and the demands have been enormous, sometimes feeling insurmountable. That would not have been my preference, of course, yet it makes the endeavor's successful outcome that much more gratifying.
At the same time I have felt all the pressure, I have long been so overjoyed with the nature of the mission, I will miss it. There is nothing quite like controlling a spacecraft well over a thousand times farther than the Moon, farther even than the Sun. Silly, trite, perhaps even mawkish though it may seem, when spacecraft I have been responsible for have passed on the far side of the Sun, I have taken those opportunities to use that blinding signpost to experience some of the awe of the missions. I block the Sun with my hand and contemplate the significance, both to this particular big, starry-eyed kid and to humankind, of such an alignment. I -- we -- have a spacecraft on the far side of the Sun!
Every day I feel exhilarated knowing that, as my car's license plate frame proclaims, my other vehicle is in the main asteroid belt. It won't be the same when that vehicle is no longer operating.
But I will always have the memories, the thrills, the deep and powerful personal gratification. And I have good reason to believe they will persist, just as some prior space experiences still fill me with gratitude, pride, excitement and pure joy. (I also hope to have many more cool out-of-body experiences.)
And long after I'm gone and forgotten, Dawn’s successes will still be important. Its place in the annals of space exploration will be secure: a wealth of marvelous scientific discoveries, the first spacecraft to orbit an object in the asteroid belt, the first spacecraft to visit a dwarf planet (indeed, the first spacecraft to visit the first dwarf planet that was discovered), the first spacecraft to orbit a dwarf planet, the first spacecraft to orbit any two extraterrestrial destinations, and more.
For now, Dawn is continuing to operate beautifully (and you can read about it in subsequent Dawn Journals). The end of the mission, when it comes, will be bittersweet for me, a time to reflect and rejoice at how fantastically well it has gone, and a time to grieve that it is no more. I will have many powerful and conflicting feelings. Like Walt Whitman, I am large, I contain multitudes.
Thanks to Dawn, we now have Vesta and we now have Ceres. Soon, very soon, Dawn will be only a memory (save for those who visit Ceres and find it still in orbit) but the worlds it revealed will forever be a part of our intellectual universe, and the capabilities to explore the solar system that it advanced and devised will be applied to exciting new missions. And the experience of being intimately involved in this grand adventure will remain with me for as long as I am able to see the night sky and marvel at the mysteries of the universe that captivated me even as a starry-eyed child.
Dawn is 1,500 miles (2,400 kilometers) from Ceres. It is also 3.46 AU (322 million miles, or 518 million kilometers) from Earth, or 1,275 times as far as the Moon and 3.42 times as far as the Sun today. Radio signals, traveling at the universal limit of the speed of light, take 58 minutes to make the round trip.
Dr. Marc D. Rayman
10:00 pm PDT August 22, 2018
TAGS: DAWN, CERES, VESTA, DWARF PLANET, ASTEROID BELT, ASTEROIDS, SPACECRAFT, SOLAR SYSTEM
Rocks and Stars with Amy | November 15, 2010
This Asteroid Inspected by #32
Over the course of the nine months we've been operating WISE, we've observed over 150,000 asteroids and comets of all different types. We had to pick all of these moving objects out of the hundreds of millions of sources observed all over the sky -- so you can imagine that sifting through all those stars and galaxies to find the asteroids is not easy!
We use a lot of techniques to figure out how to distinguish an asteroid from a star or galaxy. Even though just about everything in the universe moves, asteroids are a whole lot closer to us than your average star (and certainly your average galaxy), so they appear to move from place to place in the WISE images over a timescale of minutes, unlike the much more distant stars. It's almost like watching a pack of cyclists go by in the Tour de France. Also, WISE takes infrared images, which means that cooler objects like asteroids look different than the hotter stars. If you look at the picture below, you can see that the stars appear bright blue, whereas the sole asteroid in the frame appears red. That's because the asteroid is about room temperature and is therefore much colder than the stars, which are thousands of degrees. Cooler objects will give off more of their light at longer, infrared wavelengths that our WISE telescope sees. We can use both of these unique properties of asteroids -- their motion and their bright infrared signatures -- to tease them out of the bazillions of stars and galaxies in the WISE images.
Thanks to the efforts of some smart scientists and software engineers, we have a very slick program that automatically searches the images for anything that moves at the longer, infrared wavelengths. With WISE, we take about a dozen or so images of each part of the sky over a couple of days. The system works by throwing out everything that appears again and again in each exposure. What's left are just the so-called transient sources, the things that don't stay the same between snapshots. Most of these are cosmic rays -- charged particles zooming through space that are either spat out by our sun or burped up from other high-energy processes like supernovae or stars falling into black holes. These cosmic rays hit our detectors, leaving a blip that appears for just a single exposure. Also, really bright objects can leave an after-image on the detectors that can persist for many minutes, just like when you stare at a light bulb and then close your eyes. We have to weed the real asteroid detections out from the cosmic rays and after-images.
The data pipeline is smart enough to catch most of these artifacts and figure out what the real moving objects are. However, if it's a new asteroid that no one has ever seen before, we have to have a human inspect the set of images and make sure that it's not just a collection of artifacts that happened to show up at the right place and right time. About 20 percent of the asteroids that we observe appear to be new, and we examine those using a program that we call our quality assurance (QA) system, which lets us rapidly sift through hundreds of candidate asteroids to make sure they're real. The QA system pops up a set of images of the candidate asteroid, along with a bunch of "before" and "after" images of the same part of the sky. This lets us eliminate any stars that might have been confused for the asteroids. Finally, since the WISE camera takes a picture every 11 seconds, we take a look at the exposures taken immediately before the ones with the candidate asteroid -- if the source is really just an after-image persisting after we've looked at something bright, it will be there in the previous frame. We've had many students -- three college students and two very talented high school students -- work on asteroid QA. They've become real pros at inspecting asteroid candidates!
Meanwhile, the hunt continues -- we're still trekking along through the sky with the two shortest-wavelength infrared bands, now that we've run out of the super-cold hydrogen that was keeping two of the four detectors operating. Even though our sensitivity is lower, we're still observing asteroids and looking for interesting things like nearby brown dwarfs (stars too cold to shine in visible light because they can't sustain nuclear fusion). Our dedicated team of asteroid inspectors keeps plugging away, keeping the quality of the detections very high so that we leave the best possible legacy when our little telescope's journey is finally done.
TAGS: UNIVERSE, SOLAR SYSTEM, ASTEROIDS, WISE
Rocks and Stars with Amy | July 20, 2010
Milestones
It's hard to believe that we've just crossed the six-month mark on WISE -- seems like just yesterday when we were all up at Vandenberg Air Force Base, near Santa Barbara, shivering in the cold at night while watching the countdown clock. But the time is flying (literally!) as WISE whips by over our heads. We're analyzing data ferociously now, trying to get the images and the data ready for the public release next May. Even though the mission's lifetime is short, we've gotten into a semblance of a routine. We receive and process images of stars, galaxies and other objects taken by the spacecraft every day, and we're running our asteroid-hunting routine on Mondays and Thursdays. We've got a small army (well, okay, three -- but they do the work of a small army!) of extremely talented students who are helping us verify and validate the asteroid detections, as well as hunt for new comets in the data. Plus, there is an unseen, yet powerful, cadre of observers out there all over the world following up our observations.
And so it's come to pass that we've achieved some milestones. We completed our first survey of the entire sky on July 17 -- and we just discovered our 100th new near-Earth object! That's out of the approximately 25,000 new asteroids we've discovered in total so far; most of these hang out in the main belt between Mars and Jupiter and never get anywhere near Earth's orbit. These new discoveries will allow us to conduct an accurate census of both the near-Earth and main belt asteroid populations. We're really busy chewing on the data right now and calculating what it all means.
Because it's so short, this mission reminds me a little bit of what the first days of college felt like -- a tidal wave of new ideas, new sights and new thoughts. The pace of learning has been incredibly quick, whether I'm trying to get up to speed on asteroid evolution theories or tinkering with the software we use to write papers.
Speaking of papers, we're in the process of preparing to submit several to science journals; in fact, I've already submitted one. The gold standard of science, of course, is the peer-review process. We submit our paper to a journal, and the scientific editor assigns another scientist who is an expert in the field but not involved in the project (and who usually remains anonymous) to read it and offer comments. The referee's job is to "kick the tires," so to speak, and ask tough questions about the work to make sure it's sound. We get a chance to respond, and the referee gets a chance to respond to our responses, and then when everybody's convinced the results are right, the paper is accepted and can be published. So stay tuned -- we should have some of the first papers done soon telling us what these milestones mean for asteroid science.
› Read more from "Rocks and Stars with Amy"
TAGS: UNIVERSE, SOLAR SYSTEM, ASTEROIDS, WISE
JPL | November 10, 2009
2012 - A Scientific Reality Check
There apparently is a great deal of interest in celestial bodies, and their locations and trajectories at the end of the calendar year 2012. Now, I for one love a good book or movie as much as the next guy. But the stuff flying around through cyberspace, TV and the movies is not based on science. There is even a fake NASA news release out there… So here is the scientific reality on the celestial happenings in the year 2012.
Nibiru, a purported large object headed toward Earth, simply put - does not exist. There is no credible evidence - telescopic or otherwise - for this object's existence. There is also no evidence of any kind for its gravitational effects upon bodies in our solar system.
I do however like the name Nibiru. If I ever get a pet goldflish (and I just may do that sometime in early 2013), Nibiru will be at the top of my list.
The Mayan calendar does not end in December 2012. Just as the calendar you have on your kitchen wall does not cease to exist after December 31, the Mayan calendar does not cease to exist on December 21, 2012. This date is the end of the Mayan long-count period, but then – just as your calendar begins again on January 1 - another long-count period begins for the Mayan calendar.
There are no credible predictions for worrisome astronomical events in 2012. The activity of the sun is cyclical with a period of roughly 11 years and the time of the next solar maximum is predicted to occur about May 2013. However, the Earth routinely experiences these periods of increased solar activity – for eons - without worrisome effects. The Earth’s magnetic field, which deflects charged particles from the sun, does reverse polarity on time scales of about 400,000 years but there is no evidence that a reversal, which takes thousands of years to occur, will begin in 2012. Even if this several thousand year-long magnetic field reversal were to begin, that would not affect the Earth’s rotation nor would it affect the direction of the Earth’s rotation axis… only Superman can do that.
The only important gravitational tugs experienced by the Earth are due to the moon and sun. There are no planetary alignments in the next few decades, Earth will not cross the galactic plane in 2012, and even if these alignments were to occur, their effects on the Earth would be negligible. Each December the Earth and Sun align with the approximate center of the Milky Way Galaxy but that is an annual event of no consequence.
The predictions of doomsday or dramatic changes on December 21, 2012 are all false. Incorrect doomsday predictions have taken place several times in each of the past several centuries. Readers should bear in mind what Carl Sagan noted several years ago; "extraordinary claims require extraordinary evidence."
For any claims of disaster or dramatic changes in 2012, the burden of proof is on the people making these claims. Where is the science? Where is the evidence? There is none, and all the passionate, persistent and profitable assertions, whether they are made in books, movies, documentaries or over the Internet, cannot change that simple fact. There is no credible evidence for any of the assertions made in support of unusual events taking place in December 2012.
For more information on the silliness surrounding December 2012, see:
- http://www.badastronomy.com/bad/misc/planetx/nutshell.html
- Wikipedia: look under “Nibiru collision.”
- http://www.nasa.gov/topics/earth/features/2012.html
TAGS: SOLAR SYSTEM, COMETS, ASTEROIDS, STARDUST, NEAR-EARTH, NIBIRU
JPL | August 17, 2009
Comets and Life On Earth
With the recent discovery of the amino acid glycine in the comet dust samples returned to Earth by the Stardust spacecraft, it is becoming a bit more clear how life may have originated on Earth. Water is a well-known ingredient in both comets and living organisms, and now it appears that amino acids are also common to comets and living organisms. Amino acids are used to make proteins, which are chains of amino acids, and proteins are vital in maintaining the cell structures of plants and animals.
Amino acids had previously been identified in meteorite samples, and these samples are thought to be the surviving fragments from asteroid collisions with the Earth. So now it appears that both comets and asteroids in the Earth's neighborhood, the so-called near-Earth objects, delivered some of the building blocks of life to the early Earth.
Impacts of comets and asteroids with the early Earth likely laid down the veneer of carbon-based molecules and water that allowed life to form. Once life did form, subsequent collisions of these near-Earth objects frustrated the evolution of all but the most adaptable species. The dinosaurs checked out some 65 million years ago because of an impact by a six mile-wide comet or asteroid off the coast of the Yucatan peninsula. Fortunately, the small, furry mammalian creatures at the time were far more adaptable and survived this impact event. Thus, present day mammals like us may owe our origin and current position atop Earth's food chain to these near-Earth objects, one of which took out our dinosaur competitors some 65 million years ago.
Today, most of the attention directed toward near-Earth objects has to do with the potential future threat they can pose to life on Earth. However, the recent Stardust discovery of a cometary amino acid reminds us that, were it not for past impacts by these objects, the Earth may not have received the necessary building blocks of life, and humans may not have evolved to our current preeminent position on Earth. While giving thanks to these near-Earth objects, we still need to make sure we find the potentially hazardous comets and asteroids early enough so we don't go the way of the dinosaurs.
For more information on near-Earth objects, see: http://www.jpl.nasa.gov/asteroidwatch/index.cfm
TAGS: SOLAR SYSTEM, COMETS, ASTEROIDS, STARDUST, NEAR-EARTH