Why Go to Pluto?
Planetary exploration is a historic endeavor and a major focus of NASA. New Horizons is designed to help us understand worlds at the edge of our solar system by making the first reconnaissance of Pluto and Charon - a "double planet" and the last planet in our solar system to be visited by spacecraft. Then, as part of an extended mission, New Horizons would visit one or more objects in the Kuiper Belt region beyond Neptune.
Science at the Frontier
Our solar system contains three zones: the inner, rocky planets; the gas giant planets; and the Kuiper Belt. Pluto is the largest body of the icy, "third zone" of our solar system. The National Academy of Sciences placed the exploration of the third zone in general - and Pluto-Charon in particular - among its highest priority planetary mission rankings for this decade. New Horizons is NASA's mission to fulfill this objective.
In those zones, our solar system has three classes of planets: the rocky worlds (Earth, Venus, Mercury and Mars); the gas giants (Jupiter, Saturn, Uranus and Neptune); and the ice dwarfs of the Kuiper Belt. There are far more ice dwarf planets than rocky and gas giant worlds combined - yet, no spacecraft has been sent to a planet in this class. The National Academy of Sciences noted that our knowledge of planetary types is therefore seriously incomplete. As the first mission to investigate this new class of planetary bodies, New Horizons seeks to fill this important gap and round out our knowledge of the planets in our solar system.
Ancient Relics
The ice dwarfs are planetary embryos, whose growth stopped at sizes (200 to 2,000 kilometers across) much smaller than the full-grown planets in the inner solar system and the gas giants region. The ice dwarfs are ancient relics that formed over 4 billion years ago. Because they are literally the bodies out of which the larger planets accumulated, the ice dwarfs have a great deal to teach us about planetary formation. New Horizons seeks those answers.
Binary Planet
Pluto's moon, Charon, is half the size of Pluto. The pair form a binary-planet, whose gravitational balance point is between the two bodies. Although binary planets are thought to be common in the galaxy, as are binary stars, no spacecraft has yet explored one. New Horizons will be the first mission to a binary object of any type.
A Mission with Impact
The Kuiper Belt is the major source of cometary impactors on Earth, like the impactor that wiped out the dinosaurs. New Horizons will shed new light on the number of such Kuiper Belt impactors as a function of their size by cataloging the various-sized craters on Pluto, Charon, and on Kuiper Belt Objects.
Pluto and the Kuiper Belt are known to be heavily endowed with organic (carbon-bearing) molecules and water ice — the raw materials out of which life evolves. New Horizons will explore the composition of this material on the surfaces of Pluto, Charon and Kuiper Belt Objects.
The Great Escape
Pluto's atmosphere is escaping to space like a comet, but on a planetary scale. Nothing like this exists anywhere else in the solar system. It is thought that the Earth's original hydrogen/helium atmosphere was lost to space this way. By studying Pluto's atmospheric escape, we can learn a great deal about the evolution of Earth's atmosphere. New Horizons will determine Pluto's atmospheric structure and composition and directly measure its escape rate for the first time.
The Need to Explore
As the first voyage to a whole new class of planets in the farthest zone of the solar system, New Horizons is a historic mission of exploration. The United States has made history by being the first nation to reach every planet from Mercury to Neptune with a space probe. The New Horizons mission to Pluto and the Kuiper Belt - the first NASA launch to a "new" planet since Voyager nearly 30 years ago - allows the U.S. to complete the reconnaissance of the solar system.作者: 跨越地平线 时间: 2006-3-4 21:03 标题: part2 The PI's Perspective
Last week, details of discoveries and early interpretation of Pluto's two small moons, formally called S/2005 P1 and S/2005 P2, were published in a pair of papers (Weaver et al. 2006 and Stern et al. 2006) in the scientific journal Nature. Nature is very much the Rolling Stone of the scientific community, and the discovery and interpretation of "P1" and "P2" won the cover of Nature. This is probably as close as the nine of us will ever get to making the cover of Rolling Stone. Although this was my 10th scientific publication in Nature, it was a real rush of accomplishment that vindicated long years of searching the Pluto system for companions to Pluto and Charon.
In our two back-to-back papers, we described the orbits and sizes of our two newly discovered moons, discussed the unique architecture of the Pluto satellite system, predicted that similarly complex satellite systems will be found routinely in the Kuiper Belt, predicted that P1 and P2 probably generate ephemeral rings around Pluto, and argued that P1's and P2's orbits argue strongly that they were born in the cataclysmic collision of a large Kuiper Belt object into Pluto that created Charon, billions of years ago. In fact, we believe the presence of P1 and P2 in Charon's orbital plane is very much the discovery that checkmates the 20-year-old hypothesis that Charon was born because of a giant impact onto Pluto.
On the same day (Feb. 23) that Nature published our papers and an accompanying "News and Views" piece by New Horizons co-investigator Rick Binzel, the nine of us on the discovery team also published a scientific bulletin called an IAU Circular. This brief communication, edited by Max Mutchler and Andrew Steffl, revealed the results of brand-new Hubble Space Telescope (HST) observations of P1 and P2, made just days before on Feb. 15. The new Hubble images confirm the discovery (see above for an image from this run on HST); the new imagery also shows that our published orbital predictions, made in the fall, were almost bang on. We expect to get one more HST observation on March 2, from which we hope to further refine the orbits of P1 and P2 and obtain the first high-quality color measurements of P1 and P2.
You may have also heard that we're working on official names for P1 and P2. We hope to submit those to the International Astronomical Union (IAU) for formal approval this spring. In the meantime, we're referring to the pair as "Boulder" and "Baltimore," in honor of the hometowns of eight of the nine people on the discovery team (and, we note, the respective locations where HST's instruments were built and where HST's scientific institute is located). S/2005 P1, which is the larger moon, is the one we call "Baltimore." S/2005 P2, being smaller, is the one we call "Boulder."
And what about New Horizons? Well, it's halfway to the orbit of Mars now, and the flight mission is continuing smoothly. Last week, we conducted the "Launch Plus 35 Day" review of the engineering and operational aspects of the mission. In this formal daylong review, the engineering leads and the operations team presented the status and lessons learned from the first five weeks of flight to a review team consisting of experienced spacecraft engineers and project managers.
Also last week, we conducted the first testing of instruments in our scientific payload. In total, three instruments were tested last week: ALICE, PEPSSI, and LORRI. (And there is no truth, dear reader, to the rumor that we chose these three to begin with because they spell, A-P-L.)
Although "first light" for each of these three instruments is still in the future, the early tests we performed last week proved that all three instruments survived launch and have good power and command interfaces to the spacecraft. Additionally, each instrument put their microprocessors through various paces, and ALICE unlatched and successfully tested her front door by opening it to space. All of this testing went well and we're very happy with the engineering data returned to Earth by all three instruments.
This week, SWAP and SDC will be turned on and tested similarly to the work done last week with ALICE, PEPSSI, and LORRI. In fact, SDC will even begin collecting data, as will PEPSSI. Starting in March, we plan to use SDC, PEPSSI, and SWAP a great deal during the flight to Pluto in order to trace out conditions in the interplanetary environment across the space of 5 billion-plus kilometers from here to the Kuiper Belt.
In March, we will continue instrument commissioning with increasingly complex testing of our optical and plasma instruments. Additionally, both copies of our radio science instrument, REX, will receive their initial checkouts in mid-April.
Also in March, we'll undertake four very important activities with the New Horizons spacecraft itself. One I've discussed before is a course correction called TCM-3. This roughly 1.2 meter/second trajectory correction maneuver will trim up our course to the Pluto "keyhole" at Jupiter even more precisely than TCM-1A and 1B did. TCM-3 is planned for Thursday, March 9. The other major activities for March are an upload of a few post-launch fixes to our Command and Data Handling (C&DH) software, the checkout of our High Gain Antenna (HGA) and the installation of something called CLTSN.
CLTSN stands for Command Loss Timer Safety Net. It's a new feature of the spacecraft's autonomous fault detection and protection system designed to act as a backup ("last ditch") recovery of the mission if the spacecraft determines it has failed to hear from the ground controllers for too long a time (about 135 days). If this unlikely happenstance ever occurs, autonomy's CLTSN switches the entire spacecraft avionics chain to the backup side, turns the spacecraft back to a good communications attitude with the HGA dish pointed toward Earth, sets the downlink beacon to "Red 6" and fires up the receivers to await new instructions from Earth.
CLTSN, which we colloquially call the "catcher's mitt," is a new layer of autonomous spacecraft recovery smarts designed to take over if the normal autonomous fault detection and recovery procedures have failed to recover the mission. This is something I insisted on seeing added to the mission before I signed off on launch. I hope we never have to use CLTSN, because it'll mean we're down to the last play in our playbook. But I do think it's an important new capability. After all, without CLTSN, we wouldn't have a last-ditch recovery to take over if some "unknown, unknown" prevented the normal recovery processes from working as expected.
Well, you can see the next few weeks are going to be busy for New Horizons. Pluto, Charon, Boulder, and Baltimore lie ahead.作者: 跨越地平线 时间: 2006-3-4 21:06 标题: part-问答部分 Frequently Asked Questions
TOP 10
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Is Pluto a planet?
There is no firm definition of a planet. The debate about whether Pluto is a planet was generated by recent detections of hundreds of planetary objects in the outer solar system. Officially, the International Astronomical Union classifies Pluto as a planet. Most people call Pluto a planet because it orbits the Sun and it is large enough that its own gravity has pulled it into a spherical shape. Click here for more discussion on this issue.
Yes. Pluto's moon, named Charon, is half the size of Pluto. Side by side Pluto and Charon would fit across the diameter of our Moon. It is possible that Pluto has more, smaller moons that we have not yet detected. Click here for more information on Charon.
The surface of Pluto is extremely cold, roughly 40 degrees above absolute zero (minus 387 Fahrenheit or minus 233 Celsius), so it seems unlikely that life could exist there. At such cold temperatures, water, which is vital for life as we know it, is essentially rock-like. Pluto's interior is warmer, however, and some think there could even be an ocean deep inside.
Visit the Science section for more discussion on the possibility of life on Pluto.
Yes, though Pluto's atmosphere is not very thick. The pressure at the surface of Pluto is about 3 to 100 microbars or 3 to 100 millionths of the surface pressure of the Earth's atmosphere. The main constituent is molecular nitrogen, (N 2) the same as on Earth. Molecules of methane and carbon monoxide have also been detected at Pluto. But no oxygen has been detected at Pluto yet.
Click here for further discussion on Pluto's atmosphere.
A typical scene on Pluto would probably be an icy, frosty, dimly lit landscape. It might be similar to a view on Earth in the winter in the arctic, lit by a full moon. Go here for further discussion of views from Pluto.
Pluto is the only planet in our solar system, unexplored by space probes. A mission to Pluto-Charon and the Kuiper Belt will explore the mysterious, icy worlds at the edge of our solar system and tell us about the origin and evolution of our planetary neighbors.
Check out the science overview section for more on New Horizons' mission objectives.
Traveling to Pluto using the minimum amount of fuel would take longer than 30 years. NASA's Voyager mission demonstrated the advantages of using the gravity of the giant planets, particularly Jupiter, to "boost" a spacecraft and reduce travel times to the outer solar system. If New Horizons is launched in January 2006 and uses a flyby of Jupiter in spring 2007, it should arrive at Pluto in 2015. The shortest journey would take 9½ years.
Click here to see the New Horizons mission timeline.
Pluto's gravity is weak so that it takes a large amount of fuel to go into orbit around the planet- and with New Horizons expected to zip past Pluto at nearly 14 kilometers per second (more than 30,000 miles per hour), there is no practical way to store the tremendous amount of fuel the spacecraft would need to slow down enough to begin an orbit mission. A flyby mission provides many images and other kinds of information about Pluto and Charon, as well as an opportunity to fly on to another Kuiper Belt Object.
The cost of the mission, including the launch vehicle and operations through the Pluto-Charon encounter, will be roughly $650 million. Divided among the population of the United States (according to the U.S. Census clock at http://www.census.gov/main/www/popclock.html) over the 10-year duration of the mission, this comes out to about 20 cents per person, per year.
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FUN FACTS
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Is there enough light to read a book on Pluto?
Yes. During daylight on Pluto, the Sun would be almost 300 times as bright as the full Moon on Earth (1/900 times dimmer than full daylight on Earth).
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What does the Sun or Earth look like from Pluto?
The Sun would still be the brightest object in the sky, by far. Although a mere fraction of the brightness of sunlight on Earth, the Sun seen from Pluto would still be about 20 million times brighter than the brightest star.
Pluto is 14th magnitude as seen from Earth - meaning it's hundreds of times fainter than the naked eye can see. Earth has 36 times the surface area, 1,000 times the illumination and a similar reflectivity as Pluto, so, Earth should be about 3rd magnitude (as bright as an easily visible star) as seen from Pluto. The biggest problem with seeing Earth from Pluto is that it would be located close to the Sun, but it might be briefly visible to the naked eye when Charon eclipses the Sun.
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What would you weigh on Pluto or Charon?
Just below 7% of your Earthly weight on Pluto, and just over 3% of your terrestrial weight on Charon. To be a little more accurate, every 100 pounds of weight on your bathroom scale on Earth would weigh just 6.7 pounds on Pluto and 3.4 pounds on Charon.
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Does a human have enough energy to jump into orbit around Pluto?
No. To escape Charon's gravity you need to get up a lot of speed. The escape velocity from Charon is near 1,351 miles per hour (610 meters per second). Not even the fastest runner or the strongest person could reach those speeds on Charon.
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Would I be able to ski on Pluto?
Yes, in a fashion. Let's look at some of the key questions . . .
We're not sure, but there are bound to be at least a few craters with slopes that are hundreds of meters high. Neptune 's satellite Triton is about Pluto's size and in roughly the same region of the solar system, and the Voyager spacecraft found hundreds of craters on Triton.
On Earth, a skier heading straight down a 30-degree slope will accelerate to 11 miles per hour in one second if his wax is really good and he's got a good tuck. Gravity on Pluto is about 15 times less than on the Earth, so the same skier on Pluto would "tear" down the same hill at 0.7 mph after one second.
Pluto's surface is mainly covered with nitrogen ice at about minus 387 Fahrenheit (or minus 233 Celsius). It's not known whether solid nitrogen ice is as slippery as water ice, but a skier/boarder on Pluto may want to take advantage of another property of nitrogen ice - its vapor pressure.
All ices are slowly sublimating, which means that some of their surface molecules escape into the atmosphere. You've seen puddles of water evaporate on a sunny day, and sublimation is the same, except the molecules jump directly from the solid ice to the atmosphere. Carbon dioxide ice (dry ice) sublimates when you have a chunk of it in a room, for example.
The sublimation rate increases rapidly when the ice temperature gets little warmer. This is very important on Pluto, where the entire atmosphere depends on the average temperature of the nitrogen ice. A small increase in the nitrogen ice temperature of 2 degrees Fahrenheit will double Pluto's atmosphere!
With heated skis, our Pluto downhiller skier would float on a cushion of nitrogen gas. Since the nitrogen condenses right onto the surface (as opposed to falling as snow), the surface is probably hard and icy, not champagne powder.
The idea that we slide on water ice (either on skates or skis) because the pressure of the skate or ski causes melting is a myth - see the San Francisco Exploratorium's site on the science of hockey for more on that topic. We will have to wait for laboratory data before we can discuss the slipperiness of nitrogen ice at temperatures typical of Pluto's surface.
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What would a human see on Pluto?
An astronaut (Plutonaut?) stepping from their spaceship onto Pluto's surface would quickly notice many unusual qualities of this alien environment. Perhaps the first impression would be the overall sense of darkness. The Sun, just a bright pinpoint in the sky, provides only a thousandth as much illumination to Pluto's surface as it does to Earth's, making daytime on the distant planet much darker than a cloudy, stormy day here at home. But Pluto's sky is strikingly clear, and in addition to the Sun, thousands of stars are visible, even in daytime. There are no clouds and Pluto's air is far too thin to cause the sky to be bright and blue, as it is on Earth. Depending on which part of the planet the astronaut landed, they might see Pluto's moon, Charon, looming in the sky some seven times larger than our own Moon appears in Earth's sky. Charon is smaller than Earth's Moon, but it is much closer to the planet, making it appear far larger. Remarkably, from a given location on Pluto, Charon remains motionless in the sky, going through its cycle of phases in 6.4 days without rising or setting.
If the astronaut landed on the opposite side of Pluto, they wouldn't see its moon no matter how long they waited, because Pluto and Charon keep their same sides toward one another all the time. On this side of Pluto, the moon never rises above the horizon.
Pluto's solid surface, with its hills, valleys, craters and other topographic features, is primarily made of ice, perhaps similar to environments near the North and South poles of our own planet. But the ice on Pluto's surface is primarily frozen nitrogen, not water. At Pluto's extremely low daytime temperature of about minus 380 degrees Fahrenheit (minus 233 Celsius or 40 degrees above absolute zero), nearly everything familiar to us, even gas, is frozen solid. When frozen, nitrogen (which is also the most abundant gas in Earth's atmosphere) forms large, transparent crystals several inches across. Much of Pluto's surface must be an amazing and fantasy-like crystalline world, unlike any other place except Neptune 's largest moon, Triton, where frozen nitrogen also makes up most of the landscape. On both Pluto and Triton, small amounts of methane (CH 4, natural gas on Earth) are frozen into the nitrogen crystals. Some regions of Pluto's surface have exposures of ordinary water ice and small amounts of frozen carbon monoxide (CO).
In the dim Pluto daylight, the astronaut may be able to see that the landscape has a yellowish or pinkish color caused by particles of haze that slowly fall from the thin, cloudless atmosphere. The feeble sunlight falling on the thin envelope of nitrogen, methane and carbon monoxide gases that make up Pluto's atmosphere causes chemical reactions that form a thin layer of smog over the entire planet. These smog particles are a mixture of complex organic compounds fashioned by Nature from carbon, nitrogen, hydrogen, and oxygen atoms. These atoms are broken apart from the molecules of gas in Pluto's atmosphere by ultraviolet light from the Sun. Over vast expanses of time, some of the particles making up the smog accumulate on the surface and are incorporated into the ices, giving them a faint yellow or pink color that can be clearly seen from Earth.
Some regions on Pluto have a dark gray tone, as seen from pictures and other observations made with large telescopes. These regions may hold a concentration of carbon-rich materials, or perhaps rocky minerals similar to those found on Earth, the Moon and other rocky planets. If the regions turn out to be icy landscapes tinged with carbon-rich materials, the carbon may have come from collisions with comets, which are rich in complex molecules inherited from the giant molecular cloud of dust and gas from which they (and the Sun and planets) formed. Similarly, if the darker gray regions consist of a rocky mineral coating on the ice, those minerals may also have come from the impacts of comets, which are known to be rich in silicate minerals.作者: 天狼星YAN 时间: 2006-3-5 18:32
为什么去冥王星?
行星的探险是一个历史性的努力和一个美国航空暨太空总署的主要焦点。 新的地平线被设计藉由作冥王星和 Charon 的第一个侦察在我们的太阳系的边缘帮助我们了解世界 - 我们的太阳系的 " 两倍的行星 " 和最后行星被太空船拜访。 然后，如一个广大的任务部份，新的地平线会在海王星之外的 Kuiper 带子区域中叁观一或较多的物体。