author: none
Jun 7, 2015
astro New HorizonsOn July 14 the New Horizons spacecraft will fly past Pluto. At the time the mission was planned, Pluto was the only planet not explored by instrumentation sent from Earth. It is very unlikely we send another probe to Pluto for several decades; nothing is currently planned. Alan Stern, the Principal Investigator for the New Horizons Pluto mission, has been working on it since 1989 .. this is close to a once-in-a-lifetime event.
In 1930, we knew our solar system was made up of eight planets in orderly, near-circular orbits around the Sun. From our viewpoint, on the third planet from the sun, five other planets can be watched passing though the background of stars in the night sky. Three millions years ago, as the human mind emerged and became aware, our relatives would have seen these five “wandering stars” as special. For almost all of human history, Mercury, Venus, Mars, Jupiter and Saturn have been deities. Their passage though the constellations is still seen today, by many, as predicting the future, their alignments as portents of doom, and, in at least one well remembered case, possibly, a guide to three men from the east.
The word “planet” is derived from the Greek “wandering star.” The planet’s slow gliding paths through the starry background of the fixed stars is hard to comprehend; they loop, traveling backwards for months, pass each other, and regularly disappear for a while into the light of the sun. Efforts to understand this motion have advanced, albeit on an equally wandering path, our understanding of the universe. Aristarchus, around 300BC, could explain this motion only if the planets revolved around the sun, not the prevalent belief at the time, and not one destined to last. In about 100AD Ptolemy summarized the astronomical work of the Greeks and put the Earth back in the middle of things!
It took nearly 1500 more years before Copernicus proposed a sun-centered solar system. Though written decades earlier, his work finally appeared in print in 1543, the year he died. He dedicated it to Pope Paul III, and a preface was added (without Copernicus’ knowledge) saying that the theory was intended only for making the calculations of planetary positions easier and not meant to be a statement of reality. It must have worked; it didn’t make the list of banned books until 1616. The Italian scientist Giordano Bruno was burned at the stake for teaching, among other heretical ideas, Copernicus’ heliocentric view of the Universe.
But the tide was turning. On the nights of January 7-13, 1610, Galileo used his crude telescope to watch four “stars” which travelled with Jupiter perform a dance, changing positions in a way most easily explained by them rotating around the planet. Galileo also suffered for his challenge to prevailing dogma, but Io, Europa, Ganymede and Callisto are still called Jupiter’s Galilean moons. Johannes Kepler took the basic heliocentric idea of Copernicus and turned it into a system that worked. Using the extremely accurate positions measured by the Danish astronomer Tycho Brahe, Kepler formulated three laws of planetary motion.
Galileo Galilei died in 1642, the same year that Isaac Newton was born. Newton entered Cambridge University, and his mid-twenties was named Lucasian Professor of Mathematics at Cambridge, the post now held by Stephen Hawking. Around that time, Cambridge was closed because of the plague and Newton returned to his rural home for two years. Newton’s lifetime accomplishments were many, but in those two years he developed the mathematical theory and that proved the truth of Kepler’s laws. Getting there necessitated a new force of nature, the concept that masses attract each other at a distance .. Newton called the force “gravity.”
In 1781, William Herschel discovered Uranus, the first planet discovered with a telescope, expanding the known boundaries of the Solar System for the first time in history. Now we are seven! Newton’s work enabled planetary positions to be predicted with very high accuracy and it was apparent that Uranus’ observed position varied slightly from those predictions. Assuming that another unseen planet was perturbing Uranus’ track allowed prediction of the position such a possible object. The competition was fierce and even the criterion for “discovery” was argued about .. should it be attributed to the person who predicted the planet’s position and persuaded astronomers to look for it, or the person who looked there and saw it! A planet was located in September 1846 and called Neptune.
As we entered the twentieth century, the solar system, out to Neptune, was pretty well understood. There was a ball of fire at the center of the system and eight planets ranging from Mercury out to Neptune. We measure distances in the solar system in “astronomical units (AUs)” .. 1AU is the average distance from the sun to the Earth (149,597,871 kilometers). None of the planets’ orbits is exactly circular; they are elliptical so their distance from the sun varies as they orbit. All the planets orbit in the same plane, pretty much, Mercury at about 0.4AU rips around the sun in 88 days; Neptune, at 30AU, takes a leisurely 165 years.
In the early 1800’s the question was asked why there was no planet between Mars and Jupiter. Playing with numbers, looking for patterns in the planetary orbits, suggested a gap in a tidy outward progression (Neptune doesn’t fit the pattern, but nobody knew it existed yet). The hunt for another planet was on! A small object in “the gap” was located almost immediately, then another, and another. These so-called asteroids were small, 500Km across and smaller, and as detection technology improved, more and more were located. One hundred asteroids had been located by 1868, a thousand by 1921, 10,000 by 1981 and 100,000 by 2000. The alleged symmetry that led to thinking there was a gap was probably a coincidence, but the asteroid belt is very real.
The early solar system was a hellish, chaotic place, planets forming out of a disk of dust swirling round the sun, growing by accumulating rubble, melting under their increasing weight, crashing into each other, sweeping out paths for themselves, jockeying for stable orbits and generally being unpleasant to each other. The asteroid belt is probably a planet that never formed, maybe massive Jupiter’s proximity made the gravitational environment too turbulent, but we now have millions of pieces of rock, ice and metal all the way from inside the Earth’s orbit to outside Jupiter’s. About 65 millions years ago, a 10Km asteroid took out the dinosaurs along with half of the all the species living on Earth.
.. next, the twentieth century
2015 is a special year for interplanetary exploration. By the end of the 1970’s, humans had sent spacecraft to every planet in the Sun’s family, except one. Missions to orbit Mercury, Venus, Mars, Jupiter and Saturn, and to land four rovers on Mars followed. More recently, many of the lesser objects in the solar system have been studied up close: asteroids, the moons of Jupiter and Saturn, and even comets.
Observations of Uranus’ motion continued to show discrepancies from Newtonian predictions, even with Neptune’s gravitational pull taken into account. Was there another, yet more distant member of the sun’s family also tugging on Uranus? Percival Lowell started a project in 1906 to search for this so-called “Planet X.” Lowell died in 1916 and the search petered out, mainly because of arguments over his estate, including the Lowell telescope. Interest resumed in 1929 and the work was handed to Clyde Tombaugh, a 23-year-old Kansan who had impressed the observatory director with his astronomical drawings.
Tombaugh’s job was to photograph areas of the sky twice on nights about a week apart. Using a ‘blink comparator’ which allowed two plate images to be rapidly flipped back and forth, so anything that moved would stand out, Tombaugh worked for a year on this grunt-work till in early 1930 when plates taken on January 23th and 29th showed a likely candidate. Checking a lower quality image from a few days earlier confirmed the observation.
In fact, Planet X had been photographed several times before this, including by Lowell himself the year before he died, but its motion was missed. The earliest known image is from 1909. Further plates were exposed at the Lowell Observatory to confirm the discovery which was telegraphed to the Harvard College Observatory on 13 March 1930.
The Lowell Observatory had the naming rights to Planet X and many suggestions were made from around the world. Venetia Burney, an eleven-year-old schoolgirl in Oxford who was interested in classical mythology, suggested Pluto, the god of the underworld, in a conversation with her grandfather, a former Oxford University librarian. He passed the name to an astronomy professor friend, who cabled it to colleagues in the United States, where it was accepted unanimously. Venetia received a £5 reward, but her true reward was living just long enough to see New Horizons launched.
As telescope technology improved over the years since Pluto’s discovery, estimates of its size and mass declined (ultimately by a factor of about 500). An accurate mass was settled when Pluto’s moon, Charon, was discovered and it was far too little to account for Uranus’ perturbations. When Neptune’s mass was also refined after Voyager II few past it, the need for a Planet X to account for the Uranus discrepancies disappeared, Neptune was enough. Locating Pluto near Lowell’s predicted position for Planet X had been a coincidence!
Pluto was found nearly one and a half billion kilometers beyond Neptune but its 250 year orbit is unusually elongated, dipping down to less than 30AU from the sun, closer than Neptune, for part of its year and rising out to 50AU. As Pluto climbs away from the sun, as it is doing now, it gets colder and its mostly nitrogen atmosphere starts to fall like snow.
Pluto’s orbit is tipped out of the plane of the other planets and its polar axis is tilted by nearly 90 degrees, Because its rotational axis is so tilted, Pluto’s south pole saw sunlight for the first time in 120 years in 1987. Just to completely remove it from your list of vacation spots, the temperature of the surface is only 35-55 degrees above absolute zero. Some have speculated rivers of neon might flow there!
Pluto is about 1200Km across, much smaller than the Earth’s Moon, but its own biggest moon, Charon, discovered in 1978, is about the same size as it is. Pluto and Charon are locked in a gravitational embrace; as they orbit each other every 6 days, they keep the same hemispheres facing each other. In the last few years the Hubble Space Telescope has spotted four more small moons (Nix, Hydra, Kerberos and Styx) in addition to Charon.
Clearly, Pluto isn’t a planet like the others; it is a member of the Kuiper Belt, a cloud of material past Neptune that never joined in the serious planet making at the beginning of the solar system. Some clumping of rocky and icy material happened to create objects like Pluto, but probably none of the violence of melting, bombardment, destruction and collisions that created the inner planets, or the immense gravitational influences that pulled Jupiter, Saturn, Uranus and Neptune into gaseous giants. The Kuiper Belt has been called the dust bunny collection of the solar system. It contains a few larger objects, like Pluto, and probably tens of thousands of objects about down to about 100Km across, and many more that are smaller than that.
Following the discovery of more Kuiper Belt objects (KBO’s), and similar objects further away, by Hubble and other powerful telescopes, Pluto joined the new designation of ‘dwarf planets’ in 2006. Arguments revolve about this reclassification but Pluto is now in a class containing more named objects than there are classical planets (and there are hundreds more observed candidates for the class awaiting confirmation of their orbits and naming).
Planning for a mission to Pluto began in 1989, and the New Horizons spacecraft was launched in January 2006. It will fly though Pluto’s space (about 10,000Km above its surface) on July 14, 2015. Recently, after a long search, another small KBO was found by Hubble, far beyond Pluto, but reachable, with little expenditure of fuel, by New Horizons in 2019.
.. next, the New Horizons mission
“New Horizons’ core science goals reflect what the science community has wanted to learn about Pluto for the past two decades. The craft will map the surfaces of Pluto and Charon with an average resolution of one kilometer. It will map the surface composition across the various geological provinces of the two bodies. And it will determine the composition, structure and escape rate of Pluto’s atmosphere. NASA has also outlined a list of lower priorities, including the measurement of surface temperatures and the search for additional satellites or rings around Pluto..” — PI, Alan Stern
Planning a spacecraft mission is a major effort. They are expensive, so politics matter; there is intense scientific competition to get experiments on board; the engineering involved is difficult; and the planets don’t cooperate! New Horizons exemplifies all these challenges.
Planning for this mission started in 1989; the scientific measurement objectives were developed by NASA’s Outer Planets Science Working Group in 1992 and 1996, and adopted by NASA for the mission “Announcement of Opportunity” (AO) that led to the selection of New Horizons as as its Pluto mission in Nov 2001.
The 2002 National Research Council “Decadal Survey” sought to examine the big picture of solar system exploration, survey the current knowledge of our solar system, and compile the scientific questions that should guide solar system exploration in the next decade. The National Academy of Sciences ranked the exploration of Pluto-Charon and the Kuiper Belt among the highest priorities for space exploration, citing the fundamental scientific importance of these bodies to advancing understanding of our solar system.
Work began on the spacecraft in 2001; the science payload, includes imaging infrared and ultraviolet spectrometers, a multi-color camera, a long-range telescopic camera, two particle spectrometers, a space-dust detector and a radio science experiment. The dust counter was designed and built by students at the University of Colorado, Boulder.
Spacecraft design and construction is a constant battle between science and engineering! The nine New Horizons science instruments are the reason to fly, but the engineering systems to deliver them to the right place, power them, point them in the right direction, keep them warm, and transmit their data back to the home planet are critical .. in fact only 30 Kg of New Horizon’s total 450 Kg is science instruments, and 75 Kg is fuel.
Features of the Pluto environment have an impact: the sun is too faint to power solar panels, so New Horizons carries a 200 watt nuclear generator; communication over 30 AU needs New Horizons to carry a 2.1 meter dish antenna on its back; systems powered constantly for nine years become unreliable, so New Horizons will “hibernate” for most of the flight; if anything fails at Pluto commands from earth wouldn’t get there it time to fix it, so New Horizons has to fix itself.
Timing was critical. Pluto is a long way from here and a “direct” flight would take about 15 years. If Jupiter could be used to provide New Horizons with a “gravity assist” that time would be reduced to under 10 years. Solar system geometry demanded that to gain that advantage New Horizons would have to launch in the second half of January 2006 to arrive at Pluto in mid 2015. By the end of January a year would lost; by mid-February the Jupiter sling-shot would be lost and arrival would be in 2020.
New Horizons launched on January 19, 2006 on NASA’s most powerful rocket; it was the fastest spacecraft ever to leave the Earth. It passed the distance of the Moon in a nine hours (most flights to the Moon last more than three days), and it reached Jupiter in thirteen months. At Jupiter, it engaged all its instruments in gathering data and calibration, then it was time to hibernate. With navigation systems turned off, and the spacecraft in slow spin to keep its antenna pointed at the earth, it just emitted an unmodulated status tone once a week. Each year it was woken for a seven week checkout, and any necessary course corrections. During the 2014 checkout, there was also an end-to-end simulation of the Pluto flyby .. for seven days, ground stations, mission operations, and New Horizons performed the exact sequence that would happen at Pluto, perfectly.
In January 2015, New Horizons came out of hibernation for the last time in preparation for its encounter with Pluto. The last 150 days to Pluto are divided into three, increasingly active “Approach Phases.” AP1 started on Jan 15 using the long range camera to image Pluto and the background stars for precise optical navigation, take a series of pictures for 6 days (one Charon orbit), and another long exposure series for 38 days (one Hydra orbit), and ended of March 6. New Horizons then turned its antenna toward Earth to transmit all the data gathered.
AP2 started on Apr 5 with a repeat of the imaging of one Pluto/Charon revolution and one Hydra orbit. AP2 adds numerous new and significant observations of the Pluto system, including the first color and spectral observations of Pluto and its moons, and series of long-exposure images that will help the team spot additional moons or rings in the Pluto system. During AP2 New Horizons will start to get better images of Pluto than Hubble can, and by the end of AP2, on Mar 15, will able to see all five of Pluto’s moons.
As I post this, AP3 is under way. It is nearly 28 days, and 34 million kilometers, to Pluto; long range images are starting to tantalize!
.. next, Approach Phase 3 and Pluto Encounter
In June 1978, U.S. Naval Observatory astronomer James Christy noticed something unusual. He was studying highly magnified photos of Pluto, and Pluto wasn’t round. A small bump marred one side of blurry Pluto. That bump turned out to be Pluto’s largest moon, Charon, whose discovery Christy (working with late colleague Robert Harrington), made on June 22, 1978.
As I write this, New Horizons is barreling down the final stretch of its path Pluto approaching the target at 13.8 Km/s. The second and third sets of hazard-search observations were made May 29-30 and June 5, using the telescopic Long-Range Reconnaissance Imager (LORRI) camera on New Horizons. For these observations, LORRI is commanded to take hundreds of long-exposure (10-second) images, which are combined to enable a highly sensitive search for faint satellites, rings or dust-sheets in the system.
The latest hazard observations easily detected Pluto and all five known moons, but no rings, new moons, or hazards of any kind were found. The next New Horizons hazard search will start June 15, and the team will report on the results on approximately June 25, after completing a detailed analysis of the new and still more sensitive data.
LORRI frames are now showing some real surface variations on Pluto and Charon. It appears that Pluto will present a more textured hemisphere for imaging, during flyby, and Charon shows significant polar darkening .. [clipped from a Jun 22 LORRI image]
In the last week of June, the Pluto approach enters its third and final far encounter science phase - called Approach Phase 3, which runs until one day before Pluto close approach. AP3 highlights include taking additional images of the Pluto system for final navigation purposes; mapping Pluto and Charon in increasing detail, with LORRI, color and UV cameras, watching for variability in color, surface composition and atmospheric patterns as the small planets rotate; and searching for new moons and rings with even greater sensitivity. New Horizons will also continue sampling of the interplanetary environment – measuring both solar wind and high-energy particles, as well as dust-particle concentrations.
New Horizons’ approach to, passage through, the Pluto system on the morning of July 14 is carefully tailored. Pluto makes one revolution in 6.4 days (and Charon is gravitationally locked to Pluto, so the same is true for Charon). In order to image as much of Pluto’s surface as possible, New Horizons will survey both objects 3.2 days before flyby at about 40 Km resolution, the last opportunity to see that hemisphere in sunlight. Immediately after that, at the end of AP3, on July 12 and 13, New Horizons will download a “safety net” collection of data. There isn’t time send everything, only key images and other top priority data will be sent to Earth, in case New Horizons is lost during flyby.
Then New Horizons will turn to aim its instruments at Pluto and begin a complex dance to gather as much information as possible from every instrument through the encounter. New Horizons can turn quickly (about 1.5 degrees per second), and can stop and stabilize itself quickly. As it dives into the Pluto system, its twists and turns get larger as it trains one experiment after another on targets it passes, until it is looking back at them as it leaves the Pluto system behind.
New Horizons closest approach to Pluto (13,700 Km) is designed so that Charon will be in Pluto’s background, so turns to change pointing from one the other will be small. As New Horizons recedes from Pluto, Charon’s sunlit side will be well positioned to illuminate Pluto’s shadowed hemisphere so allowing imaging of Pluto in moonlight. Looking back towards the sun and the Earth, only a quarter of a degree apart in its sky, New Horizons is aimed to pass through Pluto’s shadow for a few minutes. When Pluto blocks out the sun, any dust rings not previously seen in reflected light may be more apparent while backlit, while UV scanning will pickup atmospheric composition signatures.
A few minutes later, Pluto passes between New Horizons and the Earth. New Horizons will be listening for a radio signal sent more than four hour earlier from two of NASA largest Deep Space Network (DSN) dishes in Australia and California. Distortions in that signal as it fades and reappears as Pluto blocks out the Earth will reveal something of any atmosphere that Pluto might have, and provide precise dimensions for the planet. And then, everything is timed so that Charon, also, will occult the sun and the Earth and the experiments will be repeated for that body. Threading those two needles requires New Horizons to fly through an imaginary box in space approximately 200 Km across timed to the minute. This ‘dance’ was choreographed 2004, before launch, though minor adjustments were made when Pluto’s faintest moons were discovered.
A little while after that, as New Horizons’ activity starts to calm down, it will turn its big antenna to point at the Earth again and emit a beacon signal to indicate it made it through the Pluto system intact. No data will be sent, but that ‘beep’ is what everyone will be waiting to hear .. expected at 9:02pm EDT. Pluto science will continue to be gathered through three “Departure Phases” into Jan 2016.
On July 15, New Horizons will send a carefully redetermined tiny subset of the gathered data. This has been named the “New York Times” transmission because it will contain a few ‘beauty shots’ .. images for the NYT front page; Pluto at 100 m/pixel! Because New Horizons will be nearly 5 billion (and increasing) kilometers from Earth, and transmitting with about 160 watts, the rate of data return will be very slow, 700 bits/second. The 16 gigabytes of data, gathered mostly in the few hours around encounter, will be sent to Earth very slowly. Given that many planetary probes use the DSN services (and the prosaic fact that rain at the DSN sites causes data loss), New Horizons cannot transmit full-time and will have to re-transmit some data. Given all this, if New Horizons gets an optimistic access to DSN, it will be sending data home for about 6-9 months.
.. next: This four-part story has been background and historical information. From here on in, it’ll be ‘news’ ..