Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.stsci.edu/~mutchler/documents/PlanetX_Ch8_NixHydra.pdf Дата изменения: Mon Mar 1 21:49:26 2010 Дата индексирования: Wed Apr 7 21:28:44 2010 Кодировка: Поисковые слова: earth's atmosphere

Chapter 8

Nix and Hydra

Max Mutchler's son Sawyer is sitting on his father's lap. It is time for him to go to bed, but first Max has to read him another story. From one of Sawyer's favorite books, about the planets of the solar system. Max leafs through the book quickly until he finds the page about Pluto, the planet Sawyer likes best. `Look, it says it right here: Pluto has one moon, just like the Earth.' Sawyer starts to beam and looks at his father full of pride. `But is that right, what the book says?' `No!' cries Mutchler junior. `So how many moons does Pluto have?' `Three!' `And who discovered the other two?' `Daddy!' `But it's still a secret, ok? You mustn't tell anyone!' Another hug and Sawyer is tucked up in bed. No, he won't tell anyone, but of course his daddy is the best astronomer in the whole world. Max Mutchler was still a child himself when he became interested in the solar system. His grade four teacher had a copy of the book The Search for Planet X which had left a deep impression on little Max, especially the wonderful drawings. He now has a copy on his own bookshelf, bought recently from Amazon for $2.25. Published in 1962 by Basic Books, as part of the Great Mysteries of Science series, with text by Tony Simon and illustrations by Ed Malsberg. And with wonderful, inspiring passages. Like, for example: `Within your own lifetime, you may yet see pictures of planet number 10 and have the fun of debating what to name it.' Mutchler did not discover a tenth planet, but he did discover two small moons around the ninth. And if it ever took a long time to decide on a name for something, it was these two small frozen lumps of rock. It all started in 2003. NASA had just given the go-ahead for the New Horizons project, the first unmanned space probe to Pluto. John Spencer of the Southwest Research Institute in Boulder, Colorado, submitted an observing proposal on behalf of a large group of Pluto researchers to use the Hubble Space Telescope to search for as yet unknown moons around Pluto. Hubble is of course excellently equipped for such a search, but the Space Telescope Science Institute in Baltimore receives so many proposals that it always has to apply a
G. Schilling, The Hunt for Planet X, DOI 10.1007/978-0-387-77805-1_8, с Springer ScienceЧBusiness Media, LLC 2009 65


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Max and Sawyer Mutchler, sharing the secret of Pluto's second and third moons (Courtesy Max Mutchler)

strict selection procedure. And Spencer's proposal was rejected, as was a second proposal submitted early in 2004 by Spencer's colleague Hal Weaver of Johns Hopkins University, also in Baltimore.

Hubble Photos
Of course, there had already been searches for new moons around Pluto, with large ground-based telescopes. New Horizons' principal investigator Alan Stern had initiated the search at the end of the 1980s. And Brett Gladman and Philip Nicholson studied Pluto in 1999 with the 5-meter Hale Telescope on Palomar Mountain. But none of these searches bore fruit. It would be fantastic to make a new attempt with the Hubble Space Telescope, but that seemed impossible. On September 28, 2004, however, Weaver and his colleagues received an unexpected message from the Space Telescope Science Institute. One of the science instruments on board Hubble, the Space Telescope Imaging Spectrograph, had stopped operating on August 3 as a result of an electronic fault. Since many of the planned measurements could now not take place, and to make the best use of the telescope time that had become available, some of the rejected proposals were being given a second chance. In May 2005, Hubble would therefore start searching for new moons around Pluto. One man's loss is another man's gain.


Hubble Photos

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Max Mutchler knew nothing of all this. He had been working at the Space Telescope Science Institute since Hubble was launched in 1990, in more recent years as an instrument analyst on the Advanced Camera for Surveys, the extremely sensitive on-board camera. He is a real magician when it comes to `scientific photoshopping' ­ cleaning up the rough images produced by the camera so that all undesirable noise is removed and every tiny detail is made visible. On Monday, June 13, 2005 Hal Weaver brought Mutchler the electronic images of Pluto taken by Hubble on May 15 and 18. Weaver had not had time to examine the images in detail himself, but wondered whether Max could take a look? He told him to unleash all his magical skills to improve them. Max didn't get around to it immediately. On Tuesday, June 14, his daughter Sierra and son Sawyer started their summer vacation and Max stayed home with the family. There didn't seem to be any great hurry. Surely Weaver's colleagues had also studied the images? Max was under the impression that they just wanted him to give a kind of second opinion. But no one had actually found the time to analyze the Hubble images, largely because team leader Weaver was much too occupied with the space probe Deep Impact, which was due to arrive at comet Tempel 1 on July 4. Mutchler did not therefore call up the images on his computer monitor until Wednesday, June 15. His practiced eye almost immediately saw something interesting: a minute point of light just outside the overexposed images of Pluto and Charon. And, a little further away, another one. To record extremely small and faint satellites, the Hubble photos had been exposed for a relatively long time: 8 minutes. In that time, Pluto had moved very slightly in the sky, at approximately the speed of a snail at 1,500 kilometers distance. Any satellites it may have would of course move along with it and their faint images would therefore be spread out over a few pixels. To prevent that, Hubble was programmed so that it stayed focused on Pluto while the photos were exposed. Faint moons would then remain point-like (in those 8 minutes they would hardly move along their orbits) while all stars in the background would appear as short stripes of light.

Discovery images of the new satellites of Pluto. Left: Full Hubble Space Telescope frame. Middle: Zoom-in on Pluto, surrounded by many bad pixels. Right: Official discovery release photo (Courtesy Max Mutchler/NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI), and the HST Pluto Companion Search Team)


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The spots that Mutchler saw were not extended, and could therefore not be background stars. And they were not as sharp as the many `bad pixels' on the Hubble photos ­ points of light that occur when the CCD chip in the camera is hit by high-energy cosmic ray particles. And the two specks were also visible on the photos taken on May 18, but then in a slightly different position with respect to Pluto. If you then assumed that they were small satellites revolving around Pluto in the same orbital plane as Charon, you could calculate their distances and orbital periods, where they would be in the future and where they must have been in the past. Could it be that easy to discover new planetary satellites?

Double Discovery
Mutchler did not of course take any chances. He applied all his knowledge and skills to the Hubble photos until he finally ended up with clear, clean images. But the points of light were still there. That evening, as he was about to go home to bed after another long day, Max sent a quick email message from Baltimore to Ted Biro, an old friend from his college days who now lived in Minneapolis. His last sentence was: `I may have just found another moon around Pluto.... zzzzzzzzzz.' On Thursday morning, Mutchler mailed Weaver to tell him about the find, but he did not reply. Unbelievable as it may sound, Max also forgot about Pluto and its moons for a while. All kinds of other projects demanded his attention, like the Deep Impact mission and a newly discovered supernova. And in between working, he also took a vacation with his wife and children in Montreal, Canada. But on the other side of the US, in Boulder, someone else was also looking for satellites orbiting Pluto. Alan Stern of the Southwest Research Institute had just taken on a new postdoc ­ Andrew Steffl ­ and thought it would be a good idea to let him gain a bit of practical experience by analyzing Hubble images. Stern had planned and prepared the Hubble Pluto observations together with Weaver, but because Weaver was officially the team leader he had to check with him first. Weaver thought it was fine, of course, and Steffl started working on the images at the end of July. Weaver did not mention the small satellites that had already been found on the Hubble images. He probably thought that there couldn't be any harm in getting independent confirmation. Andrew Steffl had little experience and tackled the job very thoroughly. He did not find the two satellites until Wednesday, August 17, 9 weeks after Mutchler had discovered them. That evening, under the shower, he suddenly had that unique feeling of discovery: Pluto has two extra moons and I am the only person in the world who knows about them. The next day he reported his find to Stern, who was of course very proud of his new postdoc. Together they conducted a series of checks to be absolutely sure that the points of light were not errors on the photos, and on August 24 they called Weaver to tell him that they had discovered two new satellites around Pluto.


Confirmation

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It was a strange call. The festive mood in Boulder was of course dampened by the news that the moons had already been found in Baltimore. Unfortunately, only one person can be first and Max Mutchler would eventually go down in history as the man who discovered the two small satellites. Mutchler also finally received a reply from Weaver, who told him that he was `probably right' about his discovery. Of course, new photographs had to be taken to provide absolute certainty. Preferably, they should be taken by the Hubble Space Telescope, and if possible without having to go through the usual prolonged procedures of official observing proposals and committee meetings. Fortunately the director of the Space Telescope Science Institute has a certain amount of `own' time on Hubble for such special cases. So, on August 30, Weaver and Stern submitted a Director's Discretionary Time Proposal. In the meantime they agreed with Mutchler and Steffl that the news must not be leaked to the outside world. This was an important discovery that needed further confirmation and would then be announced in a wave of publicity. Mutchler hoped that his friend Ted would not pass on the last sentence of his message to a journalist. It was no problem that he had talked about it at home; his wife Julie would not tell anyone and he knew that Sawyer could keep their secret, too. .

Orbits of the moons of Pluto (Wil Tirion)

Confirmation
But finding confirmation proved more difficult than they had expected. On August 29, two of the four still operational gyroscopes aboard the Hubble Telescope were deactivated as a precautionary measure. These sensitive


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instruments, which regulated the exact position of the telescope, displayed a sudden tendency to give up the ghost. Scientific observations were still possible with two gyros, but no longer in every corner of the night sky. Pluto would not come into range again until February 2006. So what next? Together with Bill Merline of the Southwest Research Institute, Weaver and Stern contacted the directors of the largest ground-based telescopes, like the Keck Telescope and the Gemini North Telescope on Mauna Kea, Hawaii, and the European Very Large Telescope in Chile. They had better luck there. On Friday, September 9, the 10-meter Keck Telescope, the largest in the world, would take photographs to see if the two new moons were indeed visible in the predicted position. And later in the month, the Gemini North and the Very Large Telescope would also make observations of Pluto. In the meantime, Max Mutchler was starting to get nervous. He knew that Mike Brown of the California Institute of Technology (Caltech) in Pasadena would be using the Keck Telescope a day later, on September 10. Early in 2005, Brown had discovered a large icy object well beyond Pluto's orbit (see Chapter 22) and wondered if this `tenth planet' also had a moon. Would the Keck technicians keep quiet about the new Pluto moons? If they said anything about the photographs made the previous day, Brown might also try to observe the Pluto moons and it was not inconceivable that he might then announce the discovery himself. Fortunately Mutchler's fears were ungrounded. But it also very quickly became clear that the big telescopes had been unable to detect Pluto's small, faint satellites. They were drowned out by the light of Pluto itself, which was a hundred thousand times brighter and had a much more disruptive effect on ground-based observations than on Hubble because of the influence of the Earth's atmosphere. More than 3 months after their discovery, the existence of the two small satellites had not yet been confirmed independently. At the beginning of October, the whole affair started to become even more painful. During his observations with the Keck Telescope on September 10, Mike Brown had discovered that his `tenth planet' did indeed have a relatively large moon. Caltech announced the discovery in a press release on September 30. On the basis of a single grainy image, and with no independent confirmation. So why couldn't Mutchler's discovery, which was based on far better quality photographs taken by Hubble on two different dates, also not be made public? But Weaver and Stern didn't want to take the slightest risk. They didn't feel certain enough until October 24, when they got confirmation from Pluto researchers Marc Buie and Eliot Young. Stern had told them about the find and they had taken another look at their old Hubble photographs of Pluto from 2002. Because they now knew what they were looking for, they found the two new moons in the exact position they had expected them to be. A week later, on Monday, October 31, 2005, the Space Telescope Science Institute issued a press release and Sawyer Mutchler could finally tell his secret to his school friends. Pluto had two new moons, provisionally named S/2005 P1 and S/2005 P2 (the


Names

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S stands for satellite and the P for Pluto) and both around a 100­150 kilometers across. They were 64,700 and 49,400 kilometers from Pluto, and had orbital periods of just about 40 and a little over 25 days.

Names
Alan Stern had insisted that the news be announced in October. On November 1 and 2, there was a meeting planned at the Kennedy Space Center in Florida for all scientists involved in the New Horizons project and it would of course be an excellent opportunity to discuss the new discovery. New Horizons was to be launched early in 2006 and now had four objects to study when it passed Pluto in 2015 instead of two. At Stern's request, Max Mutchler also went to Florida, where he met Andrew Steffl for the first time in person. Max knew exactly how Andrew must be feeling. He himself had once discovered a supernova in a remote galaxy only to find out later that it had already been discovered by someone else. So he could understand and empathize with Andrew ­ and admire him: if you can make such a discovery only 3 months after gaining your doctorate, you'll go far. The weeks that followed meant a lot of hard work preparing scientific publications on the new Pluto moons but, of course, a lot of thought was also given to finding suitable names. Keith Noll, an astronomer at the Space Telescope Science Institute, had already spoken to Mutchler about possible names for the moons in September. Because of Steffl's `shadow discovery,' they joked about calling them Baltimore and Boulder, but that was never a serious proposal.

Pluto satellite discoverers. From left to right: Andrew Steffl, Jim Christy, and Max Mutchler (Courtesy Andrew Steffl)

Mutchler knew about Jim Christy, who had in effect named the large moon Charon after his wife, but he felt that suggesting Julie, or Sierra and Sawyer, was


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going a little far. In any case, the moons had to be given appropriate names from Greek mythology. In other words, it was time for some serious brainstorming. All the team members were asked to submit suggestions and the most popular names would be selected through a series of votes. Andrew Steffl liked the sound of Persephone and Cerberus. Persephone was the queen of the underworld and Cerberus the three-headed dog that guarded the gates of Hades. But those two names had already been given to asteroids. Max Mutchler thought Aether and Hemera would be good names. Firstly because they were the son and daughter of Erebus and Nyx, the god of darkness and the goddess of the night. But especially because the initials A and H also stood for Alan and Hal, who had led the search for the two satellites. The names Obolus and Danake were also considered ­ in Greek mythology these were the coins the dead had to pay the ferryman Charon to cross the River Styx. But eventually, the team decided on Nyx (for the inner moon) and Hydra. Night goddess Nyx was not only the mother of Aether and Hemera, but also of Charon. Hydra was the nine-headed monster defeated by Heracles. The nine heads fitted in neatly with Pluto's status as the ninth planet, while the letters N and H referred to the space probe New Horizons. In February 2006, when the New Horizons probe was well on its way, the Hubble Telescope finally took new photographs of the two small Pluto moons. They would be available at the Space Telescope Science Institute on the 16th of the month, and Mutchler had invited Weaver to come and take the first look at them. Imagine that there turned out to be a third moon on the photos ­ the team leader would at least have the honor of being the first to see it. But the images were ready a day in advance and Mutchler could not contain his curiosity. To his great relief, the photographs showed both moons exactly in the predicted positions. The press release about the new Hubble observations was issued almost at the same time as the scientific publication of the discovery, on February 23 in the British weekly magazine Nature.

Quibbling
Now that the original discovery had been confirmed so convincingly it was time to submit the proposed names officially to the Outer Solar System Nomenclature task group of the International Astronomical Union. And it soon became clear that the race was far from over. First of all, there was apparently already an asteroid called Nyx. And Hydra was not only the name of a constellation, but also of the German prostitutes' union, which led some of the members of the working group to argue that it wasn't a suitable name for a planetary satellite. The discussion also rekindled an old dispute. In the spring of 2006 the debate on Pluto's true nature became more heated than ever. Alan Stern had always been firmly in favor of categorizing Pluto as a planet, but a lot of astronomers felt that the little odd man out did not merit such an elevated status. And many


Quibbling

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of them found it unacceptable that Stern and his colleagues now wanted to call one of the small moons of the `ninth planet' after a nine-headed monster. And so the members of the group came up with their own creative suggestions. What about Cerberus and Typhon, whose first letters formed the initials of Clyde Tombaugh, who discovered Pluto? Or Typhon and Ladon which, together with Pluto and Charon, formed the letters P, L, C, and T, the initials of Percival Lowell and Clyde Tombaugh? The quibbling continued by email for several weeks. Never had there been such a fierce debate about the names of small planetary satellites. Obviously professional astronomers are very sensitive about anything to do with the distant dwarf planet Pluto. Finally, after several rounds of voting, the names proposed by the team that had discovered the moons were adopted, with Nyx now spelt the Egyptian way (Nix) to avoid confusion with the asteroid. The names of the two new moons were officially announced on Tuesday, June 20, a little more than a year after Max Mutchler had discovered them. After the discovery of Nix and Hydra, Pluto became even more of an odd man out. Even though some people see the small body in the outer regions of the solar system as a full-fledged planet, it is completely different from the other eight. They fall into two clear categories: the relatively small `earthlike' planets (in addition to the Earth: Mercury, Venus, and Mars) which consist of rock and metal, and the four giant planets (Jupiter, Saturn, Uranus, and Neptune) which comprise mainly light elements such as hydrogen and helium. Pluto is clearly not a gas giant. But it is also much smaller than the four terrestrial planets, comprises more than 50% ice, and now proves to have a complicated system of moons ­ one large one and two small ones, as though the moons of the Earth and Mars were all thrown together in one heap. Max Mutchler remains stoical about the whole affair. He may go down in history as the man who discovered Nix and Hydra, and his son Sawyer may rightly be proud of him, but actually it was just a matter of chance. He was the right man in the right place at the right time, with the right photographs in his hands. It was nice of course to think back to that children's book by Tony Simon and Ed Malsberg and the words that had inspired him so long ago: `Within your own lifetime, you may yet see pictures of planet number 10.' Or of two small moons circling the ninth ­ that's also something to tell your kids about.