Sneaking Up on Pluto (Part 1)
Steve Campbell December 2015 homepage
From 1930, when it was discovered by High School Graduate Clyde Tombaugh until recently, Pluto remained a dot of light in a telescope. The way to find a planet is to see it move amongst the “fixed stars”. The further from the Sun the planet is, the slower it moves. In Figure A you will see the original “Discovery Images” of Pluto.
Figure A: The “Discovery” photos of Pluto.
Lest it seem too easy, Tombaugh spent 10 months photographing the majority of the sky and poring through pairs of images like those above. Computer generated “blink comparisons” are now common and you have probably seen examples. In 1930 two photos (glass plates with silver-based photo-emulsions) were put into a contraption with two optical paths that were alternated to the eyepiece by means of a moving mirror. He was probably looking at the original negatives, not prints. Not only was this system far from perfect, but there were also asteroids that exhibit the same behavior as the targeted planet. Those had to be tracked down and eliminated by arguments based on their apparent velocity or brightness or perhaps by looking them up in the records, if they were known. There was a similar moving pair of dots in these very images – they are cropped out here. Those moved a bit slower, which would indicate an even greater distance from the Sun, but were brighter, which would indicate a smaller distance. The apparent slowness could be caused by an asteroid in a place along its elliptical orbit where it was moving mostly toward or away from Earth. Since nobody called it a planet then, I assume it was eliminated for one of those reasons. There are some dots that appear in one photo and not in the other, you should be able to see at least 5 examples of that in Figure A. That may be due to a difference in atmospheric conditions between successive photos. That is confirmed by the fact that the stars in the January 23 photo are a bit bigger (which means brighter in star images on photographic plates). Another thing might account for single appearances would be a meteor falling through the atmosphere in a direction nearly straight at the telescope. So, you see that Tombaugh’s task was far from simple. One annoyance he did not have to deal with was the vast number of spacecraft now in orbit around the Earth.
Pluto was named by a contest, which was won by an 11year-old girl named Venetia Burney, from Oxford, England. She purportedly received a Five Pound Note for her prize. That does not sound like much, but it would be the inflation-adjusted equivalent of about 250 dollars at today’s exchange rates. She had kept to the tradition of choosing names from Greek mythology. I will just quote (5) an abbreviated explanation of those to put this in context:
- Mercury (Hermes) is the god of commerce, travel and thievery in Roman mythology…
- Venus (Aphrodite) is the Roman goddess of love and beauty…
- Earth…is the only planet whose English name does not derive from Greek/Roman mythology.
- Mars (Ares) is the Roman god of War.
- Jupiter was the King of the Gods in Roman mythology…
- Saturn (Cronus) is the Roman god of agriculture…
- Uranus is the ancient Roman deity of the Heavens…
- Neptune (Poseidon), was the Roman god of the Sea…
- Pluto (Hades) is the Roman god of the underworld…
Let me just note here for you conventional people – I refer to Pluto as a planet. I know they decided to make a new classification of “dwarf planet”. So, if you object to me calling Pluto a “planet” please remember that Earth is a “rocky planet” and Jupiter is a “gas giant planet”. But they are all planets, are they not?
Back to Venetia: As I remembered, she chose Pluto because the first two letters would honor Percival Lowell, which was the name of a notable Astronomer and of the Observatory where Tombaugh made the discovery. Some say that it was because Pluto is a dark and far-away place like the underworld, and that might be another reason. However, I found that there was an interview with the lady herself in 2006 (2) in which she says:
“Yes, I don’t quite know why I suggested it. I think it was on March the 14th, 1930 and I was having breakfast with my mother and my grandfather. And my grandfather read out at breakfast the great news and said he wondered what it would be called. And for some reason, I after a short pause, said, “Why not call it Pluto?” I did know, I was fairly familiar with Greek and Roman legends from various children’s books that I had read, and of course I did know about the solar system and the names the other planets have. And so I suppose I just thought that this was a name that hadn’t been used. And there it was.”
Perhaps the other reasons were why the judges chose her as the winner. The interview seems to be on solid ground, but watch out on the internet. I found one source that said Clyde himself named the planet and I have known that not to be so, since I was young (back in the Cretaceous, when dinosaurs ruled the Earth). In another case, when I searched “who named Pluto”, the first hit was “The boy who named Pluto”. Let’s be charitable and assume that was about the Disney cartoon dog. By the way, did you ever notice that Pluto was a dog and Goofy was a dog, but Goofy stood on two feet, wore clothes and talked, in vivid contrast to poor Pluto? Some Disney dogs are more equal than others, it seems.
Back to the planet, now.
A Better View – Just Barely
In Science Fiction, Pluto was usually described as a nearly featureless ball of rock covered by layers of frozen gasses. After being examined by the Hubble Space Telescope the public image of Pluto was enhanced to a resolution of several dots. Some assumptions were made about what happens between the pixels and the processed image in Figure B is the result. This would seem to indicate that the SciFi characterization is erroneous. We will see.
I should mention that in 1978, a moon of Pluto was discovered, called Charon:
It is a remarkable satellite, being the largest – relative to its planet – in the Solar System.
Figure B: The interpreted version of the Hubble Space Telescope image of Pluto.
Sneaking Up – Quickly
The New Horizons probe was launched on January 19, 2006. It is a relatively small spacecraft by modern standards and it was launched on one of the most powerful rockets available today. Even so, its speed toward Pluto was not nearly enough to get it there in ”merely” ten years. So, it was launched on a carefully chosen trajectory that would take it past Jupiter. There, it was accelerated by Jupiter’s gravity and redirected on a path toward Pluto. This is not a free ride, though. Jupiter gave a boost to New Horizons, but lost the same amount of energy (and didn’t miss it at all) from its revolution about the Sun. This sort of thing happens with many asteroids and comets that pass near Jupiter. Some are slung outward and gain speed, others are slowed and fall into orbits that take them closer to the Sun (a few, to collide with the inner planets) – and Jupiter gains a little. A few are captured into orbits around Jupiter itself. One comet (Shoemaker-Levy) famously was torn into multiple pieces by the tidal forces involved in a “close-encounter”. Those fragments were captured into an elongated orbit. The orbit – at the low end – happened to intersect the planet. That is another fascinating story, but I digress. Those of you who know me are not surprised.
New Horizons went speeding on toward Pluto. It was now the fastest known object in the Solar System – natural or manmade. Although it will not be in the Solar System much longer and will join four other spacecraft that are on their way to the stars. In January of 2015, the resolution of the photos from New Horizons became better than the Hubble images. Yet, still they were not much to see. In fact, Figure C, below was taken in early April and is the first color rendering of Pluto and its big moon Charon.
Figure C: Pluto and Charon April 9, 2015 Credits: NASA/JHUAPL/SwRI
Are you disappointed? I was, too. I had the January “better than Hubble” date on my calendar for about six years and this was the best they had in April. But I understood that they did not do all that complicated image processing that they applied to the Hubble picture. Why not? Because, in the New Horizons case, they had only to wait a few months to see far better resolution, so why bother? With Hubble, it was all they could hope for years and they had to have something to write papers about, in the meantime. I have been to the Lunar and Planetary Science Conference, several times. Those guys have to publish or die. It is not like Geophysics where you can accomplish actual profits by your efforts. Planetary Scientists are sadly dependent on Academic and Government funding. I found their attitudes to be shockingly predatory toward one another as compared to the polite, collegial attitudes of Geophysicists to which I was accustomed. But, I digress again. If you think I get sidetracked easily, imagine being me trying to get through college.
The Pluto imaging situation did improve as time went by and I can share with you another image, this time from early July of ‘15. The images cover most of what can be imaged by New Horizons. Charon and Pluto always show the same face to each other in their orbits around a common center. They are “tidally locked” which is an erudite way of saying the same thing. Also, the plane of their common orbit is not in the same plane as their orbit around the sun. That means that there are dark areas on both bodies that will not be seen by New Horizons. There was a time when we could have seen all of both, but that was in about 1985. I know that because I saw a lecture by a NASA Scientist about the subject by Dr. Paul Schenk (3). The good Doctor is a very good presenter – near as good as your humble Narrator. I had invited my family to travel the hour down to the Clear Lake area with me to see this public lecture and my niece gave me a provisional acceptance. I advised her that the dress code would be “business casual” (based on my Geophysics experience). This illusion was shattered when Dr. Schenk showed up in jeans and a polo shirt.
Figure D shows what will be seen, in greater detail.
Figure D: Pluto and Charon – July 1, 2015 Credits: NASA/JHUAPL/SwRI
These images reveal that the earlier much-processed image from the Hubble Telescope is valid in its depiction of Pluto as varied in color and brightness. I see that this will need to be a series of at least two parts. But I assure you, Astute Readers, that there is much more and better to come.
Pluto stopped being a dot or a smudge and became a planet, with five (count ‘em, five) satellites – one that is near half Pluto’s size. It has craters, as you would expect out of most planets, but it also has vast smooth plains and mountains unassociated with any craters. The dot is now a fascinating variegated world. All this will be discussed in Part Two of this series.
- Clyde Tombaugh: http://starchild.gsfc.nasa.gov/docs/StarChild/whos_who_level2/tombaugh.html
- Venetia Burney Interview: http://www.nasa.gov/multimedia/podcasting/transcript_pluto_naming_podcast.html
- New Horizons Mission: Cosmic Explorations – A Speaker Series, Lunar and Planetary Institute, September 3, 2015 NASA’s exploration of Ceres and Pluto: An Update, Dr. Paul Schenk
- Charon: https://en.wikipedia.org/wiki/Charon_(moon)
- Planetary Names: http://greek-mythology-gods.com/planets.html