Clyde Tombaugh Discovers Pluto

*click* ….. *click* …. *click* …. *click*

On this day in 1930, a 24-year-old man named Clyde Tombaugh was squinting into the Lowell Observatory’s Zeiss Blink microscope. The unique device, also known as a blink comparator, held two photographic plates that each contained the image of a star field taken the previous month–the images showing the same section of sky, taken a few days apart. Tombaugh could rapidly switch between the two images by rotating a dial, allowing him to quickly compare the images and watch for any variations between the two that would indicate a body moving more rapidly than the background stars (eg. planets, asteroids, etc.).

*click* ….. *click*

Late into that February afternoon, a subtle difference between the two images caught his eye.

Animation comparing Tombaugh's star fields.

Can you spot Pluto?  Click on the image to see a version with Pluto identified.   Image Credit: National Air & Space Museum


*click* … *click* … *click* .. *click* . *click* *click*

He spent 45 minutes comparing the two images. Convinced of his findings, he contacted his supervisors. Over the next couple of weeks, the observatory focused its attention to the object before confirming Tombaugh’s discovery. On May 1st, 1930, a new planet was introduced to the world: Pluto.

And of course, in 2015, we got to see Pluto in a way that Mr. Tombaugh himself could only have imagined.

NASA’s New Horizons spacecraft captured this high-resolution enhanced color view of Pluto on July 14, 2015.

NASA’s New Horizons spacecraft captured this high-resolution enhanced color view of Pluto on July 14, 2015.        Source: NASA

Dwarf Planet Ceres


Previously, I told you the fascinating story of Ceres’s discovery and complicated identity crisis; now I’m ready to tell you about the dwarf planet specifically.


Let’s take a quick trip out beyond our Moon, past Mars (if you find yourself at Jupiter, you’ve gone too far), and into the realm that is commonly known as the asteroid belt. Now, contrary to popular depictions, the asteroid belt isn’t crammed full of asteroids. As a kid, I remember seeing illustrations of the asteroid belt that made it look as densely packed as Saturn’s rings. That depiction is a gross exaggeration. In fact, while there are billions of bodies orbiting out in the asteroid belt–it is believed that there are somewhere between 1 and 2 million asteroids with a diameter of 1 kilometer or more–the area is still mostly just empty space. If you were to board a rocket that would fly through the asteroid belt, the chances of actually smacking into anything are extremely slim. Of the asteroids in the belt that have a diameter of 10 km or more, a collision is only likely to occur about once every 10 million years. So anyway, this is the home of one such body, the dwarf planet Ceres.

Compared to the other bodies in the asteroid belt, Ceres is huge. Ceres has a diameter of 950 kilometers (590 miles). This is a little smaller than the width of Texas or Montana. Ceres comprises between a quarter and a third of the mass of the entire asteroid belt. King among the asteroid belt, Ceres falls short when facing up against the other planets in our solar system. Compared to Earth and the Moon, Ceres has the mass of .00015 that of the Earth and .0128 that of the Moon. (For some perspective, it would take almost 80 Cereses to equal the mass of just the Moon.)

Size comparison of Earth, the Moon, and Ceres.

Size comparison of Earth, the Moon, and Ceres.


Ceres orbits the Sun at an average distance of 415 million kilometers (257 million miles), in a nearly circular orbit. At this distance from the Sun, and at the speed that Ceres is traveling, one year on Ceres is equivalent to 4.6 years on Earth.

Ceres is believed to consist of a thin, dusty crust situated above a fairly thick layer of water-ice. At the center of the dwarf planet is a thick rocky core.

Cutaway image showing Ceres's layers.

Cutaway image showing Ceres’s layers.
“Ceres Cutaway” by NASA, ESA, and A. Feild (STScI)


Ceres, of course, has less mass than the Earth, and thus you would weigh less standing on a scale on Ceres than you would on Earth.  If you weigh 150 pounds on Earth, then you weigh a mere 4.2 pounds on Ceres!

Ceres is one of the latest planets to be explored by high-tech modern spacecraft. NASA’s Dawn spacecraft is currently orbiting the dwarf planet at a just recently arrived distance of only 2,700 miles above its surface. For about a month, Dawn will orbit and study Ceres from this location. The spacecraft will complete an orbit every three days, constantly kicking images and other important data back to Earth. For some perspective, the resolution Dawn can obtain while imaging Ceres is somewhat comparable to what it would be like for you to observe a soccer ball from 10 feet away. Subsequent to the 2,700 mapping orbit, Dawn will venture even closer to the dwarf planet providing better and better views of Ceres. By the end of 2015, Dawn will be concluding its mission at an altitude of only 230 miles. Dawn’s cameras at this distance will be able to produce images with a resolution 850 times greater than that of what Hubble would be able to produce. Now, that soccer ball is a mere 3.3 inches away! At this distance, Dawn will be in a fairly stable orbit around the dwarf planet and is expected to become its satellite into perpetuity.

I’ll have more to share about Dawn soon.

For now, let’s all celebrate the fact that we’re still exploring–exploring not just planets and asteroids and moons, but exploring actual worlds. Let’s celebrate the fact that we’re learning new things about this particular world on a daily basis and that this will continue for many months to come. And, let’s celebrate the fact that with all that we know today, it’s a tiny amount compared to what we still get to learn in the future. Having a lot to learn, I think, is much more exciting than already knowing it all.

Ceres–Either the Most or Second-Most Popular Dwarf Planet

It has been nearly a decade since the International Astronomical Union (IAU) formally defined the word ‘planet’, resulting in the reclassification of Pluto as a ‘dwarf planet’. Some people still remain upset about the decision, considering the new classification as a demotion. If you roll with the kinds of people that I do, battle-lines have been drawn around the issue and many a friendship have been lost in the process. I don’t want to rekindle those debates (this is likely inevitable, however, as Pluto will be in the news quite a bit in the coming months as New Horizons is finally about to have its encounter with the dwa… whatever-you-want-to-call-it), so let’s take a look at a dwarf planet that appears to have finally found comfort in its classification: Ceres.

Color view of Ceres as imaged by Hubble in 2004 - Credit: NASA, ESA, J. Parker (Southwest Research Institute), P. Thomas (Cornell University), L. McFadden (University of Maryland, College Park), and M. Mutchler and Z. Levay (STScI)

Color view of Ceres as imaged by Hubble in 2004 – Credit: NASA, ESA, J. Parker (Southwest Research Institute), P. Thomas (Cornell University), L. McFadden (University of Maryland, College Park), and M. Mutchler and Z. Levay (STScI)

If you thought Pluto’s designation was complicated and controversial, just wait until you Ceres’s story.

Ceres has had a bit of an identity crisis of its own. Italian astronomer Giuseppe Piazzi discovered Ceres on New Years Day, 1801. He at first thought it was a star, but observed its movements against the stellar backdrop over the course of a few days and determined it to be a planet. He took a conservative approach in his announcement however, by referring to it as a comet.

I have announced this star as a comet, but since it shows no nebulosity, and moreover, since it had a slow and rather uniform motion, I surmise that it could be something better than a comet. However, I would not by any means advance publicly this conjecture. – Giuseppe Piazzi in a letter to fellow Italian astronomer Barnaba Oriani

With the help of other astronomers and using a method for calculating orbits developed by Carl Friedrich Gauss, it was confirmed that the object was not a comet, but in fact some sort of small planet. German astronomer Johann Bode had been promoting his hypothesis that planets orbited their host stars at distances that could predicted by mathematics. This hypothesis predicted a planet should exist between Mars and Jupiter. When Bode heard news of Piazzi’s discovery of an object at precisely that location, he rushed to announce that the missing planet had been located and even went as far as to name it himself. The name he gave: Juno. Piazzi, however, had taken the liberty as the new planet’s discoverer to give it the name ‘Ceres Ferdinandea’, honoring the patron goddess of Sicily and King Ferdinand of Bourbon. Piazzi rightfully objected to Bode’s stake on naming rights:

“If the Germans think they have the right to name somebody else’s discoveries they can call my new star the way they like: as for me I will always keep it the name of Cerere and I will be very obliged if you and your colleagues will do the same.” Piazzi in a letter to prominent astronomer and editor of scientific journals, Franz Xaver von Zach.

Piazzi’s name ultimately won out, though it was shortened to its currently-accepted name: Ceres.

"Giuseppe Piazzi" by F. Bordiga - Image from Smithsonian Institute Library

“Giuseppe Piazzi” by F. Bordiga – Image from Smithsonian Institute Library

After more objects were discovered orbiting in the same area, Sir William Herschel, in 1802, labeled these new objects, including Ceres, as asteroids (though the term asteroid, which means “star-like”, wasn’t commonly accepted until the early 1900s).

So thus, Ceres became the first, and largest, of the asteroids that orbit between Mars and Jupiter in a loose collection that we collectively refer to as the asteroid belt. But Ceres’s identity crisis wasn’t over just yet. Ceres was king of the asteroids until 2006, when that controversial IAU reclassified it as a dwarf planet. 1

From star, to comet, to planet, to asteroid, and finally to dwarf planet, Ceres looks to Pluto and remarks, “Psh… and you think you had it bad.”

Now that this introduction is out of the way, stay tuned for more information about Ceres. I’ll tell you about this fascinating world and get you up to speed on NASA’s Dawn spacecraft that will be arriving at Ceres in March of this year.

Animation of Ceres as viewed by the Dawn spacecraft on January 13, 2015. - Source: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

Animation of Ceres as viewed by the Dawn spacecraft on January 13, 2015. – Source: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

(Much of the information in this post came from Giuseppe Piazzi and the Discovery of Ceres, G. Foderà Serio, A. Manara, and P. Sicoli, published in Asteroids III by the University of Arizona Press)

  1. Since Pluto’s reclassification from planet to dwarf planet was viewed by many as a demotion, I wonder if it’s safe to refer to Ceres’s reclassification from asteroid to dwarf planet as a promotion.

New Horizons Awakens

If everything has gone according to its meticulous plan, by the time you are reading this NASA’s New Horizons spacecraft will have awoken from its electronic hibernation for the last time and begun its careful preparations to encounter Pluto in July of 2015.

Maybe I should back up for those that aren’t familiar with New Horizons, or just want a little recap:

New Horizons is the name of a NASA spacecraft and mission to complete a fly-by mission of Pluto and its moons, and then on to view other Kuiper-Belt objects. New Horizons will give us shiny new photos of our favorite dwarf planet and a wealth of other scientific data. It’s about time, too. I mean, just look at the current best image we have of what we–at least  used to–consider 1/9th of our solar system’s planetary awesomeness:

Pluto as imaged by Hubble in 2010.

Pluto as imaged by Hubble in 2010.

Yuck! And NASA was impressed enough to brag about these “most detailed and dramatic images ever taken of the distant dwarf planet“. I’m looking forward to which adjectives they’ll use when we get real images courtesy of New Horizons. But I digress.

On January 19, 2006, New Horizons lifted-off from its Cape Canaveral launchpad and screamed into the heavens. In fact, nothing before or since has left the Earth with such a sense of urgency. New Horizons holds the record for the fastest launch of any spacecraft. It left the Earth with a velocity of 36,373 miles per hour (58,356 kilometers/hour), fast enough to propel it not just out of the Earth’s orbit, but completely out of the solar system (referred to as a solar escape velocity).

Subsequently, New Horizons continued to voyage towards its 2015 encounter with Pluto. Along the way, it came within 1.4 million miles (2.3 million kilometers) of Jupiter, on February 28, 2007, and actually used its proximity to gain a gravity assist boost from the massive gas giant. This gave New Horizons a speed boost of about 9,000 miles per hour (14,000 kilometers/hour). Taking advantage of that graviational slingshot, the voyage to Pluto was shortened by three full years. Score! Free energy!

New Horizons zoomed along, passing Saturn’s orbit in June of 2008, Uranus’s in March of 2011, and then Neptune’s in August of this year.

Next up: Pluto.

Throughout its journey, New Horizons has gone through hibernation/wake cycles more than a dozen times, in fact, spending about 2/3 of its time in an electronic slumber. During hibernation, most of the craft’s systems are powered down or entered into an extremely low-functioning state. This “reduced wear and tear on the spacecraft’s electronics, it lowered operations costs and freed up NASA Deep Space Network tracking and communication resources for other missions”.  Today, however, New Horizons is waking for good.

Beginning in February, the main observation objectives begin. Around the beginning of May, New Horizons will be capturing images of Pluto exceeding the resolution that Hubble was able to produce. For the next two months, Pluto will become more accessible to all of the spacecraft’s instruments. The closest approach is projected for July 14, where New Horizons will be within 6,200 miles (10,000 kilometers) of Pluto. New Horizons’s Long Range Reconnaissance Imager (LORRI) is expected to capture images on the scale of 50 meters per pixel and accomplish a handful of other primary and secondary scientific objectives.

But wait, there’s more!

In addition to Pluto, New Horizons will be observing and recording images and data from Pluto’s known moons: Charon, Hydra, Nix, Styx, and Kerberos.

And that’s still not all. Remember how I mentioned that New Horizons is on a solar system escape trajectory? That means the craft is going to continue hurtling away from the Earth and Sun, away from Pluto, and out beyond the ends of our solar system and into intergalactic space. Included in the craft and mission design, is fly-by opportunities for one ore more Kuipier-Belt Objects (KBOs), the residents of the Kuiper Belt. If you’re not familiar with the Kuiper Belt, think asteroid belt except much larger but instead of rocky asteroids, these bodies consist more of frozen gases such as methane, ammonia, and water. (Some of the moons of our solar system are believed to be former residents of the Kuiper Belt, but that’s another story for another time.) The ability to complete this mission will depend on targetable candidates and remaining fuel supplies.

After all of this, New Horizons slips into the furthest reaches of the Sun’s influence, the fascinating realm known as the outer heliosphere, including the heliosheath and heliopause (again, another story/another time). If the craft is still alive at this point, New Horizons will continue the work of the Voyagers in mapping this interesting environment.

That’s it for today. Stay tuned for updates on this historical mission, and much, much more!