What You Need To Know About The New Horizons Mission To Pluto In 10 Infographics

The New Horizons mission team has released some very cool infographics that illustrate the amazing journey to Pluto and the science we will do at this new frontier.

If you'd like to follow along with NASA's New Horizons Mission to Pluto and the Kuiper Belt, please download our FREE Pluto Safari app for iOS and Android.  It is available for mobile devices. Simulate the July 14, 2015 flyby of Pluto, get regular mission news updates, and learn the history of Pluto.

Simulation Curriculum is the leader in space science curriculum solutions and the makers of Starry Night, SkySafari and Pluto Safari. Follow the mission to Pluto with us on Twitter @SkySafariAstro, Facebook and Instagram. 

Where's Pluto? How to See it Through A Telescope

With NASA’s New Horizons probe zeroing in on Pluto, due to pass it on July 14, attention of astronomers all over the world is focusing in on Pluto.

Let’s leave aside the question of whether Pluto is the smallest planet of the Sun or the largest of the Kuiper Belt Objects, and agree that it is an interesting and mysterious member of the solar family.

Many amateur astronomers are interested in seeing Pluto with their own telescopes, and this is what we will discuss here. Pluto is at present around 14th magnitude, requiring a telescope with at least 8 inches (200mm) aperture to be seen. The good news is that it is traveling in front of a rich part of the Milky Way, so there will be plenty of guide-posts among the stars to help you find it. The bad news is that it is easily lost amongst those stars, because it will look no different from a 14th magnitude star.

With Starry Night, I have plotted a series of charts zooming in on this tiny target. The first chart is what you will see with naked eye and binoculars at 3 a.m. this week: the familiar “teapot” of Sagittarius. Use Ascella and Nunki in the “handle” of the teapot to locate the two 4th magnitude stars Omicron and Xi2 in your telescope.

With naked eye and binoculars, locate Pluto in relation to the well-known “teapot” asterism of Sagittarius. It is close to the stars Chi2 and Omicron Sagittarii, just north of the “handle” of the teapot.  Credit: Starry Night software.

Switch to a low power eyepiece to see the view in the second chart. To give you some idea of scale, this chart shows the position of the New Horizons probe, although it is too faint to be visible in even the most powerful telescopes on Earth.

With a low power eyepiece in your telescope, zero in on Xi2 and Omicron Sagittarii. Credit: Starry Night software.

Notice the wide triangle of 9th magnitude stars just below Pluto’s location which points to it. The left two stars of the triangle point to a wide pair of stars at about 10th magnitude, and these will serve as a reference to locate Pluto’s position in the third chart.

This chart shows the position of Pluto for the next eight nights, as seen in a high-power eyepiece. The left-most dot, labeled “Pluto,” is its position tonight at 3 a.m., the rightmost dot, its position on June 4. Remember that the date changes at midnight. Credit: Starry Night software.

The brown dots in this chart show Pluto’s position at 3 a.m. EDT on the nights of (from left to right) May 28 through June 4. These dates are for the second half of the night (after midnight), when the date has changed to the next day’s date.

Pluto will resemble a tiny star, and the only way to make sure you are looking at the right star is to make a careful plot of the stars in the field. Comparing positions the next night will tell you for sure which “star” is Pluto: It’s the one that has moved.

So, to positively identify Pluto, is essential to observe it on at least two successive nights.

Because of the great interest in Pluto with the impending fly-by of New Horizons, Simulation Curriculum has released a free app for iOS and Android called Pluto Safari. This will be updated with new information as the fly-by approaches.

If you'd like to follow along with NASA's New Horizons Mission to Pluto and the Kuiper Belt, please download our FREE Pluto Safari app.  It is available for mobile devices. Simulate the July 14, 2015 flyby of Pluto, get regular mission news updates, and learn the history of Pluto.

Simulation Curriculum is the leader in space science curriculum solutions and the makers of Starry Night, SkySafari and Pluto Safari. Follow the mission to Pluto with us on Twitter @SkySafariAstro, Facebook and Instagram. 

The Next Pluto Mission: Part III

Continued from Part II ...

PEOPLE ON PLUTO

Now let’s have some fun.  Suppose, this coming July, New Horizons were to discover something truly wild as it flashed past Pluto.  What if it revealed a bizarre surface chemistry that - like the oxygen in Earth’s atmosphere - could only be the result of some biological process?  What if its imager recorded a clearly artificial set of markings on its surface - a giant pyramid, the ruins of an alien civilization.  (What if the cameras revealed a large, goofy-smiling dog?)

In light of such a monumental discovery, we might very well skip the next logical step of a robotic Pluto lander, and instead mount a manned mission.  I’ll put aside questions of cost for now, and assume that for the sake of this speculation, a manned Pluto mission - like the Apollo program - is just something that we were going to do, no matter what.  Is a manned Pluto mission within our near-term technological grasp, at any cost?

The most advanced propulsion systems we have today require 10 - 15 years to deliver a 1.6 kilogram spacecraft into Pluto orbit.  The international space station, though lacking significant propulsion, has been continuously orbiting the Earth, manned, for 14 years, since 31 October 2000.  There is, of course, an enormous difference between the ISS and a manned Pluto spacecraft.  The ISS has been resupplied and occupied by rotating crews from Earth’s surface several times per year for the past 14 years.  The Pluto astronauts would be utterly isolated; their life support systems would have to be completely self-contained.  The longest period one human being has ever spent in space is 437 days.  And no small, closed, self-contained biosphere capable of supporting human life has survived more than two years.

Tracy Caldwell Dyson aboard the International Space Station (ISS).

What if we put our Pluto-bound astronauts into hibernation?  Aside from the possibility of the mission control computer becoming homicidal during wakeup phase, there’s another objection: we don’t currently know how to hibernate human beings for more than a decade and have them come back alive.  For that reason, I’m forced to relegate hibernation scenarios to science fiction, and rely on technologies which are known at the present time.

DROPPING THE BOMB

Is there any known spacecraft propulsion technology capable of delivering a multi-hundred-ton manned mission to Pluto within a year?  It turns out that the answer is yes, and that the technology has been with us since the 1950s.  Science fiction buffs reading this piece will probably have guessed that the answer is Project Orion.  For everyone else, the Wikipedia article on that topic gives a good overview.  Briefly, the concept is to propel the spacecraft by exploding thousands of small nuclear bombs behind it.  Each detonation drives a “pusher plate” attached to the spacecraft by an enormous set of shock absorbers.  The exhaust velocities are tens to hundreds of kilometers per second, but with millions of tons of thrust.

An artist's conception of the NASA reference design for the Project Orion spacecraft powered by nuclear propulsion.

The original Project Orion physicists worked out the essentials in the early 1960s.  NASA revisited the concept again in 2000, this time under the name “External Pulsed Plasma Propulsion”.  The smallest Orion nuclear spacecraft have a mass of about 900 tons.  The original team developed an “advanced interplanetary” configuration capable of delivering a 10,000-ton spacecraft to Saturn and back again in three years.  While such a spacecraft could be launched directly from the Earth’s surface, nuclear fallout concerns would make this course of action untenable.  Instead, it would have to be constructed in Earth orbit - like the ISS - and depart for Pluto from there.

A year or two later, our nuclear-bomb-firing mothership would decelerate into orbit around Pluto, and turn its engines off.  A manned descent to Pluto’s surface would take place using more conventional chemical rockets.  Pluto’s surface gravity is about 1/12 of the Earth’s, or half of the Moon’s.  Landing on Pluto’s surface from a low orbit at 100 kilometers’ altitude requires half the delta-V of a landing on the Moon from the same height (800 meters/sec vs. 1700 meters/sec.)

Landing any spacecraft - let alone a manned spacecraft - on Pluto would present some unique challenges.  Unlike the Moon, Pluto has a very thin atmosphere of nitrogen, methane, and carbon monoxide.  Its surface pressure is varies from 6.5 to 24 micro bars - about as thick as Earth’s atmosphere 50 miles up, or about 1/1000th the density of Mars’s atmosphere at its surface.  This is probably just enough to require some kind of heat shield, but not enough to provide any useful aerobraking capability (like a parachute).  Elon Musk’s Dragon V2 capsule combines a heat shield with propulsive landing rockets, and is probably a step in the right direction.  The Dragon V2 stores enough fuel for 300 meters/second delta-V, so extra fuel tanks would be needed to land, take off, and rendezvous with the orbiting mothership.  But the technology seems feasible.

The SpaceX Dragon V2, during a test of its abort system.

There might be other hazards.  The Moon’s surface is mostly made of silicate rock.  Pluto, on the other hand, is covered with ice - not just water ice, but frozen methane, carbon monoxide, and nitrogen.  On contact with hot rocket exhaust at several thousand degrees, there’s a real danger that the landing site might vaporize.  Some care would have to be taken to land our first Pluto explorers on a stable, rocky outcropping.

THE VIEW FROM PLUTO

Imagine you’re one of those first human Pluto explorers, stepping out of your lander.  Pluto’s moon Charon would hang motionless in your sky.  The two are tidally locked, always presenting the same face to each other as they orbit over a 6.37 day period.  But at only 19,600 kilometers away - closer than our geosynchronous satellites - Charon would appear nine times larger in Pluto’s sky than the full Moon appears from Earth.  Pluto’s other four moons Nix, Hydra, Kerberos, and Styx would be visible as slowly-moving stars, gradually rising and setting, while Charon remained fixed in the heavens.

Charon as seen from the surface of Pluto.

The Sun would be the brightest object in the sky, but would look nothing like it does in ours.  Pluto’s Sun is only an arc minute across, and would appear starlike.  But what a star!  At magnitude -19, it would appear 650 times brighter than our full Moon, will all that brightness packed into an icy, diamond-like point.

Jupiter would be the brightest planet in your sky, around magnitude 2.5, somewhat fainter than the stars in the Big Dipper.  Saturn would vary in and out of naked-eye visibility, from about magnitude 4.5 to 8.5.

And if you looked carefully, appearing about three full-Moon diameters away from the starlike burning Sun, you might notice another, much fainter, bluish “star”.  That pale blue dot would be the Earth: at magnitude 3.7, still visible to your unaided eye, but difficult to pick out from the Sun’s glare.  That’s home.  You’ve come a long way to this cold, lonely outpost at the edge of the Solar System.  And unlike New Horizons, you’re coming back.

Science fiction?  Possibly.  But let’s not forget that Pluto was discovered only 85 years ago.  Today, a spacecraft carrying the ashes of its discoverer is speeding toward that planet: a fact unimaginable in 1930.  What will the next 85 years hold?  If there’s anything you should count on, it’s not to count anything out.

If you'd like to follow along with NASA's New Horizons Mission to Pluto and the Kuiper Belt, please download our FREE Pluto Safari app.  It is available for iOS and Android mobile devices. Simulate the July 14, 2015 flyby of Pluto, get regular mission news updates, and learn the history of Pluto.

Simulation Curriculum is the leader in space science curriculum solutions and the makers of Starry Night, SkySafari and Pluto Safari. Follow the mission to Pluto with us on Twitter @SkySafariAstro, Facebook and Instagram. 

The Next Pluto Mission: Part II

Continued from Part I ...

ROCKET SCIENCE 101

All spacecraft are limited by Tsiolkovsky’s rocket equation, named after Konstantin Tsiolkovsky, the 19th century Russian founding father of astronautics.  Tsiolkovsky’s rocket equation determines the speed a rocket can attain, based on the rocket’s exhaust velocity, the “dry mass” of the rocket (without fuel), and the amount of fuel it carries.  Here’s the essential take-away: for your rocket to go faster than the exhaust from its burning fuel, it needs to carry a lot more fuel.  You need exponentially more fuel the faster you want it to go.  And this is also true in reverse: if you are already going very fast, your rocket will need exponentially more fuel to slow down.

Here’s the math:

DV = Ve * ln ( (Md + Mp) / Md )

DV = Delta-V, total velocity change produced by rocket after all fuel is exhausted
Ve = Exhaust velocity
Md = Dry mass of rocket
Mp = Mass of propellant (fuel) carried by rocket

New Horizons has a dry mass of 400 kilograms, and carries about 78 kilograms of hydrazine fuel.  That fuel has an exhaust velocity of about 2.2 km/sec.  Plugging those numbers in, that means New Horizons’ rocket motors can change its speed by at most 390 meters per second.  New Horizons is moving past Pluto at 14 kilometers per second.  So this is not nearly enough to slow down and achieve orbit around Pluto.  To shed 14 km/sec, New Horizons would need to burn 580 times its own weight - or 232 metric tons - of hydrazine fuel.

A more efficient fuel, like the liquid hydrogen and oxygen in the Centaur upper stage, has an exhaust velocity around 4.4 km/sec.  That improves things quite a bit, but New Horizons would still have to carry 24 times its dry weight in fuel to slow down from 14 km/sec.  For comparison, the Centaur upper stage which boosted New Horizons toward Pluto has a dry mass of 2.2 tons, and carried 20.8 tons of fuel.  Its maximum theoretical delta-V is 10.2 km/sec.  In other words, the fully fueled Centaur booster, carrying no space probe at all, could not slow itself down enough to enter Pluto orbit.  Some even larger rocket would have to be built to slow the Pluto orbiter into orbit around Pluto.

And don’t forget - that Pluto orbiter and its 50+ tons of fuel would still have to be launched from Earth, on an escape trajectory toward Pluto, in the first place.  The Atlas V that launched New Horizons couldn’t possibly do this.  The largest rocket under construction - NASA’s Space Launch System - can deliver a maximum payload of 7 tons to Jupiter.  That is hopelessly insufficient to deliver a Pluto probe with 50 tons of fuel to Pluto.

GOING SMALL

Instead of making a larger rocket, how about making a smaller payload?  During the early 2000s, while New Horizon was in its early design phases, a (literally) small revolution was taking place in satellite design.  The first CubeSats packed all the essential elements of an functioning satellite into a 10x10x10 cm cube weighing less than 1 kilogram, and were launched in 2003.  To date, hundreds of CubeSats have been launched, and are becoming increasingly capable as technology advances.  On the first SLS launch, scheduled for late 2018, NASA plans to deploy three CubeSats from the SLS upper stage as it passes the Moon, to see how well they’ll perform at interplanetary exploration tasks.  (By the way: want to win $5 million?  You can compete for the chance to launch your own lunar CubeSat on the maiden SLS launch.  Here’s how: http://www.nasa.gov/cubequest/details

CubeSats in space.

In 2014, a San Francisco startup called Planet Labs launched a constellation of several dozen CubeSats.  These CubeSats are capable of imaging the entire Earth’s surface at a resolution of 5 meters, every day.  Each of those “3U” CubeSats weighs about 4 kilograms.  Suppose the next Pluto mission wasn’t a single orbiter, but rather 10 tiny 3U CubeSat Pluto orbiters.  This whole fleet of Pluto orbiters would weigh 40 kilograms - about 1/10th the mass of New Horizons, and the same as a 6th grader.  A single ton of liquid hydrogen/oxygen fuel could decelerate this tiny payload into Pluto orbit.  Launch from Earth on an SLS, with Jupiter flyby and gravity assist, then direct orbit insertion around Pluto, becomes thinkable.

With this approach, there are a lot of benefits - you get ten Pluto orbiters instead of one - but there are a lot of challenges to overcome.  Solar panels are useless at 40 times the Earth’s distance from the Sun.  So our hypothetical Pluto CubeSat orbiters would need require tiny nuclear reactors for electricity.  A large radio dish several meters across would be needed to beam any useful amount of data back home.  Deploying such a dish from a spacecraft 10 centimeters across is not easy, although inflatables are currently in development.  Interplanetary laser communication systems, as demonstrated in 2013 by NASA’s LADEE moon orbiter, may be the right answer.  These are the cutting edges of today’s spacecraft technology, and you can see why NASA is funding competitions to spur their development.

A NEW DAWN

On September 28th, 2007, eighteen months after New Horizons launched, another NASA dwarf planet explorer lifted off.  Less than four years later, in July 2011, the Dawn mission entered orbit around the asteroid Vesta.  It departed Vesta in September 2012, and entered orbit around the dwarf planet Ceres just this past March.

Dawn spacecraft at Ceres.

Dawn is NASA’s first interplanetary mission to be propelled by electricity.  Instead of a high-thrust rocket engine burning many tons of chemical fuel over the course of a few minutes, Dawn’s electric engines emit tiny amounts of electrically charged ions - at much higher velocities than chemical rocket exhaust.  Dawn’s engines only consume three milligrams of fuel per second.  But the ions emitted from Dawn’s engines have an exhaust velocity over 31 kilometers per second.  That produces a thrust of 91 millinewtons, or about the force a piece of paper exerts on your hand when you pick it up.  Still, the thrust is constant, adding 24 km/hour per day, day after day, to the spacecraft’s velocity.  Over the course of 67 days, the accelerations adds up to a velocity of 1,000 mph.  Dawn carried 425 kilograms of propellant (as opposed to the Centaur booster’s 20.8 tons), and yet Dawn can perform a velocity change of more than 10 km/sec over the course of its mission.

What if New Horizons were equipped with an ion engine, like Dawn’s?  Some modifications would be needed.  Again, solar panels are near-useless at Pluto’s distance from the Sun, so “New Dawn” would need a nuclear reactor quite a bit more powerful than New Horizon’s (whose nuclear reactor produces 250W of electrical power vs. Dawn’s 1400W solar panel array.)  Dawn also weighs about twice as much as Hew Horizons (780 vs 400 kg dry.)  It seems reasonable to guess that a nuclear-powered, ion-propelled Pluto orbiter with the same 30 km/sec exhaust velocity as Dawn could be built with a total spacecraft dry mass of one metric ton.  Plugging those numbers into Tsiolkovsky’s rocket equation, our “New Dawn” Pluto orbiter would need 620 kilograms of Xenon fuel to decelerate from 14 km/sec cruise velocity into orbit around Pluto.  Something like this could conceivably be launched from Earth by the same Atlas V 551 that actually launched New Horizons.  It could certainly be launched by the Falcon Heavy or SLS.

How long might this spacecraft take to reach Pluto?  Again, we’ll assume a gravity-assist slingshot by Jupiter, barreling toward Pluto at ~14 kilometers per second.  “New Dawn”, firing its ion engine continuously, would need 1.7 years to shed this velocity as it approached Pluto.  Given an 8-year cruise from Jupiter to Pluto, this seems amply doable.

Dawn was launched little more than a year after New Horizons.  Its technological development schedule paralleled New Horizons’.  The next time Jupiter and Pluto properly align for a gravitational slingshot maneuver will be in 2018-2019.  Using today’s ion propulsion technology, NASA could conceivably mount a New Dawn-like Pluto orbiter mission in just a few years.

 

Continued in Part III...

 

 

If you'd like to follow along with NASA's New Horizons Mission to Pluto and the Kuiper Belt, please download our FREE Pluto Safari app.  It is available for iOS and Android mobile devices. Simulate the July 14, 2015 flyby of Pluto, get regular mission news updates, and learn the history of Pluto.

Simulation Curriculum is the leader in space science curriculum solutions and the makers of Starry Night, SkySafari and Pluto Safari. Follow the mission to Pluto with us on Twitter @SkySafariAstro, Facebook and Instagram. 

The Next Pluto Mission: Part I

On July 14th, NASA’s New Horizons spacecraft will fly by Pluto.  It’s among NASA’s most impressive achievements to date.  But what might come next?

New Horizons was launched on January 19, 2006, atop an Atlas V 551 rocket with a Centaur upper stage.  That upper stage, and the New Horizons probe inside, had highest launch speed of any man-made object leaving Earth.  New Horizons crossed the Moon’s orbit just 9 hours after launch - the Apollo astronauts took three days - and reached Jupiter in just over a year  (the Voyager spacecraft took nearly three years).  

Launch of New Horizons. The Atlas V rocket on the launchpad (left) and lift off from Cape Canaveral. New Horizons‍ ' launch was the fastest ever to date, at 16.26 km/s.

New Horizons then used Jupiter’s gravity to slingshot itself onto a hyperbolic trajectory that intersects Pluto just over eight years later.

A composite image of Jupiter and Io, taken on on February 28 and March 1, 2007 respectively. Jupiter is shown in infrared, while Io is shown in true-color.

By the time New Horizons reaches Pluto this July, it will be moving at nearly 14 kilometers per second relative to the planet.  That’s 30% faster than the ISS orbits the Earth.  The probe will flash by Pluto in just a few hours.  New Horizons can’t slow down.  It doesn’t carry enough fuel to enter orbit around, or land on, Pluto.  Nor was it designed to.  Instead, New Horizons will keep flying past Pluto, into a vast outer region of our solar system called the Kuiper Belt.  New Horizons may fly by a few Kuiper Belt Objects after its Pluto encounter, a few candidate KBOs are being selected now.

New Horizons flyby of Pluto and Charon on July 14, 2015. Created with Pluto Safari, a free app for iOS and Android.

But what if New Horizons had been intended to stay longer at Pluto?  After a flyby, the next step in planetary exploration is an orbiter to perform extended surface observations, and then a lander.  Are these things even possible, within current technology?  Pluto is forty times farther from the Earth, than Earth is from the Sun.  Transmissions radioed back by New Horizons take four and a half hours to reach us.  Is there any hope of catching anything more than a fleeting glimpse of such a distant place?

Continued in Part II...

If you'd like to follow along with NASA's New Horizons Mission to Pluto and the Kuiper Belt, please download our FREE Pluto Safari app.  It is available for iOS and Android mobile devices. Simulate the July 14, 2015 flyby of Pluto, get regular mission news updates, and learn the history of Pluto.

Simulation Curriculum is the leader in space science curriculum solutions and the makers of Starry Night, SkySafari and Pluto Safari. Follow the mission to Pluto with us on Twitter @SkySafariAstro, Facebook and Instagram. 

Pluto Is A Planet, And So Is Eris

Is Pluto a planet?  What is a planet, anyhow?  We hope you’ll agree that the IAU's current answers to these questions are unclear and confusing.  Here, we propose clear and unambiguous answers to these fundamentally unclear problems.  Above all, we hope you have fun with the debate, no matter what side of it your heart may lay on.

The Planet Definition Mess

 As astronomers began to discover objects similar in size to Pluto, culminating with the discovery of Eris in 2005, it quickly became clear that if Pluto was a planet, so should Eris.  And if Eris was a planet why not some of the other newly discovered  objects. Our solar system might have dozens of planets.  One camp felt that a line needed to be drawn somewhere, and another camp felt that the newly discovered objects should be added to the list of solar system planets.

 

Illustration of the relative sizes, albedos, and colours of the largest trans-Neptunian objects.

Illustration of the relative sizes, albedos, and colours of the largest trans-Neptunian objects.

In 2006 the International Astronomical Union (IAU) met with the intention of solving the debate once and for all.  The goal was to come up with a definition for “planet”, which had never been done before.  After many days of contentious debate, the IAU passed the following resolution:

RESOLUTION 5A

The IAU therefore resolves that planets and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way:

(1) A "planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

(2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape [2], (c) has not cleared the neighbourhood around its orbit, and 

(d) is not a satellite.

(3) All other objects, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".

This is a poor definition that has only served to add more confusion.  With resolution 2c,  “has cleared the neighborhood around its orbit”, the IAU is trying to express that a planet should be the dominant gravitational force in its local region of the solar system.  That's not an unreasonable position.  Certainly the Earth and Jupiter are the dominant objects in their local regions.  But have any of these planets actually "cleared the neighborhood" around their orbits?  No.  Pluto is still clearly in Neptune's "neighborhood".  For that matter, Jupiter has two well-known groups of asteroids, the "Trojans", which lead and follow Jupiter along in its orbit.  For that matter, the Earth hasn't quite "cleared the neighborhood" around its orbit, either, as anyone who was near Chebalyink, Russia on Feb 15th, 2013 or Tunguska, Siberia on June 30th, 1908 can attest to.  So are Earth, Jupiter, and Neptune the dominant gravitational objects in their local neighborhoods?  Yes.  Have they "cleared their neighborhoods"?  No.

The Thousand Kilometer Rule

 Here is what the IAU should have resolved in 2006:

 (1) A "planet" [1] is a celestial body that (a) is in orbit around the Sun, (b) has a maximum surface radius greater than 1000 kilometers.

 (2) All other objects orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".

 "But that's completely unscientific" you say. "Why 1000 kilometers?  Why not 1200, or 750?"  I submit to you that the precise definition of a planet as an object at least 1000 kilometers in radius is no less "scientific" than the definition of a "kilometer" as being a unit of distance equal to 1000 meters, or a "degree" being 1/360th of a circle.

 Here is a list of the largest known objects orbiting the Sun, and their radii in kilometers:

Jupiter - 69,911
Saturn - 58,232
Uranus - 25,362
Neptune - 24,622
Earth - 6,378
Venus - 6,052
Mars - 3,390
Mercury - 2,440
Pluto - 1,184
Eris - 1,163
Makemake - 715
Haumea - 620
Quaoar - 555
Sedna - 498
Ceres - 475
Orcus - 458

By the 1000-kilometer definition, all eight classical planets would remain planets.  As would Pluto, and we add Eris.  The solar system would have exactly ten planets. Those fond of keeping Pluto's planetary status for historical reasons would retain its dignity.  And elevating Eris to a first-class planet would be an honorable nod to the cutting-edge astronomers whose work led to a need for this new definition in the first place.

And as to the "cleared the neighborhood" part of the definition?  This it the most unclear and least popular part o the IAU's 2006 definition.  It's best dealt with by being eliminated entirely.  The end game is to define the term "planet" in a manner that's simple, understandable, and satisfying.  The 1000-kilometer rule does this aptly.

If you'd like to follow along with NASA's New Horizons Mission to Pluto and the Kuiper Belt, please download our FREE Pluto Safari app.  It is available for iOS and Android mobile devices. Simulate the July 14, 2015 flyby of Pluto, get regular mission news updates, and learn the history of Pluto.

Simulation Curriculum is the leader in space science curriculum solutions and the makers of Starry Night, SkySafari and Pluto Safari. Follow the mission to Pluto with us on Twitter @SkySafariAstro, Facebook and Instagram.