Pluto - the planet of surprises

Pluto - the planet of surprises

We explore because we are human

Prof. Stephen Hawking

Pluto is in the news this week and with good reason. Pluto, as most people are aware, is more than just a beloved Disney character or Roman God of the underworld. However, most people would struggle to tell you whether Pluto is even a planet - which it is… sort of. For most, we remember the science textbooks of a long forgotten youth, telling us that Pluto was the last thing of significance in our solar system before the long journey to our next nearest star (Alpha Centauri at 4.4 light years away. If your mind does not work in light years in kilometres that is 41 followed by 12 zeroes).

Children around the world, have been able to name Pluto for 80 years or more now, but in this time Pluto has remained a quirk in science. We know it is there, very accurately in fact, and we know the way it moves, but we do not really understand what is there.

The problem is that Pluto is a long way away.

No, really! A really, really long way away.

7.5 billion kilometres (or about 4.7 billion miles).

Now that’s a stupidly big number that human brains cannot really deal with, so let me try and put that into terms that your brain has a better chance of dealing with. If you tried to drive in a car (that somehow worked in space), travelling at a typical 100 km/h (62 MPH), you would be just about getting to Pluto in 8556 years.

Or to put it another way, if Earth was a basketball, then at that scale Pluto would be about the size of a golf ball… 80 km (50 miles) away!

When you start to understand the scales involved you can start to see the problems in finding out basically anything about Pluto.

“But wait!”, you might say. “We have the Hubble Space Telescope, and we can see all these galaxies, and stars, that are a whole universe away!”

To understand why a rock two-thirds the size of the moon is so difficult to see requires a little understanding of the history of Pluto.

Pluto’s discovery

Back in 1840, astronomers were observing Uranus’ orbit. According to Newton’s laws of physics, the planet should be moving in a great big circle around the Sun, as the Sun’s gravity should be the only real factor in consideration. But this was not the case. Little wobbles in Uranus’ orbit could be seen. These distortions in the circular orbit could only be from the gravity of another, unseen planet.

After some searching, a French mathematician in 1846, Le Verrier, calculated the position of where this new planet should be. He sent the coordinates to his German astronomer friend in Berlin, Galle, who with a little help from his apprentice, became the first man to (knowingly) observe Neptune.

Following the discovery of Neptune, a lot of time was spent studying Neptune’s orbit. With careful observation, astronomers found that Neptune’s orbit did not account for the wobbles seen in Uranus. There must be another planet! And so began the great hunt for Planet X.1

Pluto remained very difficult to spot for a long time. The mathematicians, from observation of the gas giant planets, had calculated several possible locations to look in the night sky. With photography becoming more popular, astronomers started taking photographs of the night sky at identical positions spaced a few days apart. By rapidly superimposing the pictures from a few days apart, any differences should jump out from the slides.

Finally, in 1929 Pluto was seen for the first time in these slides2

!(Pluto discovery plates)[]

With the benefit of hindsight, astronomers could look back at previous observations, and it actually turned out that there had been 16 unwitting observations of Pluto, dating as far back as 1909.

From first sighting…

For a long time, Pluto remained exactly the way it had first been observed back in 1929.

Due to the extreme distances involved, even the best telescopes in the world struggled to get a good look at Pluto. All we saw was a faint twinkle in the night sky. Almost everything relating to Pluto has based more upon speculation rather than real science.

In 1972, Pioneer 10 and 11 probes were launched to look at Jupiter and Saturn. By getting good data on Jupiter, it allowed better estimates of Pluto to be inferred. But 1977 saw the launch of the Voyager space probes - making use of the favourable alignment of Jupiter, Saturn, Uranus, and Neptune, allowed scientists the first real data of what the gas giants really looked like. Again, getting data on planets far away really helped with the estimates of Pluto’s size, location and orbit.

Voyager 1 could have made a trip to Pluto back in the 1980s, but with limited fuel onboard, NASA engineers instead decided to investigate Saturn’s moon Titan instead.

2006 saw the Hubble Space Telescope focus its mirrors towards the distant planet, returning our first real look at what Pluto really might look like. As you can see the images are much improved, but are still sorely lacking3

Animated Pluto rotation4

Is Pluto even a planet?

Pluto’s distance from just about everything made its classification difficult for many years. Afterall, in the night sky Pluto is 50,000x fainter than Mars.

The initial observations made of the orbits of Uranus and Neptune estimated Pluto to have a mass approximately equivalent of Earth. But as mathematicians refined their methods and understanding of planetary movement improved throughout the 1940s, Pluto’s size was downgraded to the size of Mars in 1948.5

In 1976, with careful analysis of the sunlight being reflected by the surface of Pluto, scientists in Hawaii concluded that the surface must be made of methane ice. Methane ice is highly reflective, meaning that the estimates of Pluto’s size, at the time, based on reflected light were vastly overblown. This new information reduced the estimated size of Pluto to 1 % of Earth.

Then the moon of Pluto, Charon, was discovered (approximately a third of the size of Pluto). This information combined with new data on Neptune (thanks to Voyager 2), further reduced Pluto’s estimated size to 1/459th that of Earth.

Suddenly, a planet originally thought to be the same size of Earth, had been reduced to being not even as wide as the US.

But size was only one problem confronting Pluto’s planet status.

Size isn’t everything

From 1992, many new objects were discovered orbiting in the same area as Pluto. The more astronomers looked, the more they found. These objects, as it turned out, belonged to what is now called the Kuiper Belt - an asteroid belt on the outside of the solar system, extending between 4.5 - 7.5 billion km from the Sun. The Kuiper Belt is essentially the same as the asteroid belt between Mars and Jupiter, except only 20x bigger.

Solar masses diagram by Robert Ziche6

As several objects in the Kuiper belt were found in sizes approaching that of Pluto’s calls began to be made to reclassify Pluto as just another asteroid.

Then in July 2005, Eris was discovered. On discovery, Eris had an orbit around the Sun similar to Pluto, except it appeared to larger in mass than Pluto. At the time, several experts and the press were calling Eris the 10th planet,7 whilst others were calling for the Eris to replace Pluto as the 9th planet.

Finally in 2006 the International Astronomical Union (IAU) could ignore the problem no longer and set about drawing up new definitions for what constitutes a planet.8

  1. A planet must orbit the Sun
  2. A planet must be large enough such that the planet’s gravity creates a nearly spherical shape with only minor distortions
  3. A planet must over time, use its gravity to clear the space around its orbit of small objects

Pluto passes criteria 1 and 2 because it orbits the Sun and it does have a spherical shape.

However, Pluto does not pass the third criterion. Earth has a mass approximately 1.7 million times that of the all the other objects in Earth’s orbit combined. Pluto’s mass by comparison is only 7 % of the combined mass of all the objects in its orbit.

As a result from 2006, Pluto along with Eris and a few other objects have been reclassified as dwarf planets. The simple truth is that, if Pluto were discovered today it would not be classified as a planet, the fact that it is, is a result of a historical error.9

Whilst Pluto is no longer officially a formal planet, it is important to note that only 5 % of the IAU actually voted for this reclassification in 2006. There are also some very loud opponents to the new guidelines of what makes a planet. These opponents claim that by these definitions, the Moon should be considered a planet, whilst Earth, Mars, Jupiter and Neptune should not.10

Why is NASA interested?

Pluto and the Kuiper Belt represent a big gap in the knowledge of how our solar system works. For 80 years, a planet existed in our solar system and yet, we had no idea what it looked like, or how big it was.

From even the earliest observations, Pluto was the exception to so many rules. The inner planets of the solar system are all rocky, yet all the other outer planets are gas giants. Pluto (and its moon Charon) belong to a third category known as ice dwarfs - they have solid surfaces, but the majority of the surface is ice.

The American National Academy of Sciences has long ranked exploration of the Kuiper Belt and Pluto as of the highest priority for solar system exploration.11 It is hoped by taking a close-up look at the outskirts of the solar system that we will learn of its nature and that we may reveal the origins of the solar system.

Of course, NASA has a little bit of pride invested as well. The US has been the first nation to reach every other planet in the solar system and when New Horizons was launched, Pluto has still a planet.

NASA New Horizons group photo

New Horizons

Pluto is a long way away. A really long way away. To get any information about it, really required a trip.

After much political fighting, then years of designing and building, the New Horizons space probe left Earth from Cape Canaveral in 2006. New Horizons immediately set a record as it departed at an amazing 58536 km/h (36,373 MPH).

If you’re struggling to remember the start of 2006, then James Blunt’s Your Beautiful and Shakira’s Hips Don’t Lie, all came out months after New Horizons launch.

New Horizons stopped by Jupiter in 2007, recording a huge amount of new data - including the first detailed images of Jupiter’s spot. After 4 months of observation, in June 2007, New Horizons used Jupiter’s gravity as a gigantic slingshot, accelerating up to 82,000 km/h (51,000 MPH) towards Pluto.12

Even with these immense speeds, it took New Horizons, 9 years, 5 months and 25 days to get to Pluto from Earth.

Remember our analogy from earlier, if Earth was a basketball, then Pluto would be about the size of a golf ball 80 km away? At that same scale, the New Horizons probe is a tiny fraction of a grain of sand.

Phoning home

The distance between Earth and Pluto, is so big, that even at the speed of light, transmissions take 4.5 hours to get back to Earth. Then, when the signals do get back to Earth, they are so weak that NASA has to use a 60 m (200 ft) antenna to detect them.

Even then the transmissions are impossibly slow. Due to the distances involved and the amount of possible interference, transmission has a 1 kbps download speed - 50x slower than a dial-up modem. That means, a picture the size of the screen that you are reading this on, would take about an hour to download.

As New Horizons approached Pluto, at some 49500 km/h, everything went silent. The piano-sized space probe has all of its scientific instruments mounted on the same side as the radio antenna to protect them as much as possible from the dangers of space. On approach, it simply could not face Pluto and transmit at the same time.

All NASA could do was wait. For 21 hours. New Horizons had to pass within 12,500 km (7,800 miles). 150 km too high or too low and the instruments onboard would not be effective.13

New Horizons mission control flight team

As you can see those guys were pretty happy when New Horizons finally called in.

New Horizons collected so much data in its fly-by that NASA is predicting that it will take 16 months to send it all back to Earth.

Where next?

What is next for New Horizon is really a question of money and politics.

The initial $700m project was really designed for Pluto exploration with possible extension. Whilst New Horizons will continue to broadcast for months to come, exploration of the Kuiper Belt and one or two of its icy mini-worlds will require an extended mission fund approval from NASA brass to keep ground crew and access to radio-antenna (to send and receive the data).14

If New Horizons is approved for an extended mission, then it will be due to fire its main thruster in around 2 weeks and set a trajectory towards the first Kuiper Belt Object. This object will be a giant rock, with a diameter of around 50 km, and will take around 2 to 3 years to get there. The object will then be extensively mapped and tested, before possibly moving onto a second Kuiper Belt Object - depending upon fuel and favourable trajectories.

At this current moment scientists are still trying to decide which object to set course for. So little was known about this region of space prior to New Horizons getting there, scientists will be frantically pouring over all of the long-range data collected on the approach to Pluto to select the best target.

Why go to all this effort?

Space travel has always had an ability to inspire the best in humanity. Satellites, space probes and space stations are constantly pushing the boundaries of our abilities and constantly improving the world in which we live. From communications and navigation to diaster relief and weather prediction. All would be impossible without advances in space exploration, even with very valid criticisms of NASA bureaucracy and wasted development.15

The New Horizons mission might at first seem to have little relevance other than for the US to flaunt its technological prowess with a few HD pictures. That being said, when Christopher Columbus set sail, he was only looking to further himself with a few riches.

1 trip around the Sun for Pluto equates to 248 Earth years. Last time Pluto was in this location, the US did not exist, nor did the theory of evolution, nor the knowledge that germs transmitted disease.

The last time we saw Pluto here, humanity’s first attempts at aviation were decades away - space travel almost 2 centuries.

As we explore more and more of the universe, the more, we as a species, become mesmerised by it. The future is always uncertain, but who is to say that by the time Pluto comes around again we will not be waiting with just a probe but a whole colony?

Other random trivia

To impress your friends here are 5 bonus facts I found in my research that had no good place in the main article, but are still amazing!

  1. New Horizons payload includes
  • ashes of Clyde Tombaugh (the man who discovered Pluto back in 1930)
  • 2 US flags
  • 2 state quarters from Maryland (the build site) and Florida (the launch site)
  • a CD-ROM of 434,000 names of signees from 2005
  • a DVD-ROM with pictures of New Horizons team
  • a piece of SpaceShipOne (the first privately owned space craft)
  • US stamp from 1991 saying “Pluto not yet explored”
  1. This is NASA’s first mission that is investigator-led. The leaders of the project are all university professors and not a member of the military or NASA administrator.
  2. New Horizons is the first space probe to include an instrument built by students, from the University of Colorado.
  3. The whole space probe operates on a single radioisotope generator, which supplies just over 200 watts, about 2 old-fashioned light bulbs.
  4. The images from 10 weeks before New Horizons got to Pluto were already better than those obtained by Hubble.

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