Derby and District Astronomical Society

The Journal of the Derby and District Astronomical Society
January - April 2006

Into the Unknown Ė New Horizonsí Voyage to Pluto
By Anthony R. Southwell

The launch of New Horizons
The New Horizons spacecraft beginning its voyage to Pluto.  Image Credit: NASA/JPL
At 19:00 UTC on January 19th 2005 an Atlas V launch vehicle left its launch pad at Cape Canaveral carrying a very special item of cargo. The cargo in question, the New Horizons spacecraft, was hitching a lift on the Atlas V on the first leg of its incredible journey into the unknown, for New Horizons had an appointment set for Summer 2015, an encounter with the Solar Systemís most mysterious inhabitant, the frozen planet Pluto. The launch of New Horizons was originally set for January 17th, but the launch was scrubbed due to low-level gusting winds at the Cape, so the launch team was stood down for twenty-four hours. The next launch attempt on January 18th was also scrubbed, and yet again, the weather played a role. The weather conditions at the launch site were perfect, the wind speeds were within mission rules and everything looked like it was ready to go, and then the scrub was called. A storm that was in the vicinity of the New Horizons main control centre, the Applied Physics Laboratory at the Johns Hopkinsí University in Maryland, caused a power outage. So with New Horizonsí Mission Control Centre out of action, the launch could not take place. The launch was stood down again for another twenty-four hours.

It was at this point that the author began to feel a little bit worried, getting home as fast as I could so I could get NASA TV up on the Internet to watch the launch for two nights in a row, and seeing that the launch vehicle was still 'rooted' to the spot for two nights in a row, I began to get the feeling that this mission could be jinxed and that the malevolent beast that haunted many a planetary mission, 'The Great Galactic Ghoul' (those who know the history of the Jet Propulsion Laboratory will be familiar with this creature! Ė Editor) was waiting to claim another victim. So I resigned myself to another launch scrub and realised that I would not be back home in time to watch the launch 'live' over the Internet, as the new launch time was for 6:05 pm GMT on the 19th January, and I would still be stuck on the bus home. So the next launch day arrived and I got home from work at around 6:50 pm, fully expecting to have missed the launch, I didnít turn on the news until about 7:00pm, and what did I find on Sky News? Live coverage of the New Horizons launch! The launch was an hour late! The Atlas V had already left the pad and was now steadily accelerating away (I use the term 'steadily accelerating' in the loosest possible way! This vehicle really moves!). By this point the Atlas V was about two minutes into its flight and was at the point of jettisoning its strap-on solid rocket boosters (the launch vehicle had five of them). I immediately dropped everything and rushed to the PC and got NASA TV up and running, I watched the rest of the launch phase over the Internet.

Before too long the tracking cameras at the Cape lost the launch vehicle and NASA TV switched to a live telemetry feed from the vehicle. A computer graphic of the launch vehicle above the Earth was displayed, the graphic changed as the Atlas V went through its various programmed manoeuvres during the launch phase. At T + 4 minutes 34 seconds, the Atlas V core stage had shut down and separated from the rest of the launch vehicle. At T + 4 minutes 44 seconds, the Centaur upper stage fired and raised New Horizons orbit and added to it's velocity. Following the first Centaur burn, the combined Centaur/New Horizons spacecraft went on to cruise in orbit for about 10 minutes. During the cruise phase the Centaur fired its own control thrusters to keep a good trim during this coasting phase of the launch. You could actually watch these thruster firings 'live' via the computer graphic. You saw a thrusters fire and the graphic would slightly alter its orientation. It was almost like being there, you felt that you were riding along with the New Horizons spacecraft during its launch. Then after the coast phase of the launch the Centaur fired again at around 16 minutes or so after launch for 32 minutes and 23 seconds and added more velocity to New Horizons. At T + 42 minutes the Centaur shut down for the last time and separated from New Horizons, then the Star 48B solid motor third stage lit and fired for 52 seconds. At T + 47 minutes and 22 seconds the Star 48B separated from New Horizons after giving it the final push the probe needed to leave the Earth, and boy did New Horizons leave the Earth, from launch to Earth escape velocity took a little over 47 minutes.

The Atlas V/Centaur/Star 48B combination gave New Horizons and enormous amount of energy. The probe did not even make one orbit of the Earth before it headed out, it left the Earth at a speed of around 36,000 mph! Earth escape velocity is 25,000 mph, so New Horizons was really motoring. The most amazing thing I learnt on the night of the launch was that at the speed New Horizons was going, it would cross the orbit of the Moon in nine hours! Nine hours! It took the Apollo astronauts four days to travel the 240,000 miles to the Moon! To a child of the Apollo era such as myself this was just mind-blowing, I just sat there looking at these facts and figures on the screen before me feeling absolutely dumbfounded. New Horizons is the fastest spacecraft that has ever been launched. But the record breaking does not stop there, oh no! To get to Pluto, New Horizons will need a little bit more velocity, so it will pay a flying visit to Jupiter for a gravity assist, passing by Jupiter and using that giant planetís gravity to 'throw' it onward to Pluto. Travelling at 36,000 mph, it will take New Horizons just 13 months to get to Jupiter! It took the Galileo and Cassini spacecraft six and four years respectively to reach that behemoth of the Solar System (via much less direct routes than New Horizons, I grant you!) Once the gravity assist manoeuvre has been completed, Jupiter will have increased New Horizons velocity to a staggering 47,000 mph! This means that New Horizons will reach Pluto in 'only' nine years and after travelling a little over 3 billion miles, it is one fast little spacecraft!

I must admit once it was confirmed that New Horizons had separated from its final stage and was in good health and safely on its way, I did punch the air and yell "YES!". We were on our way to Pluto! As I have alluded to earlier in this very edition of Aries I do feel personally connected to this mission. A year or so ago I received an e-mail from a member of the Society informing me that the mission planners had invited people to submit their names so that they could be sent to Pluto on a CDROM that would be attached to the spacecraft. You could send our name to Pluto! I was not going to miss-out on that. So on January 19th 2006 435,000 people left the Earth on-board the New Horizons spacecraft bound for the unknown, I donít know how many DDAS members also signed up for the trip, but just in case there are any of them reading this article, hello there shipmate! I still find it exciting to think that my name is going to Pluto and beyond. I think that sending your name on a spacecraft to a celestial body is a wonderful way of involving people in the exploration of the Universe. It connects you with the mission in a very personal way. If I cannot physically visit any of these fascinating worlds, then at least my name can go there! This is the third spacecraft that I have submitted my name to, the first was Stardust, and then Deep Impact. So Iíve been to two comets so far, and New Horizons is my first planetary mission.

The launching of New Horizons was critical, the spacecraft had a launch window that opened on January 17th and closed on February 13th 2006. If the launch slipped during that period it could add years to New Horizonís journey as follows:

Launch Date Pluto Arrival Date
January 17th - 27th 2005 July 14th 2015
January 28th 2005 August 15th 2015
January 29th - 31st 2005 July 12th 2016
February 1st - 2nd 2005 July 11th 2017
February 3rd - 8th 2005 July 10th 2018
February 9th - 12th 2005 June 7th 2019
February 13th - 14th 2005 July 20th 2020

With New Horizons launching on January 19th, it secured an encounter date with Pluto of July 14th, 2015.

Percival Lowell
Before we take a look at the New Horizonís spacecraft and its mission, it would be a good idea to review what we already know about the probeís eventual destination. The discovery of Pluto actually came out of the hunt for 'Planet X'. Not long after the discovery of Neptune in 1846, it was noted that the planet was 'wobbling' in its orbit. This raised alarm bells in astronomerís minds, for it was the strange 'wobble' of the planet Uranus in its orbit, which pointed towards the presence of a planet beyond. So, with Neptune exhibiting a similar wobble, the search for a trans-Neptunian planet was on. The hunt for Planet X attracted none other than Percival Lowell and in 1905 he began a search for the ninth planet. By 1908, no planet had been found, and over the next eight years the search continued. By 1915 the Lowell Observatory had been upgraded, but, by the end of that year, Planet X still eluded Lowell. The disappointment that Lowell felt at not finding Planet X was deepened by the American Academy of Arts and Sciences refusing to publish his paper detailing his theoretical and observational efforts to find the ninth planet. By July of 1916, the photographic searches for Planet X had been terminated. Ironically, Planet X appeared twice in a set of plates around this time, but it was much dimmer than predicted, as such, it went unrecognised. Lowell was to die of a massive heart attack at the observatory on November 16th 1916, not realising how close he came to discovering his elusive Planet X.

It is at this point in our story that the 22-year old Clyde Tombaugh first appears. Tombaugh was an amateur astronomer, and had built three telescopes, using parts from farm machinery. He also sent sketches of Mars, Jupiter, and Saturn he had drawn using his home-built instruments to the Lowell Observatory for appraisal. Even though Tombaugh had not attended university or had any kind of formal training, Vesto Slipher, the Director of the Lowell Observatory was impressed enough to invite him out for a three-month trial period. This trial period actually stretched to a thirteen-year career at the Lowell Observatory for Tombaugh. Tombaugh arrived in Flagstaff in January, 1929, and by April the search for Planet X was underway again. Tombaugh spent around ten months photographing the sky and carefully viewing the resulting plates. If a planet was captured, it would show a slight displacement in position between two plates photographed a week apart. Percival Lowell had started out inspecting his plates with a magnifying glass, but Tombaugh used a blink comparator, which allowed him to view two plates simultaneously and look for any movement that would betray the existence of a planet. By September, Tombaugh decided that Lowell's predictions had little use, and that he would conduct a thorough search of the sky himself. In January, he exposed two plates of an area in the constellation Gemini. It was a few weeks before he could check them, and on February 18, 1930, Tombaugh was comparing the two plates when he saw a star shifting position. It was fainter than expected, but he knew he had finally found Planet X. The observatory followed the object for a few weeks to confirm it was Lowell's planet. The discovery of the ninth planet was then officially announced on March 13, 1930 - Percival Lowell's birthday. As it turned out, Lowell was very nearly right in his predictions as to where his planet would be in the sky. Planet X was discovered only six degrees from where Lowell said it would be.

Clyde Tombaugh
As Planet Xís orbit was refined, the process of naming the newly-discovered world began in earnest. Not to be outdone, Percival Lowellís widow, Constance, felt that she had the natural right of naming the new planet, so she suggested 'Zeus', then 'Percival' in honour of her late husband, and then 'Lowell', but the astronomical community preferred a classical name. Planet X was finally given its name by Venetia Burney, an 11-year old schoolgirl from Oxford. Venetia had been studying Greek and Roman mythology at school and decided that as the new planet was far from the Sun in its own dark part of the Solar System, it should be named after the Greek god of the underworld. Incidentally, Venetia is still alive and well into her nineties and was interviewed by NASA TV prior to the launch of New Horizons. The Lowell Observatory officially proposed the name Pluto on May 1, 1930, with the symbol being PL, the first two letters of Pluto and coincidentally the initials of Percival Lowell. It was at this point that Plutoís strangeness began to make itself known. Lowell had predicted that Pluto's mass would be 6.6 times that of the Earth, while Pickering believed it to be twice the mass of the Earth. As more observations were made it became obvious that Pluto had to have a mass much less than Lowell's or Pickering's predictions. This presented a dilemma. Pluto's mass could not account for the gravitational disturbances in Neptune's motion, yet those very disturbances had been used to predict Pluto's position.

Little was discovered about Pluto over the next forty years. Telescopes improved, but the planet remained a mystery. In 1955, Merle Walker and Robert Hardie at the Lowell Observatory measured Pluto's brightness, which varied regularly, to determine that Pluto's period of rotation was 6.4 Earth days. But what of its size? In 1950, Gerard Kupier used the 200-inch telescope at Mount Palomar to see Pluto as a globe for the first time. He concluded that Pluto's diameter was less than 5,900 kilometres or 3,666 miles (the Earth's diameter is 12,756 kilometres or 7,927 miles). Then, in 1965, Pluto was observed as its orbit passed near a star. If Pluto was large enough, it would cover the star - an occultation. It missed. This event confirmed that Pluto's diameter had to be less than 6,800 kilometres or 4,226 miles, independently verifying Kuiper's measurements. Pluto was determined to be even smaller than Mars. Another weird aspect of Pluto is its orbit, it is inclined to the ecliptic (the plane in which all the other planets orbit the Sun) by 17 degrees, which means that Pluto moves above and below the plane of the Solar System as it orbits the Sun. The planet takes 248 years to make one orbit of the Sun and is 39.48 AU from the Sun (an AU is the mean distance from the Sun to the Earth), which is 5.9 billion kilometres or 3.7 billion miles. It has been calculated that, based on Plutoís size, it has a mass of 0.002 of the Earth, and is 0.36 as dense as the Earth. Plutoís axis of rotation is inclined at 122 degrees, this means that Pluto orbits the Sun a little more than on its side, with its south pole pointing towards us.

Nothing was known of Pluto's composition until 1976, when astronomers at the Kitt Peak Observatory made the first near-infrared photometric measurements of the planet. The spectroscopic analysis showed Pluto to be covered with frozen methane. In 1988 Pluto passed in front of a star, the first true occultation observed since Pluto had been discovered. But rather than winking out suddenly, the star gradually disappeared and reappeared. Pluto had given up another of its secrets, it possessed an atmosphere. This was assumed to be a thin layer of methane gas, but observations in 1992 revealed it to contain mostly nitrogen, with trace amounts of methane and carbon monoxide.

But Pluto is not alone in that cold, dark part of the Solar System, and in 1978 it was found that Pluto had a companion. American astronomer James Christy was studying photographs of Pluto at the U.S. Naval Observatory and found that the images of Pluto were elongated, so much so that the pictures were thought to be defective. However, Christy found other photographs in which the bump on Pluto was in different positions. In fact, he found a series of photographs from 1970 that showed the bump progressing around Pluto in a 6-day period. Christy reasoned that Pluto must have a satellite. On June 22, 1978 the announcement of a moon around Pluto was made. Christy decided to name the Moon Charon, who in Greek mythology was the ferryman who transported the souls of the dead across the River Styx and into Plutoís realm. The discovery of Charon made the accurate measurement of Plutoís mass possible, and as mentioned earlier, it turned out to have a mass of 0.002 of the Earth. There was just no way that tiny Pluto could causing the gravitational effects observed at Uranus and Neptune.

Other discoveries came quickly. Charon orbits 18,800 kilometres above Pluto, and circles Pluto every 6.4 days, exactly matching Pluto's rotation. Charon and Pluto are dynamically locked in their orbits, each keeping the same face to the other (in the same way our Moon keeps the same face to the Earth). Relative to Pluto, Charon is quite large, being about half the diameter and one-seventh the mass of Pluto. Before 1978, the Earth and Moon were the closest thing the Solar System had to a double planet, due to their relative sizes. Now Pluto and Charon hold that honour. In 1980, Charon passed in front of a star, which disappeared for fifty seconds and showed that Charon's diameter is about 1,200 kilometres or 746 miles. In 1985, Charon began a series of eclipses of Pluto that lasted five years. From these, astronomers measured the time it takes Charon to pass in front of and behind Pluto. Result: Pluto's diameter is 2,300 kilometres, this makes Pluto smaller than our own Moon! These eclipses began just seven years after Charon was discovered, which was extremely fortunate. The next series of eclipses won't occur until the 22nd century. Had Charon been discovered twelve years later, astronomers would have missed this rare opportunity, and we might still be in the dark about Pluto.

But Charon is not the only satellite of Pluto. It turns out that Pluto has a further two moons. These new satellites, named S/2005 P1 and S/2005 P2 are between 45 and 160 km (30 and 100 miles) and they appear to orbit Pluto at least twice the distance Charon does. P2 orbits 49,000km or 30,448 miles from Pluto and P1 is even further away at a distance of 65,000 km. The moons were 'discovered' by the Hubble Space Telescopeís Advanced Camera for Surveys on May 15th 2005. Three days later, Hubble was instructed to observe Pluto again, and once more, Hubble found the two new moons were still there and had slightly changed position, confirming that they were indeed in orbit of Pluto. Hubble images of Pluto were re-examined and on a set of images taken on 14th June 2002, both P2 and P2 can be seen (see the Astro News Desk item in this issue of Aries).

In part two of this article I will round off our brief review of what we know about Pluto and take a look at the current debate concerning Plutoís status as a planet. We will then go on to take a detailed look at the New Horizons spacecraft and its mission.