Derby and District Astronomical Society

Aries
The Journal of the Derby and District Astronomical Society
May - August 2006
[Contents]

Into the Unknown – New Horizons’ Voyage to Pluto - Part 2
By Anthony R. Southwell

In part one of this article I described the launch of New Horizons on its journey to Pluto, back in January 2006, and then went on to describe Pluto’s discovery and included a ‘potted’ history of what we have learnt about Pluto since its discovery, much of which has been gleaned in recent decades. We now know Pluto to be an enigmatic world, which does not seem to fit its place within the Solar System. It is an oddball of a world. Pluto is tiny, smaller than our own Moon, so it could not be responsible for the gravitational ‘wobbles’ of Uranus and Neptune, which prompted the search for Pluto in the first place. Its orbit is strange, for a time, it comes within the orbit of Neptune and then swings out again, in its 248-year dance about the Sun. Also its orbit is inclined to the plane of the orbits of the other planets of the Solar System, by 17°, so it orbits both above and below the plane of the ecliptic. This sounds the alarm bells. Did Pluto form at the same time as the other planets in the Solar System? Its orbital characteristics would seem to say not. If that is the case, what is Pluto? The more we learnt about Pluto, the more of an oddball this small frozen world became.

I finished the first part of this article by saying that I would round off what we already know about Pluto and take a look at the continuing debate concerning Pluto’s status as planet. Following that, the second part of the article would take a look at the New Horizons spacecraft and its mission. When this author was writing the first part of that article, we were living in a Solar System that comprised nine worlds, then the International Astronomical Union (IAU) Congress occurred on August 24th 2006, and everything changed! So much for writing a simple second part to this article! At the moment there seems to be a controversy raging in the aftermath of the IAU Congress and its decision as to Pluto’s present status, more on that later.

For the last 25 years there has been a debate going between two factions within the astronomical/planetary science community in regards to Pluto’s status as a planet. The doubt over whether Pluto can be classified as one of the ‘major’ planets of the Solar System was cast in response to its strange orbital behaviour, one school of thought said that because of Pluto’s wild path around the Sun, it must be a captured object from the outer reaches of the Solar System, captured into its present orbit by the gravitational influence of Neptune. This would seem to be plausible, could Pluto be a member of the theorised Kuiper Belt? The Kuiper Belt is often called the 'Third Zon' of our planetary system, and is the largest structure in our Solar System, holding an estimated 100,000-plus miniature worlds with diameters larger than 100 kilometres, and is the source of short-period comets. The Kuiper Belt was theorised by Dutch-American astronomer Gerard Kuiper in the 1950s, who suggested that Pluto was not just a lone icy odd-ball world, but could, in fact, be a member of a vast collection of bodies located out beyond the orbit of Neptune. The name Kuiper Belt was coined for this region and the name stuck. The Kuiper Belt appeared in the scientific literature for many years, but not one Kuiper Belt Object was found.

Then in the late 1980s, scientists determined that only something like the Kuiper Belt could explain why short-period comets orbit so close to the plane of the Solar System. This circumstantial evidence for a distant belt of bodies in the same region as Pluto drove observers back to their telescopes in search of undiscovered, faint objects. In 1992, astronomers at the Mauna Kea Observatory in Hawaii discovered the first Kuiper Belt Object (KBO), which was about 10 times smaller and almost 10,000 times fainter than Pluto. Since then, observers have found more than 1,000 KBOs, with diameters ranging from 50 to 2,000 kilometres (30 to 1,240 miles) – and researchers estimate that the Kuiper Belt contains more than 100,000 objects larger than 100 kilometres (about 60 miles) across. In fact there may be KBOs that could be the size of the planet Mars.

In many ways, the Kuiper Belt has turned out to be the big brother of the asteroid belt, with more mass and objects, and a greater supply of ancient, icy and organic material left over from the birth of the planets than previously imagined. The Kuiper Belt’s discovery made it clear that Pluto is not an anomalous body, but instead moves within a swarm of smaller bodies orbiting 5 billion kilometres (and beyond) from the Sun. This far-off region may hold important clues to the early development of the Solar System. So what does all this have to do with Pluto’s status as a planet? Well quite a lot really! If Pluto was to be a member of such an assemblage of objects, how could it possibly be labelled as a ‘major’ body of the Solar System? Up until recently, all the KBOs discovered had sizes that were smaller than Pluto, Quaoar is 600 miles in diameter, Sedna is between 700 and 1,100 miles in diameter, so, by some tenuous reasoning, Pluto could cling on to its status as a planet, even though the debate concerning Pluto’s status was still raging, but then came 2003 UB313 to stir things up!

Pluto is 1,400 miles in diameter, but 2003 UB313 (unofficially known by its discoverers as ‘Xena’ - they were fans of the Xena: Warrior Princess TV show) was bigger than Pluto, but only just. Xena was 1,491 miles in diameter, it beat Pluto in size by a mere 91 miles! The discovery of 2003 UB313 really inflamed the debate surrounding Pluto, if 2003 UB313 is classed as nothing more than a KBO, and is very slightly larger than Pluto, then how can Pluto be classified as a planet? The debate lead to the IAU to set up an ‘Planetary Definition’ committee to come up with the definitive definition of what makes a planet, and this would finally lay to rest the argument over Pluto. After about two years of work, the committee presented a plan to the IAU Congress, held in Prague, Czech Republic, in August of 2006. Their plan would allow Pluto to keep its planetary status and elevate three more bodies to being planets. Sedna, being one of them, another would be Ceres (the largest of the main belt asteroids and is spherical in shape), also 2003 UB313 would be classed as a planet. This would mean that we would be living in a Solar System of 12 planets. The model was fine as far as it went, except for one thing, when more KBOs were discovered, and many being bigger than Pluto, who was to say that in few decades time, we could be living in a planetary system of 200 or more worlds! That would be confusing, how would you keep track of the names? This plan was voted down at the IAU meeting, and a new one was sought. Two days later a new planetary definition was put forward and was voted on at the IAU meeting on August 24th 2006 and was carried by those present. The definition was as follows.

For a body to be classed as a planet it must:

So, with that definition, as vague as it sounds, Pluto was stripped of its planetary status and joined Sedna, Quaoar, and 2003 UB313 in the family of ‘Dwarf Planets.’ So we now live in a Solar System of eight ‘classical’ planets and a number of dwarf planets. At the time of writing this article, the author is unsure as to Ceres’ status - is it classed as a dwarf planet, or is it still a mere asteroid? The author is about as confused as he ever wishes to be! If the readership knows better, then please by all means let the author know! Recently, actually, on 13th September 2006, the IAU finally officially named 2003 UB313, as of that date that icy body will be known as ‘Eris’, named for the Greek goddess of discord and strife, quite an appropriate name for body that caused Pluto to lose its planetary status! But that is not all, Eris has a companion, it too has been given a name, Dysnomia, daughter of Eris and goddess of lawlessness!

So, that is that. Or is it? Even though the decision has been made, a lot of astronomers and planetary scientists are not very happy with the IAUs actions in regards to this matter, so much so, that a group of 300 eminent astronomers and planetary scientists, have banded together to get a petition going to get the IAU to change its decision, and come up with a better definition of what a planet is. No doubt this continuing argument will make repeated appearances within the pages of Aries. So, let’s leave all the ‘planetary politicking’ behind and take a look at the New Horizons spacecraft and its mission to Pluto and its Kuiper Belt cousins.

The New Horizons mission trajectory
The New Horizons mission trajectory      Credit: NASA/JPL

By launching on 19th January 2006, New Horizons has secured for itself a Pluto encounter date of 15th January 2015, but however, Pluto will not be the only Solar System body it will visit. In March 2007, New Horizons will pay a short visit to Jupiter for a ‘gravity-assist.’ The spacecraft will use Jupiter’s gravity to increase its velocity to 47,000 mph, and the spacecraft will be 1.4 million miles from the planet at this point. Thus ensuring that the spacecraft will make its 15th January 2015 date with Pluto. Now this is a remarkable fact, for it means that New Horizons will have made the journey from the Earth to Jupiter in thirteen months! Quite remarkable. What makes this fact all the more astounding is that New Horizons has been taking images of Jupiter while still six months away from its closest approach to the giant of the Solar System. The image this author has seen recently, via an e-mail alert from the Universe Today website, was absolutely breathtaking (se below). We have an image of the planet Jupiter from a spacecraft that is still six months out from the planet, and that spacecraft only left the Earth in January of this year! Amazing, truly amazing!

Jupiter imaged by New Horizons
Jupiter imaged by New Horizons on 4th September 2006
Credit: NASA/JHUAPL

So what is the New Horizons mission all about, what are the mission objectives, and what makes up the spacecraft? The science objectives of the New Horizions mission are split into three sections:

Required

Important

Desired

New Horizons will begin its study of the Pluto system five months before the closest approach to the planet. In the weeks leading up to closest approach, the mission team will be able to map Pluto-Charon in increasing detail and observe phenomena such as Pluto’s weather by comparing the images of the planet over time. It will take high-resolution views of Pluto and its moons to decide which geological features are worthy of intensive scrutiny. The highest-resolution images will be near LANDSAT-class (an Earth-observation family of satellites) in quality, with resolution in the tens of meters.

During closest approach, New Horizons’ imagers will map the entire sunlit faces of Pluto and Charon and also map their outer surface compositions. The team hasn’t yet determined exactly how close New Horizons will come to Pluto. Pre-launch planning is in the range of 10,000 kilometers (6,200 miles). Once the spacecraft passes Pluto, it will turn around and map the planet’s night side, which will be softly illuminated by the reflected moonlight from Charon. And the spacecraft’s antenna will receive a powerful radio beam from Earth, aimed so that it passes through Pluto’s atmosphere. By measuring the effects of atmospheric refraction on the radio beam as it travels to the spacecraft, and similar effects on ultraviolet sunlight passing through the atmosphere, scientists will be able to plot the temperature and density profile of the atmosphere down to the surface. New Horizons will also sample the density and composition of material escaping from Pluto’s atmosphere, map surface temperatures across Pluto and Charon, study Pluto’s ionosphere, refine the radii and masses of Pluto and its moons, search for dust particles in the Pluto system and search for rings and additional moons – among other studies. After the Pluto-Charon encounter, New Horizons will manoeuvre to begin a series of what the team hopes could be one to two encounters with other Kuiper Belt Objects over the following five to seven years. Funding that extended mission will require NASA approval.

The New Horizons spacecraft is 0.7 meters (27 inches) tall, 2.1 meters (83 inches) long and 2.7 meters (108 inches) at its widest. A 2.1-meter (83-inch) diameter antenna dish is attached to the top deck. The spacecraft measures 2.2 meters (87 inches) tall from the payload attachment fitting on the bottom deck to the top of the dish antenna stack. At launch the spacecraft weighed 478 kilograms (1,054 pounds), which included 77 kilograms (170 pounds) of hydrazine propellant and a 30-kilogram (66-pound) science instrument payload.

The science instruments on-board the New Horizons spacecraft include:

Alice

Mass: 4.5 kilograms (9.9 pounds)
Average Power: 4.4 watts

Alice is a sensitive ultraviolet imaging spectrometer designed to probe the composition and structure of Pluto’s dynamic atmosphere. An imaging spectrometer both separates the different wavelengths of light and produces an image of the target at each wavelength. Alice’s spectroscopic range extends across both extreme and far-ultraviolet wavelengths from approximately 500 to 1,800 Angstroms. The instrument will detect a variety of important atomic and molecular species in Pluto’s atmosphere, and determine their relative abundances, giving scientists the first complete picture of Pluto’s atmospheric composition. Alice will search for an ionosphere around Pluto and an atmosphere around Pluto’s moon Charon. It will also probe the density of Pluto’s atmosphere, and the atmospheric temperature of Pluto, both as a function of altitude.

Ralph

Mass: 10.3 kilograms (22.7 pounds)
Average Power: 6.3 watts

Ralph is the main 'eyes' of New Horizons and is charged with making the maps that show what Pluto, its moons, and other Kuiper Belt Objects look like. The instrument is so named because it’s coupled with Alice in the New Horizons remote-sensing package! Ralph consists of three panchromatic (black-and-white) and four color imagers inside its Multispectral Visible Imaging Camera (MVIC), as well as an infrared compositional mapping spectrometer called the Linear Etalon Imaging Spectral Array (LEISA). LEISA is an advanced, miniaturized short-wavelength infrared (1.25-2.50 micron) spectrometer provided by scientists from NASA’s Goddard Space Flight Center. MVIC operates over the bandpass from 0.4 to 0.95 microns. Ralph’s suite of eight detectors – seven charge-coupled devices (CCDs) similar to those found in a digital camera, and a single infrared array detector – are fed by a single, sensitive magnifying telescope with a resolution more than 10 times better than the human eye can see. The entire package operates on less than half the wattage of a night light.

Ralph will take images twice daily as New Horizons approaches, flies past and then looks back at the Pluto system. Ultimately, MVIC will map landforms in black-and-white and color with a best resolution of about 250 meters (820 feet) per pixel, take stereo images to determine surface topography, and help scientists refine the radii and orbits of Pluto and its moons. It will aid the search for clouds and hazes in Pluto’s atmosphere, and for rings and additional satellites around Pluto and other Kuiper Belt Objects. It will also obtain images of Pluto’s night side, illuminated by 'Charon-light'. At the same time, LEISA will map the amounts of nitrogen, methane, carbon monoxide, frozen water and other materials, including organic compounds, across the sunlit surfaces of Pluto and its moons (and later Kuiper Belt Objects). It will also let scientists map surface temperatures across Pluto and Charon by sensing the spectral features of frozen nitrogen, water and carbon monoxide.

Radio Science Experiment (REX)

Mass: 100 grams (3.5 ounces)
Average Power: 2.1 watts

REX consists only of a small printed circuit board containing sophisticated signal-processing electronics integrated into the New Horizons telecommunications system. Because the telecom system is redundant within New Horizons, the spacecraft carries two copies of REX. Both can be used simultaneously to improve the data return from the radio science experiment. REX will use an occultation technique to probe Pluto’s atmosphere and to search for an atmosphere around Charon. After New Horizons flies by Pluto, its 2.1-meter (83-inch) dish antenna will point back at Earth. On Earth, powerful transmitters in NASA’s largest Deep Space Network antennas will beam radio signals to the spacecraft as it passes behind Pluto. The radio waves will bend according to the average molecular weight of gas in the atmosphere and the atmospheric temperature. The same phenomenon could happen at Charon if the large moon has a substantial atmosphere, but Earth-based studies indicate this is unlikely.

REX will also measure the weak radio emissions from Pluto and other bodies the spacecraft flies by, such as Jupiter and Charon. Scientists will use the data to derive accurate globally averaged day-side and night-side temperature measurements. Also, by using REX to track slight changes in the spacecraft’s path, scientists will measure the masses of Pluto and Charon and possibly the masses of additional Kuiper Belt Objects. By timing the length of the radio occultations of Pluto and Charon, REX will also yield improved radii measurements for Pluto and Charon.

Long Range Reconnaissance Imager (LORRI)

Mass: 8.8 kilograms (19.4 pounds)
Average Power: 5.8 watts

LORRI, the 'eagle eyes' of New Horizons, is a panchromatic high-magnification imager, consisting of a telescope with an 8.2-inch (20.8-centimeter) aperture that focuses visible light onto a charge-coupled device (CCD). It’s essentially a digital camera with a large telephoto telescope – only fortified to operate in the cold, hostile environs near Pluto. LORRI images will be New Horizons’ first of the Pluto system, starting about 200 days before closest approach. At the time, Pluto and its moons will resemble little more than bright dots, but these system-wide views will help navigators keep the spacecraft on course and help scientists refine their orbit calculations of Pluto and its moons. At 90 days before closest approach – with the system more than 100 million kilometers (60 million miles) away – LORRI images will surpass Hubble-quality resolution, providing never-before-seen details each day. At closest approach, LORRI will image select sections of Pluto’s sunlit surface at football-field-size resolution, resolving features at least 50 meters across.

This range of images will give scientists an unprecedented look at the geology on Pluto, Charon, and additional Kuiper Belt Objects – including the number and size of craters on each surface, revealing the history of impacting objects in that distant region. LORRI will also yield important information on the history of Pluto’s surface, search for activity such as geysers on that surface, and look for hazes in Pluto’s atmosphere. LORRI will also provide the highest resolution images of any Kuiper Belt Objects New Horizons would fly by in an extended mission.

Solar Wind at Pluto (SWAP)

Mass: 3.3 kilograms (7.3 pounds)
Average Power: 2.3 watts

The SWAP instrument will measure interactions of Pluto with the solar wind – the high-speed stream of charged particles flowing from the Sun. The incredible distance of Pluto from the Sun required the SWAP team to build the largest-aperture instrument ever used to measure the solar wind. Pluto’s small gravitational acceleration (approximately 1/16 of Earth’s gravity) leads scientists to think that about 75 kilograms (165 pounds) of material escape its atmosphere every second. If so, then the planet behaves like a comet, though Pluto is more than 1,000 times larger than a typical comet nucleus. The atmospheric gases that escape Pluto’s weak gravity leave the planet as neutral atoms and molecules. These atoms and molecules are ionized by ultraviolet sunlight (similar to the Earth’s upper atmosphere and ionosphere). Once they become electrically charged, the ions and electrons become 'picked up' and are carried away by the solar wind. In the process, these pick-up ions gain substantial energy (thousands of electron-volts). This energy comes from the solar wind, which is correspondingly slowed down and diverted around Pluto. SWAP measures low-energy interactions, such as those caused by the solar wind. By measuring how the solar wind is perturbed by the interaction with Pluto’s escaping atmosphere, SWAP will determine the escape rate of atmospheric material from Pluto.

Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI)

Mass: 1.5 kilograms (3.3 pounds)
Average Power: 2.5 watts

PEPSSI, the most compact, lowest-power directional energetic particle spectrometer flown on a space mission, will search for neutral atoms that escape Pluto’s atmosphere and become charged by their interaction with the solar wind. It will detect the material that escapes from Pluto’s atmosphere (such as molecular nitrogen, carbon monoxide and methane), which break up into ions and electrons after absorbing the Sun’s ultraviolet light, and stream away from Pluto as 'pick up' ions carried by the solar wind. The instrument will likely get its first taste of Pluto’s atmosphere when the planet is still millions of kilometers away. By using PEPSSI to count particles, and knowing how far New Horizons is from Pluto at a given time, scientists will be able to tell how quickly the planet’s atmosphere is escaping and gain new information about what the atmosphere is made of.

Student Dust Counter (SDC)

Mass: 1.9 kilograms (4.2 pounds)
Average Power: 5 watts

Designed and built by students at the University of Colorado at Boulder, the SDC will detect microscopic dust grains produced by collisions among asteroids, comets, and Kuiper Belt Objects during New Horizons’ long journey. Officially a New Horizons Education and Public Outreach project, SDC is the first science instrument on a NASA planetary mission to be designed, built and flown by students. The SDC will count and measure the sizes of dust particles along New Horizons’ entire trajectory and produce information on the collision rates of such bodies in the deep outer solar system. SDC will also be used to search for dust in the Pluto system. Such dust might be generated by collisions of tiny impactors on Pluto’s small moons.The instrument includes two major pieces: an 18-by-12-inch detector assembly, which is mounted on the outside of the spacecraft and exposed to the dust particles, and an electronics box inside the spacecraft that, when a hit occurs on the detector, deciphers the data and determines the mass and speed of the particle. Because no dust detector has ever flown beyond 18 astronomical units from the Sun (nearly 1.7 billion miles, about the distance from Uranus to the Sun), SDC data will give scientists an unprecedented look at the sources and transport of dust in the solar system.

So, all we can do now is wait for New Horizons to reach Pluto, and at long last, that frozen world will finally, if not grudgingly, give up its secrets. We will also learn about the early history of our Solar System by studying Pluto and a few KBOs close-up. But New Horizons will not be Pluto’s only visitors, for on-board New Horizons are 435,000 names on a CD-ROM, my own name is among them. I am going to Pluto, a part of me is flying out of the warm life-giving centre of the Solar System to the cold outer reaches of our part of the Cosmos, and what an adventure to become involved with, if only in name! See you all at Pluto in July 2015!

Anthony Southwell

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