Pluto: A new makeover

6:22:03 am IST; July 15, 2015.

Thirteen hours since New Horizon’s close flyby to Pluto and subsequent Radio silence. The spacecraft, after nine and half years of space journey, had reached Pluto flew-by at 12,472km on July 14that about 17:19 hrs IST. To make most of this opportunity scientists had oriented New Horizons towards Pluto and away from Earth. The Plutonium powered spacecraft had only limited power and computing, which to make most of it, scientists had diverted to gathering data rather than sending it back to Earth. As a result the spacecraft was on autopilot for about eight hours with no signal sent back to Earth.

Pluto and its moons, planetary geologists, had speculated formed out of collision millions of years ago. If that is true such a cataclysmic event would have resulted in large number of small and big rocks whirling posing danger to the spacecraft and hence space scientists were anxious and worried. Will the New Horizons phone back home?

The clock ticked away.. and the tension was building. Seconds felt minutes and minutes hours. Strain and stress were writ large on every face.

Across the NASA TV screen one could witness sudden explosion of joy. Everyone in the mission control room at the Johns Hopkins University Applied Physics Laboratory got up; clapped. One could hear sigh of relief, joyous shrills, boisterous boos. Electrified atmosphere was evident. Signal from arrived as anticipated, confirming that the probe had indeed survived its history-making Pluto flyby. The Deep Space Network antenna in Spain received the signals sent across 4.88 billion km bby the spacecraft and received at Earth after four hours and 25 minutes. The audience stood up, applauded and waved American flags as, one by one, mission controllers reported “nominal” status for the hardware that was their responsibility. The spacecraft is in good health.


The Mission

So far even with best of the telescopes Pluto and its companion Charon were merely just blobs of light, hiding their surface feature, if any. The New Horizon, first ever mission to Pluto, is expected to remove the veil and reveal its full glory. Launched way back on January 19, 2006, .New Horizon mission was to study Pluto and Kuiper belt object. Spacecraft is small, just a size of a piano, weighing under 480 kg and just 2.5 meter across. Yet the suite of seven powerful scientific instruments on-board including the first ever ‘student’ built and ‘flown’ scientific payload, Student Dust Counter (SDC) along with three optical instruments, two plasma instruments, a dust sensor and a radio science receiver, are potent. vAfter making a five billion kilometre long journey the spacecraft has to reach a point near Pluto with an accuracy of about 100-150 km on July 14, 2015. It flew by Pluto at 12500 km and its biggest moon Charon at 28,800 km. These close-up shots are expected to reveal surface features and provide geological, atmospheric data.


Bulletproof vest

New Horizon hurling at a breakneck speed of about 54000 km/h would break into pieces if it collided with space rocks or debris. Further collision with micro meteoroids could puncture the instruments. To protect the instruments, the spacecraft is adorned with a bulletproof vest made out of 18 layers of Dacron mesh cloth sandwiched between aluminized Mylar and Kapton film, which also acts as a thermal blanket. Space is a very very cold place. For electronic equipment to work a minimum heat is required and New Horizons’ unique “thermos bottle” design retains heat and keeps the spacecraft operating at room temperature without the need for power hungry large heaters.

Why just a brief rendezvous?

One may wonder why after this arduous ten month journey the spacecraft spends just 30 minutes close by Pluto. Why cannot it orbit once or twice around the planet, as India’s MOM does around Mars? It will take 700 years for even the fastest jet to reach Pluto at 750 crore km. After its launch from Cape Canaveral on January 19, 2006, the probe entered into an escape trajectory featuring a speed of 16.26 kilometers per second (58,536 km/h), setting a new record for the highest launch speed of a human-made object flung from Earth. Flinged by the gravity of Jupiter subsequently, it attained an additional 4 km/s (14,000 km/h). Once at the Pluto-Charon system, the spacecraft will pass through at a velocity of about 13.8 km/s relative to the dwarf planet (49,680 km/h) At this speed one would be able to traverse the distance between Chennai and Trichy is just one minute. If you have to make this racing spacecraft to orbit around Pluto then one has to reduce its speed by about 90%, for that one would require 1000 times more fuel in the spacecraft.



Inner planets such as Mercury, Venus, Earth and Mars are terrestrial planets rich in metals such as iron. The Gaseous planets such as Jupiter and Saturn are less dense and are essentially filled with lighter elements such as Hydrogen and Helium. Uranus and Neptune are frozen gaseous planets mostly made up of hydrogen and helium, but rich in frozen methane.

Pluto, at the far reach of solar system at 750 crore kilometre, is a icy planet frozen at about minus 233 degrees, at with large percentage of ammonia, water ices, and other compounds such as methane. This is the same composition that comets have. Pluto has a very thin atmosphere, just one part in one lakh of Earth’s atmosphere. Pluto’s atmosphere is rich in nitrogen just like Earth, but Pluto’s atmosphere is rich in methane and carbon-dioxide and is devoid of oxygen. Its largest moon is Charon which is locked with Pluto gravitationally and hene one face of it always turned towards it. Four additional moons were discovered only recently.


To be or not be a planet?

Since the discovery of Neptune in 1846, astronomers have predicted that there would be a ninth planet in the Solar System, which they called Planet X. After a massive hunt with ingenious contraptions, Clyde W. Tombaugh, then just 22 years old, discovered Pluto using the Lowell Observatory in Flagstaff, Arizona. At the time of discovery and subsequently, this tiny celestial object has remained an odd curiosity.

Alas, although it was still considered a planet when the New Horizon was launched, within a few months, astronomers from across the world, who gathered for the congress of the International Astronomical Union in 2006, removed Pluto from the list of planets and re-classified it as a ‘dwarf planet’.

Over the last few decades, powerful new ground and space-based observatories have unraveled hitherto unknown parts of our solar system. Instead of being the only planet in its region like the inner Solar System, Pluto and its moons are now known to be just one among the large collection of objects called the Kuiper Belt objects (KBOs), found in a region beyond Neptune and extending out to 55 astronomical units (55 times the distance of the Earth to the Sun).

One estimate places the number of KBOs to be 70,000 icy objects, all with the same composition of Pluto and many measuring more than 100 km across. The discovery of a bigger KBO called Eris – 2,600 km across and about 25% more massive than Pluto – nailed the debate.

Although bats fly, it is a mammal; Penguins cannot fly, but are expert swimmers, yet they are birds. The re-classification of Pluto as dwarf planet is similar to this. A Planet has to go around the Sun and have enough mass (gravity) to form a spherical shape. Pluto meets both these criteria. However according to astronomers, for an object to be classified as a Planet it had to satisfy another crucial criteria – that it needs to have “cleared its neighbourhood”. This is where Pluto fails.

What does “cleared its neighbourhood” mean? When a proto-planet forms and evolves, it becomes the dominant gravitational body in its orbit, sweeps up the rest of the materials around and grows in size. In the end either it consumes almost all the objects near it or slings them away with its gravitational interactions. However Pluto is only 0.07 times the total mass of the other objects in its orbit. The Earth, in comparison, has 1.7 million times the mass of other objects in its orbit.

Any object that does not meet the astronomers’ third criterion is classified as a dwarf planet. And so, Pluto is a dwarf planet.


All eyes and ears

The instruments on New Horizons were all designed to work together to give us a comprehensive picture of the Pluto system. Seven instruments, Venetia Burney Student Dust Counter (SDC), Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI), Solar Wind Around Pluto (SWAP), Long Range Reconnaissance Imager (LORRI), Radio Science Experiment (REX), Ralph and Alice. Together they will help scientists study Pluto’s geology, surface composition, temperature and atmosphere—as well as its five moons. All the instruments aboard the spacecraft were in sleep mode until April 2015 and one by one they were woken-up.


It is official: Pluto is the bigger one

Detailed picture of Pluto by New Horizons

It is not an exaggeration to say that we have learnt more about Pluto since April 2015, when the instruments on-board New Horizons were awakened than in the last 85 years since its discovery in 1930. Even with the trickling in data analyzed researchers on the New Horizons team have processed enough flyby data to start nailing down many new details about Pluto.

To begin with we have been able to nail its diameter as 2,370 kilometres with an accuracy of +/- 12 km, somewhat larger than many prior estimates using stellar occultation method. When Pluto occults- hides- a distant star, the duration of the eclipse is precisely measured to find the size. However such methods for estimating Pluto’s size were fraught with uncertainty because the thin atmosphere, that acted as a mirage, blurring the boundaries of just how big the dwarf planet is.

Images acquired with the Long Range Reconnaissance Imager (LORRI) were used by the astronomers to make this new estimate for Pluto and its various moons. The measurement obtained by LORRI and by earlier stellar occultations tallied at 1,212 +/- 1.6 km for Charon lacking atmosphere enhancing the reliability of this technique.

Earlier estimates of Pluto at 2,302 was much less than 2336 +/- 12 km of Eris lacking in atmosphere, making it the largest KBO, partly influencing the decision to demote Pluto’s planetary status.

LORRI images were also used to compute the size of Nix and Hydra, two other moons of Pluto. Their diameter are estimated to be 35 km and 45 km respectively. However a new mystery has cropped up as to how such small objects have such an high albedo, fueling speculation that perhaps surfaces are quite bright, possibly due to the presence of ice cover on their surface.

Even the dimensions of Kerberos and Styx tiniest moons could be teased out of the data that we would receive. The complex dynamic ‘cosmic dance’ these five moons do around Pluto would also come to light once we are able to download sufficient data from the spacecraft.


X-raying Pluto

Further confirmation awaits return of the data from the exciting Radio Science Experiment, (REX) to determine the sizes and densities of Pluto, Charon, and a future Kuiper Belt object as well as measure the temperature (both during the day and at night) and density of Pluto’s atmosphere at various heights as well as the density of Pluto’s ionosphere.

The “uplink radio experiment” technique of REX involves powerful radio signals from NASA’s Deep State Netwok aimed at New Horizons when it is bang behind Pluto. The radio waves will pass through Pluto’s atmosphere and will bend certain ways depending on the temperature and molecular weights of gasses encountered. These signals would be received by the New Horizons and relayed back to Earth for further analysis.

The current estimates, unless overturned by the REX, makes the Pluto the largest object discovered in the Kuiper blet so far and put an end to the debate as to whether Eris was actually bigger than Pluto.


Pluto in new light

If indeed Pluto is slightly larger than we thought then it leads to a whole train of new conclusions. Bigger size paired with the mass that we already knew very well, connotes lower density which in turn implies higher proportion of ice than we previously thought. If indeed Pluto has has more ice layered on its rocks then it hint troposphere lower than we had thought. A larger diameter than thought also makes its atmosphere thinner than anticipated.

These results would impel a complete revision of the atmospheric as well icy and rocky interior models of Pluto. Smaller-but- 27% more heavier Eris means greater density contrast between it and Pluto hinting at very different histories for the two worlds.


Pluto is not dead

Being that far, so small, planetologist, considered Pluto to be dead or only active perhaps due to tidal influences of its moons. First of high resolution close-up images of the Pluto show number of mountains which are about young, icy and about 11,000ft tall. Astronomers estimate that the mountains, ascend from bedrock of water-ice and are likely less than 100 million years old; younger than the Himalayas.

Scientists have expected to find ice geysers and cryovolcanoes, but no evidence seems to be present in these images. New Horizons has confirmed the existence of a polar cap on Pluto. Spectral measurements have shown that the methane absorption bands are much weaker in the dark regions as compared to the polar regions indicating that polar regions are compositionally very different from the dark regions. Further images reveal a potential snow cap, a mysterious elongated dark feature at the equator, which has been dubbed the “whale”, and a large heart-shaped bright region measuring about 2000 km across. To honour the discoverer of Pluto, the heart shaped region has been named after Clyde W. Tombaugh. Zoom into these high-res images surprisingly show not a single impact crate. Topography as revealed by the high-res images indicate internal activity implying Pluto is not a dead plane but active.


New Face of Charon

Detailed picture of Charon

The high tech spacecraft has also pictured Pluto’s largest moon, Charon, and given scientists their clearest ever look at the giant ball of ice. Surprisingly it is young and varied topography. The new images reveal a barren landscape of vast craters and chasms – the largest of which is believed to be miles deeper than Earth’s own Grand Canyon.The most prominent crater on Charon is about 100 km across and lies near the South Pole of the moon. There are the incipient evidence of geologic activity such as faulting and surface disruption on Charon, which was thought to be a nearly featureless ball of ice until now.

Scientists suspect Pluto, Charon and their four small moons, all discovered in Hubble images after New Horizons was launched, formed after an ancient collision of two icy bodies. That theory will be tested with the new evidence of the tumbling, wobbly moons, and observations by New Horizons.


The Nitrogen mystery

Pluto is one of only three objects in the solar system known to have a nitrogen-rich atmosphere, joining Earth and Saturn’s moon Titan. Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI), a compact, lowest-power directional energetic particle spectrometer aboard the New Horizons was to search for neutral atoms that escape Pluto’s atmosphere and become charged by their interaction with the solar wind. Chemical molecules such as molecular nitrogen, carbon monoxide and methane become ionized after absorbing the Sun’s ultraviolet light were expected to blown away from Pluto by the solar wind.

Astronomers had expected this instrument to get the first taste of Pluto’s atmosphere when the spacecraft was close to Pluto at 1 to 2.5 million Km. Yet to much surprise of the planetary-geologists PEPPSI started to sniff nitrogen escaping from Pluto five days before the close-flyby at about 6 million km. This perhaps indicate Pluto’s thin atmosphere may be escaping the planet faster than expected or concentration of nitrogen in Pluto is many times stronger than we had estimated. It could also means something more exotic, like a yet-to-be-determined process concentrating the escaped gas and our probe just coincidentally intercepting the stream. Further data from the spacecraft would help us learn what else is in Pluto’s atmosphere, and if Charon and Pluto actually share an atmosphere within their odd little system.


Patience is virtue

The mission is not complete after its date with Pluto on July 14th 2015. It is only the first step for the spacecraft in its quest. As it recedes away from Pluto, just like the famous Raja Ravi Verma’s Shakunthala painting, it turned and take a look at the night side of Pluto. In the silhouette of the Sun’s rays examined the thin atmosphere as well as searched for rings around the planet. The probe gathered lots of scientific data during the flyby on Pluto’s atmosphere, temperature, and geology, but we will be able to access them only in the next 16 months.

The spacecraft uses 2.1 meter-wide high-gain antenna is used by the spacecraft to send signals to Earth. But the high gain beam is only 0.3 degrees wide, means New Horizons must be pointing precisely straight at the Earth in order for us to receive its signal. 70-meter dishes which farm part of the Deep Space Network are deployed to receive weak signals from the spacecraft.

Pluto is far away — very far away, more than 30 times Earth’s distance from the Sun – hence signals from New Horizons’ are weak. Weak signal means low data rates: at the moment, New Horizons can transmit at most 1 kilobite per second! A typical hig-res image is about 2.5 Megabits and at 1 kilobit per second: it takes42 minutes to return one photo to Earth. Hence we have to wait with patience to download all the data from the spacecraft. The pace of discoveries will only quicken over the next few months as New Horizons starts sending home its observations.


Picture abi baki hai

Pluto is not the last stop for New Horizons mission. The mission is to venture into unexplored territory of Kuiper belt which is thought to be consisting of objects left-over from the building blocks of the solar system. But where to head was a challenging question. The target beyond pluto has to be somewhere near the path the spacecraft is taking and the remaining fuel should be sufficient to reach there. To the scientists’ relief, in October 2014 the search team announced three potential targets named PT1(2014 MU69), PT2 (2014 OS393)and PT3( 2014 PN70). About billion kilometres beyond Pluto two of them are brighter and so probably bigger (40-70 km) and the third although smaller (25 km) will be easier to reach. Currently the preferred flyby target is PT1, a 40–70 km object, but PT3, a slightly bigger object, could also be targeted for a flyby, with the decision to be made in August 2015. PT2 is no longer under consideration. In coming months, scientists will decide the spacecraft’s next target and send signals from Earth to New Horizons to thrust its rockets to tweak its trajectory. If all goes well, it may then head into the Kuiper Belt for a possible flyby of a second object in 2019.


Author: T V Venkateswaran (Vigyan Prasar)

© 2015 The Johns Hopkins University Applied Physics Laboratory LLC.

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