This global view of Pluto is made up of four images from New Horizons’ Long Range Reconnaissance Imager (LORRI)
along with colour data from the Ralph instrument
Pluto has fascinated humankind from the time it was discovered by American astronomer Clyde Tombaugh in 1930. Now known to be a dwarf planet and the first object to be discovered in what is now identified as the Kuiper belt, Pluto was considered a planet from its discovery until the definition of planet was changed in 2006. Pluto’s orbit allows it to be closer to the sun than Neptune for nearly 20 years during its 248-year revolution, and it is recognised now as a fascinating and scientifically important object at the end of the solar system.
But for decades, however, Pluto’s surface and appearance was a mystery as it remained nothing more than a dot of light for observers from Earth. As Pluto’s disc is much smaller than what can be resolved from beneath the Earth’s atmosphere, even the largest Earth-based telescopes cannot discern much about Pluto. Imagine trying to see the print on a football that is placed 64 km away! Resolving the surface of Pluto, which is both small and distant from Earth, is as difficult as that.
Hubble outside the bubble
Things, however, changed with the advent of the Hubble Space Telescope. Launched and inserted into low Earth orbit in 1990, the telescope, which continues to remain in operation, is one of the largest space telescopes that has served as a vital research tool.
With a relatively unobstructed view of Pluto, Hubble was put to the task of observing and imaging it in late June and early July 1994. As Pluto completed one rotation along its axis in 6.4 Earth days, Hubble snapped photographs of the entire surface of the object from its vantage point, 4.3 billion km away! The European Space Agency’s (ESA) Faint Object Camera aboard the Hubble clicked the pictures.
The first image based surphase map of Pluto that was released on March 7,1996
Reasons for delay
It took more than a year-and-a-half to put together Hubble’s pictures before they were eventually released on March 7, 1996. While the time required to process and computer-enhance the images was one reason for the delay, the fact that astronomers were working on other higher priority research also contributed to the time taken. Another reason for releasing it in 1996 was to pay tribute to Tombaugh, who turned 90 just a month earlier.
The two small inset pictures at the top of the main image were actual images from Hubble with each square pixel about 160 km across. Hubble was able to identify 12 major regions on Pluto’s surface that were either bright or dark at this resolution.
Global map of Pluto
Based on computer image processing of the Hubble data, the larger images were produced – a global map of the surface of Pluto. The tile pattern that can be seen in these images are the result of the image-enhancing technique used. The pictures, taken in blue light, show opposite hemispheres of Pluto as seen in these two views. Hubble’s images were able to reveal surface features of Pluto hitherto unknown.
The Hubble Space Telescope took photographs of Pluto again from 2002-03 and it took years to process these. A technique called dithering was used during the computer-image process to combine multiple, slightly offset pictures to generate a higher resolution view. By comparing the 1994 Hubble images with those taken in 2002-03, astronomers were able to gather evidence about the fact that Pluto’s northern polar region got brighter, while the southern hemisphere got darker.
Hubble’s pictures not only remained the sharpest views of Pluto until NASA’s New Horizons probe was within six months of its Pluto flyby in 2015, but these images also helped scientists to plan the details of that flyby. Over five years on after that flyby, which helped us see Pluto’s surface in spectacular detail, the encounter has shown that the dwarf planet is anything but an inert ball of ice that it was once presumed to be. Scientists agree that Pluto is an active and exciting world that is scientifically important, possibly holding the keys to our understanding of other bodies in the far reaches of the solar system.