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10. Best Images Yet of the Dwarf Planet Pluto
John Austin
images of dwarf planet pluto

Images of Pluto obtained by NASA's New Horizons spacecraft between 8 and 12 May 2015. The distance on each occasion is indicated below the image. On the different days, the central longitudes were 350o, 238o and 125o. The closest approach is still two months away and the images, taken from a distances of about half the distance between the Earth and the Sun are still very blurred, although some features are starting to emerge. Image Credit: NASA.

The New Horizons spacecraft, after its long trip via Jupiter, has sent back the best images yet of Pluto. It is due for a flyby of the dwarf planet on 14 July. In the above images, taken over a period of time, the dwarf planet has rotated presenting a different face to the LORRI (Long Range Reconnaissance Imager) camera. With a period of rotation of about 6.4 days the images cover most of the surface.

The Route to Pluto

The New Horizons spacecraft was launched on 19 January 2006 and passed by Jupiter on 28 Feb 2007. The flyby of Jupiter enabled the spacecraft to be accelerated by an additional 4 km/s from its 16 km/s Earth launch speed. During the Jupiter flyby, the instruments on board were given a test run on the Jovian system and provided important new data on the Jupiter's moons, atmosphere and magnetosphere. It is now approaching Pluto and the Kuiper belt.

Of the Kuiper Belt Objects (KBOs) Pluto, with a diameter of 2340 km is not the largest. Eris is another KBO with a diameter slightly larger at 2400 km, but there are large uncertainties in this measurement. As part of the New Horizons mission other KBOs will be observed, although precisely which ones is currently undecided, as the orbits and details of most of the KBOs are unknown. Once the mission is over, New Horizons will be only the 5th artificial satellite to leave the solar system, after Pioneer 1 and 2, and Voyager 1 and 2.

The Pluto System

Of course since 2006, Pluto has been considered a dwarf planet, as unlike the planets it has not cleared its orbit from asteroid debris. Also, it is much smaller than a typical planet. Nonetheless, it has its attendant moons. Indeed with Charon it acts almost like a double dwarf planet with Charon and Pluto rotating about their common centre of gravity (barycentre). Charon has a diameter of about 1210 km, about half that of Pluto, but its density is about two thirds that of Pluto, and its mass is about one fifteenth of Pluto's. So the centre of rotation is close to Pluto itself.

Pluto and its satellites (2005), by H. Weaver (JHU/APL), A. Stern (SwRI), and the HST Pluto Companion Search Team. Image licensed under Public Domain via Wikimedia Commons.

Pluto moon P5 discovery with moons' orbits by NASA, ESA, and L. Frattare (STScI). Image licensed under Public Domain via Wikimedia Commons.

Four other moons of Pluto are also previously known from images of the Hubble Space Telescope (Nix, Hydra, Kerberos and as recently as 2012, Styx). In the left hand image, a Hubble photograph from 2005 is shown, while in the right hand image, the discovery of a 5th moon is shown with schematic diagram of the orbits. New Horizons, as it approaches Pluto, will have much higher spatial resolution, so may well enable more satellites to be discovered.

According to the New Horizons Principal Investigator Alan Stern, the surface images of Pluto obtained to date hint at what may be complex surface geology. The dwarf planet is also likely to have a polar cap with the extent of the cap varying with longitude. Spectroscopic measurements due to be made in July will allow the iciness of the bright regions to be measured.

Rendezvous with Pluto

Rendezvous with Pluto is timed for mid July, and the closest approach will be about 10,000 km so the images will be thousands of times better. Unlike with the recent Ceres dwarf planet encounter by the Dawn Spacecraft, New Horizons does not have the capability of orbiting Pluto. Nonetheless, as we know very little about the dwarf planet data will be collected which will have a unique view of Pluto itself and unique insights into the Kuiper Belt and the formation of the early solar system. With the success of the Dawn satellite to Ceres, the European Rosetta spacecraft success in landing on comet 67P/Churyumov-Gerasimenko, and now the promising delivery of much new information about Pluto, it is tempting to suggest that this is the best mode of scientific discovery and exploration of the solar system. Can we really learn anything new about Mars by sending humans there at enormous cost?

Will Pluto become a Planet Again?

There has been renewed debate about the status of Pluto as a planet. The original decision to rename Pluto as a dwarf planet was a controversial one, and probably not very popular with the public. The "demotion" to dwarf planet hinged on whether Pluto had "cleared its orbit", or removed all other material from its orbital vicinity. Although a clear definition of this does not seem to be available, nonetheless, there is a strong sense that, as Pluto resides in the Kuiper Belt, it regular passes through other debris such as asteroids and comets and therefore should remain a dwarf planet. To change this definition would, in my view, reopen a "can of worms".

Some of the largest KBOs currently known. If Pluto is reinstated as a planet, all of these objects will need to have their status as a planet considered. Image credit NASA Hubble Space Telescope/Wikimedia Commons.

Some of the Kuiper Belt material will presumably continue to become part of Pluto as it evolves, or indeed will be acquired in the form of moons in orbit. If the "clearing the orbit" condition were to be removed, it would mean that Pluto would be declared a planet again, but merely one passing through the Kuiper Belt. In addition, we would have to acknowledge Ceres as another planet, but one which happens to pass through the main asteroid belt between Mars and Jupiter. Indeed when Ceres was first discovered in 1801 it was initially declared a planet (the 8th since Neptune and Pluto were not known). Declaring Pluto and Ceres as planets may not be the end of it. Already, we know of 7 other potential objects (see above image) and there may others within the Kuiber Belt which have so far elluded discovery because of their size and distance. New Horizons may encounter more such objects. It is now the duty of the International Astronomical Union to confirm or change its opinion regarding Pluto, with the appropriate consequences for Ceres and all the largest Kuiper Belt Objects.

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Article initially prepared 29 May 2015; revised 31 May 2015

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Website revised by John Austin, 31/5/2015. © Enigma Scientific Publishing, 2015.