Will Pluto regain its status after spacecraft fly-by?

Tariq Majeed unpacks Pluto's questionable status as a planet

As the New Horizon Nasa mission approaches Pluto, its status as a planet is questionable. AFP Photo via NASA

Since the discovery of the dwarf planet Eris in the Kuiper Belt – a region of our solar system beyond the orbit of Neptune – the status of Pluto as a planet has been in question. The surface of Eris is composed of the same ice and rock mixture as Pluto and is considerably bigger.

One of the criteria for large objects to acquire planethood status in our solar system adopted by the International Astronomical Union (IAU) is that the objects must hold their gravitational dominance and must have cleared the neighbourhood around their orbits. Unfortunately, both Pluto and Eris are so tiny compared to the masses of other materials in their orbits that they fall into the category of dwarf planets – an object that is neither a planet nor a natural moon.

The prospect of demoting Pluto has been poorly received by the astronomical community. Dr Alan Stern, of the Southwest Research Institute in Colorado and the principal investigator with Nasa’s robotic New Horizons mission to Pluto, publicly denounced the IAU ruling, calling it “sloppy science”.

The New Horizons spacecraft was launched in 2006 to explore how the Pluto system and Kuiper Belt have evolved. During its closest approach, which will occur on Tuesday July 14, the spacecraft will possibly pass within 13,000 kilometres of Pluto. With sophisticated instruments on-board, New Horizons will be able to map surface compositions and morphology of Pluto and Charon, one of its largest moons, along with their surface temperatures.

Furthermore, it will measure Pluto’s atmosphere and its interaction with solar wind particles and look for an atmosphere around Charon. It will also study Pluto’s smaller moons.

Recently, Nasa’s science mission directorate released processed images taken by the telescopic Long-Range Reconnaissance Imager showing Pluto with its five orbiting moons.

The images show “a complex world with discrete equatorial bright and dark regions – some that may also have variations in brightness”, says Dr Stern.

The New Horizons team also discovered an icy polar cap on Pluto and a mysterious “dark pole” – a kind of anti-polar cap – on Charon. Dr Stern said that Charon “starts to show mysterious features which we were not be able to see from the ground and space telescopes. Obviously, nothing like this kind of frontier, outer solar system exploration has happened since Voyager 2 was at Neptune way back in 1989”.

After exploring the Pluto system, New Horizons will continue its journey into the Kuiper Belt, where it will encounter a very large body of debris, rocks and dust encircling our solar system.

Three potential targets (PT1, PT2, and PT3) have been identified by the New Horizons team. The sizes of these targets are in the range 30-55km in diameter and in a deep-freeze zone at distances in the vicinity of 7 billion kilometres from the sun.

According to Nasa, communication constraints restrict the ability of New Horizons to visit these potential targets and it hasn’t been decided how close this visit will be. It is expected to visit these identified targets in early 2019, assuming all goes well with the Pluto fly-by. The characterisation of these objects under close observation will provide clues about the origin of our solar system.

A visit to Eris has not been planned because it’s too far away – beyond the capability of New Horizons’s communication with the Earth. It would be great if the spacecraft could explore Eris and its single moon Dysnomia, their atmospheres and surface morphologies, so that a direct comparison with the Pluto system could be made to understand dwarf planetary science.

Following the first-ever mission to explore the Pluto system and potential targets in the Kuiper Belt, it is possible that Nasa may announce a mission to Eris and other planetary bodies in the belt – those that are known today and those that will be discovered during the Pluto mission by New Horizons.

The question remains of whether the New Horizons exploration of the Pluto system will be able to soothe the painful memory of demoting its planetary status?

Tariq Majeed is an associate professor in the department of physics at the American University of Sharjah