New Horizons prepares for a close encounter with mysterious Pluto

Much of our understanding of Pluto is educated guesswork. That should change when a probe completes its flyby of the dwarf planet on July 14th

On July 14th, the Nasa space probe New Horizons will make its closest flyby over the dwarf planet Pluto and begin its first scientific observations in a mission that has been nine years and 4.67 billion miles in the making.

The spacecraft hopes to shed light on one of the least-known bodies in our solar system. It primary mission will be to chart both the surface and the atmosphere of Pluto. It also marks the first in-depth space exploration of its kind since Voyager 2 explored Neptune in 1989.

"This is probably the last opportunity for such a significant step, from knowing nothing about one of the traditional worlds to knowing quite a lot about it in a very short space of time" says Kevin Nolan, lecturer in physics at Institute of Technology Tallaght. "This will fill in the last link in terms of major solar-system bodies. It can add a complete picture, and that can help us in terms of how we travel in space in the future. This mission – combined with others such as Rosetta – are starting to fill in a complete picture of the solar system."

“It adds to the broader understanding of who we are in the cosmic context.”

The probe awakens

The probe, which was launched at Cape Canaveral in January 2006, is the first in Nasa's $700 million New Horizons programme.

The spacecraft carrying the probe used the gravity of Jupiter to “swing” itself forward through space, saving itself three years of flight time. For most of its seven years travelling through space, the probe remained mostly dormant, sending only the occasional message back to mission control that all was well, and running tests to ensure its equipment was working correctly. It was only fully woken up in December last year as it began its final stages of approach to Pluto.

"New Horizons is continuing from where the Voyager missions left off in the 1970s," says Dr Apostolos Christou of Armagh Observatory. "It's like finishing a chapter of solar system exploration."

Although now considered a dwarf planet (it was downgraded just seven months after the probe was launched), Pluto still holds a lot of intrigue for astronomers. Much of our current understanding of Pluto’s surface and atmosphere is educated guesswork. It was only in April that the probe was able to get the very first close-range colour image of Pluto and its moon Charon.

“We have actually visited most other locales in the solar system,” says Christou. “But we’ve never been to one of those ‘trans-Neptunian’ objects. It’s a real voyage of discovery because nobody knows what we’re going to find. The best telescopes on the Earth can’t put [together] more than a few pixels.”

Ice, rock and methane

When it makes its closest pass at 11.57am, the probe will be 12,500km from the surface of Pluto. It is thought to be composed mostly of ice and rock, although astronomers think the atmosphere is rich enough in methane gas to create winds on Pluto. It can then characterise specific details on the surface as well as the various temperatures of its surface.

“If you look at the instruments on board, they range from viewable cameras to infrared cameras, to radio monitoring to particle detectors,” says Nolan. “We will get a huge amount of signs about what Pluto is made of and therefore how it came about.”

The probe carries links with its past: on board are some of the ashes of Pluto’s discoverer, Clyde W Tombaugh. One of the scientific devices has been named after Venetia Burney Phair, the woman who named the planet when she was 11 years old.

After charting Pluto, the probe could go on to investigate other objects in the area that interest Nasa. It could also travel outwards beyond the solar system much in the way Voyager 1 travelled beyond the solar system after completing its mission of charting Neptune.

When this final phase of space exploration ends, a new one will open up. “Going to Pluto now sets a new goal for a new generation” says Nolan. “The next generation will have to think far wider, and indeed even into interstellar space.”


Pluto was discovered in 1930 by American astronomer Clyde W Tombaugh, based on the hypotheses of two previous astronomers who wanted to explain an orbital “wobble” observed in Pluto’s closest neighbour, Neptune. However, just 76 years later, it would lose this honour to become what is now known as a dwarf planet. What went wrong?

The main problem stemmed from the very term “planet”. The word had been left almost intentionally vague for most of human history. Until recently, many were content to put that label on whatever rotated around the sun and could be found with the naked eye or, later, telescopes.

It was Pluto that caused the modern debate about what exactly constitutes a planet. After it was discovered at an Arizona observatory, initial calculations put Pluto’s mass as equal to the Earth’s. However, as astronomical equipment and techniques improved throughout the century, this calculation was lowered to less than 1 per cent: smaller than our moon.

Things became worse in 1978, when Pluto’s moon Charon was discovered. Charon is more than half Pluto’s size and remarkably close to it; a flight from Ireland to Australia covers roughly the same distance. This led some astronomers to call Pluto a double dwarf planet: two mini-planets that act as one. As the number of moons rose to five, serious debate about the future of planet Pluto began to be discussed.

In 2006 the International Astronomical Union released its formal definition of a planet as something that:

(1) revolves around the sun

(2) has enough mass to form a round shape

(3) has cleared the neighbourhood around its orbit.

It is this third condition that Pluto failed to meet. Pluto moves around countless large asteroids and debris along its path, an area in the outer solar system known as the Kuiper Belt.

While New Horizons won't help to disprove Pluto as being a dwarf planet, astronomers will soon find out just how planet-like it is.