FLUG REVUE April 2006: New Horizons probe is on its way

April 2006

NEW HORIZONS HEADS FOR THE FRINGES OF THE SOLAR SYSTEM

By Matthias Gründer

The first space probe of NASA's New Frontier programme was launched on 19 January 2006, bound for Pluto. Experts are divided over this celestial body on the outskirts of our solar system: is it really a planet or just an oversized object in the Kuiper Belt?

Ever since its discovery on 18 February 1930 by the then 24-year-old astronomer, Clyde Tombaugh, Pluto has puzzled the scientists. The existence of a “planet X” had been surmised for almost 100 years before that date because some researchers – wrongly, as we know now – claimed to have observed perturbations in the orbits of the ice giants, Uranus and Neptune. According to their theory, these could only have been caused by another planet on the fringe of the solar system.

Tombaugh discovered this tiny dot of light, but then it turned out that this celestial body, with a diameter of only 2,300km, was even smaller than our Earth. A planet that small could hardly have perturbed the orbits of its giant neighbours, and although Tombaugh went on to investigate another 90 million or so other stars in later years, he found nothing new. Apart from Pluto he could not find any other planet so far away.

But whether Pluto really is a planet is a contentious issue to this day among astronomers. The International Astronomical Union said it must be a planet as the tiny icy sphere fulfilled the most important criteria: it had a spherical shape, it orbited around the Sun, albeit on a highly elliptical trajectory, and it was itself encircled by three moons, the biggest of which was named Charon. But then in 1951 the counter theories at last received fresh impetus when the Dutch astronomer, Gerard Kuiper, formulated the hypothesis that our Sun is encircled by a “gigantic pile of debris”. This, it was argued, was a thick band of asteroids, comets and other lumps of ice and stone dating back to the era when our solar system was formed, and even Pluto could be just one mini-planet among many.

For many years no evidence was produced to support the existence of the Kuiper Belt, as earthbound observation technology was not yet capable of detecting such small celestial bodies. Opponents of the planet theory had to wait until 1992 when evidence was found for the first time of a piece of debris that was one-tenth the size of Pluto. Since then, over 850 such Kuiper Belt Objects (KBOs) have been observed, and new ones are constantly being added. Among them are some which are hardly any smaller than Pluto, and when in 2005 the tiny planet “2003 UB313” with a diameter of around 2,700km was discovered, everyone was perplexed. Did all these celestial bodies, and possible additional ones still to be discovered, all need to be promoted to the status of planets, or should one simply downgrade Pluto to an asteroid?

Whether NASA can resolve this dispute with its 478kg space probe, New Horizons, is questionable, as it will take this spacecraft about nine years to get close enough to Pluto to send any pictures and data about its surface back to Earth. Until then there are two possibilities: either Pluto acquires further brothers and sisters from among the KBOs, but retains its special status as a planet, or else it is downgraded. By the time the mission comes to an end, in around 2020, we will know more.

When NASA published its “New Frontiers in the Solar System” research plan in July 2002, Pluto was definitely one of the priority objects for observation, along with Jupiter, Venus, the Moon and the comets. The only condition was that the cost must not exceed $700 million. But the Pluto mission soon threatened to get out of hand, and the costs were already approaching $800 million, so that it looked as if NASA would have to abandon the project. Scientists, supporters of Pluto and even children protested in tens of thousands, so NASA gave in.

But now the conditions were made even tougher: anyone who managed in a nationwide competition to implement the project for no more than $500 million would get the grant. In actual fact, one team, headed by Alan Stern of the Southwest Research Institute of Colorado, fulfilled all the criteria, so nothing further stood in the way of launching the space probe. The winning team even managed to work sufficiently quickly to allow New Horizons to be launched before the window of opportunity within which the spacecraft had to be sent on its way came to an end on 3 February 2006.

This means that the space probe will now be able to fly directly past Jupiter and gain enormous momentum from its gravitational forces. Without this boost the journey would take almost three years longer and the craft would hardly be able to catch up with Pluto as the latter would have moved further away from the Sun on its heavily elliptical orbit. It will not approach again for another 100 years – too long a time for astronomers hell-bent on discovery.

Meanwhile it will steadily get darker and colder on the (still) ninth planet – and its current surface temperature of minus 230 degrees Celsius is already a record. There, beyond the outskirts of our solar system where the Sun appears just as one star among many, conventional solar cells would no longer be capable of maintaining an adequate supply of onboard energy. A plutonium battery was therefore installed onboard the spacecraft, and during the journey most of the instruments will be in a deep sleep, awakened only once a year for some brief checks.

The scientists are optimistic that New Horizons will complete all its mission tasks and that after flying past Pluto it will then be able to study the Kuiper Belt as well. Two potential asteroids with diameters of 40 to 90 kilometres have already been identified. After the successful launch, NASA could send a twin space probe to study further KBOs in 2009 or so. Only the funding remains to be clarified. In any case, in a few years' time we will know a lot more about conditions on the edge of our solar system. Whether Pluto will continue to be recognised as a planet in the future will then no longer actually matter.

From FLUG REVUE 4/2006

Last updated 10 March 2006

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