Once you've sent the first space probe to Pluto, what do you do for an encore? As NASA’s New Horizons spacecraft races to its encounter with the dwarf planet next year, the Hubble Space Telescope Time Allocation Committee have given its recommendation that the Hubble be used to find the Horizons’ next destination in the outer reaches of the Solar System.
The US$650 million New Horizons mission was launched January 19, 2006 atop an Atlas V rocket from Cape Canaveral, Florida. The unmanned 478 kg (1,054 lb) spacecraft was sent on a 9.5-year mission to fly by Pluto – a distance so far that radio signals from the nuclear-powered probe take four hours to reach Earth. Sent on a slingshot trajectory using the gravitational pull of Jupiter, New Horizons is scheduled to pass the orbit of Neptune on August 24 and will rendezvous with Pluto on July 14 of next year, which it will pass at a distance of 13,000 km (8,000 mi).
The question is, what to do next with New Horizons after it has finished its primary mission? Once past Pluto, there’s light years of nothing in front of the spacecraft, so a decision of where to go next must be made soon, so that mission control can plan for the appropriate maneuvers to use Pluto’s gravity to slingshot it in the right direction.
The obvious target is one of the thousands of objects that make up the Kuiper Belt; the detritus left over from the formation of the Solar System that extends beyond the orbit of Pluto to a distance of about 8 billion km (5 billion mi) from the Sun. It’s similar to the asteroid belt between Mars and Jupiter, but it’s 20 times wider and has up to 200 times more material in it.
The Belt is extremely interesting to astronomers because it’s a huge repository of examples of what the Solar System was like 4.6 billion years ago. The trick is to find one of the objects that New Horizons can reach. The objects aren't the mixture of stony or iron-nickel asteroids found in the inner Solar System, but icy lumps made up of ice, methane, and other volatile chemicals.
For the most part, our knowledge of the Kuiper Belt is the result of deduction and speculation rather than direct observation. It’s a bit like the giant squid that inhabit the depths of the Earth’s oceans; we know they’re there, but not much else. If New Horizons is going to visit one of these objects, then NASA has to find one that’s within the probe’s reach. That’s where Hubble comes in.
Hubble has already been used to support the New Horizons mission, including the creation of low-resolution maps of the surface of Pluto, the discovery of four new moons, and a survey of dust clouds around the planet that might endanger the flyby. With its powerful optics, it has a much better chance of spotting a candidate for New Horizons to visit than ground-based scopes.
However, it’s not easy to find Kuiper Belt objects. They aren't large in cosmic terms and they don’t reflect much light. According to NASA, they tend to be smaller than Manhattan Island and as black as a lump of coal – not to mention extremely cold. Hubble will look for these objects using the same methods asteroid hunters have used for two hundred years.
The Hubble will look at a small arc of the sky in the constellation of Sagittarius that’s within reach of New Horizons. The background of stars will remain fixed, but if a possible Kuiper belt object is present, it will appear as a streak as it moves across the image during the time of exposure. These can then be further investigated for verification.
The goal is to find two objects in the designated area, which is a field of view roughly the angular size of the full moon. If these are found, it will indicate that there’s a statistical chance of objects being present that New Horizons could reach. If so, then Hubble will be directed to make a more detailed search. According to NASA, this step by step approach is necessary because the Hubble telescope is currently overbooked and time needs to be carefully allocated.
"The planned search for a suitable target for New Horizons further demonstrates how Hubble is effectively being used to support humankind's initial reconnaissance of the solar system," says Matt Mountain, director of the Space Telescope Science Institute (STScI) in Baltimore, Maryland. "Likewise, it is also a preview of how the powerful capabilities of the upcoming James Webb Space Telescope will further bolster planetary science. We are excited by the potential of both observatories for ongoing solar system exploration and discovery."