The first age of deep space planetary exploration came to an end today as NASA's New Horizons spacecraft flew by Pluto at 7:49 am EDT. The nuclear-powered unmanned probe sped past the dwarf planet at a distance of 12,500 km (7,750 mi) and a speed of 14 km/sec (31,000 mph), making it the final classical planet to be visited by a spacecraft.
The New Horizons flyby is not only historic, but also a bit paradoxical. Because the US$750 million spacecraft is on a ballistic trajectory, NASA mission control is in no doubt that it has flown past Pluto. The question is, did it survive the attempt and what is its present status? Because Pluto is currently 4.77 billion km (2.97 billion mi) from Earth, radio signals from New Horizons take 4 hr and 25 min to reach us.
Worse, during the flyby, New Horizons is positioning itself to bring its suite of seven scientific instruments to bear, so the probe's main antenna isn't pointing in the right direction to transmit to NASA's Deep Space Network on Earth. According to NASA, this means that news of the mission's success or failure will not be received until about 9:00 pm EDT.
If all is going to schedule, New Horizons should have hit its 300 km (200 mi) diameter target circle at the correct moment, has sped past Pluto and its five moons, and is now receding into deep space. However, at a press conference yesterday, a NASA spokesman cautioned that there is a one in 10,000 chance that the spacecraft may have encountered a dust particle or larger piece of debris. At the probe's hypersonic velocity, even the smallest grain would hit like bullet; crippling or even destroying the craft.
Assuming this remote accident did not happen or a technical malfunction did not occur, New Horizons has spent the encounter autonomously carrying out 30 scientific objectives that will included mapping, atmospheric studies, and looking for new rings and moons. In addition, it gathered data that will help to pin down such vital statistics as Pluto's exact size, density, magnetic field (if any), and the nature of its largest moon, Charon.
One particular irony is that though the flyby is over and its next encounter with a Kuiper belt object is years away, New Horizons still has a lot of work ahead of it. Because of its distance and limited transmitting power, the probe can only return data at a sluggish 2 kilobits per second, so the mass of data collected today will take 16 months to download.