Like a racehorse stumbling at the finish line, NASA's New Horizons deep space probe gave mission control a moment of anxiety on July 4 as communications were temporarily lost. The unmanned nuclear-powered spacecraft, which is only nine days from its historic flyby with the dwarf planet Pluto, lost contact with the Deep Space Network at 1:54 pm EDT and came back online at 3:15 pm.

According to NASA, the anomaly that caused the loss of communications wasn't due to a hardware or software failure, but rather a timing flaw in the spacecraft command sequence that occurred while preparing for the close flyby of Pluto. The probe went into safe mode as it is programmed to do in such an event, and a backup computer reestablished communications with Earth and transmitted telemetry to allow engineers to assess the status of the probe.

The fact that it takes for a radio signal nine hours to reach New Horizons and return is of represents something of a challenge, but mission control is satisfied with the results. New Horizons is expected to return to full operational status on July 7 without any loss to the schedule of scientific studies. NASA stresses that the anomaly is extremely unlikely to occur again.

The US$650 million New Horizons mission was launched January 19, 2006 atop an Atlas V rocket from Cape Canaveral, Florida and has so far traveled 3 billion miles (4.8 billion km). The 1,054 lb (478 kg) nuclear-powered probe is on a 9.5-year mission to fly by Pluto and then on to study selected objects in the Kuiper Belt. Sent on a slingshot trajectory using the gravitational pull of Jupiter, New Horizons passed the orbit of Neptune on August 24 last year and will rendezvous with Pluto on July 14 2015, which it will pass at a distance of 8,000 miles (13,000 km).

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