So New Horizons has been travelling across the Solar System for the last 9 ½ years or so, and on the day of writing (11 June 2015) finds itself 34 days and about 40 million km (25 million miles) from its destination. Travelling at a speed relative to Pluto of about 13.8 km/sec (8.6 miles/sec) it will finally reach its closest approach to its target on 14 July 2015. Note that this is a fly-by mission – New Horizons does not have the rocket power or propellant to allow it to slow down and enter orbit around Pluto. So this means that operations to observe and measure the Pluto system is a rather brief affair, beginning on 4 July and concluding on the 20th. After the encounter the spacecraft will continue on its trajectory and will ultimately leave the Solar System.
The mission poses many challenges to the spacecraft designers and operations team. The thermal control design needs to accommodate the extreme variation in environmental temperature – from the significant solar heating experienced at Earth orbit to the intense cold at Pluto. The relatively low light intensity at Pluto impacts the imaging operations. One of the main factors is the light-time between the Earth and the spacecraft. A signal from Earth to New Horizons will take about 4 ½ hours, so the close approach operations cannot be controlled from Earth. This means that all imaging and other data gathering tasks have to be carefully planned prior to the encounter, and then all the time-sequenced commands required to carry out these tasks have to be uplinked to the spacecraft, so that whole process can be controlled automatically.
After the encounter this stored data will need to be downlinked to Earth. The spacecraft has a 2.1 m (83 inch) diameter high gain antenna to help with this task. However, because of the extreme distance of propagation and the low power availability onboard the spacecraft, the data rate will be typically about 2000 bits per second (2 kbps). Comparison of this with your typical broadband data rate, say 20 million bits per second (20 Mbps), shows that your broadband’s performance is about 10,000 better than the downlink from New Horizons. The task is also limited because the large ground-based antennae of the DSN (Deep Space Network) need to be shared with other spacecraft missions. As a consequence, the process of downlinking the data products will probably take until the back end of 2016 – so don’t hold your breath!
But it will be worth the wait!
* UT (Universal Time) is the time system used by the astronomical community. It is essentially equivalent to GMT (Greenwich Mean Time). For those in the UK, British Summer Time BST = UT + 1, so the closest approach to Pluto will occur at 12.50 BST on 14 July 2015.