New Horizons is on a one-way journey to the Kuiper Belt and beyond. Unlike some missions that return back to the Earth, New Horizons sends back all of its data using a radio transmitter and its 83-inch (2.1-meter) diameter radio antenna. It receives commands over this link, and returns both science data and information on the spacecraft's temperature, health, and power.

Sending Commands to the Spacecraft

All commands sent to the New Horizons spacecraft must first pass a rigorous development and review process to ensure the safety of the spacecraft. The science team will work closely with the instrument mission operations and spacecraft teams to develop the commands that trigger New Horizons' scientific activity. After the command sequences are tested on the ground, they will be sent by the New Horizons Mission Operations Center at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland, to NASA's Deep Space Network (DSN), which is operated and managed by the Jet Propulsion Laboratory.


Spacecraft Pointing

Usually, New Horizons must be oriented in a particular direction to take data with its scientific instruments. For example, its various telescopes must be accurately pointed at a specific target (such as a location on the surface of Pluto). New Horizons has an advanced guidance and control (G&C) system for determining its orientation. An inertial measurement unit (IMU), which is a sophisticated gyroscope, provides relatively coarse positional information and keep the spacecraft stable. Star tracker cameras employing charge coupled devices (CCDs) image the sky, and the positions of the detected stars are used to acurately determine the orientation of the spacecraft. The star tracker feeds star-position information to the G&C computer, which compares the observed position to the commanded position. If the difference is outside some predetermined tolerance, small hydrazine thrusters will fire to re-orient the spacecraft to the desired position.

The thrusters provide the only mechanism for maneuvering the New Horizons spacecraft, and the amount of hydrazine thruster fuel will be carefully watched to ensure that the mission's scientific objectives are fullfilled. Besides the small thrusters that are used to fine-point the spacecraft, thrusters that are approximately five times more powerful will be used during trajectory correction maneuvers (TCMs) that keep New Horizons on the proper path to its targets.


Science Instruments

New Horizons carries seven scientific instruments, which will collect several types of data. (The instrument names and main functions are described in the science payload section of the Web site.) As an instrument makes an observation, data will be transferred to a solid state recorder (similar to a flash memory card for a digital camera), where they are compressed (if necessary), reformatted and transmitted to Earth through the spacecraft's radio telecommunications system.


The Data Rate Challenge

A major challenge for the New Horizons mission will be the relatively low "downlink" rate at which data can be transmitted to Earth, especially when you compare it to rates now common for high-speed Internet surfers.

During the Jupiter flyby in February 2007, New Horizons will send data home at about 38 kilobits per second (kbps), which is slightly slower than the transmission speed for most computer modems. The situation gets even more challenging when New Horizons reaches Pluto, where the downlink rate falls to about 300-600 bits per second. At 300 bps, mission operators would need nearly 12 hours to downlink a single image from New Horizons' long-range imager, and nearly 40 days of continuous downlink to gather the entire 10 gigabits (or so) of data during the Pluto-Charon encounter.

Since NASA's Deep Space Network has to track other missions besides New Horizons, the team plans to produce compressed "browse" data - using an average compression ratio of about 20 - that will be sent back to Earth within 10 days of Pluto closest approach. At that compression rate some of the original data will be lost. However, New Horizons scientists will examine the browse data carefully and set priorities for downlinking "losslessly compressed" data that doesn't lose any original content. Some of the browse data will good enough for immediate public release, so everyone can share in the excitement of the flyby. Initial analyses of the so-called losslessly compressed data should appear in peer-reviewed scientific journals with a year after the Pluto-Charon flyby.


New Horizons Mission Operations Center

Data received on Earth through the Deep Space Network will be sent to the New Horizons Mission Operations Center at APL, where data will be "unpacked" and stored. The mission operations and instrument teams will scour the engineering data for performance trend information, while science data will be copied to the Science Operations Center at the Southwest Research Institute in Boulder, Colorado. At the Science Ops Center, data will pass through "pipeline" software that converts the data from instrumental units to scientific units, based on calibration data obtained for each instrument. Both the raw and calibrated data files will be formatted for New Horizons science team members to analyze. Within nine months of receipt, both the raw and calibrated data, along with various ancillary files (such as documents describing the pipeline process or the instruments) will be archived for use by the general scientific community at the Small Bodies Node of NASA's Planetary Data System.