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Emily LakdawallaJanuary 20, 2006

The good news for New Horizons keeps coming

Alan Stern just posted a new "PI Perspective" on the New Horizons website, and though brief it contains one tidbit of extremely good news:
Initial trajectory solutions indicate our launch was almost perfect, needing just perhaps 20 meters/sec or so of makeup delta-V. This is far less than the 100 meter/sec we had budgeted for, meaning we have much more fuel for Pluto and Kuiper Belt encounters than our "3 sigma" planning had to allow for.
To explain, every single spacecraft that launches from Earth carries with it a supply of fuel that is used for trajectory correction maneuvers (often abbreviated TCMs). That supply of fuel is one of the most critically limiting resources to what you can do with a spacecraft once its nominal mission has ended. It can't be regenerated in space. Rather than talk about the volume of fuel remaining, like you might with a car, mission planners refer to the amount of remaining fuel using the term "delta V" or "delta vee" -- "delta" being the Greek symbol used to apply to the change in a quantity, and "vee" meaning "velocity." That is, the amount of fuel remaining can change the velocity of New Horizons by a total of some number of meters per second.

When the New Horizons team laid out their mission plan for their Pluto/Charon encounter, they had to plan for all of the worst-case scenarios for the launch and Jupiter assist. That is, they expect that the launch vehicle will deliver them on a certain track, but statistics tells them how close they are likely to be to that planned track; the "3 sigma" part means that the team had to plan for a worst case scenario in which New Horizons would be three standard deviations, in statistical terms, off the planned track. Getting them back on track from that distance would take up to 100 meters per second of precious delta vee. But New Horizons will only need to use 20% of that, leaving a surplus of 80 meters per second in their fuel budget. The more fuel they have left after the Pluto encounter, the more they can shift their path after Pluto to encounter Kuiper belt objects. So Alan's good news means that they will have a wider range of choices of targets than they might have, and could even mean that they could get to more different targets than they had originally planned. One of the most interesting things about New Horizons' otherwise uneventful cruise period (uneventful except for the Jupiter flyby) will be waiting and seeing how the planning develops for the post-Pluto Kuiper belt object encounters.

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Emily Lakdawalla

Solar System Specialist for The Planetary Society
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