Small course corrections

You don’t send a probe to Pluto by pointing really well.

Pluto is about one thousand kilometers across and a few billion kilometers away. It’s not possible to calculate, much less execute, a trajectory so accurately that we could fire a projectile at Pluto and hit it. But space probes are not simply passive projectiles. They make lots of small course corrections along the way. New Horizons is on its way to Pluto and will arrive on July 14, 2015, not because the rocket that launched it was aligned precisely but because its course has changed slightly all along the way. (How do probes do this? Kalman filters and their extensions.)

The human world is far less predictable than celestial mechanics. It’s foolish to think we can plan far ahead without any course corrections.

Artist conception of New Horizons via NASA

3 thoughts on “Small course corrections

  1. Heh. this bit on trajectory accuracy reminds me of an old E.E.”Doc” Smirh novel.

    From the Wikipedia synopsis of “Spachouds of IPC”:

    When the Inter-Planetary Corporation’s (IPC) crack liner, “IPV Arcturus”, took off on a routine flight to Mars, it turned out to be the beginning of an unexpected and long voyage. There had been too many reports of errors in ship’s flight positions from the Check Stations and brilliant physicist Dr. Percival (“Steve”) Stevens is aboard the Arcturus on a fact-finding mission to find out what’s really happening, and hopefully save the honor of the brave pilots of the space-liner Arcturus from the desk-jockeys’ in the Check Stations implications of imprecision – the nastiest insult you could cast at a ships pilot. He and the pilots are right, it was the Check Stations that were out of position, not the ships.

  2. … not because the rocket that launched it was aligned precisely but because its course has changed slightly all along the way.

    Would it not be more correct to say that the spacecraft will successfully arrive at Pluto because it was launched precisely and because of the course corrections? You can’t just launch at any random time, pointing any which-way, because it would be impossible to carry enough fuel for the course corrections.

    And if you look at the comments by the principal investigator, e.g. here:
    you can see him noting how well the launch was pointed:

    “Fortunately, our launch was so accurate that only about 40 miles per hour of trajectory change needed to be made; this is less than one quarter of our post-launch trajectory correction budget. Compare that 40 miles/hour number to our 36,254 mile-per-hour exit from Earth, and you’ll see just how fantastic a job our Atlas V/STAR-48 launcher did. This allows us to bank the difference in fuel as savings for future Jupiter, Pluto and Kuiper Belt Object encounters.”

  3. After reading the prior post entitled “Academic freedom”, I was thinking this one would contain errata for a tutorial.

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