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Thread: Dawn probe peak speed

  1. #1
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    Dawn probe peak speed

    I know it made the record as the highest delta v change in a space probe (10 km/s), but what was the peak speed it achieved as it accelerated towards Ceres or Vesta? Some people on the internet have takem the whole meaningless "0 to 60 mph in 6 days" factoid to mean that Dawn is the "slowest spacecraft ever" and that it was actually approaching Vesta at the speed of 60 mph which I know is very incorrect but just how fast were the peak cruise speeds, that is, before it started decelerating as it was preparing for a orbit insertion?

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    Quote Originally Posted by m1omg View Post
    I know it made the record as the highest delta v change in a space probe (10 km/s), but what was the peak speed it achieved as it accelerated towards Ceres or Vesta? Some people on the internet have takem the whole meaningless "0 to 60 mph in 6 days" factoid to mean that Dawn is the "slowest spacecraft ever" and that it was actually approaching Vesta at the speed of 60 mph which I know is very incorrect but just how fast were the peak cruise speeds, that is, before it started decelerating as it was preparing for a orbit insertion?
    See Google results of "Dawn Vesta" search:

    https://www.google.com/?gws_rd=ssl#q=dawn+vesta

    Top speed 25,635 mph or about 11.45km/sec, which as I understand it was at the moment of launch vehicle cutoff upon liftoff from Earth. When approaching Vesta, and later in transit from Vesta to Ceres, its speed would have been close to that of main belt asteroids the same distance from the Sun, with its ion thruster gently tweaking the trajectory.

    That figure of 60 mph, if reckoned relative to Vesta, may be realistic.

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    Quote Originally Posted by Hornblower View Post
    That figure of 60 mph, if reckoned relative to Vesta, may be realistic.
    Wouldn't that mean it would take more than half a century to get there? Or is it that Dawn was going in a trajectory that meant it was not going to Vesta, but instead, Vesta came to it? Did it actually use the engine to slow down to assume a trajectory that made it encounter Vesta? That doesn't seem right as orbital speed of Vesta is 19.34 km/s so if it was going at a speed similiar to main belt asteroids it must have been going at roughly that speed. Also, is the 11.45 km/s figure relative to Earth? If so, what was it's peak speed relative to Earth.

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    I used the NASA's eyes simulation app and while the Vesta encounter speed is just 161.76 km/h, the speed relative to Vesta it achieved right after the Mars gravity assist and 270 days of thrust is as you can see in the picture I attached, very high (132 000 km/h or 36.6675 km/s). So why it was not registered as a speed record, considering even New Horizons only got around 17 km/s? I know it is not a wrong decimal point, the arrival speed is clearly 161.76 km/h as the other picture shows.

    To add to the confusion, when I divide the 1.73 billion miles trajectory to Vesta by the roughly 4 year travel time, I get ~22 km/s. And now I am really confused.
    Attached Images Attached Images
    Last edited by m1omg; 2015-Nov-23 at 11:39 AM.

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    Is speed is defined such that it can have negative values?
    Is moving away from a body a positive relative speed, and
    moving closer a negative relative speed? Is moving in an
    eastward orbit a positive relative speed and moving in a
    westward orbit a negative relative speed?

    If so, then Dawn's speed relative to Earth and Vesta and
    just about anything else will vary continuously over time
    from positive to negative and back. Only its speed relative
    to the Sun would always be positive, since its orbital speed
    is much greater than its radial speed toward or away from
    the Sun.

    Edit:

    Hmmmm. Negative orbital speeds don't sound like they
    make any sense. A polar orbit would have the same speed,
    but would be neither eastward nor westward. Hmmmm.

    -- Jeff, in Minneapolis
    Last edited by Jeff Root; 2015-Nov-23 at 12:33 PM.

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    I meant to illustrate in a simple way just how wildly speeds
    change due to their relative nature and the fact that all the
    bodies are moving relative to each other. But I must have
    made it too simple.

    Sorry, Einstein.

    -- Jeff, in Minneapolis
    Last edited by Jeff Root; 2015-Nov-23 at 01:49 PM. Reason: minor typo

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    Now that I think about it, that speed I gave that showed up in my Google search would have been relative to the launch site in Florida, with an initial bearing away from the Sun as seen from Earth. In the next few hours it would have curved around on a roughly parabolic path relative to Earth and asymptotically approaching tangency to Earth's orbit, slowing down as it goes.

    I don't know your source of that 60mph figure, or where the spacecraft was at the time. It could have been from a popular media author who was oversimplifying things. My educated guess for the approach to Vesta is that the spacecraft was in something resembling a Hohmann transfer, with the thruster enabling formation flying with the target and gradually converging with it.

    Edited to add: I don't think this craft was anywhere near as fast as New Horizons was when it passed through the main asteroid belt. The latter had to be hyperbolic to get to Pluto as soon as it did. Once again, a popular media dilettante could have misconstrued a large cumulative delta v from the ion thruster as a new speed record.
    Last edited by Hornblower; 2015-Nov-23 at 01:17 PM.

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    At some point they must switch from speed relative to the
    launch site to speed relative to the center of the Earth, and
    then to speed relative to the center of the Sun.

    -- Jeff, in Minneapolis

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    Just talking to the OP... maybe you already know this, but not all readers will. The Delta-V that Dawn set the record for was Delta-V by the spacecraft itself, not gravity assists, not other boosters. Also not all of that Delta-V went into one effort to increase speed away from one object in particular. It went into maneuvering and orbit matching, so the net speed for Dawn might never have been very high compared to New Horizons, or the real man-made speed champion so far: Helios (70 km/sec). What Dawn has done is great, but it is not about speed.
    Forming opinions as we speak

  10. #10
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    Quote Originally Posted by m1omg View Post
    I used the NASA's eyes simulation app and while the Vesta encounter speed is just 161.76 km/h, the speed relative to Vesta it achieved right after the Mars gravity assist and 270 days of thrust is as you can see in the picture I attached, very high (132 000 km/h or 36.6675 km/s). So why it was not registered as a speed record, considering even New Horizons only got around 17 km/s? I know it is not a wrong decimal point, the arrival speed is clearly 161.76 km/h as the other picture shows.
    There is no meaning in such a speed record. It didn't change its velocity by anything close to that amount. Its orbital speed is almost unchanged from when it was near Vesta, it is simply far from it and moving in a different direction.

    Every satellite orbiting Earth changes in velocity with respect to Mercury by 100 km/s every 44 days. Why don't we hand out records? Because they haven't done anything.

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    Quote Originally Posted by m1omg View Post
    Wouldn't that mean it would take more than half a century to get there?
    I think the 60 kph is the speed it was going when it got relatively close to Vesta. Of course, when it left the earth it would be going faster, and then gradually slow its speed to match that of Vesta.

    Quote Originally Posted by m1omg View Post
    Or is it that Dawn was going in a trajectory that meant it was not going to Vesta, but instead, Vesta came to it?
    I think that in physics, the two are equivalent. When two things approach one another, it's OK to say that A is approaching B or B is approaching A. They are both true. So it's not an either/or issue.

    Quote Originally Posted by m1omg View Post
    Did it actually use the engine to slow down to assume a trajectory that made it encounter Vesta?
    Yes, I'm sure it did. I presume that it started out from the earth in a trajectory that would make it quickly approach Vesta, and then would slow down, in the same way that the lunar orbiters approached the moon at high speed and then used their engines to slow down as they approached, allowing them to make a soft landing. The process must have been similar to that.

    Quote Originally Posted by m1omg View Post
    That doesn't seem right as orbital speed of Vesta is 19.34 km/s so if it was going at a speed similar to main belt asteroids it must have been going at roughly that speed.
    Yes, as you say, it would have to go at a speed similar to that (relative to the earth), but of course, relative to Vesta it would be going very slowly.
    As above, so below

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