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See other posts from February 2013

Headshot of Emily Lakdawalla

When will New Horizons have better views of Pluto than Hubble does?

Posted by Emily Lakdawalla

2013/02/18 04:22 CST

Topics: explaining science, trans-neptunian objects, Pluto, dwarf planets beyond Neptune, New Horizons, Hubble Space Telescope

Last week, I posted an explainer on why Hubble's images of galaxies show so much more detail than its images of Pluto. In brief, it's because galaxies can appear a lot bigger in the sky than Pluto does; the one I used in my example spans a bit of sky a thousand times wider than the disk of Pluto. Then I set you all a homework problem: when will New Horizons be able to see Pluto better than Hubble does?

New Horizons at Pluto, July 2015

Dan Durda, SwRI

New Horizons at Pluto, July 2015
Artist's concept of New Horizons as it reaches Pluto.

Here are the relevant facts.

  • Hubble's angular resolution -- the width of one Wide Field Camera 3 pixel, projected onto the sky -- is 0.04 arcseconds, or 0.19 microradians. (I discussed what this means and why I might want to use microradians in last week's post.)
  • Pluto appears fewer than 3 Hubble WFC3 pixels across.
  • New Horizons' highest-resolution camera, LORRI, has an angular resolution of about 5 microradians.

So New Horizons' vision is is about 25 times less sharp than Hubble's. Therefore, New Horizons will have to be about 25 times closer to Pluto than Earth is before its images will begin to improve on Hubble's. Right now it's "only" 4 times closer to Pluto than Earth is. It still has a long way to go!

Pluto will appear 3 pixels across to LORRI when New Horizons is about 160 million kilometers away. How did I get that number? Pluto is 2400 kilometers across, so 3 pixels means each pixel is 800 kilometers. Divide that by 5 microradians -- which is the same as dividing it by 5 and then tacking the word "million" on the end -- and you get 160 million kilometers.

New Horizons is currently a thousand million kilometers from Pluto. When will it have closed to 160 million? The simplest way to answer this question is to use the Solar System Simulator. Set it to view Pluto as seen from New Horizons, then play with dates until you make the distance to Pluto go to 160 million kilometers. (Note that you don't have to keep returning to the form to change dates -- you can change them directly, in the URL.) It looks like it'll be almost exactly two more years for New Horizons to get close enough to Pluto for the mini-planet to appear three pixels across to LORRI. Hubble's view actually spans fewer than three pixels, so New Horizons' view will actually be better than Hubble's a bit earlier than that. A good round date to use is January 1, 2015. So it makes perfect sense that January 1, 2015 is when New Horizons' approach science observations begin!

New Horizons Science Overview

New Horizons Science Overview
New Horizons will spend most of 2015 training its package of seven imagers, spectrometers, and in situ plasma instruments on the Pluto system.

 

Or read more blog entries about: explaining science, trans-neptunian objects, Pluto, dwarf planets beyond Neptune, New Horizons, Hubble Space Telescope

Comments:

Michael Mayer: 02/18/2013 06:12 CST

Emily, I cheated: Wikipedia says "May 5, 2015 Better than Hubble Images exceed best Hubble Space Telescope resolution."

Derek Szymanski: 02/18/2013 09:23 CST

Instead of 3 pixels, I used 2.68 pixels and ended up with about 180 Million miles. Meh...... Close enough for government work, I suppose.

Derek Szymanski: 02/19/2013 12:22 CST

Opps...... Not miles but kilometers.

Ted Hartley: 02/19/2013 08:40 CST

I was foiled by a missing 0. My calculation for a better picture should have been 133,333 km, BUT, at 13,333 km the pictures should be awesome.

Emily Lakdawalla: 02/19/2013 05:11 CST

Derek, if you're going to use more significant figures, then we'll also have to start talking about the actual performance of Hubble WFC3 versus LORRI -- and that's too much detail for me! Back-of-the-envelope calculations like the ones I did get you close enough for blogger work :)

Enzo: 02/20/2013 02:22 CST

Would it be possible to repeat the same exercise for Dawn and Ceres ? Another interesting question would be if Cassini will be able to resolve at least some of the particles in the rings during the close orbits to Saturn at the end of the Solstice Mission. Thanks.

David Kemp: 03/17/2013 05:33 CDT

I understand miles better than kilometers.

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