In the question Where is Lucy going? (asteroid mission) I've asked for more clarification on the trajectory of the Lucy mission's trajectory.

I believe that Lucy will simply do a close fly-by of each of these asteroids, rather than stop to smell the roses. Lucy will be in an elliptical heliocentric orbit with a periapsis of 1AU and an apoapsis of about 5.6 AU to go out a little farther than Jupiter's L4/L5 distance from the Sun (see image in that question). That gives Lucy a semi-major axis of 3.3 AU We can calculate the velocities using the vis-viva equation:

$$v^2 = GM \left( \frac{2}{r} - \frac{1}{a} \right).$$

The Sun's standard gravitational parameter is about 1.33E+20 m^3/s^2, and 1 AU is about 1.5E+11 meters, so Lucy's slowest velocity at apoapsis should be only about 6.9 km/s.

On April fool's day 2028 the asteroid 11351 Leucus's state vector will be:

(-3.62, -7.64, -0.20) x 1E+08 km 
(10.8,  -4.99,  2.41) km/s

which puts it at about 5.6 AU and and 12 km/sec, or 5.1 km/s faster than Lucy assuming they are moving in the same direction.

This means Lucy could be within a million km of the asteroids for four or five days, and so will have to take advantage of good optics to photograph them at long range while they slowly rotate.

I'm guessing this means that Lucy's imaging optics will be very good. These asteroids are much smaller than planets, and so the fast flyby's are really pushing it compared to deep space planetary flyby's.

The Wikipedia article about Lucy mentions:

  • L'Ralph - panchromatic and color visible imager and infrared spectroscopic mapper. L'Ralph is based on the Ralph instrument on New Horizons and will be built at Goddard Space Flight Center.
  • L'LORRI - high-resolution visible imager. L'LORRI is derived from the LORRI instrument on New Horizons and will be built at the Johns Hopkins University Applied Physics Laboratory.

But "based on" and "derived from" leaves room for substantial improvements since there are 15 years between the two launches.

Question: I'd like to ask about the resolution of Lucy's cameras, and their ability to work during the closest approach of flyby, and what improvements are been made or contemplated in their design and operation based on lessons learned from New Horizons.


1 Answer 1


I fear there is not much room left for substantial improvements of the resolution of L'LORRI. Optical limits for angular resolution of such a camera did not change during these 15 years. Weight limits are still very narrow.

Increasing the image sensor pixel count only would very slightly increase the weight of the camera, but does not change the maximal angular resolution of the telescope. The sensor with smaller pixels will be less sensitive and noise will increase. Exposure time should be longer. Transfer time for one image will be much longer, less images could be transmitted. You get images with more pixels but not more details when the optical resolution of the telescope is the limit.

Increasing the focal length of the telescope but not its aperture does not change angular resolution. Weight will slightly increase. With a smaller angular field of view and the same angular resolution you get even less image information. The light intensity delivered to the image sensor will decrease and longer exposure times will be necessary. Image noise will increase.

Doubling the aperture diameter of the telescope will improve the angular resolution and an image sensor with more pixels could be used. There will be enough light for the smaller pixels of the sensor but exposure time should be shorter to limit motion unsharpness. But doubling the diameter of the mirror of the telescope will at least quadruble its weight.

I agree to Steve Linton's and uhoh's comments, improvements of detector readout, dynamical range and noise are possible.

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    $\begingroup$ What about criteria other than resolution? Given the mission, short exposure times and rapid adjustment of the optics would seem to be at a premium, as would fast readout from the camera to the onboard storage, all so as to get as many images (perhaps with different filters etc.) as possible during each encounter. $\endgroup$ Commented Dec 4, 2018 at 10:38
  • $\begingroup$ Parallel instead of serial capture of images at different wavelength would be a big thing. But exposure times and focus adjustment should be equal for all wavelengths imaged simultaneously. $\endgroup$
    – Uwe
    Commented Dec 4, 2018 at 10:56
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    $\begingroup$ Yes, detector readout technology can improve, so can dynamic range, noise, and frame buffering for anti-blurring and on-board data storage and processing. 15 years is a long time! While these may not affect the theoretical resolution of a fixed camera in a laboratory test, they can all contribute to the resolution of actual images collected during a 5 km/s flyby of an asteroid in deep space with only 4% of the sunlight we have at 1 AU. $\endgroup$
    – uhoh
    Commented Dec 4, 2018 at 10:57
  • $\begingroup$ LORRI at Pluto had only about 0.06% of the sunlight we have at 1 AU, 60 times less than L'LORRI will have at the asteroids. That should help doing many images with short exposure times during the flyby. $\endgroup$
    – Uwe
    Commented Dec 4, 2018 at 11:52
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    $\begingroup$ @Uwe 11351 Leucus is 50 times smaller than Pluto. This is a picture of Hydra, a moon of Pluto, almost twice as big as 11351 Leucus en.wikipedia.org/wiki/Hydra_(moon). I wouldn't say that this resolution is good enough to justify a deep space mission. Let's hold for some factual information on Lucy's capabilities. There are too many variables to design a mission here in Stack Exchange. $\endgroup$
    – uhoh
    Commented Dec 4, 2018 at 12:45

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