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Headshot of Emily Lakdawalla

Looking Backward: Curiosity gazes upon the setting Earth

Posted By Emily Lakdawalla

05-02-2014 11:58 CST

Topics: pics of Earth by planetary missions, pretty pictures, amateur image processing, Earth, Mars, Curiosity (Mars Science Laboratory), many worlds, explaining image processing

Look again at that dot. That's here. That's home. That's us.

Carl Sagan wrote those words about a view of Earth from beyond Neptune, captured by Voyager 1 on February 14, 1990. No matter how far we go, Earth pulls at us, tempting us to gaze homeward. Curiosity looked skyward to photograph Earth a few days ago, on the 529th day of her mission to Mars.

I've been visiting hourly for the last few days, waiting for the full-resolution images to come down. The other amateurs at were looking for the images, too, and since the images hit the web 7 hours ago, several of them have produced their own unique takes on the data set. Here it is, as processed by Damia Bouic: a bright point of light in the twilit sky, setting toward the distant rim of Gale crater. Earth is the little isolated bright dot, left of center.

Earth sets over Gale crater

NASA / JPL / MSSS / Damia Bouic

Earth sets over Gale crater
On sol 529 (January 31, 2014), the rover looked westward after sunset to see a brilliant evening star -- Earth -- setting toward the horizon.

Here is an annotated version, processed by James Sorenson. You are here.

You Are Here: Earth sets over Gale crater

NASA / JPL / MSSS / James Sorenson

You Are Here: Earth sets over Gale crater
On sol 529 (January 31, 2014), the rover looked westward after sunset to see a brilliant evening star -- Earth -- setting toward the horizon.

The actual observation consisted of three Mastcam-100 images taken within the space of one minute (18:47 local time) of Earth among some stars, followed by a 3-by-2 Mastcam-34 mosaic of the twilit horizon. The original image data is noisy, a result of the low light levels; quite a bit of artistic license was involved in the creation of the pretty pictures. Here's an example bit of data cropped from the mosaic, displayed at its full resolution without any correction, so you can see what they had to start with. Earth is not in this part of the photo (I wanted to show you a bit that included the horizon). You can see all the original images here. This is the one that contains Earth.

Raw data from Curiosity sol 529 mosaic of western horizon

How did they figure out which dot was Earth? Partly by subtracting one frame from another (which eliminates "hot pixels" that are bright in all frames); partly by using software like Stellarium to figure out which dot should have been Earth; and partly by an understanding of how an actual light source looks a bit different from a hot pixel, though that difference is a bit hard to discern in the JPEG-compressed data that is shared with the public via the raw images website.

The Mastcam-100 image sequence is particularly interesting. Earth moves slightly from frame to frame -- that's Mars' rotation making it set toward the western horizon.

Earth setting over Gale crater (animation)

NASA / JPL / MSSS / fredk

Earth setting over Gale crater (animation)
Curiosity captured three images of Earth setting in the western sky after sunset on sol 529 with its higher-resolution Mastcam-100. Here, the images have been subtracted to cancel out noise in the camera detector, revealing Earth's bright dot in motion.

One of the amateurs (who goes by a pseudonym, fredk) noticed a tiny dot preceding Earth in the setting motion. Could that be the Moon? Let's do the numbers. A look at the solar system simulator shows that the Moon was very nearly as far from Earth as it gets at the time of the observation -- 360,000 kilometers from Earth. At the same time, Earth is relatively far from Mars, about 159 million kilometers. So Earth and the Moon were separated by about (360k/159000k)=2.3 milliradians. The angular resolution of the Mastcam-100 is 0.074 milliradians. So they should be separated by 30 pixels. I measure 25. Could be the Moon, though the math didn't work out quite as close as I'd've liked it to. This animation is less pretty than the one above, because the Moon (if that's what it is) is a much, much dimmer target than Earth, so it's nearly lost among the noise and JPEG compression artifacts. A better version can be made from the science data once it becomes available.

Earth and Moon on sol 529

Stacking the images together, fredk achieved this view of Earth and the Moon, as seen by a telephoto camera mounted on the head of a rover built on Earth but sitting on Mars.

Earth and the Moon from Curiosity, sol 529

NASA / JPL / MSSS / fredk

Earth and the Moon from Curiosity, sol 529
See other posts from February 2014


Read more blog entries about: pics of Earth by planetary missions, pretty pictures, amateur image processing, Earth, Mars, Curiosity (Mars Science Laboratory), many worlds, explaining image processing


stevesliva: 02/05/2014 03:39 CST

You are here. Yutu is here.

Scott: 02/05/2014 05:02 CST

But surely the angular distance merely places an *upper bound* on how far the moon will appear to be from Earth; depending on the relative positions of the three bodies, it might appear closer. In a limit case, suppose the moon were directly between Earth and Mars: the apparent angular distance from a Martian's perspective is 0 pixels. So I think you're right to identify this dot as the moon, and you shouldn't worry about the seeming 5-pixel discrepancy.

Ned: 02/06/2014 07:40 CST

Looking at the Stellarium projection of the view of Earth from Gale crater shows a pretty much dead on match for separation and orientation of the moon.

Mark: 02/07/2014 09:20 CST

Anyone know what the apparent visual magnitude of Earth would have been in this picture? It would be interesting to know how the brightness of Earth as seen from Mars compares to our view of Mars from Earth (other than just brighter and bluer).

Jonathan Ursin: 02/07/2014 03:38 CST

Great! Another homework problem for my Pre-Calculus students!

telluric: 02/08/2014 03:46 CST

My question is the same as Marks? What is the Moon and Earth's visual magnitude at Mars? Not just Earth but also the Moon. Is Venus or the Earth the brightest object after the Sun, Phobos and Deimos? How bright is the Moon? Albedos - Moon=.136, Earth=.367, Venus=.67. I'd say that Venus being about twice as far despite nearly twice as shiny, gives Earth the edge. Sorry Moon - too dull but must still be about mag 1 or 2. Yes must be a pretty sight! And what alignments of Earth-Moon and Venus from Mars. Pretty nice I guess. And Stellarium -- what a great and free planetarium program. They should make it available on IOS and Android (obstacle - written in QT). I'll have to try it out from viewpoint Mars!

Michael Mann: 03/16/2014 08:16 CDT

Emily 529 days on Mars & many billions spent have any of the below mission objectives been met? Or are you guys now just glorified tourists taking "happy snaps" 1)Determine the nature and inventory of organic carbon compounds 2)Investigate the chemical building blocks of life (carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur). 3)Identify features that may represent the effects of biological processes (biosignatures) 4)Investigate the chemical, isotopic, and mineralogical composition of the Martian surface and near-surface geological materials 5)Interpret the processes that have formed and modified rocks and soils 6)Assess long-timescale (i.e., 4-billion-year) Martian atmospheric evolution processes 7)Determine present state, distribution, and cycling of water and carbon dioxide. We all know what the great Carl Sagan said & we don't need to gaze homeward whilst on a multi billion dollar mission of discovery to Mars, please focus on your mission objectives, with budget cuts to other program's already announced you people can't afford to indulge yourselves "gazing homeward" as you say, but need to focus on maximising the return of scientific data of the Curiosity mission

Emily Lakdawalla: 03/16/2014 04:37 CDT

Michael, they have accomplished all of these, but not yet at the rocks they really want to get to at the base of the mountain. So they are still driving, and will be for the next several months. They can only drive during specific times of the day and certainly not at night. What's wrong with capturing a few happy snaps among the thousands of science images? Stay tuned this week for more science results, if you care to learn about them. However, since you can tell very quickly with an Internet search that it's false that "they are just glorified tourists taking happy snaps," I assume you are just trolling here.

KinkyAfro: 02/06/2015 03:49 CST

Beautiful. Question, though... Where is Venus? From Earth's prospective at the moment they are in the same part of the sky, which must mean Venus is at the side of the sun to us and Mars is on the far side (ish). So from Mars, surely Venus must be in view. And as Venus is closer to us than Mars, and we see a crescent Venus, then so must Mars. Where is it?

Kinkyfro: 02/06/2015 08:45 CST

A ahhh.... Please ignore last question/comment. Just seen date stamp! as I stated, though - beautiful. Still I wonder what that sight WOULD look like - Venus and Earth - in a Martian sky..?

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