Emily LakdawallaFeb 28, 2019

Fun With a New Data Set: The OSIRIS-REx Earth Flyby

The OSIRIS-REx team recently issued their first data release to the Planetary Data System. This release doesn’t include any closeup pictures of asteroid Bennu, but it does include all the pictures they took during their September 2017 Earth flyby. Looking at familiar targets like Earth and the Moon is helpful for getting into a new data set. This is true for amateurs and professionals both -- read Vicky Hamilton’s article for The Planetary Report on how the Earth flyby helped the science team.

OSIRIS-REx

NASA's OSIRIS-REx mission will collect a sample from asteroid Bennu and return it to Earth in 2023.

It’s a relatively small data release, so I dove straight in. I downloaded everything and converted it from FITS to PNG format using Björn Jónsson’s IMG2PNG software to see what there was. I uploaded all the OSIRIS-REx Earth flyby PNG files here with a little bit of metadata -- browse it for yourself and see what you can find!

What did I find? First of all, there’s a neat set of photos taken with the NavCam of Earth, its size diminishing as OSIRIS-REx flew away from its flyby.

NavCam image animation from OSIRIS-REx’s Earth flyby
NavCam image animation from OSIRIS-REx’s Earth flyby OSIRIS-REx tested most of its instruments on Earth and the Moon following the 22 September 2017 Earth flyby. This animation includes 29 images of Earth taken between 21:08 and 22:43 on that day as OSIRIS-REx shifted to place the image of Earth in different places on the NavCam detector. NASA / GSFC / UA

To get a better look at Earth’s rotation and diminution, I aligned all the pictures on one of the cloud features. All we should be seeing in the animation below is Earth rotating and receding. So why does it look like a squishy stress ball?

Squish squash Earth
Squish squash Earth 29 images of Earth taken by the NavCam on OSIRIS-REx have been aligned on a cloud feature. Earth's apparent squishiness results from geometric distortion in the camera optics. NASA / GSFC / UA / Emily Lakdawalla

It was time for me to read more about OSIRIS-REx’s cameras. OSIRIS-REx has a lot of cameras. For that reason, I made sure that we made a good cheat sheet to accompany the aforementioned Planetary Report article. Take a look and compare the detector sizes and fields of view of the six different cameras.

OSIRIS-REx's instruments
OSIRIS-REx's instruments OSIRIS-REx’s instrument deck is crowded. Most are science instruments, including OCAMS, OLA, OTES, OVIRS, and REXIS, but TAGCAMS and LIDAR are for navigation purposes. Loren A. Roberts for The Planetary Society

You can see that there are two main sets of cameras on OSIRIS-REx. First, there are the OCAMS, the science cameras SamCam, MapCam, and PolyCam. They have smaller detectors (1024 by 1024 pixels), which is a common size for space science cameras. Each one has a field of view about 5 times narrower than the next one, providing resolving power 5 times greater. A set of images of the Moon taken 3 days after the Earth flyby shows you how the 3 cameras fields’ of view and resolving power compare to each other.

OSIRIS-REx OCAMS fields of view compared
OSIRIS-REx OCAMS fields of view compared OSIRIS-REx took these 3 images of the Moon with its 3 science cameras within about an hour of each other on 25 September 2017. The pictures illustrate the differences in field of view and resolving power among the cameras. SamCam has a 21 degree field of view; MapCam has a 4 degree field of view; and PolyCam has a 0.82 degree field of view. NASA / GSFC / UA / Emily Lakdawalla

The other set of cameras is called TAGCAMS. There are three of them: two NavCams and a StowCam. (One of the NavCams is often called the NFTCam, for “natural feature tracking;” I will have to research more about what that means.) These have larger detectors than the OCAMS and very wide fields of view. They see a lot more area on the sky than any of the science cameras, at slightly higher resolution than SamCam but lower resolution than MapCam or PolyCam.

OSIRIS-REx NavCam field of view compared to OCAMS
OSIRIS-REx NavCam field of view compared to OCAMS OSIRIS-REx's NavCams have a wide field of view (44 by 32 degrees) and large detectors (2592 by 1944 pixels). This is a photo of the Moon taken a few days after OSIRIS-REx's Earth flyby. The squares on the image show the fields of view of the mission's science cameras. NASA / GSFC / UA / Emily Lakdawalla

It’s the wide field of view that’s responsible for Earth’s stress-ball-like squishiness in the animation above. There is geometric distortion in the NavCam images, as explained in this paper describing the TAGCAMS instruments. (When data sets get released to the PDS, they also provide documentation, papers about how the instruments work, and this paper is open-access.) Here’s an illustration showing what a NavCam photo of a checkerboard looks like without (left) and with (right) geometric correction. You can see that squares near the center of the uncorrected image appear larger than those nearer the edges, and that the farther you get from the center, the more distorted the squares get.

Correcting optical distortion in OSIRIS-REx TAGCAMS images
Correcting optical distortion in OSIRIS-REx TAGCAMS images The left image was taken with an engineering model of an OSIRIS-REx NavCam. On the right, the image has been calibrated, with its optical distortion removed. Bos et al. 2018

By playing with the left image in Photoshop I figured out that I could use the Lens Correction filter and set a custom value of 12.00 in the “Remove Distortion” field o dramatically reduce the distortion. Having done that, I recreated the receding-from-Earth animation, below. It’s not perfect, but it’s pretty good, I think!

Flying away from Earth
Flying away from Earth As OSIRIS-REx departed from its 22 September 2017 Earth flyby, it took a sequence of images of the diminishing, rotating planet with its NavCam. NASA / GSFC / UA / Emily Lakdawalla

Björn Jónsson, who’s more skilled than I am at image processing, produced a few really nice views of Earth and the Moon from these data.

OSIRIS-REx MapCam view of Earth, just after flyby
OSIRIS-REx MapCam view of Earth, just after flyby OSIRIS-REx’s MapCam looked at the Pacific Ocean through color filters just hours after the flyby when Earth filled its field of view. The photo was taken on 23 September 2017 at 00:03 from a range of 179,000 kilometers. NASA / GSFC / UA / Björn Jónsson
OSIRIS-REx MapCam view of the Moon, 3 days after flyby
OSIRIS-REx MapCam view of the Moon, 3 days after flyby Color image enlarged by a factor of 2. OSIRIS-REx took this photo on 25 September 2017 from a range of 1,227,000 kilometers. NASA / GSFC / UA / Björn Jónsson

And here is Jacint Roger Perez' take on some of the MapCam data:

MapCam views of Earth from OSIRIS-REx flyby
MapCam views of Earth from OSIRIS-REx flyby Six days after it flew past Earth, OSIRIS-REx pointed its color MapCam at our home planet. NASA / GSFC / UA / Jacint Roger Perez

What can you do with the OSIRIS-REx Earth flyby data? Check them out for yourself!

OSIRIS-REx Earth flyby timeline
OSIRIS-REx Earth flyby timeline OSIRIS-REx observed Earth and the Moon with its cameras during four opportunities following its Earth gravity assist (EGA). Loren Roberts for The Planetary Society

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