DART: Trick Shot by OpNav, and a Successful Test

Image from NASA's Hubble Space Telescope: Asteroid Dimorphos, 285 hours after intentional impact by DART spacecraft.
Asteroid Dimorphos and comet-like dust trail, 28 hours after DART spacecraft impact on September 26, 2022. (Image taken October 8. 2022) Image from NASA/ESA/STScI/Hubble, used w/o permission

NASA/Johns Hopkins Applied Physics Lab's DART (Double Asteroid Redirection Test) poster. (2021)On September 26, 2022, the NASA/APL DART mission changed the orbit of an asteroid: Dimorphos, a satellite of 65803 Didymos.

NASA Confirms DART Mission Impact Changed Asteroid’s Motion in Space
Josh Handal, Justyna Surowiec; press release; NASA, Johns Hopkins Applied Physics Laboratory (October 11, 2022)

Prior to DART’s impact, it took Dimorphos 11 hours and 55 minutes to orbit its larger parent asteroid, Didymos. … the investigation team has confirmed the spacecraft’s impact altered Dimorphos’ orbit around Didymos by 32 minutes, shortening the 11 hour and 55-minute orbit to 11 hours and 23 minutes. This measurement has a margin of uncertainty of approximately plus or minus 2 minutes.

“Before its encounter, NASA had defined a minimum successful orbit period change of Dimorphos as change of 73 seconds or more. This early data show DART surpassed this minimum benchmark by more than 25 times.…”
[emphasis mine]

Dimporphos and Didymos weren’t going to hit Earth before, and they aren’t going to now.

Not unless gravitational interactions with the tens of thousands of other near-Earth objects we’ve spotted so far change their orbit around the Sun. Which could happen, and is why developing planetary defense tech makes sense.

Transferring momentum from one object to another is simple. We do it every time we play pool or billiards.

I could try talking what happens by using phrases like vector quantities and the product of the mass and velocity of an object.

But I won’t.

Partly because there’s an awful lot of math involved. And partly because I figure you get what I mean from the “pool or billiards” reference.

Math matters, though. Particularly when figuring out how much the DART spacecraft might shift that asteroid’s orbit.1 There’s more to it than mass, velocity and vector. And that’s another topic.

So instead, I’ll take a look at how DART managed to hit an asteroid nearly head-on.

DART: Improving on Deep Impact’s Smart Impactor

Screenshot of NASA/JPL's 'Eyes on Asteroids' interactive real-time visualization. (September 28, 2022)This wasn’t the first time NASA crashed a spacecraft on purpose.

Back on July 4, 2005, Deep Impact’s “Smart Impactor” hit comet Tempel 1.

That mission’s flyby spacecraft’s ‘eye’ was an MRI (Medium Resolution Imager). It had spotted the comet 69 days earlier.

The Smart Impactor had an ITS (Impactor Targeting Sensor). The ITS was just like the MRI, but without a filter wheel.

Tempel 1 was close to Earth when the Deep Impact spacecraft reached Tempel 1, but that’s “close” on a cosmic scale. A signal would have taken seven and a half minutes to travel from the spacecraft to Earth and back again.

That’s not nearly fast enough for controlling the impactor with a joystick in mission control. So the Deep Impact flyby and impactor spacecraft did their own last-minute course corrections.

Spotting a comet or asteroid against a starry backdrop is anything but simple. Partly because we didn’t know what comet Tempel 1 looked like.

And so, folks at mission control couldn’t tell the Deep Impact spacecraft exactly what to look for. Comet nuclei can be round or shaped like lumpy potatoes. One even looked a bit like a rubber duck.

Folks planning the DART mission had a similar problem. When the DART spacecraft reached Didymos and Dimorphos, its moon, they were about 11,000,000 kilometers, 7,000,000 miles, from Earth.2

Size and Distance Comparisons
NASA/Johns Hopkins APL's illustration, comparing sizes of the two asteroids in the Didymos system relative to objects on Earth. (2022)
NASA/APL’s size comparison chart: Didymos, Dimorphos and familiar landmarks. (2022)

That’s close on an astronomical scale. But scientists figured Didymos was about 780 meters, 2,560 feet across and Dimorphos roughly 170 meters, 560 feet, across.

Or was, before the impact. And apparently Dimorphos was a tad smaller than expected. Around 525 feet across.

Either way, if I scaled the distance to Dimorphos, 7,000,000 miles, down to the length of an American football field, the asteroid would be about one five-thousandth of an inch across. A typical human hair is maybe 75 μm across. That’s around two thousandths of an inch.

You’re not going to see Didymos and Dimorphos without a good-sized telescope. Something bigger than the eight-inch scopes many amateur astronomers use, anyway.3

Magnitude, Observations and a Cubesat

Italian Space Agency's image of Dimorphos, taken by their LICIACube spacecraft a few minutes after the DART impact. (September 26, 2022) from ASI/NASA, via NASA, used w/o permissionAnd now, getting technical.

For earthbound observers, Didymos and Dimorphos was a 14th magnitude object when DART hit the asteroid’s moon.

That’s “object,” singular. Even with larger telescopes, the binary asteroid looks like a single speck of light.

That’d make measuring the pair’s orbital period pretty much impossible. Unless they eclipsed each other from Earth’s viewpoint. Which they do. At the moment, that is.

And that’s no coincidence. Narrowing possible targets down to something that was just right for the DART mission ended with a list of one.

Each time Didymos and Dimorphos eclipse each other, the Didymos-Dimorphos blob in earthbound telescopes gets a little dimmer. Timing the interval between dimmings told scientists what their orbital period was before and after the impact.

Up-close pictures of the Didymos-Dimorphos system, post-impact, came from LICIACube, ASI’s cubesat. DART released LICIACube 15 days before it aimed itself at Dimprphos.4

Autonomous Optical Navigation and Acronyms

NASA/Johns Hopkins APL/Ed Whitman's photo. The DART spacecraft's DRACO instrument, at Johns Hopkins Applied Physics Laboratory. (last updated August 13, 2021)
The only instrument on DART: the Didymos Reconnaissance and Asteroid Camera for Optical navigation. (2021)

Okay! So: how hard would it be, really, to run DART into Dimorphos by guiding it in from mission control, back on Earth?

Well-nigh impossible, as it turns out.

Again, Dimorphos is less than 170 meters, 560 feet, across. That’s big, at least compared to — say — a vending machine or refrigerator.

But Didymos-Dimorphos was roughly 11,000,000 kilometers, 7,000,000 miles, from Earth when DART reached it.

Mission control knew about where DART and the asteroids were.

But using traditional radiometric (Doppler and range) tracking data, they could narrow the asteroid’s position down to being somewhere in a zone 25 kilometers across.

Even back when they figured that Dimorphos was 180 meters across, that wasn’t nearly accurate enough.

So the folks at a remarkable number of places helped design something that’s smarter than Deep Impact’s “Smart Impactor.”

They came up with DRACO: the Didymos Reconnaissance and Asteroid Camera for OpNav.5

Double Asteroid Redirection Test (DART)
“…It [the DART spacecraft] will carry a single instrument, the Didymos Reconnaissance and Asteroid Camera for OpNav (DRACO), which will provide images for the Small-body Maneuvering Autonomous Real-Time Navigation (SMARTNav) algorithm to be used for guidance, navigation, and control operations in targeting the asteroid, assisted by a star tracker and 5 Sun sensors….”

OPNAVs and a Flow Chart

Shuang Li,
Ruikun Lu, Liu Zhang, Yuming Peng's Figure 1 'a sketch of the image processing procedure' from 'Image Processing Algorithms For Deep-Space Autonomous Optical Navigation' - The Journal of Navigation, published online by Cambridge University Press (22 April, 2013)All OPNAVs are not created equal.

OPNAV, for example, is the Office of the Chief of Naval Operations.

OPNav is Proprietary Software Platform Powered by Skai/Orca Pacific.

And OpNav is Joseph E. Riedel et al’s optical navigation system for the Altair lunar lander.

DART’s DRACO OpNav is an optical navigation system, too. Or maybe software. I didn’t find a solid definition for that particular acronym. Frustrating.

Finally, OpNAV is the Orion Optical Navigation Image Processing Software.

That last, OpNAV, Data and Image Processing Reference Number MSC-26456-1, runs on a Linux operating system and is for “U.S. Release Only.”

From which I gather that optical navigation systems, some of them at least, are sensitive technology. Which is, I suppose, understandable. Although I’d have preferred finding more detail on just how the DART OpNav works.

On the ‘up’ side, I found an image processing flow chart for Deep-Space Autonomous Optical Navigation in a paper by Shuang Li, Ruikun Lu, Liu Zhang and Yuming Peng.6

My number-one daughter pointed out that, although the text refers to “four steps” in the diagram, there are five or six. Seven or eight, counting “Grey image” and “Nav measurement (Line-of-sight).”

She’s got a point. But I suspect that we’re supposed to see “Pseud0-edges removal,” “Least squares based fitting” and Levenberg-Marquardt based ellipse fitting” as one step.

Still, I think this is another example of why “technical writer” and “science writer” are occupational titles. Or should be. And that’s yet another topic.

Reminiscing, B Movies — and a Really Big Deal

NASA/Johns Hopkins APL/Ed Whitman's photo. The DART spacecraft, at Johns Hopkins Applied Physics Laboratory. (last updated August 13, 2021)
Preparing the DART spacecraft for launch at Johns Hopkins Applied Physics Laboratory. (2021)

Doug Ellison and NASA's composite image of asteroid Dimorphos, taken by NASA's DART spacecraft shortly before impact. (September 26, 2022)I have a very active imagination.

And during my teens, I watched a whole mess of movies with titles like “Warning from Space” and “The Green Slime.”

I had the house to myself, apart from two cats, weekdays from about 3:30 p.m. to 4:45 or so, a local television station had (cheap) afternoon movies; and I’m drifting off-topic.

The point is that my visual memory is fairly well-stocked with video clips of spaceships approaching Asteroid Flora, planets with improbable names, and space aliens who were less than ecstatic about encountering humans.

Echoes from B Movie Science Fiction and an Asteroid Deflection Method that Works

So, as I watched Didymos slide past the lower left corner of DART’s video feed, and Dimorphos grew from a dot to a blob, and then an oval gravel pile — my imagination dropped a wildly-unlikely scenario on my mind’s front desk. Metaphorically speaking.

What if — as DART got close enough to see features on Dimorphos — we noticed, lying on the asteroid’s rocky surface, a few clusters of spheres and cylinders. One of which started moving away from the projected impact point.

Italian Space Agency's image of Dimorphos, taken by their LICIACube spacecraft a few minutes after the DART impact. (September 26, 2022)By that time, no message from mission control would stop a probe from Earth from crashing into someone’s — research station? Communications relay? Equivalent of Mars 2020’s sample caches?7

That didn’t happen, of course.

I’m not convinced that DART hitting an installation set up by folks from another world is impossible. But I think the odds of such an incident happening are — well, are slim to virtually none.

Echoes from my adolescent imagination aside, we’ve shown that we can change an asteroid’s orbit. Granted, Dimorphos is a small asteroid. But the DART mission has shown that the kinetic impact asteroid deflection method works.

This is, by any reasonable standard, a big deal.

I’ve talked about that, and almost-related topics, before:

1 Mass, motion and a mission:

2 Astronautical trick shots:

3 Measurements and comparisons:

4 More measurements and picking a target:

5 DART details:

6 Acronyms and more:

7 Old movies, new missions:

About Brian H. Gill

I was born in 1951. I'm a husband, father and grandfather. One of the kids graduated from college in December, 2008, and is helping her husband run businesses and raise my granddaughter; another is a cartoonist and artist; #3 daughter is a writer; my son is developing a digital game with #3 and #1 daughters. I'm also a writer and artist.
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