# What can be learned from NASA's choice of Didymos B for the Double Asteroid Redirection Test

The NASA News item NASA’S First Asteroid Deflection Mission Enters Next Design Phase describes a test the kinetic impactor technique that could someday be used to adjust the orbit of an asteroid in order to avoid a collision with Earth in the future by making a small change at a time much earlier than the predicted event.

The article says:

The target for DART is an asteroid that will have a distant approach to Earth in October 2022, and then again in 2024. The asteroid is called Didymos -- Greek for “twin” -- because it’s an asteroid binary system that consists of two bodies: Didymos A, about one-half mile (780 meters) in size, and a smaller asteroid orbiting it called Didymos B, about 530 feet (160 meters) in size. DART would impact only the smaller of the two bodies, Didymos B.

The Didymos system has been closely studied since 2003. The primary body is a rocky S-type object, with composition similar to that of many asteroids. The composition of its small companion, Didymos B, is unknown, but the size is typical of asteroids that could potentially create regional effects should they impact Earth.

[...]“A binary asteroid is the perfect natural laboratory for this test,” said Tom Statler, program scientist for DART at NASA Headquarters. “The fact that Didymos B is in orbit around Didymos A makes it easier to see the results of the impact, and ensures that the experiment doesn’t change the orbit of the pair around the sun.”

Can someone explain the physics of this explanation to me more completely? Is it a small change in the orbital period of the pair that will be detected? Wouldn't this be predictable from just conservation of linear and angular momentum and Energy? Radar would only work during close Earth approach, so can the rotation be phased by optical means? What if the impact changes the rotation rate of the primary about its own axis and brings it out of tidal lock. Wouldn't this distort the brightness curve unpredictably?

Or is is the details of how the impact process itself proceeds, how efficiently momentum is transferred. In this case wouldn't the unknown composition of the B component make this more difficult to interpret? Will the spacecraft release an "observer" spacecraft to record the collision from a safe distance, say a few hours behind the impactor?

I don't understand what the observables are here, and what can be learned from them.

above: Delay-doppler data from Arecebo in 2003, found here. The small line above the primary is caused by the relative orbital motion of the secondary.

• We don't have a generic tag yet for "doing stuff to asteroids to see what happens in hopes of learning how to possibly saving the Earth some day". orbital-modification? astro-civil-engineering? saving-the-world? bruce-willising or ben-afflecking? Advice welcomed. – uhoh Jul 2 '17 at 10:32
• Did you read en.wikipedia.org/wiki/AIDA_(mission) and the links to the references? Maybe there is more information about the observables. But the impact should be done during a close aproach to the earth in october 2022. In my opinion the orbit of Didymos B is the observable. But the calculated delta V is only 0.4 mm/s. – Uwe Jul 2 '17 at 16:44
• I made some calculations using the third law of Kepler. The delta v is 0.4 mm/s, the period is 0.4958 days and the orbit axis is 1.18 km. The result is a change of the period by 244 seconds. According to this page johnstonsarchive.net/astro/astmoons/am-65803.html the uncertainity for the period is 0.0002 days or 17 seconds. A change of the period by 244 seconds should be observable. – Uwe Jul 2 '17 at 21:34
• From meetingorganizer.copernicus.org/EPSC2012/EPSC2012-935-1.pdf : " The DART impact will change the period by 0.5% – 1%, and this change can be determined to 10% accuracy within months of observations." – Uwe Jul 3 '17 at 8:55
• I will write an answer with the information found, but this will take some time. – Uwe Jul 3 '17 at 10:28