Hello!

One of the things I enjoy most about the hobby is learning new things. If that comes at the expense of correcting something I said or did incorrectly, that's fine.

I recently posted on Twitter about how fortunate I had been to be in the right place at the right time to record an entire 2-hour fly-by of the ISRO Chandrayaan-2 spacecraft which is working towards a moon landing in a few weeks.

https://twitter.com/scott23192/status/1155528793831944193

... in that post, I referred to the situation as an Earth "gravity-assist". That was not the correct term for the flight of Chandrayaan-2 and if you'd like to hear what the difference is, please read on.

Edgar, DF2MZ, was kind enough to fill me in on the distinction. To accomplish a gravity-assist, an object (Chandrayaan-2 in this example) needs to fly by another object (Earth in this case) in a path ideally described as "hyperbolic". Put simply, it's a one-time fly-by as opposed to a recurring orbit. You can read about the theory of this type of alignment here:

https://en.wikipedia.org/wiki/Gravity_assist

This is not the case with Chandrayaan-2, which is why the ISRO is using a schedule of rocket firings to boost its orbit farther and farther from Earth. When an object like Chandrayaan-2 is in a recurring orbit (often described as "elliptical"), it's more-or-less going to stay there unless outside forces change that. I guess the most common forces would be gravity or rocket firings.

For an amazing, mathematically correct animation of Chandrayaan-2's journey (complete w/ on-schedule rocket firings), check out:

http://sankara.net/chandrayaan2.html

(the last "big loop" that eventually makes the lunar capture possible will blow your mind)

So that's the difference between the two circumstances and I wanted to set the record straight. I greatly enjoyed being able to record the spacecraft's S-Band downlink throughout the entire pass but didn't want to leave an incorrect description of the pass hanging out there. In the end it was a nice bonus to learn something new about the nature of orbits since they play such a major role in our hobby.

Thanks,

-Scott, K4KDR