Insider’s Cassini: Radio Science at Solar Conjunction

Todd J. Barber, Cassini lead propulsion engineer



During this more relaxed annual hiatus that is solar conjunction, I thought it would be a good time to interview Dr. John Armstrong, a Cassini team member specializing in radio science experimentation. Many members of the Cassini flight team are enjoying a well-earned break, but this is the busy season for John and his colleagues. During conjunction periods, the radio signal to and from Cassini passes very near the sun on the way from and to Earth, and this offers a chance to do some unique science. Two of the most notable experiments include an exquisitely accurate test of Einstein’s theory of relativity during the 2002 conjunction as well as probing of interplanetary plasma during these nearly annual conjunction events, this year included.

The groundbreaking paper from the 2002 relativity experiment, published in 2003 by Drs. Bruno Bertotti, Luciano Iess, and Paolo Tortora in the prestigious journal “Nature,” is one of the most heavily cited Cassini publications, an instant classic. Simply put (thanks to John, for otherwise I would have been lost!), this test of Cassini’s X-band radio uplink and Ka-band radio downlink basically confirmed Einstein’s prediction for space-time curvature (due to the sun’s gravity) with a 50-fold increase in experimental accuracy! The most accurate Doppler data ever recorded was realized during this test, to a level of accuracy of one micron per second, or about 3.4 inches per DAY. This speed makes a snail look like an Olympic sprinter in comparison! Most folks think Einstein is probably correct, but experiments like these confirm his theories (remember, at one time, most people thought Isaac Newton was 100% correct, too, erroneously).

In an example of the reuse capability of these data, the multi-link (X-band and Ka-band) feature on Cassini allowed John and his colleagues to solve exactly for the 3D position and extent of interplanetary plasma in our inner solar system. As John told me, “I love what other people think is noise” but oh what wonderful “noise” this is. John proceeded to show me graphs and even movies mapping the interplanetary plasma contours in a process called tomography, likely familiar to medical buffs or “CSI” fans. A very surprising recent result is that the plasmas are apparently much thinner closer to the sun, and more variable than expected. John’s exact words were “these data blow my socks off” and by the end of our time together, I found his enthusiasm and groundbreaking science results more than a bit infectious. I think it’s time for me to read some papers and learn more!

The one theme that kept recurring during our talk, other than John’s giddy excitement, was how marvelous the Cassini ground system, the Deep Space Network, and the fantastic radio science system on board Cassini work together to bring these unique science data to Earth. John also wanted to make sure to spread credit widely for these results, from the Italian professors who wrote the “Nature” article, the Radio Science Systems group at JPL headed up Sami Asmar, and a summer student that worked with John named Adam Richie-Halford.

As I turned to leave, I noticed an Albert Einstein bauble-head on top of John’s filing cabinet, so I asked about it. Some of John’s family friends received it in a “Happy Meal” from Mickey-D’s, but they thought John “needed it more than the kids.” If you’ll pardon the pun, Bauble-Head Al seemed to nod his head in approval at the mind-blowing science results from Cassini, so many millions of miles away. It was wonderful meeting you, John, and I wish you all the best during every Cassini conjunction and opposition. Thanks for opening my mind to the wonders of Cassini radio science! I now know conjunction isn’t just for taking vacations and catching up on sleep!