Help Shape the Future of Space Exploration

Join The Planetary Society Now  arrow.png

Join our eNewsletter for updates & action alerts

    Please leave this field empty

Headshot of Emily Lakdawalla

Hey, Moon! Get out of the way of Cassini!

Posted by Emily Lakdawalla

16-07-2007 19:50 CDT


Here's a funny Cassini item that Dave Seal just pointed out to me. Go to and ask to see Earth as seen from Cassini, and here's what the view looked like just a couple of hours ago:

Earth as seen from Cassini, July 16, 2007

NASA / JPL / Solar System Simulator (Dave Seal)

Earth as seen from Cassini, July 16, 2007
That is to say, Cassini saw the Moon transiting Earth. Which, on its own, is neat. However, it could have been a big problem. Look at where the Moon is -- and consider the reason why Cassini turns to Earth. Dave tells me that as of the moment simulated in this image, Cassini was, in fact, in radio communication with one of the giant antennas of the Deep Space Network situated at Goldstone, right here in southern California. Just a tiny tweak to the geometry of this lunar transit would have resulted in the Moon being bang in the way of Cassini's attempt to radio valuable science data to Earth.

Dave also said that if the Moon is sufficiently close to Cassini's radio beam, it's possible they'll see higher "system noise temperature from the radio reflections from the Moon being near the primary beam / in the sidelobes." Sadly, I can't say I know enough about radio transmissions to quite understand what Dave is talking about here, but I reproduce the information for those of you who do. Dave also said that the navigation team -- which uses Cassini's radio signals to track the speed of the spacecraft -- might see a signal from the occultation in their Doppler data, one when the downlink signal gets "lensed" by its passage near the Moon, and another one that'll happen one round-trip light time period later (which is I think about 170 minutes right now), after the uplinked commands are radioed back to Earth (something that Cassini does as a check to make sure it received the uplink correctly).

I wonder if the Goldstone operators looked out their windows and said, "hey, it looks like the Moon's right where we're pointing our antennas..." It's probably the closest lunar occultation of a Deep Space Network station that Cassini has had. Do the Goldstone operators have windows?

See other posts from July 2007


Or read more blog entries about:


Leave a Comment:

You must be logged in to submit a comment. Log in now.
Facebook Twitter Email RSS AddThis

Blog Search

Essential Advocacy

Our Advocacy Program 
provides each Society member 
a voice in the process.

Funding is critical. The more 
we have, the more effective 
we can be, translating into more 
missions, more science, 
and more exploration.


Featured Images

BEAM with expansion grid overlay
Huge color Voyager 1 Ganymede mosaic
Saturn and its moons from Voyager 2
Voyager 2 Ganymede mosaic: Osiris crater
More Images

Featured Video

The Planetary Post - A Visit To JPL

Watch Now

Space in Images

Pretty pictures and
awe-inspiring science.

See More

Join the New Millennium Committee

Let’s invent the future together!

Become a Member

Connect With Us

Facebook, Twitter, YouTube and more…
Continue the conversation with our online community!