One Gravity-Assist to go, Please!

On April 26, 1998 at 1:44:41 PM Coordinated Universal Time (UTC), the Cassini spacecraft made its closest approach to Venus. This was the first swingby of four swingbys required to reach Saturn. For those interested, I've tried to put together some Navigation-related information about the swingby. (Please note that I do not officially represent the Cassini project, JPL, or NASA. Any opinion expressed herein is mine)

Feel free to email me or sign my guestbook to give feedback on this page.

Background

I'm assuming you already know all about Cassini and have read the V-1 Significant Events Report, and the April 26th and April 29th press releases.

Graphics

JPL always seems to produce some great graphics in support of its missions. Cassini is no exception. The DView movie gallery has five(!) nice Quicktime animations (15.0 MB) (4.2 MB) (2.2 MB) (4.5 MB) (27.0 MB). I've also made an animation that shows how Cassini's osculating orbit changes during the flyby.

The Cassini Navigation team put on a Friends & Family Event during the swingby. I was the host of the morning, in a round-a-bout way. Our center-piece was a real-time data stream of Doppler-effect measurements of the spacecraft velocity. To try and help people follow the plot of that data and correlate that with mission events, I put together these handouts:

The Earth-Venus-Sun Triangle as it appears during the flyby (30KB)

"Doppler Plot" prediction & mission events (55KB)

A plot of the trajectory during the swingby (9KB), the 7 minutes and 33 seconds delay time mentioned in this handout is the time required for a radio signal to travel from the spacecraft to our antennas on Earth.

And here's some interesting parameters for the Venus-centered, hyperbolic orbit (for those of you familiar with orbital mechanics):

For the Venus-1 Minus 31 Day solution (V1M31D)

V_infinity =            6.0298094 km/s
eccentricity =          1.7091199
bending angle (delta) = 71.619609 deg [= 2*asin(1/ecc) ]

magnitude of the velocity vector "added" by Venus-1 swingby, 7.0560402 km/s  
[= V_infinity*sqrt( 2*(1 - cos(delta)))] 

For the reference trajectory

V_infinity =            6.0299770 km/s
eccentricity =          1.7151190
bending angle (delta) = 71.330688 deg
velocity "added"      = 7.0315551 km/s

The Venus-1 Reconstruction

[Coming Soon!]

The following is a message from the solar system dynamics group at JPL giving what is effectively information on planetary masses. I just thought this might be interesting for you...

Subject: GM of Venus
Date:    4/23/98  11:53 AM

April 23, 1998

Here are the mass uncertainties for the planets.

                   GMsun/GMplanet     sigma

[1] Mercury         6023600.           250.
[2] Venus            408523.71           0.06
[3] E+M              328900.56           0.02
[4] Mars            3098708.             9.
[5] Jupiter            1047.3486         0.0008
[6] Saturn             3497.898          0.018
[7] Uranus            22902.98           0.03
[8] Neptune           19412.24           0.04
[9] Pluto         135000000.       7000000.


For Venus and DE405, this implies GM2=324858.599 km**3/sec**2,
with an uncertainty of 0.05 km**3/sec**2.

The reference for this is

Sjogren,W.L., Trager,G.B. and Roldan,G.R.: 1990, "Venus: A Total Mass Estimate", Geophys. Res. Let., (10), 1485-1488.