Launched 15 October 1997, the Cassini Mission is in its 20th year. It reached Saturn and entered orbit on 1 July 2004. Details of the mission can be read at this Wikipedia summary. This post is mostly about the maneuvers by Cassini to change its orbit and make 22 close encounters with Saturn in what is called the Grand Finale. End of mission is scheduled for 15 Sep 2017 when the spacecraft plunges into the atmosphere of Saturn ending a long and brilliant exploration of the famous ringed planet, its rings, and 62 moons.
The Orbits of Cassini
The image below shows all of the orbits of Cassini since it reached Saturn in 2004. Saturn is too small to see in the center. The orbits are color coded for different aspects of the mission over the years. The spacecraft did not have a large amount of fuel nor a powerful engine to enable it to change orbits radically. Instead, it closely encountered the moon Titan 126 times and used the strong pull of gravity by Titan to alter the speed and direction of the orbit of Cassini. Such gravity assists are a commonly used tool in precision orbital mechanics.
Buried deep within the image below is the orbit of Titan colored in red. It lies between the two largest orbits colored white. The white orbits are those of some of the other larger moons of Saturn including Iapetus, Rhea, Dione, Tethys, Mimas, and Enceladus.
The Grand Finale Orbits
The image below shows the recent ring-grazing orbits of Cassini flown from 30 Nov 2016 to 22 Apr 2017. Note how they skimmed the outermost edge of the rings. Also shown is the orbit of Titan. On 22 April 2017, Cassini and Titan were quite close together in their respective orbits at the right side of this image. The pull of gravity by Titan altered Cassini’s speed and direction shifting it from the grey colored ring-grazing to the blue Grand Finale orbits. This maneuver put Cassini on a path taking it between the rings and the cloud tops of the planet Saturn.
The orbits in blue are the final 22 passes Cassini is scheduled to make near the planet. Use this link to follow updates on the progress of Cassini as it completes each of the scheduled orbits. On the final orbit 15 Sep 2017 it will enter the dense atmosphere and clouds where it will heat intensely and vaporize.
Scientists do not want to risk any chance that Cassini could contaminate the moons of Saturn, especially Enceladus. This moon has a system of hydrothermal vents and geysers. The conditions under the ice of Enceladus are such that they could support life. This moon will likely be visited on a return mission. Details of that story from 13 April 2017 are here.
Each time Cassini passes close to Saturn and through the ring plane there is the possibility for a collision with dust or rocks or ice. It would not take a very large object to destroy Cassini due to the very high speed (77,000 mph) of the spacecraft. The first time Cassini made that pass was on 27 April 2017.
The spacecraft craft was oriented with the large radio antenna in front to act as a shield as depicted in this video. As this 360 view video plays you can drag the window of the video around or use the small controller in the upper left to see other parts of the view. Give it a try. If Cassini survives each orbit, the antenna might not be the leading part on future orbits. This will enable views and measurements in different directions for data gathering. The successive 21 orbits over the next few months should produce some dramatic results.
First Images Returned
Mission scientists were elated to acquire the Cassini signal 27 April 2017 after its first dive close to Saturn. Much more is to come as long as Cassini survives each close encounter with Saturn. Click for larger views.
The Huygens Probe
The mission originally included the Huygens Probe of the European Space Agency ESA. Huygens is the large gold disc on the side of Cassini in the photo at the top of this post. Huygens released from Cassini 25 Dec 2004 and descended to the surface of the largest Saturnian moon Titan. The video below uses imagery during the descent and landing. Cassini flew by Titan 126 times during its mission and gathered evidence of lakes, rivers, clouds, and precipitation of methane.
End of Mission
The Grand Finale marks the official end of the operation of the Cassini spacecraft. If all goes well for the spacecraft, it will survive all the orbits and reach the 15 September date. Data from the past years and these final months will provide scientists with decades of work. Our knowledge of the planets and moons will give us insights into the history of our small region of space. The accomplishments of the scientists and engineers are amazing and exciting. I can’t wait to see what comes next.