Alternatives and Problems: Defying the Great Space Goblins

"There are no strings on me."

Pinocchio

In a mission as complex as Cassini, there are many efforts conducted here on Earth that are essential to the success of the mission. People create, design, and build all of this technology, and it is people who plan, execute and monitor the mission. Also, long journeys into deep space to distant planetary environments we only partially understand (or else, why would we be going?) may harbor surprises we had not planned for. Perhaps these are some reasons why things don't always proceed as expected. There are goblins lurking everywhere - on Earth, on the spacecraft, in deep space, even at Saturn - and they are constantly scheming to trip us up, to interrupt the wealth of data streaming back from our ambassador to outer space.

An ounce of prevention...

In this section we talk about what is called contingency planning. A contingency is an unexpected event which can cause a reduction in the quantity and/or quality of returned data. The purpose of contingency planning is to preclude goblins in the first place, and to preserve the science data return when some goblins do sneak through our defenses. Let's see... you could say it's a lot like fire prevention. Think about it: the chance of a fire happening where you live is very small on a day-to-day basis, but smoke alarms and a few fire extinguishers lying around can easily make the difference between a burnt pot roast and an expensive neighborhood disaster. Just imagine how different things would be to homeowners of the Oakland hills if someone happened to be standing right where the fires of 1992 started with a bucket of water. (You can begin to see how difficult it is to predict where the goblins are lurking, too. We can't have people just standing around everywhere all the time with buckets of water!)

Nearly all of the contingencies we plan for have very low chances of happening; most are estimated to be less than a few percent, while some are as low as one in a million! Nevertheless, it is prudent to take some actions to plan against problems, and we are not so naive as to think that everything will be perfect all the time. During JPL's Voyager missions to the outer planets there have been all sorts of aches and pains, though none so serious as to cause a significant loss of science. The Voyager team's great success record is due primarily to the great planning and design that went into the mission, and (perhaps as importantly) the creative thinking that kept things running smoothly when a problem did occur.

Goblins generally come in three varieties. There are those in the "human element" category, which can all be traced to the same source: we humans simply make mistakes every once in a while. The more common "glitches," where seemingly perfect programs or hardware hiccups, can usually be solved by a detailed examination of what is happening on the spacecraft during a problem sequence. A third species of goblin include the "acts of nature" -- natural environmental effects of travelling through space: radiation, temperature extremes, dust and debris, uncertainty of location, and the like. Natural factors on Earth such as stormy weather (which can interfere with communication) and earthquakes influence the contingency plans as well.

Alternate mission trajectories

One area we need to do contingency planning involves problems as we near the opening of the launch period. To enable recovery from launch delays, the Cassini project has more than one trajectory to Saturn, each with a different launch date. Multiple missions typically protect us against launch slips that occur after the spacecraft assembly is complete, but can't be diagnosed and fixed quickly enough to meet the primary launch schedule. Typical problems are technical difficulties with the spacecraft or launch vehicle, recent launch vehicle failures, and inclement weather.

The primary mission, which has the best trajectory, launches in October of 1997 and has a compatible launch period duration of more than a month. The alternate trajectory set contains two families of opportunities, secondary and backup trajectories. Cassini's secondary opportunity is a VEEGA trajectory which has one Venus and two Earth flybys (thus the term VEEGA, or Venus-Earth-Earth Gravity Assist) as opposed to the primary trajectory which has two Venus flybys, an Earth and a Jupiter flyby (a VVEJGA). The secondary mission launches in December 1997, just two months after the primary launch period. A deep space maneuver is executed after the first Earth flyby to properly phase the spacecraft for the second Earth flyby, finally arriving at Saturn on October 13, 2006.

  Primary, Secondary, and Backup Trajectories
Classification        Primary       Secondary      Backup   
Trajectory Type       VVEJGA         VEEGA          VEEGA   
Launch Period      10/97 - 11/97  12/97 - 1/98   3/99 - 4/99
Arrival Date         7/1/2004      10/13/2006     12/22/2008
Cruise (yr)            6.7            8.8            9.8    
Saturn Tour (yr)        4              4              4

The backup mission opportunity also launches on a VEEGA trajectory in March 1999. Both the secondary and backup missions suffer from missing the rapid Venus-Earth transition, followed by the Jupiter gravity assist which the primary mission uses to get to Saturn earlier. This is primarily because Venus, Earth and Jupiter have moved and the "phasing" isn't right for these missions to get us to Saturn. Unlike the secondary mission, the backup is designed to accomodate longer launch delays - usually from programmatic or serious technical issues - which is why it's about 1.5 years after the primary mission. A deep space maneuver is executed also after the first Earth flyby to properly phase the spacecraft for the second Earth flyby, arriving at Saturn on December 22, 2008.

The secondary and the backup trajectories have enough performance to carry out the mission with some but not serious degradation to the scientific return. The significant difference between these missions and the primary is the longer interplanetary cruise time. These longer cruise times cause a decrease in available power since our power source weakens slowly with time. This results in fewer instruments being allowed to operate at a given time, or less engineering support to suit all the instruments. Also, the tilt of the rings degrades slightly over time, decreasing the amount of illumination that falls on the ring material. These and other reasons make the primary trajectory superior in terms of mission return, but not nearly so much so that it's not worth going at all if we should slip past October 1997. Getting to Saturn at all is far superior to just giving up and going home when the going gets tough. And even a secondary or backup mission is still well worth the investment that the U.S. and Europe have made in Cassini.

How we do contingency planning

With so much operational experience behind us, it is not terribly difficult to anticipate the most plausible anomalies that might occur. The challenge, however, is not merely to catalog a list of all possible problems and responses (which would take forever and, undoubtedly, not cover every possible thing that could go awry), but to develop an effective defense against goblins in general and prepare strategies to respond to any problem when it occurs. This is one of humanities greatest strength: the ability to apply knowledge gathered from a finite set of experiences, and apply it to new situations. In any emergency, clear thinking can easily be as important as knowing the circumstances of the problem.

Keep in mind how difficult contingency planning can be. Our job here at JPL is not so unlike building a car that must be working perfectly after eleven years without an oil change, new tires, or a single trip to the mechanic. And it has to survive extreme heat, extreme cold, fast moving space debris, and deadly radiation. And it has to drive about five billion kilometers (three billion miles) on a single "tank of gas!"

So, keeping in mind the nominal plan of reaching Saturn with everything in good working order, here are some of the more interesting examples of contingency planning that have been pondered to keep the goblins in the closet and under the bed.

Loss of a flight or attitude control computer. The Cassini spacecraft carries a lot of redundant equipment. Redundancy is when you have a backup set of equipment ready to go in case something goes wrong with the primary stuff. In this case, Cassini has two flight computers and two attitude control computers. If one is experiencing problems, we can just switch to the other one and take the time to figure out - carefully - what the problem is.

Spokes in Saturn's rings. (Image P-23925)

Avoiding the environmental hazards. There's a lot of space junk zipping around out there, but fortunately most of it is either very small, or in known locations. Saturn's rings, for example, have specific gaps that we could use to fly through (though nearly all of our time there is spent far away from the hazardous areas of the rings). Most of the sensitive components of the spacecraft are shielded by other equipment or thermal blanketing, which would vaporize small particles of space debris before they have a chance to cause any serious damage.

Missing a Maneuver. It is highly unlikely for a problem on the spacecraft to prevent the execution of a maneuver. Even if one were to occur, all maneuvers can still be performed up to at least a few days later without losing the mission. Those maneuvers just before and after a Titan flyby at Saturn, however, are particularly sensitive to timing. Backup windows have been put in place as emergency opportunities to retry the maneuver. And even if one is missed, it's not as if the mission is over. Cassini would be off its nominal trajectory around Saturn, but we'd still be able to perform good science on a different trajectory.

Saturn Orbit Insertion. There's one maneuver which simply cannot be missed and still give us a mission at Saturn, and that's the Saturn Orbit Insertion (SOI) burn. This maneuver gets us into orbit around Saturn, and without it, we'd just go zipping right by, never to return. A tremendous amount of contingency planning is required for us to be absolutely sure that this maneuver will happen, especially since it must happen without our involvement. That is, by the time we could fix any problems that crop up, it'd be too late. The spacecraft must handle any problems by itself.

Loss of a Digital Recorder. Again, Cassini carries two Solid State Recorders (SSRs) on board, so if one should fail, we can still use the second to return data to the ground, though we clearly wouldn't be able to store as much. But two billion bits of information a day would still delight the anxiously awaiting scientific community on Earth.

A Major Earthquake in California. Yes, we even have plans for this goblin! If a major earthquake were to strike, communication lines between JPL and the remote Deep Space Network, which we use to communicate to the spacecraft, might be broken for hours to possibly days. And with a probability of about 2 percent per year of a "major" earthquake, we'd better be ready for it if it happens. Our strength in this area is Cassini's distributed network, with scientists planning observations all over the country. The spacecraft sequence files, not to mention the science observations, will be stored in more than one location. Of course, after an earthquake we'd have to make sure our operations team can get somewhere secure to continue to plan spacecraft events, but with sequences being 28 days long, it's likely we'll have some time to recover and get a team to develop the next 28-day sequence before the current sequence ends.

Prof. Manuel Grande
Last updated: 1 July 2004