Why Must Cassini Crash Into Saturn?

Why does Cassini have to crash into Saturn?
In this screenshot from the short animated film Cassini's Grand Finale, the spacecraft is shown breaking apart after entering Saturn's atmosphere. The planned end of Cassini will occur on Sept. 15, 2017. Image credit: NASA/JPL-Caltech

In this screenshot from the short animated film Cassini's Grand Finale, the spacecraft is shown breaking apart after entering Saturn's atmosphere. The planned end of Cassini will occur on Sept. 15, 2017. Image credit: NASA/JPL-Caltech

Originally posted on Forbes!

Cassini has served us well over the past twenty years, giving us incredible images of Saturn since 2004; thirteen years of imagery from its vantage in the outer solar system. With any lengthy mission, it is sad to see a spacecraft reach its end, and Cassini has been exceptional. Cassini has truly changed the way we see Saturn and its moons. With such an illustrious explorer, it’s natural to wonder if we really had to say goodbye.

We must - there’s no escaping the finite nature of all of our robotic explorers of the solar system. 

Cassini, like all orbiting spacecraft far from Earth, has limited fuel reserves, on both of its two fuel sources. The first is the power generator, which replaces the usefulness of solar panels in the outer solar system, where sunlight is too weak. Onboard Cassini, this is a piece of plutonium, which generates heat, and is converted into electrical power. This is the source of the power to operate Cassini’s instruments, and has gradually decreased in power over time, but not enough to incapacitate Cassini. But then there’s the fuel for the thrusters. The thrusters are responsible for changes in Cassini’s orbit, both to direct the craft towards a moon for a gravitational assist, or to correct its path afterwards, so that the craft continues onwards in exactly the right position. These thrusters have been 90 percent empty since 2009, and Cassini has been extremely economical with the remaining propellant for its thrusters for a long time.  Unlike the International Space Station, which can be refueled by the shuttling crafts between the Earth and low orbit, we can’t refuel Cassini.

This graphic shows Cassini's final plunge toward Saturn, with tick marks representing time intervals of 2 minutes, leading to the spacecraft's entry into the atmosphere.  Image credit:  NASA/JPL-Caltech

This graphic shows Cassini's final plunge toward Saturn, with tick marks representing time intervals of 2 minutes, leading to the spacecraft's entry into the atmosphere. Image credit: NASA/JPL-Caltech

We can’t afford to run Cassini dry, either. It’s true that right now the spacecraft still has fuel, and we could have opted to let Cassini continue to orbit, which might have let us squeeze a few more images out of the spacecraft, but by doing that, we consign Cassini to an end-of-mission which leaves it wandering derelict around Saturn, completely uncontrolled, once its fuel reserves are exhausted.

An uncontrolled Cassini is an unacceptable end. It’s unacceptable because we cannot control at what point it might crash into one of Saturn’s many moons, and Saturn has some extremely special places that we cannot risk even the slightest chance of contaminating.  Enceladus, a moon of Saturn, was discovered (by Cassini) to have a global ocean of warm, salty water underneath its icy crust.

Places in our solar system with liquid water fall under the strongest planetary protection rules.  These are rare and delicate places within our solar system, and we have agreed as an international community that unless our robotic explorers have met the absolute highest standards for cleanliness, sterilization, and testing of that sterilization, we should stay away from the watery places. The goal is simply this: don’t contaminate worlds that might have their own unique chemistry (and, perhaps, simple life) with the ultimate invasive species from Earth.

Saturn's icy moon Enceladus hovers above Saturn's exquisite rings in this color view from Cassini. The rings, made of nearly pure water ice, have also become somewhat contaminated by meteoritic dust during their history, which may span several hundred million years. Enceladus shares the rings' nearly pure water ice composition.  Image credit :  NASA/JPL

Saturn's icy moon Enceladus hovers above Saturn's exquisite rings in this color view from Cassini. The rings, made of nearly pure water ice, have also become somewhat contaminated by meteoritic dust during their history, which may span several hundred million years. Enceladus shares the rings' nearly pure water ice composition. Image creditNASA/JPL

Cassini was sterilized before launch, but not to the specifications required for watery worlds. It is an orbiter, and it’s not designed for interactions with the atmospheres or surfaces of planets or their moons. Because landing on a watery moon wasn’t part of its mission, Cassini never needed to go through the gauntlet of extra sterilization. And while Cassini has been in the vacuum of space for 20 years, bacteria from our home planet have been shown to survive for years in outer space.

As we can’t say with confidence that Cassini has no microbial hitchhikers, we need to place Cassini carefully, while we still have enough propellant to direct it accurately. For Saturn's moons, the safest place for Cassini is Saturn itself. Saturn is not hospitable to life, and its atmosphere will destroy Cassini the way that Earth’s atmosphere destroys a meteorite. Much of Cassini will vaporize, becoming one with the clouds of Saturn, and anything that survives will melt in the heat of the deeper planetary layers, sinking gradually towards the center of Saturn.

It is through the skill and experience of Cassini’s flight crew and science team that we have been able to undertake the Grand Finale set of orbits before the spacecraft has its final encounter with the atmosphere of Saturn. It is a gift to the scientists who have studied and will study Saturn in past and future years - a wealth of new data has flooded through Cassini’s transmissions home. And for those of us who do not research Saturn, it has been a gift of tremendous imagery of the sixth planet from the Sun.

NASA's Cassini spacecraft gazed toward the northern hemisphere of Saturn to spy subtle, multi-hued bands in the clouds there.  Image credit : NASA/JPL

NASA's Cassini spacecraft gazed toward the northern hemisphere of Saturn to spy subtle, multi-hued bands in the clouds there. Image credit: NASA/JPL

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Are There Rules For How To Avoid Contaminating Other Planets? (Or Our Own?)

Is there an internationally-agreed protocol to avoid compromising the detection of life beyond Earth with living matter of earthly origin, and for quarantining any organisms crossing that boundary either way? If there isn’t such a protocol, should there be one?
The fascinating surface of Jupiter’s icy moon Europa looms large in this newly-reprocessed color view, made from images taken by NASA’s Galileo spacecraft in the late 1990s. This is the color view of Europa from Galileo that shows the largest portion of the moon’s surface at the highest resolution. Credits: NASA/JPL-Caltech/SETI Institute

The fascinating surface of Jupiter’s icy moon Europa looms large in this newly-reprocessed color view, made from images taken by NASA’s Galileo spacecraft in the late 1990s. This is the color view of Europa from Galileo that shows the largest portion of the moon’s surface at the highest resolution. Credits: NASA/JPL-Caltech/SETI Institute

Originally posted at Forbes!

There certainly is a protocol to avoid contaminating other worlds which might host life, and it is broadly described by the phrase “planetary protection.” Planetary protection boils down to not sending any spacecraft to places which might host life without pushing the spacecraft through our absolute best, most ridiculous sterilization process, to avoid even potentially contaminating that world with Earth microbes. It also means protecting our own planet from any microbes which might live over there, and carelessly letting them roam free on our home.

Any country which has signed on to the Outer Space Treaty, overseen by the United Nations (which is most of them at this point) is legally bound to try their best to avoid contaminating any world. Generally, we want to avoid bringing too many Earth contaminants along in any case, because it will muddle our measurements of the other world. The last thing you want to do when taking a very precise measurement of a moon of Jupiter is to accidentally also measure Leftover Earth Bits.

The current guardian of the planetary protection guidelines is a committee named COSPAR (Committee on Space Research), and they’ve got five tiers of “how much does your spacecraft need to be super sterile.” The first is for objects like the Sun or Mercury, where there’s no hope of life on that world. The second is for objects like the Moon or Venus, where there’s interesting chemistry but it’s pretty unlikely that you’re going to contaminate any life there. The third is for flyby missions, where you’re going past a world which could have life, like Europa or Mars.

One of the Viking landers being prepared for dry heat sterilization. Image credit: NASA

One of the Viking landers being prepared for dry heat sterilization. Image credit: NASA

The fourth category is also for worlds which might have life (in our solar system, this usually includes all worlds with water by default) but is for landers and rovers, where you’re getting much closer to the place that life might be. A flyby mission, in principle, should not get that close to the planet -- but a rover is going to be right there in the dust. All of our Mars rovers are in this category, and have to undergo really strenuous sterilization. If your rover is not searching for life, and is not in an area that might have any (a really dry part of Mars, for instance), you can get away with only having 300 bacteria per square meter of your spacecraft’s surface. (This is still very, very clean.)

If, on the other hand, you’d like to look for life, or would like to go near where there appears to be liquid, your spacecraft has to be 10,000 times cleaner. The only way to do this is to bake your spacecraft in a dry heat oven, which was done for the Viking landers, whose purpose was to search for life on Mars.

For stuff coming back from other worlds, there’s a similar “is there life?” divide for what needs to be done with the material. For something coming back from an asteroid (like the Osiris Rex mission, and Hayabusa 2) where we really don’t expect there to be any life, you just want to not contaminate your precious sample -- it’s a standard level of scientific caution, like with not wanting to measure the Earth when you’re intending to measure Io.

Recent Cassini images of Saturn's moon Enceladus backlit by the sun show the fountain-like sources of the fine spray of material that towers over the south polar region. The image was taken looking more or less broadside at the "tiger stripe" fractures observed in earlier Enceladus images. It shows discrete plumes of a variety of apparent sizes above the limb of the moon. The greatly enhanced and colorized image shows the enormous extent of the fainter, larger-scale component of the plume. Imaging scientists, as reported in the journal Science on March 10, 2006, believe that the jets are geysers erupting from pressurized subsurface reservoirs of liquid water above 273 degrees Kelvin (0 degrees Celsius). Credit: NASA/JPL/Space Science Institute

Recent Cassini images of Saturn's moon Enceladus backlit by the sun show the fountain-like sources of the fine spray of material that towers over the south polar region. The image was taken looking more or less broadside at the "tiger stripe" fractures observed in earlier Enceladus images. It shows discrete plumes of a variety of apparent sizes above the limb of the moon. The greatly enhanced and colorized image shows the enormous extent of the fainter, larger-scale component of the plume. Imaging scientists, as reported in the journal Science on March 10, 2006, believe that the jets are geysers erupting from pressurized subsurface reservoirs of liquid water above 273 degrees Kelvin (0 degrees Celsius). Credit: NASA/JPL/Space Science Institute

But if we’re not sure if there should be life, or we’re suspicious that there might possibly be life in our little sample, and we want to bring it back to Earth, you have a whole lot of quarantining to do before you even get back. First you have to make sure that your spacecraft is clean enough to safely go there and not contaminate, say, Enceladus. Then we have to make sure that Enceladus doesn’t contaminate us, and the suggested path there is to take your entire spacecraft, and put it inside a box, and then send that box back to Earth. Since the box didn't go to Enceladus, it should be safe to land the box on Earth, and then you can take the box to the cleanest clean room of all time to crack it open and retrieve your sample. Hopefully, with those precautions in place, both the Earth and Enceladus are uncontaminated, and scientists will be very happy.

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