Mohawk Valley Astronomical Society

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Hello Orbilander!

by Perry Pezzolanella

Look! Up in the sky! It’s an orbiter! It’s a lander! It’s Orbilander! That sight could be for real for anyone standing on the surface of Enceladus in the 2050’s. Cassini discovered a huge subsurface ocean on Enceladus that vents water into space. The fissures slashing the south polar region are warm enough to hint that the ocean is being heated by the core. A team of scientists at the John Hopkins University - Applied Physics Laboratory (JHU-APL) are designing a mission that would take a closer look in a way no other spacecraft has ever attempted. Meet Orbilander, appropriately named for its ability to function as both an orbiter and a lander. It will try to answer an important question: “Is there life on Enceladus?”

Enceladus was generally thought to be nothing more than a cratered moon before Voyager 1 flew past it in November 1980. It is nothing more than a star-like point of light shining feebly around magnitude +11.7 even in the best telescopes. In the frozen depths of the Solar System, the assumption was that the moons of the outer planets were too small to retain their heat of formation and would freeze fast with no geological activity. Voyager 1 revealed Enceladus to be a moon that is unusually smooth and bright. Enceladus reflects nearly 90% of the sunlight that falls on it. This is higher than pure snow and meant that Enceladus was covered with fresh crystals from an unknown process. At the time, scientists speculated that Enceladus might have geysers that were active in recent geological history, possibly to this very day. Both Voyagers 1 & 2 had state-of-the-art cameras for that era, but when they flew by, neither detected any plumes or vapors emanating from Enceladus, or anywhere along its orbit. Even though Enceladus is only 311 miles in diameter, it was thought that the strong gravitational tug between Saturn and the other moons, especially Titan, could create tides and internal heating. The discovery of active volcanoes on Io reinforced the theory. Photos of Enceladus revealed evidence of a geological past of soft or slushy ice flowing across the surface, which later solidified. There were very few craters on those ice flows indicating that area of Enceladus was created in recent geological times. Large grooves rake the southern polar region, and many craters are truncated where flowing ice partially obliterated them. A large part of Enceladus, especially the southern hemisphere, is covered with young ice flows and bright, icy deposits. Enceladus, Titan, Saturn, its rings, and the other moons were so fascinating and left so many unanswered questions that an orbiter mission to Saturn was proposed and approved despite its over $1 billion expected cost.

The Cassini orbiter mission began with launch on October 15, 1997, on an adventure of exploration with a final cost of $3.3 billion. It arrived at Saturn on June 30, 2004, and deployed the Huygens probe, which landed on Titan on January 14, 2005, for an historic first. Cassini bristled with the most sophisticated cameras and detectors; there would be no way any geyser would escape detection. Indeed, it was not long after the Titan landing that Cassini flew past Enceladus and looked back to make one of the most important discoveries of its mission: active geysers! These were no ordinary geysers as they were huge and plentiful! Several jets of water ice particles around 100 miles high spewed from the south polar region. The sources of the geysers were along a series of fissures that cut deep into Enceladus known as “Tiger Stripes” for their appearance. Cassini mapped the temperature of the fissures on later flybys and found that they are about -120ºF, much warmer than the overall -330ºF surface; therefore, Enceladus has a warm interior as theorized. Cassini flew as close as 12 miles from the surface in March 2008! It flew through the plumes without harm and took some excellent measurements of the vapor and particles. It turns out that these plumes are pure water ice that is not tainted with ammonia or methane. The water ice contains nitrogen, carbon, and organic compounds, all of which are found in Earth’s water and essential to life. Enceladus may hold a vast reservoir or ocean of liquid water beneath its icy crust.

Enceladus is at the same time an easy and difficult world to explore. The subsurface ocean is easy to sample with available technology due to the plumes. As intense as these plumes may appear, it is no worse than flying through fog so the risk of particles disabling a spacecraft is low. Low-cost missions to Enceladus have been drafted that are flybys while orbiting Saturn. Orbilander is a flagship-class orbiter-lander mission, meaning it would cost no less than $2.5 billion. It would begin its mission by orbiting Enceladus for about 200 days, which will be a difficult feat since Saturn’s powerful gravity will try to dislodge the Orbilander from its orbit around Enceladus. Careful planning and maneuvers will be necessary to keep Orbilander in orbit while collecting particles from the plumes that funnel into science instruments. A generous amount of time in orbit will be spent looking for the perfect place to land as the Cassini images are not of high enough resolution to reveal hazardous topography.

Once a safe landing site is found, Orbilander would turn on its side and convert to a lander. It will descend using terrain-relative navigation like that used by Perseverance to land on Mars, or what Dragonfly will use when flying above Titan’s surface. Orbilander will use two nuclear power sources to keep it operating on the surface for at least 1½ years. Orbilander will rely on a complex suite of instruments to determine whether Enceladus’ water has a blend of chemicals conducive for life as we know it, and search for amino acids, lipids, and cells. The instruments include mass spectrometers to weigh and analyze molecules, a seismometer, a microscope, and a DNA sequencer. For remote sensing, Orbilander will have cameras, radar sounders, and a laser altimeter. It would be simpler if there was such a thing as a “Life Detection Instrument” where it would collect a sample, analyze it, and yield a simple, life or no life result, but it does not exist, so scientists came up with six different instruments to provide indications of several different biosignatures.

Enceladus is a wonderful world to explore as it offers a unique opportunity to sample its water without having to drill through the crust. Not only will Orbilander analyze the plumes by flying through them while in orbit but also will capture plume material falling back to the surface after the spacecraft lands. If approved for flight, Orbilander will not launch until 2038 arriving at Enceladus around 2050. The tight budget will make it a challenge to get approval among other compelling missions elsewhere in the Solar System, but 2050 is when Enceladus’ south pole will be in sunlight. There may be no better place to look for life than Enceladus, and Orbilander will make it possible with all its promise of discovery.