Europa Clipper

• 1. Introduction

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  Europa Clipper is the largest interplanetary spacecraft NASA has ever launched, and its destination is a moon of Jupiter. The probe is making a voyage of 2.9 billion kilometers, or 1.8 billion miles. Its target, Europa, is one of the four Galilean moons. Scientists count it among the places in the Solar System where microbial life might conceivably exist.

  Yet the spacecraft will not orbit the moon it was built to study. It will circle Jupiter instead, and swoop past Europa again and again. Why would a mission to a moon refuse to settle beside it? What can instruments learn about an ocean hidden under a crust of ice? And how does a machine full of electronics survive in the radiation that surrounds Jupiter?

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• 2. Ocean Beneath the Ice

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  Galileo, the robotic spacecraft that orbited Jupiter from 1995 to 2003, is the probe that found the subsurface ocean under Europa's ice crust. Its primary mission ran from the 8th of December 1995 to the 7th of December 1997. On that last date it began an extended phase called the Galileo Europa Mission, which continued until the 31st of December 1999.

  That extension ran on a budget of only 30 million US dollars. A team of about 40 to 50 people replaced the 200 who had staffed the primary mission. With so few people, the team could not absorb surprises. When problems arose, it temporarily recalled former members, known as tiger teams, for intensive repairs.

  During this period Galileo made eight flybys of Europa, four of Callisto and two of Io. At each moon it collected two days of data, compared with seven days in the primary mission. Its Europa passes ranged from 196 to 3582 kilometers over two years.

  Three requirements for life frame the goals of Europa Clipper: liquid water, chemistry and energy. The first goal is to confirm water within or beneath the ice and to study how surface ice and ocean exchange material. The second is to map key compounds and link them to the ocean's makeup. The third is to understand how surface features formed, including places that are active now or were recently.

  The spacecraft will also look for signs of geothermal activity and measure the magnetic field that Europa induces. That field can point to a salty ocean below. The findings are meant to help choose a landing site for the proposed Europa Lander. But the moon holding these answers sits deep inside the magnetosphere of Jupiter, and that location shaped almost every decision about how the mission works.

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• 3. Flybys Instead of Orbit

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  Two cancelled missions, Europa Orbiter and Jupiter Icy Moons Orbiter, both called for circling Europa itself. Planners rejected that approach for Clipper because of radiation. Even a radiation-hardened spacecraft in close orbit would keep working for only a few months.

  Data return posed a second problem. Most instruments can gather information far faster than Earth's limited set of receiving antennas can accept it. So the time available to send data home matters more than the time spent making close observations.

  Flybys solve both problems at once. Between encounters, Clipper will have seven to ten days to transmit what it recorded. That adds up to as much as a year of transmission time, against just 30 days for an orbiter. Studies at the Jet Propulsion Laboratory found that the result is almost three times as much data, with less radiation exposure. They also concluded that a 2 billion dollar Clipper could make the most crucial measurements of the cancelled 4.3 billion dollar Jupiter Europa Orbiter.

  The spacecraft will enter an elliptical orbit around Jupiter, shaped by a gravity assist from Ganymede, Jupiter's largest moon. From there it will make 49 close flybys of Europa at altitudes from 25 to 2700 kilometers during a mission of about 3.5 years. Gravity assists from Europa, Ganymede and Callisto will bend its path so each pass reaches a new point. Together the passes should give a medium-quality global topographic survey, including ice thickness. On a low pass it could even fly through plumes of water vapor erupting from the ice, sampling the ocean without drilling.

  A total ionizing dose of 2.8 Mrad is expected over the mission. To endure it, the electronics sit inside a vault weighing 150 kilograms, shielded with titanium, zinc and aluminum. The vault walls are 9.2 millimeters of aluminum alloy. The electronics are also nested in the core of the spacecraft for extra protection.

  The mission's name comes from the lightweight, fast clipper ships that sailed trade routes in the 19th century. The spacecraft will pass Europa as often as every two weeks, which echoes those ships. The mission patch shows a sailing ship.

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• 4. Money and Rockets

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  In 1997, a team proposed a Europa Orbiter to NASA's Discovery Program, and the proposal was not selected. One month after the Discovery selections, the Jet Propulsion Laboratory, known as JPL, announced that NASA would conduct a Europa orbiter mission anyway. It invited the Discovery team to serve as the Mission Review Committee. Afterward JPL sketched a ladder of concepts, starting with the 16 billion dollar Jupiter Icy Moons Orbiter and ending with a multi-flyby spacecraft, Europa Clipper.

  The National Research Council recommended a Europa mission in 2013. The cost estimate was about 2 billion dollars that year and rose to 4.25 billion dollars by 2020. The mission is a joint project of the Johns Hopkins University Applied Physics Laboratory and JPL.

  In March 2013-75 million dollars was authorized to refine the mission activities and science goals and to start instrument development. In May 2014, a House bill raised the funding for what was then called the Europa Multiple Flyby Mission. It went from 15 million to 100 million dollars for the 2014 fiscal year. After the 2014 elections, bipartisan support was pledged to keep funding it, and the executive branch added 30 million dollars for preliminary studies.

  In April 2015, NASA invited the European Space Agency to propose an extra probe to fly with Clipper, with a mass limit of 250 kilograms. It could have been a simple probe, an impactor or a lander. In May 2015, NASA picked nine instruments, budgeted at about 110 million dollars over three years. A lander was added in January 2016, then cancelled in 2017 as too risky. In between, the Ocean Worlds Exploration Program was approved in May 2016, with the Europa mission as part of it.

  Congress originally required Clipper to fly on NASA's Space Launch System, or SLS. NASA asked for flexibility because it foresaw too few SLS rockets. On the 25th of January 2021, the Planetary Missions Program Office told the team to "immediately cease efforts to maintain SLS compatibility". The SLS would have flown directly to Jupiter in under three years. Commercial alternatives meant a cruise of about six years with gravity assists.

  In July 2021, NASA chose a Falcon Heavy, flown in a fully expendable configuration. Three reasons were given: a reasonable launch cost of around 178 million dollars, doubts about SLS availability, and strong vibration from the SLS solid boosters. NASA expected the Artemis program to use the SLS rockets heavily. Redesigning Clipper for that vibration was estimated to cost 1 billion dollars. The switch saved an estimated 2 billion dollars in launch costs alone.

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• 5. Built for Jupiter

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  The spacecraft bus is 5 meters long. It joins an aluminum cylindrical propulsion module, 150 centimeters wide, to a rectangular box. Clipper is a NASA Planetary Science Division mission, designated a Large Strategic Science Mission. It is the second flight of the Solar System Exploration program, and it is supported by the Ocean Worlds Exploration Program.

  Sunlight at Jupiter is only 4 percent as intense as at Earth. Even so, designers weighed solar panels against a plutonium-238 generator, a multi-mission radioisotope thermoelectric generator. That generator type had already flown on the Mars Science Laboratory mission. Five units were available, with one reserved for the Mars 2020 rover and another held as a backup. The Juno mission had already shown that solar panels work at Jupiter.

  In September 2013, planners decided solar arrays would be cheaper. On the 3rd of October 2014, NASA announced the panels were chosen. They are heavier than the generators would have been, but the spacecraft stayed within acceptable launch limits.

  Each panel covers 18 square meters and produces 150 watts continuously while facing the Sun in orbit around Jupiter. Batteries charged by the panels keep the spacecraft gathering data when it passes through Europa's shadow. Airbus Defence and Space in the Netherlands supplied the panels. The same intense magnetosphere that threatens the electronics is expected to wear down the panels gradually as the mission proceeds.

  NASA's Goddard Space Flight Center in Greenbelt, Maryland built the propulsion subsystem. The Applied Physics Laboratory in Laurel, Maryland delivered it as part of the Propulsion Module. It stands 10 feet tall and 5 feet across, making up about two-thirds of the main body. It holds nearly 6000 pounds of monomethyl hydrazine and dinitrogen tetroxide. Between 50 and 60 percent of that propellant is reserved for the Jupiter orbit insertion burn, which lasts 6 to 8 hours. Twenty-four engines, each rated at 27.5 newtons, handle both steering and propulsion.

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• 6. Nine Instruments

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  Nine scientific instruments, with a combined mass of 82 kilograms, were announced in May 2015. Two of them read heat and chemistry. The Europa Thermal Emission Imaging System, led by Philip Christensen of Arizona State University, images the surface in mid to far infrared to find warm spots such as vents venting plumes. It descends from an instrument on the 2001 Mars Odyssey orbiter, which Christensen also developed. The Mapping Imaging Spectrometer for Europa, led by Diana Blaney of JPL, maps organics, including amino acids and tholins, along with salts, acid hydrates and water ice phases.

  The Europa Imaging System, led by Elizabeth Turtle of the Applied Physics Laboratory, has two cameras. The wide-angle camera covers a 48 by 24 degree field of view. The narrow-angle camera reaches 0.5 meters per pixel from a 50 kilometer altitude, and sits on a two-axis gimbal so it can aim at targets. Together they should map more than 95 percent of Europa at 50 meters per pixel or better. Only about 14 percent of the surface had previously been mapped at 500 meters per pixel or better. Kurt Retherford of the Southwest Research Institute leads the ultraviolet spectrograph, and he belonged to the team that spotted Europa's plumes with the Hubble Space Telescope.

  The radar instrument REASON uses two frequencies, 9 and 60 megahertz, to sound the ice down to the ocean boundary, up to 30 kilometers deep. Donald Blankenship of the University of Texas at Austin leads it, and JPL built it. The magnetometer has three flux gates on an 8.5 meter boom that was stowed during launch. Margaret Kivelson of the University of Michigan leads that team. It replaced a costlier instrument, ICEMAG, which was cancelled after cost overruns. Joseph Westlake of the Applied Physics Laboratory leads the plasma instrument, which measures the plasma that otherwise masks the ocean's magnetic signal.

  Two mass spectrometers sample the material Europa throws into space. MASPEX, led by Jim Burch of the Southwest Research Institute, measures the moon's tenuous atmosphere and ejected surface material. Jack Waite, who led its development, had also led the Ion and Neutral Mass Spectrometer science team on Cassini. The Surface Dust Analyzer, led by Sascha Kempf of the University of Colorado Boulder, analyzes tiny grains from the surface and plumes. It is sensitive enough to detect signs of life even if a grain holds less than a single bacterial cell.

  The ninth instrument is the radio system itself. The high-gain antenna is 3.1 meters across, designed under Matt Bray at the Applied Physics Laboratory. It was tested at Langley Research Center and Goddard Space Flight Center in 2022. It uses X-band frequencies of 7.2 gigahertz for uplink and 8.4 gigahertz for downlink, plus a 32 gigahertz Ka-band, about 12 times higher than typical cellular communications. X-band downlink reaches about 16 kilobits per second, while Ka-band can reach up to 500. Commands arrive at roughly 2 kilobits per second.

  Erwan Mazarico of NASA's Goddard Space Flight Center leads the gravity and radio science. Doppler shifts in the signal between the spacecraft and Earth reveal tiny changes in its velocity. Those changes record how Europa's gravity tugs on the spacecraft, which exposes the moon's interior structure and how tidal forces deform it.

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• 7. Launch and the Long Road

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  On the 7th of June 2022, the main body of the spacecraft was completed, after final design began in August 2019 and assembly in March 2022. By the 30th of January 2024, every science instrument was aboard. In May 2024, the spacecraft arrived at Kennedy Space Center for final preparations.

  In June 2024, engineers discovered that some components were less radiation-hardened than believed, which raised fears of missing the launch window. Through the summer, intensive retesting of the transistors in question showed they would likely anneal enough to self-heal. In September 2024, the spacecraft was approved for a window opening on the 10th of October. On the 6th of October, NASA announced it would stand down because of Hurricane Milton, and the spacecraft went into secure storage.

  Europe Clipper lifted off on the 14th of October 2024, at 12:06 p.m. EDT, from Launch Complex 39A at Kennedy Space Center. The Falcon Heavy's boosters had flown five times before, including the launch of Psyche and an X-37B. The center stage flew only this mission. Both boosters and the first stage were expended because of the spacecraft's mass and trajectory. Originally the launch had been set two days after a Falcon 9 carried the European Space Agency's Hera toward the asteroid 65803 Didymos.

  The trajectory was announced on the 10th of February 2021 as a 5.5-year route. On the 1st of March 2025, a gravity assist from Mars slowed the probe by 2 kilometers per second, bending its orbit around the Sun. The Earth flyby on the 3rd of December 2026 then adds speed. The spacecraft reaches aphelion beyond Jupiter's orbit on the 4th of October 2029, then falls toward Jupiter for its insertion burn in April 2030.

  After entering the Jupiter system, Clipper will pass Ganymede at 500 kilometers, shedding about 400 meters per second. At 11 Jovian radii, the main engine adds about 840 meters per second, placing the spacecraft into a 202-day orbit. At the far point of that orbit, a burn of about 122 meters per second raises the closest approach. First science at Europa comes on the 6th of March 2031.

  The European Space Agency's Jupiter Icy Moons Explorer, called Juice, launched in April 2023 and arrives in July 2031. Clipper launched 18 months later but arrives 15 months earlier, thanks to a more powerful rocket and fewer gravity assists. Juice will fly past Europa twice and Callisto several times before orbiting Ganymede.

  In June 2022, lead project scientist Robert Pappalardo said planners were weighing a crash into Ganymede if no extended mission is approved. Such an impact would help Juice study Ganymede's surface chemistry. Any disposal target, whether Jupiter, Ganymede or Callisto, keeps the probe from reaching Europa. Without an extension, disposal is targeted for the 3rd of September 2034.

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• 8. Message in a Bottle

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  On the 1st of June 2023, NASA opened a campaign called Send Your Name to Europa, also described as a Message in a Bottle. People worldwide could add their names to a poem, "In Praise of Mystery: A Poem for Europa", written by U.S. Poet Laureate Ada Limón. A total of 2,620,861 people signed, most of them in the United States. Each participant received a digital ticket with details of the launch and destination.

  The poem is engraved on a tantalum plate, roughly 7 by 11 inches, that seals an opening into the vault. The inward-facing side carries the poem in the poet's own handwriting. The names are etched onto a microchip attached to the plate, within artwork showing a bottle surrounded by the four Galilean moons.

  The same inward side holds the Drake equation and the spectral lines of the hydrogen atom and the hydroxyl radical, known together as the water hole. Douglas Vakoch, president of METI International, designed that component. A portrait of planetary scientist Ron Greeley is engraved there as well.

  The outward-facing panel shows Earth's connection to Europa. Linguists collected recordings of the word water spoken in 103 languages from families around the world. METI International gathered the audio files, which were converted into waveforms and etched into the plate. They radiate outward from a symbol of the American Sign Language sign for water, and they travel toward a moon whose ocean is the entire reason for the journey.

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