Europa Clipper

Europa Clipper is an interplanetary mission in development by NASA comprising an orbiter. Set for a launch around 2022-2025, the spacecraft is being developed to study the Galilean moon Europa through a series of flybys while in orbit around Jupiter.

This mission is a scheduled flight of the Planetary Science Division, designated a Large Strategic Science Mission, and funded under the Planetary Missions Program Office's Solar System Exploration program as its second flight. It is also supported by the new Ocean Worlds Exploration Program. Europa Clipper will perform follow-up studies to those made by the Galileo spacecraft during its eight years in Jupiter orbit, which indicated the existence of a subsurface ocean underneath Europa. Plans to send a spacecraft to Europa were initially conceived with projects such as Europa Orbiter and Jupiter Icy Moons Orbiter, in which a spacecraft would be injected into orbit around Europa. However, due to the strong impact of radiation from Jupiter's magnetosphere in Europan orbit, it was decided that it would be safer to inject a spacecraft into an elliptical orbit around Jupiter and make 45 close flybys of the moon instead. The mission began as a joint investigation between the Jet Propulsion Laboratory and the Applied Physics Laboratory.

The mission will complement ESA's Jupiter Icy Moons Explorer launching in 2022, which will fly-by Europa twice and Callisto multiple times before moving into orbit around Ganymede. Launching around the same time as the Europa Clipper, the Jupiter Icy Moons Explorer will have a cruise phase some three times as long.

The Europa Clipper orbiter will be built and manufactured with a scientific payload of nine instruments, contributed by the JPL, APL, Southwest Research Institute, University of Texas, Arizona State University and University of Colorado Boulder.

History
Europa has been identified as one of the locations in the Solar System that could possibly harbor microbial extraterrestrial life. Immediately following the Galileo spacecraft's discoveries, JPL conducted preliminary mission studies that envisioned a capable spacecraft such as the Jupiter Icy Moons Orbiter (a $16B mission concept ), the Jupiter Europa Orbiter (a $4.3B concept), an orbiter ($2B concept), and a multi-flyby spacecraft: Europa Clipper.

The proposed Europa Clipper is still in its planning phase, but the approximate cost is estimated at $2 billion. The mission is a joint project between the Johns Hopkins University's Applied Physics Laboratory (APL), and the Jet Propulsion Laboratory (JPL). The mission's name is a reference to the lightweight clipper ships of the 19th century that routinely plied trade routes around the world. The moniker was chosen because the spacecraft will "sail" past Europa, as frequently as every two weeks.

In March 2013, $75 million USD were authorized to expand on the formulation of mission activities, mature the proposed science goals, and fund preliminary instrument development, as suggested in 2011 by the Planetary Science Decadal Survey. In May 2014, a House bill substantially increased the Europa Clipper (referred to as Europa Multiple Flyby Mission) funding budget for the 2014 fiscal year from $15 million  to $100 million to be applied to pre-formulation work.

Following the 2014 election cycle, bipartisan support was pledged to continue funding for the Europa Multiple Flyby Mission project. The executive branch has also granted $30 million for preliminary studies.

In April 2015, NASA offered to the European Space Agency to submit concepts for an additional probe to fly together with the Europa Clipper spacecraft, with a mass limit of 250 kg maximum. It could be a simple probe, an impactor, or a lander. An internal assessment at ESA is underway to see if there is interest and funds available,  opening a collaboration scheme similar to the very successful Cassini-Huygens approach.

In May 2015, NASA chose nine instruments that would fly on board the orbiter. They will cost about $110 million over the next three years. In June 2015, NASA announced its approval of the mission concept, allowing the orbiter to move to its formulation stage, and in January 2016 it approved a lander as well. In May 2016 the Ocean Worlds Exploration Program was approved, of which the Europa mission is part. In February 2017 the mission moved from Phase A to Phase B. Phase B is the preliminary design phase of the mission, planned to continue through September 2018. In addition, subsystem vendors will be selected, as well as prototype hardware elements for the science instruments. Spacecraft subassemblies will be built and tested as well. Phases C and D include final design, spacecraft fabrication, assembly and testing, and launch.

In 18 July 2017, the House Space Subcommittee held hearings on the Europa Clipper as a scheduled Large Strategic Science Missions class, and to discuss a possible follow up mission simply known as the Europa Lander.

Objectives
The goals of Europa Clipper are to explore Europa, investigate its habitability and aid in the selection of a landing site for the future Europa Lander. This exploration is focused on understanding the three main requirements for life: liquid water, chemistry, and energy. Specifically, the objectives are to study:
 * Ice shell and ocean: Confirm the existence, and characterize the nature, of water within or beneath the ice, and processes of surface-ice-ocean exchange.
 * Composition: Distribution and chemistry of key compounds and the links to ocean composition.
 * Geology: Characteristics and formation of surface features, including sites of recent or current activity.

Strategy
Because Europa lies well within the harsh radiation fields surrounding Jupiter, even a radiation-hardened spacecraft in near orbit would be functional for just a few months. Another key limiting factor on science for a Europa orbiter is not the time the instruments can make observations. Rather, it is the time available to return data to Earth. Most instruments can gather data far faster than the communications system can transmit it to Earth because there are a limited number of antennas available to receive the scientific data.

Studies by scientists from the Jet Propulsion Laboratory show that by performing several flybys with many months to return data, the Europa Clipper concept will enable a $2B mission to conduct the most crucial measurements of the cancelled $4.3B Jupiter Europa Orbiter concept. Between each of the flybys, the spacecraft will have seven to ten days to transmit data stored during each brief encounter. That will let the spacecraft have up to a year of time to transmit its data compared to just 30 days for an orbiter. The result will be almost three times as much data returned to Earth, while reducing exposure to radiation. The Europa Clipper will not orbit Europa, but instead orbit Jupiter and conduct 45 flybys of Europa at altitudes from 25 to 2700 km each during its 3.5-year mission. A key feature of the mission concept is that the Clipper would use gravitational attraction from Europa, Ganymede and Callisto to change its trajectory, allowing the spacecraft to return to a different close approach point with each flyby. Each flyby would cover a different sector of Europa in order to achieve a medium-quality global topographic survey, including ice thickness. The Europa Clipper could conceivably flyby at low altitude through the plumes of water vapor erupting from the moon's icy crust, thus sampling its subsurface ocean without having to land on the surface and drill though the ice.

The Europa Clipper will inherit tested technology of the Galileo and Juno Jupiter orbiters with regards to radiation protection. Shielding will be provided by 150 kilograms of titanium. To maximize its effectiveness, the electronics will be nested in the core of the spacecraft for additional radiation protection.

Power
Both radioisotope thermoelectric generator and photovoltaic power sources were assessed to power the orbiter. Although solar power is only 4% as intense at Jupiter as it is in Earth's orbit, powering a Jupiter orbital spacecraft by solar panels was demonstrated by the Juno Mission. The alternative to solar panels was a Multi-Mission Radioisotope Thermoelectric Generator, fueled with plutonium-238. The power source has already been demonstrated in the Mars Science Laboratory mission. Five units were available, with one reserved for the Mars 2020 rover mission and another as backup. In September 2013 it was decided that the solar array was the less expensive option to power the spacecraft, and on October 3, 2014, it was announced that solar panels were chosen to power Europa Clipper. The mission's designers determined that solar was both cheaper than plutonium and practical to use on the spacecraft. Despite the increased weight of solar panels compared to plutonium-powered generators, the vehicle's mass had been projected to still be within acceptable launch limits.

Initial analysis suggest that each panel will have a surface area of 18 sqm and produce 150 watts continuously when pointed towards the Sun while orbiting Jupiter. While in Europa's shadow, batteries will enable the spacecraft to continue gathering data. However, ionizing radiation can damage solar panels. The Europa Clipper orbit will pass through Jupiter's intense magnetosphere, which is expected to gradually degrade the solar panels as the mission progresses. The solar panels will be provided by Airbus Defense and Space Netherlands.

Scientific payload
The Europa Clipper mission is equipped with a sophisticated suite of 9 instruments to study Europa's interior and ocean, geology, chemistry, and habitability. The spacecraft payload and trajectory are subject to change as the mission design matures. The nine science instruments for the orbiter, announced in May 2015, have an estimated total mass of 82 kg and are listed below:

Possible secondary elements
The Europa Clipper mission considers an extra mass of about 250 kg to carry an additional flight element. About a dozen proposals have been suggested, a few of which are described next: Since the Europa Clipper mission may not be able to easily modify its orbital trajectory or altitude to fly through the episodic water plumes, scientists and engineers working on the mission have investigated deploying from the spacecraft several miniaturized satellites of the CubeSat format, possibly driven by ion thrusters, to fly through the plumes and assess the habitability of Europa's internal ocean. Some early proposals include Mini-MAGGIE, DARCSIDE, and Sylph. The Europa Clipper would relay signals from the nanosatellites back to Earth. With propulsion, some nanosatellites could also be capable of entering orbit around Europa.
 * Nanosatellites


 * Secondary orbiters
 * Biosignature Explorer for Europa
 * NASA is also assessing the release of an additional 250 kg probe called Biosignature Explorer for Europa (BEE), that would be equipped with a basic bi-propellant engine and cold gas thrusters to be more agile and responsive to the episodic activity on Europa and sample and analyze the water plumes for biosignatures and life evidence before they are destroyed by radiation. The BEE plume probe would be equipped with a proven mass spectrometer combined with gas chromatograph separation. It would also carry a UV plume targeting camera as well as visible and IR cameras to image the active region with better resolution than the Clipper mother ship instruments. The BEE probe would fly through at 2–10 km altitude, then make a quick exit and perform its analysis far from the radiation belts.


 * Europa Tomography Probe (ETP)
 * A European proposal is an independent powered spacecraft equipped with a magnetometer that would orbit Europa on a polar orbit for at least six months. It would determine the deep interior structure of Europa and provide a good determination of the ice shell thickness and ocean depth, which arguably cannot be done accurately by multiple flybys.


 * Impactor probes
 * Some proposed impactor probe concepts include those by the Netherlands, and United Kingdom.

The Europa Life Signature Assayer (ELSA) concept by the University of Colorado consists of a probe that is flown as a secondary payload. ELSA would use a small impactor to create a plume of subsurface particles and catapult them to altitudes where it would be able to pass through to collect samples and analyze them on board. A variation of this concept is the 1996 Ice Clipper, which involves a 10 kg impactor that would be jettisoned from the main spacecraft to impact Europa, thereby creating a debris cloud in nearby space about 100 km altitude, subsequently sampled by a small spacecraft on a close flyby and use Europa's gravitational force for a free return trajectory. The collection mechanism is tentatively considered to be Aerogel (similar to Stardust mission).
 * Flyby sample return

Add-on lander history
An early Europa Clipper concept calls for a small lander about 1 meter in diameter, perhaps about 230 kg with a maximum of 30 kg for instruments plus propellant. Suggested instruments are a mass spectrometer and a Raman spectrometer to determine the chemistry of the surface. The lander would be delivered to Europa by the main spacecraft and possibly require the sky crane system for a high precision, soft landing near an active crevasse. The lander would operate about 10 days on the surface using battery power. In January 2016 it was determined that including the additional mass of a lander and detachable probes, meant the Europa Clipper may need be launched with NASA's Space Launch System (SLS) heavy lift launch vehicle that could arrive at Jupiter on a direct trajectory in less than three years.

The Europa Clipper would take about three years to image 95% of the surface of Europa at about 50 meters per pixel. With this data, scientists could then find a suitable landing site. By one estimate, including a lander could add as much as $1 billion to the mission's cost.

It was determined that designing a system capable of landing on a surface about which very little is known, is too much risk, and that the Europa Clipper will lay the foundation for a future landing by performing detailed reconnaissance. This led to a stand-alone mission proposal in 2017: the Europa Lander. The NASA Europa Lander, if funded, would be launched separately in 2024 to complement the studies by the Europa Clipper mission. If funded, approximately 10 proposals may be selected to proceed into a competitive process with a $1.5 million budget per investigation.
 * Separate launch

Launch options
The current baseline profile for the mission involves launch aboard NASA's Space Launch System (SLS) heavy lift launch vehicle with a cruise to Jupiter on a direct trajectory in less than three years. Since the development and timing of the SLS may not match the mission's timing, an official alternative is a launch on an Atlas V 551. By using a Venus-Earth-Earth gravity assist trajectory the transit time to Jupiter would be about six years. Other alternatives, also requiring an indirect cruise, include launching on a Falcon Heavy, or another EELV.