Minimize Parker Solar Probe

Parker Solar Probe - former SPP (Solar Probe Plus) Spacecraft Mission

Spacecraft   Project Status   Launch   Sensor Complement   References

The Solar Probe Plus mission is part of NASA's LWS (Living With a Star) Program. The program is designed to understand aspects of the sun and Earth's space environment that affect life and society. The program is managed by NASA/GSFC (Goddard Space Flight Center). The Johns Hopkins University Applied Physics Laboratory (JHU/APL) in Laurel, MD., is the prime contractor for the spacecraft. In September 2010, NASA selected the Solar Probe Plus mission for development. A launch of the mission is planned for 2018. 1)

NASA's first mission to go to the sun, the Parker Solar Probe, is named after Eugene Parker who first theorized that the sun constantly sends out a flow of particles and energy called the solar wind.

On May 31, 2017, NASA has renamed the Solar Probe Plus spacecraft humanity's first mission to a star, which will launch in 2018 as the "Parker Solar Probe" in honor of astrophysicist Eugene Parker. The announcement was made at a ceremony at the University of Chicago, where Parker serves as the S. Chandrasekhar Distinguished Service Professor Emeritus, Department of Astronomy and Astrophysics.

In 1958, Parker — then a young professor at the university's Enrico Fermi Institute — published an article in the Astrophysical Journal called "Dynamics of the interplanetary gas and magnetic fields." Parker believed there was high speed matter and magnetism constantly escaping the sun, and that it affected the planets and space throughout our solar system.

This phenomenon, now known as the solar wind, has been proven to exist repeatedly through direct observation. Parker's work forms the basis for much of our understanding about how stars interact with the worlds that orbit them.

"This is the first time NASA has named a spacecraft for a living individual," said Thomas Zurbuchen, associate administrator for NASA's Science Mission Directorate in Washington. "It's a testament to the importance of his body of work, founding a new field of science that also inspired my own research and many important science questions NASA continues to study and further understand every day. I'm very excited to be personally involved honoring a great man and his unprecedented legacy."

"The solar probe is going to a region of space that has never been explored before," said Parker. "It's very exciting that we'll finally get a look. One would like to have some more detailed measurements of what's going on in the solar wind. I'm sure that there will be some surprises. There always are."

In the 1950s, Parker proposed a number of concepts about how stars — including our sun — give off energy. He called this cascade of energy the solar wind, and he described an entire complex system of plasmas, magnetic fields and energetic particles that make up this phenomenon. Parker also theorized an explanation for the superheated solar atmosphere, the corona, which is — contrary to what was expected by physics laws — hotter than the surface of the sun itself. Many NASA missions have continued to focus on this complex space environment defined by our star — a field of research known as heliophysics.

"Parker Solar Probe is going to answer questions about solar physics that we've puzzled over for more than six decades," said Parker Solar Probe Project Scientist Nicola Fox, of the Johns Hopkins University Applied Physics Laboratory. "It's a spacecraft loaded with technological breakthroughs that will solve many of the largest mysteries about our star, including finding out why the sun's corona is so much hotter than its surface. And we're very proud to be able to carry Gene's name with us on this amazing voyage of discovery."

NASA missions are most often renamed after launch and certification; in this case, given Parker's accomplishments within the field, and how closely aligned this mission is with his research, the decision was made to honor him prior to launch, in order to draw attention to his important contributions to heliophysics and space science.

Born on June 10, 1927, in Michigan, Eugene Newman Parker received a Bachelor of Science in physics from Michigan State University and a doctorate from Caltech. He then taught at the University of Utah, and since 1955, Parker has held faculty positions at the University of Chicago and at its Fermi Institute. He has received numerous awards for his research, including the George Ellery Hale Prize, the National Medal of Science, the Bruce Medal, the Gold Medal of the Royal Astronomical Society, the Kyoto Prize, and the James Clerk Maxwell Prize.

Table 1: NASA has renamed the Solar Probe Plus mission to Parker Solar Probe 2)


Figure 1: NASA's first mission to go to the sun, the Parker Solar Probe, is named after Eugene Parker who first theorized that the sun constantly sends out a flow of particles and energy called the solar wind (image credit: NASA, JHU/APL)

The SPP science objectives are: 3) 4) 5)

1) Determine the structure and dynamics of the magnetic fields at the sources of the fast and slow solar wind.

2) Trace the flow of energy that heats the corona and accelerates the solar wind.

3) Determine what mechanisms accelerate and transport energetic particles.

4) Explore dusty plasma phenomena in the near-sun environment and their influence on the solar wind and energetic particle formation.

Background: 6) 7) 8) 9) 10)

• The concept for a "solar probe" dates back to "Simpson's CommiIee" of the Space Science Board (National Academy of Sciences, 24 October 1958). The need for extraordinary knowledge of Sun from remote observations, theory, and modeling to answer the questions:

- Why is the solar corona so much hotter than the photosphere?

- How is the solar wind accelerated?

• SPP was a NASA concept study in 2008. The challanging objective of the mission is to explore the near-Sun environment for a better understanding of solar physics. So far, no missions have penetrated closer to the Sun than 0.3 AU (Astronomical Units).

• Helios 1 and 2 were a pair of cooperative US and German deep space probes (launch Dec. 10, 1974 and Jan. 15, 1976, respectively) which set the record for the closest approach to the Sun, at ~45 million km, slightly inside the orbit of Mercury.

• The NASA MESSENGER mission (launch Aug. 3, 2004) was the first spacecraft to orbit planet Mercury. The data of the Sun are unique representing the only in situ measurements of the inner heliosphere as close as 60 solar radii (RS). The unexplored region within this distance is where the corona is accelerated to form the supersonic solar wind, and is critical to our understanding of the Sun's impact on the solar system.

First definitions of Solar Probe missions (studies) at NASA/JPL were started in 1978. The original Solar Probe mission concept of 2005, based on a Jupiter gravity assist trajectory, was no longer feasible under the new guidelines given to the mission. A complete redesign of the mission was required to meet the mission constraints, which called for the development of alternative mission trajectories that excluded a flyby of Jupiter.

In mid-2007, NASA asked JHU/APL to consider another concept for Solar Probe that would perform all science objectives of the 2005 concept, implemented as a non-nuclear powered spacecraft, and executed under a New Frontiers-like cost cap. The resulting mission is called Solar Probe+ in recognition of the potential gains in science of the current concept over predecessors. 11) 12) 13)

In March 2012, the SPP project advanced to Phase-B. 14)

Two key technical challenges make a solar probe much more difficult than other missions: 15)

1) The extremely high temperature and harsh environment in the Sun's proximity, which the spacecraft cannot survive without adequate thermal protection

2) The extreme difficulty of getting close to the Sun, as an enormous amount of velocity must be canceled out from the Earth orbital velocity in order for a probe to get close to the Sun.

SPP will sample the solar corona to reveal how it is heated and the solar wind and solar energetic particles are accelerated. Solving these problems has been a top science goal for over 50 years. 16) During the seven-year mission, seven Venus gravity assist (VGA) maneuvers will gradually lower the perihelia to <10 RS (Radius of sun ~700,000 km), the closest any spacecraft has come to the Sun. Throughout the 7-year nominal mission duration, the spacecraft will spend a total of 937 hours inside 20 RS , 440 hours inside 15 RS , and 14 hours inside 10 RS, sampling the solar wind in all its modalities (slow, fast, and transient) as it evolves with rising solar activity toward an increasingly complex structure. SPP will orbit the Sun in the ecliptic plane, and so will not sample the fast wind directly above the Sun's polar regions (Figure 1). However, the current mission design compensates for the lack of in-situ measurements of the fast wind above the polar regions by the relatively long time SPP spends inside 20 RS.17) - This will allow extended measurement of the equatorial extensions of high-latitude coronal holes and equatorial coronal holes. At a helioradius ~35 RS , there are two periods per orbit (one inbound and one outbound) when SPP will be in quasi-corotation with the Sun and will cross a given longitudinal sector slowly. In these intervals, known as fast radial scans, the spacecraft will sample the solar wind over large radial distances within a given flux tube before moving across the sector. These measurements will yield additional information on the spatial/temporal dependence of structures in the solar wind and on how they merge in the inner heliosphere.


Figure 2: Solar wind speed as a function of heliographic latitude illustrating the relationship between the structure of the solar wind and coronal structure at solar minimum (a, c) and solar maximum (b). Ulysses SWOOPS solar wind data are superposed on composite solar images obtained with the SOHO EIT and LASCO C2 instruments and with the Mauna Loa K-coronameter. (d) Solar cycle evolution (image credit: D. J. McComas et al.) .18)

Science overview:

The SPP mission targets processes and dynamics that characterize the Sun's expanding corona and solar wind. SPP will explore the inner region of the heliosphere through in-situ and remote sensing observations of the magnetic field, plasma, and energetic particles. The solar magnetic field plays a defining role in forming and structuring the solar corona and the heliosphere. In the corona, closed magnetic field lines confine the hot plasma in loops, while open magnetic field lines guide the solar wind expansion in the inner corona. The energy that heats the corona and drives the wind derives from photospheric motions, and is channeled, stored, and dissipated by the magnetic fields that emerge from the convection zone and expand in the corona where they dominate almost all physical processes therein. Examples of these are waves and instabilities, magnetic reconnection, and turbulence, which operate on a vast range of spatial and temporal scales. Magnetic fields play also a critical role in coronal heating and solar wind acceleration. They are conduits for waves, store energy, and propel plasma into the heliosphere through complex forms of magnetic activity [e.g., CMEs (Coronal Mass Ejections), flares, and small-scale features such as spicules and jets]. How solar convective energy couples to magnetic fields to produce the multifaceted heliosphere is central to SPP science.

SPP will make in-situ and remote measurements from <10 RS to at least 0.25 AU (53.7 RS ). Measurements of the region where the solar wind originates and where the most hazardous solar energetic particles are energized will improve our ability to characterize and forecast the radiation environment of the inner heliosphere. SPP will measure local particle distribution functions, density and velocity field fluctuations, and electromagnetic fields within 0.25 AU of the Sun. These data will help answer the basic questions of how the solar corona is powered, how the energy is channeled into the kinetics of particle distribution functions in the solar corona and wind, and how such processes relate to the turbulence and wave-particle dynamics observed in the heliosphere. Cross-correlation of velocity, density, and electromagnetic fluctuations will allow a partial separation of spatial and temporal effects.

The physical conditions of the region below 20 RS are important in determining largescale properties such as solar wind angular momentum loss and global heliospheric structure. The Alfvénic critical surface, where the solar wind speed overtakes the Alfvén speed, is believed to lie in this region. This surface defines the point beyond which the plasma ceases to corotate with the Sun, i.e., where the magnetic field loses its rigidity to the plasma. In this region solar wind physics changes because of the multi-directionality of wave propagation (waves moving sunward and anti-sunward can affect the local dynamics including the turbulent evolution, heating and acceleration of the plasma). This is also the region where velocity gradients between the fast and slow speed streams develop, forming the initial conditions for the formation, further out, of CIRs (Corotating Interaction Regions).


Figure 3: SPP, shown along its orbit (dashed curve) near a perihelion pass, will measure solar energetic ions and electrons from a vantage point very near the site where these particles are accelerated. The illustration sketches the occurrence of a solar flare and a CME extending a few RS from the Sun. The shock at the front edge of the CME and the compressed sheath plasma behind the shock form as the CME, with its entrained flux rope (tangled pink lines), pushes outward from the Sun through the ambient solar wind. Swept-up magnetic field lines are refracted and compressed across the shock and draped around the CME. Energetic particles accelerated at both the flare and CME shock are shown spiraling away from the Sun (yellow spirals) along the magnetic field. For simplicity, magnetic field lines around the shock are depicted as smooth. However, it is expected that the field ahead of CME shock and in the sheath will highly structured.Waves ahead of the shock that are produced by high intensities of shock-accelerated ions streaming away from the shock are sketched for the uppermost magnetic field line connected to the CME shock (image credit: Ref.16)

SPP participation:

• 31 institutions participate in SPP science teams

- 23 in the US, 8 foreign

- 17 educational, 5 non-profit, 8 government labs

• 106 science team members

- 69 PIs and Co-Is

- 37 additinal scientists

- Next generation graduate students and post-docs.


Figure 4: Participating organizations in SPP (image credit: JHU/APL)


Figure 5: This illustration of NASA's Parker Solar Probe depicts the spacecraft traveling through the Sun's outer atmosphere (image credit: JHU/APL)

To accomplish the science objectives of addressing the fundamental questions about the Sun by acquiring critical data and measurements to answer questions that cannot be answered by observations from satellites in Earth orbit and from other interplanetary space probes, a solar probe must approach the Sun closely. A solar orbit approach to within the range of 10 solar radii (Rs) from Sun's center must be considered to conduct the necessary in situ measurements and investigations.

Getting directly to the Sun from Earth would require a launch energy C3 as large as 423 km2/s2. This is beyond the capability of launch vehicles currently available (Atlas V, Delta IV Heavy) or to be developed in the near future. The highest launch C3 ever achieved was 164 km2/s2 for the New Horizons mission to Pluto (launch Jan. 2006).

After an extensive analysis by NASA and JHU/APL, the trajectory option 5 was chosen as the baseline trajectory for the new solar probe. The redesigned mission is named SPP (Solar Probe Plus) for its significant advantages in both technical implementation and science accomplishments as compared with the original Solar Probe mission.

The mission design utilizes seven Venus gravity assists to gradually reduce perihelion (Rp) from 35 solar radii (Rs) in the first orbit to < 10 Rs for the final three orbits. The SPP orbit consists of two primary orbit phases, a science phase (0.25AU to perihelion) and a cruise/data downlink phase (0.25AU to aphelion).


Solar Probe (2005)

Solar Probe Plus (2008)

Minimum perihelion

4 Rs

9.5 Rs


90º from ecliptic

3.4º from ecliptic

Number of solar passes



Total time within 20 Rs

96 hours

961 hours

Time between passes

4.6 years

88 to 150 days

Time from launch to first perihelion

4.1 years

3 months

Mission duration

8.8 years

6.9 years


5.5 AU

1 AU

Table 2: Comparison of Solar Probe and Solar Probe Plus (Ref. 15)


Figure 6: Reference Mission: Launch and Mission Design Overview (image credit: JHU/APL, NASA)


Figure 7: Detail of solar encounter timeline for a typical orbit (image credit: NASA, JHU/APL)

Figure 7 shows the orbit within ± 10 days of perihelion, and an expansion of the region ± 20 Rs. This figure also shows the time spent in each part of the solar encounter of scientific interest for one of the final orbits. In total, SPP will spend more than 2100 hrs closer than 30 Rs, nearly 1000 hours below 20 Rs, and 27 hrs in the region below 10 Rs.

SPP is an ambitious mission, requiring significant technology development in several major areas. Table 3 is a summary of the technology readiness assessment for SPP and gives an indication of the basis for technology. For each area, technology development plans have been established, and in each case, significant progress has been made to achieve TRL (Technology Readiness Level) 6 by PDR (Preliminary Design Review).


TRL (Technology Readiness Level)


TPS (Thermal Protection System)


NASA sponsored technology development

Solar array


Combines space heritage and concentrator cells

Cooling system


Adapted from heritage systems

X-/Ka-band transponder


NASA sponsored technology development

LEON3 processor


Qualified product in new JHU/APL application

Table 3: Technology development for SPP (shows only items with TRL lower than 6)


A short presentation of the two solar missions: Parker Solar Probe of NASA and Solar Orbiter of ESA

• May 16, 2018: Two upcoming missions will soon take us closer to the Sun than we've ever been before, providing our best chance yet at uncovering the complexities of solar activity in our own solar system and shedding light on the very nature of space and stars throughout the universe. 19)

- Together, NASA's Parker Solar Probe and ESA's (the European Space Agency) Solar Orbiter may resolve decades-old questions about the inner workings of our nearest star. Their comprehensive, up-close study of the Sun has important implications for how we live and explore: Energy from the Sun powers life on Earth, but it also triggers space weather events that can pose hazard to technology we increasingly depend upon. Such space weather can disrupt radio communications, affect satellites and human spaceflight, and — at its worst — interfere with power grids. A better understanding of the fundamental processes at the Sun driving these events could improve predictions of when they'll occur and how their effects may be felt on Earth.

- "Our goal is to understand how the Sun works and how it affects the space environment to the point of predictability," said Chris St. Cyr, Solar Orbiter project scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "This is really a curiosity-driven science."

- Parker Solar Probe is slated to launch in the summer of 2018, and Solar Orbiter is scheduled to follow in 2020. These missions were developed independently, but their coordinated science objectives are no coincidence: Parker Solar Probe and Solar Orbiter are natural teammates.

- Both missions will take a closer look at the Sun's dynamic outer atmosphere, called the corona. From Earth, the corona is visible only during total solar eclipses, when the Moon blocks the Sun's most intense light and reveals the outer atmosphere's wispy, pearly-white structure. But the corona isn't as delicate as it looks during a total solar eclipse — much of the corona's behavior is unpredictable and not well understood.

- The corona's charged gases are driven by a set of laws of physics that are rarely involved with our normal experience on Earth. Teasing out the details of what causes the charged particles and magnetic fields to dance and twist as they do can help us understand two outstanding mysteries: what makes the corona so much hotter than the solar surface, and what drives the constant outpouring of solar material, the solar wind, to such high speeds.

- We can see that corona from afar, and even measure what the solar wind looks like as it passes by Earth — but that's like measuring a calm river miles downstream from a waterfall and trying to understand the current's source. Only recently have we had the technology capable of withstanding the heat and radiation near the Sun, so for the first time, we're going close to the source.

- "Parker Solar Probe and Solar Orbiter employ different sorts of technology, but — as missions — they'll be complementary," said Eric Christian, a research scientist on the Parker Solar Probe mission at NASA Goddard. "They'll be taking pictures of the Sun's corona at the same time, and they'll be seeing some of the same structures — what's happening at the poles of the Sun and what those same structures look like at the equator."

- Parker Solar Probe will traverse entirely new territory as it gets closer to the Sun than any spacecraft has come before — as close as 3.8 million miles from the solar surface. If Earth were scaled down to sit at one end of a football field, and the Sun at the other, the mission would make it to the 4-yard line. The current record holder, Helios B, a solar mission of the late 1970s, made it only to the 29-yard line.

- From that vantage point, Parker Solar Probe's four suites of scientific instruments are designed to image the solar wind and study magnetic fields, plasma and energetic particles — clarifying the true anatomy of the Sun's outer atmosphere. This information will shed light on the so-called coronal heating problem. This refers to the counterintuitive reality that, while temperatures in the corona can spike upwards of a few million degrees Celsius, the underlying solar surface, the photosphere, hovers around just 6,000ºC. To fully appreciate the oddity of this temperature difference, imagine walking away from a campfire and feeling the air around you get much, much hotter.

- Solar Orbiter will come within 26 million miles of the Sun — that would put it within the 27-yard line on that metaphorical football field. It will be in a highly tilted orbit that can provide our first-ever direct images of the Sun's poles — parts of the Sun that we don't yet understand well, and which may hold the key to understanding what drives our star's constant activity and eruptions.

- Both Parker Solar Probe and Solar Orbiter will study the Sun's most pervasive influence on the solar system: the solar wind. The Sun constantly exhales a stream of magnetized gas that fills the inner solar system, called solar wind. This solar wind interacts with magnetic fields, atmospheres, or even surfaces of worlds throughout the solar system. On Earth, this interaction can spark auroras and sometimes disrupt communications systems and power grids.

- Data from previous missions have led scientists to believe the corona contributes to the processes that accelerate particles, driving the solar wind's incredible speeds — which triple as it leaves the Sun and passes through the corona. Right now, the solar wind travels some 92 million miles by the time it reaches the spacecraft that measure it — plenty of time for this stream of charged gases to intermix with other particles traveling through space and lose some of its defining features. Parker Solar Probe will catch the solar wind just as it forms and leaves the corona, sending back to Earth some of the most pristine measurements of solar wind ever recorded. Solar Orbiter's perspective, which will provide a good look at the Sun's poles, will complement Parker Solar Probe's study of the solar wind, because it allows scientists to see how the structure and behavior of the solar wind varies at different latitudes.

- Solar Orbiter will also make use of its unique orbit to better understand the Sun's magnetic fields; some of the Sun's most interesting magnetic activity is concentrated at the poles. But because Earth orbits on a plane more or less in line with the solar equator, we don't typically get a good view of the poles from afar. It's a bit like trying to see the summit of Mount Everest from the base of the mountain.

- That view of the poles will also go a long way toward understanding the overall nature of the Sun's magnetic field, which is lively and extensive, stretching far beyond the orbit of Neptune. The Sun's magnetic field is so far-reaching largely because of the solar wind: As the solar wind streams outward, it carries the Sun's magnetic field with it, creating a vast bubble, called the heliosphere. Within the heliosphere, the solar wind determines the very nature of planetary atmospheres. The heliosphere's boundaries are shaped by how the Sun interacts with interstellar space. Since Voyager 1's passage through the heliopause in 2012, we know these boundaries dramatically protect the inner solar system from incoming galactic radiation.

- It's not yet clear how exactly the Sun's magnetic field is generated or structured deep inside the Sun — though we do know intense magnetic fields around the poles drives variability on the Sun, causing solar flares and coronal mass ejections. Solar Orbiter will hover over roughly the same region of the solar atmosphere for several days at a time while scientists watch tension build up and release around the poles. Those observations may lead to better awareness of the physical processes that ultimately generate the Sun's magnetic field.

- Together, Parker Solar Probe and Solar Orbiter will refine our knowledge of the Sun and heliosphere. Along the way, it's likely these missions will pose even more questions than they answer — a problem scientists are very much looking forward to.

- "There are questions that have been bugging us for a long time," said Adam Szabo, mission scientist for Parker Solar Probe at NASA Goddard. "We are trying to decipher what happens near the Sun, and the obvious solution is to just go there. We cannot wait — not just me, but the whole community."




At 9.5 Rs, the solar intensity is 512 times that at 1AU. SPP is packaged behind the carbon-carbon TPS (Thermal Protection System), a 11 cm thick heat shield, to protect it from this extreme solar environment and allow it to operate at standard space thermal environments while the TPS experiences temperatures of 1400ºC on its sun-facing surface. SPP utilizes actively cooled solar arrays for power generation maintaining the solar cells within required temperature limits (Ref. 3). 20)

Solar Probe Plus is a 3-axis stabilized spacecraft, shown in Figure 8, with functional block diagram in Figure 10.

TPS: The most prominent feature is the 2.3 m diameter TPS, with associated structure used to attach the shield to the spacecraft. The TPS protects the bus and payload within its umbra during solar encounter. The conceptual science instruments are mounted either directly to the bus, on a stand-off bracket near the fairing attachment, or on a science boom extended from the rear of the spacecraft.

In general, the payload is protected from the effects of solar exposure by the TPS. Two notable exceptions are the SPC (Solar Probe Cup), part of the SWEAP investigation, and the electric field antennas carried as part of the FIELDS investigation. Both sensor packages extend beyond the TPS and see the same environment as the TPS sunward-looking face. Both sensors are of high heritage; however the solar environment during solar encounter is significantly more severe than all previous experience. Therefore, technology development programs for each have been implemented to demonstrate the operation of each in the expected SPP environment.

Three deployable conceptual carbon-carbon plasma wave antennas are mounted 120º apart on the side of the bus. These antennas will partially protrude beyond the umbra during encounter. The solar array cooling system dissipates the high solar flux absorbed by solar array wings during closest approach to the sun enabling the solar cells to operate within their temperature constraints while providing the required electrical power. Water in the cooling system is pumped from the outboard-most edge of the solar array substrate, or platen, up through channels in the solar array wings into the four cooling system radiators mounted under the TPS and back through the pump located on the top deck of the spacecraft. The system can dissipate 6000 W of heat at perihelion, and is designed and operated to prevent freezing at aphelion.

The new configuration uses a single pair of arrays to generate power. The bulk of the solar array panel is filled with "primary cells" similar to cells used on the MESSENGER mission to Mercury, while the angled panel on the end of the solar arrays use cells designed to withstand the high illumination during perihelion.


Figure 8: Illustration of the Solar Probe Plus spacecraft configuration (image credit: JHU/APL)


Figure 9: Spacecraft overview (image credit: JHU/APL, Ref. 10)


Figure 10: Block diagram of the Solar Probe Plus spacecraft (image credit: JHU/APL, Ref. 4)

At aphelion, the entire array is exposed to sunlight. As the spacecraft nears the sun, the array is tilted toward the spacecraft body until at perihelion only the end of the array is exposed to sunlight in the penumbra created by the TPS knife edge. The array substrate is a titanium plate with channels running under the cells. Water pumped through this panel carries heat to radiators mounted on the TPS support structure. Figure 11 shows the configuration at perihelion.


Figure 11: Solar array illumination at perihelion (image credit: JHU/APL)

Technology development work on TPS has resulted in several changes to the design. The TPS remains a carbon-carbon and carbon foam sandwich, with a ceramic coating on the Sun-facing surface to control reflectance and emissivity properties. The shape and size of the TPS has changed to optimize mass while considering manufacturability and the need for longer knife edges for illumination control of the solar arrays. In particular, the TPS has shrunk from nearly 3 m in diameter to reflect more efficient packaging of the spacecraft.

The spacecraft in Figure 8 also reflects the new antenna configuration, including a 0.6 m HGA (High Gain Antenna) mounted on the body of the spacecraft instead of a boom. In addition to mass optimization, this change removes the need to deploy and retract the HGA each orbit to protect it from thermal damage at perihelion, thus increasing the reliability of the system.

The design uses a blowdown monopropellant hydrazine system for propulsion, with thrusters for attitude control and trajectory correction. Star Trackers and an internally redundant IRU (Inertial Measurement Unit) are included for guidance and control. The avionics suite is based on the APL IEM (Integrated Electronics Module) and PDU (Power Distribution Unit) used in most APL missions over the last decade or more. The IEM houses the command and data handling processor, solid-state recorder, interface to the guidance and control instruments, and payload interface. The PDU is an internally redundant box that includes all power switching. RIO (Remote I/O) devices are used to collect spacecraft telemetry, and communicate with the avionics suite through serial data links.

Avionics and SpaceWire Network: 21)

• SpaceWire selected over 1553: SpaceWire offers greater bandwidth and lower emissions

• Redundant processor module: (prime, hot spare, warm spare)

• Redundant electronic modules: SSRs (Solid State Recorders) are cross strapped

• Two cross strapped transponders.


Communication Coverage Profile:

Adequate communication links between ground and spacecraft are essential for mission operations. Transmissions of spacecraft operation commands, spacecraft telemetry, science observation sequences, and instrument measurement data between the SPP spacecraft and ground are through the spacecraft Telecomm system and NASA's DSN (Deep Space Network) of tracking stations located at Goldstone in the United States, Canberra in Australia, and Madrid in Spain. Besides the data transmission, navigation of the SPP spacecraft will rely on regular and periodic tracking of the spacecraft through the DSN. The communication coverage of the spacecraft over the mission duration directly affects the spacecraft's operation, science data download, navigation, and the control of the flight trajectory. Due to launch and navigation errors, the flight trajectory needs to be periodically adjusted by applying a TCM (Trajectory Correct Maneuver). Availability of adequate navigation tracking and communication links to the spacecraft dictates the placement of the trajectory correction maneuvers, which has direct impacts on the onboard ΔV budget (Ref. 20).

A comprehensive study of detailed communication coverage over the entire mission was conducted in Phase B across multiple SPP subsystem teams. Because of the unique operation environment of the SPP mission, many factors must be understood in order to maintain adequate communications with the spacecraft. First, the highly elliptical solar orbits across the inner solar system cause frequent solar conjunctions, sometimes with extended periods. And secondly, the spacecraft's TPS obstructs the view of the antenna and causes extra outage of communication times.

The X-band is baselined for spacecraft tracking for navigation and works for both uplink and downlink modes. The Ka-band is mainly for science data downlink and works only for the downlink mode.

Besides the communication outage due to the solar conjunctions attributed to the viewing geometry of Sun, Earth, and the spacecraft, the TPS of the spacecraft sometimes causes additional outage. Because of the extremely high heat radiated from the Sun, the spacecraft bus must be constantly protected from direct solar radiation to prevent overheating. When spacecraft solar distance is less than 0.7 AU, the spacecraft must be oriented with the TPS pointed at the Sun, so that the spacecraft bus and components including the antennas are behind the TPS and are protected inside the TPS umbra. Since the antennas are behind the TPS, some of the radio transmission is obstructed by the TPS. About 14° of the field of view from the center of the TPS is blocked. When the direction of Earth is near the direction of the Sun and within the 14° cone angle about the TPS center, the view from the SPP antenna to Earth is obstructed by the TPS, thereby preventing communication between Earth and the spacecraft.

The survive the extreme solar radiation conditions, the TPS must remain pointed toward the sun at all times. The flight software is required to be capable of controlling attitude within 5 seconds of a processor reset or demotion. The spacecraft has three flight processors (prime, hot spare, and backup spare) to meet this requirement. The tight TPS pointing requirements cause geometric challenges for communications with earth resulting in severely limited communication availability and bandwidth. The SPP spacecraft will use Ka-band downlink transmissions which provide high throughput with CFDP (CCSDS File Delivery Protocol) to return as much data as possible. The SPP spacecraft will use X-band uplink with CFDP to provide efficient guaranteed delivery of commands and save uplink bandwidth when deploying command loads to multiple processors. 22)

Commanding: The SPP flight software reuses heritage code from JHU/APL missions designed to use CCSDS Telecommand packets for commanding. The SPP Ground Software has a database of commands which can create telecommand packets and package them into CLTUs (Command Link Transmission Units). SPP supports the unreliable delivery Expedited Service (BD Service) of the CCSDS Communications Operations Procedure-1 (COP-1) commanding protocol. SPP does not support the reliable Sequence-Controlled Service (AD Service) of COP-1 commanding. The COP-1 AD Service is not well suited for deep space without modification as it provides a limited maximum number of commands without acknowledgment and requires significant retransmission if a single command is dropped.

SPP is a decoupled mission where each SOC (Science Operations Center) can command their instrument as they see fit. Aside from a limited set of calibration activities and earth pointing for communications (when allowed), the spacecraft pointing is fixed at the sun. There is no coordination required between the SOCs and the MOC (Mission Operations Center) to point the spacecraft. The MOC validates that instrument commands are well-formed, targeted to the right destination, and have an APID (Application Identifier) within the assigned instrument APID range prior to allowing transmission to the spacecraft. The MOC does not perform any further validation of instrument commands. The flight software will only send instrument CF contents to the target instrument interface. Instruments can only be commanded via files sent to the MOC by SFTP. These command files are queued and later uplinked to the spacecraft. A separate sequence number will be used for each instrument interface. This guarantees the ordering of files sent to the instrument interface while not impacting sequencing of CDH (Command and Data Handling) or other instrument command files. Due to power constraints, instruments are off during Ka telemetry downlinks, but files can still be uplinked during this period and later streamed to the instrument when it is powered on. Figure 12 highlights the steps involved in sending instrument commands to the target instrument.

The MOC runs a file queue management application that is responsible for initiating the uplink file transfers. The spacecraft CDH and instrument files are all stored in separate queues in this application. Instrument files have an enable time when it is considered acceptable to send them to the spacecraft and a time-out time when an opportunity would have been missed and it no longer makes sense to uplink the file. MOC files are queued in realtime by a contact plan and do not have time out times. Each queue can be enabled for file selection or disabled by the MOC. This application will select the next file by checking for priority files first and then doing a round-robin selection between each enabled command interface with a file that is ready to send.


Figure 12: Instrument command flow (image credit: JHU/APL)

Telemetry: JHU/APL uses the SLE (Space Link Extension)) Return All Frames (RAF) service to receive CCSDS telemetry frames from the spacecraft. Virtual channels are assigned for real-time telemetry, recorded data on SSR (Solid State Recorder), and realtime fill frames. The process of prioritizing and playing back SSR telemetry via CFDP has been used quite successfully on the MESSENGER and Van Allen Probes missions. SSR Housekeeping telemetry is ingested into the MOC telemetry archive. Instrument SSR telemetry files will be provided directly to SOCs with no processing by the MOC.

SPP will record telemetry immediately before a low data rate contact and use CFDP to guarantee delivery of the most critical data during this contact. Figure 13 shows the high level flow of instrument telemetry from creation to the SOC.


Figure 13: Instrument telemetry flow (image credit: JHU/APL)

Frontier Radio on SPP mission: 23)

NASA has selected the Frontier Radio DS (Deep Space) version, developed by JHU/APL, for the communication for the SPP (Solar Probe Plus) mission. The VAP (Van Allen Probes) mission successfully transitioned the Frontier Radio technology to TRL-9 in an S-band duplex configuration for Near-Earth applications (Frontier NE). The successful VAP effort and TRL-6 X/X/Ka-band development efforts provided a deep space Frontier Radio (Frontier DS) with high heritage from the TRL-9 near-Earth unit. The low-SWaP (Size, Weight, and Power) and intrinsically high radiation tolerance of the Frontier Radio DS uniquely qualified it for the SPP application and resulted in the mission baselining this radio. As with VAP for the near-Earth radio, the SPP effort supported the maturation of the deep space radio enhancements, including the necessary compatibility testing with the DSN (Deep Space Network). Flight Frontier Radios for the SPP mission (Figure 14) have completed qualification as of August 2016 and will be integrated into the spacecraft during the remainder of 2016. 24)


Figure 14: A flight Frontier Radio for SPP (image credit: JHU/APL)


Figure 15: Configuration of the Avionics and SpaceWire Network (image credit: JHU/APL)


Development status of the project:

• April 30, 2018: The Parker Solar Probe's Faraday cup, a key sensor aboard the $1.5 billion NASA mission launching this summer, earned its stripes last week by enduring testing in a homemade contraption designed to simulate the sun. 25)

- The cup will scoop up and examine the solar wind as the probe passes closer to the sun than any previous manmade object. Justin Kasper, University of Michigan associate professor of climate and space sciences and engineering, is principal investigator for Parker's SWEAP (Solar Wind Electrons Alphas and Protons) investigation.

- In order to confirm the cup will survive the extreme heat and light of the sun's surface, researchers previously tortured a model of the Faraday cup at temperatures exceeding 1650 ºC, courtesy of the Oak Ridge National Laboratory's Plasma Arc Lamp. The cup, built from refractory metals and sapphire crystal insulators, exceeded expectations.

- But the final test took place last week, in a homemade contraption Kasper and his research team call the Solar Environment Simulator. While being blasted with roughly 10 kilowatts of light on its surface—enough to heat a sheet of metal to 980ºC in seconds—the Faraday cup model ran through its paces, successfully scanning a simulated stream of solar wind.

- "Watching the instrument track the signal from the ion beam as if it was plasma flowing from the sun was a thrilling preview of what we will see with Parker Solar Probe," Kasper said.

- Roilings in the sun's atmosphere can violently fling clouds of plasma into space, known as coronal mass ejections, sometimes directly at Earth. Without precautionary measures, such clouds can set up geomagnetic oscillations around Earth that can trip up satellite electronics, interfere with GPS and radio communications and—at their worst—can create surges of current through power grids that can overload and disrupt the system for extended periods of time, up to months.

- By understanding what makes up the solar corona and what drives the constant outpouring of solar material from the sun, scientists on Earth will be better equipped to interpret the solar activity we see from afar and create a better early-warning system.

- To test the cup model, researchers had to create something new. Their simulator sits in a first-floor lab at the Smithsonian Astrophysical Observatory in Cambridge, MA, and embodies the adage that necessity is the mother of invention.

- It has the look of a makeshift operating room, with a metal frame holding up thick blue tarps around three sides creating a 16 x 8 workspace.

- Inside the area, recreating the sun's heat and light fell to a quartet of modified older model IMAX projectors that Kasper's team purchased on eBay for a few thousand dollars apiece. These are not the digital machines you find in today's Cineplexes, but an earlier generation that utilized bulbs.

- "It turns out a movie theater bulb on an IMAX projector runs at about the same 5,700 degrees Kelvin—the same effective temperature as the surface of the sun," Kasper said. "And it gives off nearly the same spectrum of light as the surface."

- Space offers essentially no atmosphere, meaning a proper testing environment for the Faraday cup would have as little air as possible. So researchers placed the cup in a metal vacuum chamber for testing.

- All four of the IMAX projectors sit atop wheeled tables, and to set up for the test, researchers rolled them into place, with their beams pointed through the vacuum tube window directly at the Faraday cup.

- The final element of the simulator is its ability to generate the kinds of particles the Faraday cup will need to sense and evaluate. To do that, the team attached an ion gun to the vacuum tube hatch, with the "barrel" of the device reaching inside and pointed at the cup.

- In this final test, the Faraday cup took the heat and delivered—putting Parker Solar Probe on track for its summer launch.

- Kelly Korreck, a U-M alumna and astrophysicist at the institute, serves as head of science operations on Parker's SWEAP investigation as well as SWEAP activities for the Smithsonian. "As for the test today, it confirmed what I had suspected—when you take an amazing team of scientists and engineers, give them a complex, difficult, interesting project and the motivation of exploring a region of the universe humankind has never been to, before remarkable things happen," she said.


Figure 16: Researchers use a quartet of IMAX projectors to create the light and heat the Parker Solar Probe cup will experience during its trips through the sun's atmosphere. The cup sits inside a vacuum chambers set up in a lab at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts (image credit: Levi Hutmacher, Michigan Engineering)

• April 6, 2018: NASA's Parker Solar Probe has arrived in Florida to begin final preparations for its launch to the Sun, scheduled for July 31, 2018. 26)


Figure 17: The custom shipping container holding NASA's Parker Solar Probe is prepared for unloading from the C-17 of the United States Air Force's 436th Airlift Wing after landing at Space Coast Regional Airport in Titusville, Florida, on the morning of April 3, 2018 (image credit: NASA/JHU-APL/Ed Whitman)

- In the middle of the night on April 2, the spacecraft was driven from NASA's Goddard Space Flight Center in Greenbelt, Maryland, to nearby Joint Base Andrews in Maryland. From there, it was flown by the United States Air Force's 436th Airlift Wing to Space Coast Regional Airport in Titusville, Florida, where it arrived at 10:40 a.m. EDT. It was then transported a short distance to Astrotech Space Operations, also in Titusville, where it will continue testing, and eventually undergo final assembly and mating to the third stage of the Delta IV Heavy launch vehicle.

- Parker Solar Probe is humanity's first mission to the Sun. After launch, it will orbit directly through the solar atmosphere – the corona – closer to the surface than any human-made object has ever gone. While facing brutal heat and radiation, the mission will reveal fundamental science behind what drives the solar wind, the constant outpouring of material from the Sun that shapes planetary atmospheres and affects space weather near Earth.

- "Parker Solar Probe and the team received a smooth ride from the Air Force C-17 crew from the 436th," said Andy Driesman, Parker Solar Probe project manager from the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland. "This is the second most important flight Parker Solar Probe will make, and we're excited to be safely in Florida and continuing pre-launch work on the spacecraft."

- At Astrotech, Parker Solar Probe was taken to a clean room and removed from its protective shipping container on Wednesday, April 4. The spacecraft then began a series of tests to verify that it had safely made the journey to Florida. For the next several months, the spacecraft will undergo comprehensive testing; just prior to being fueled, one of the most critical elements of the spacecraft, the TPS (Thermal Protection System), or heat shield, will be installed. The TPS is the breakthrough technology that will allow Parker Solar Probe to survive the temperatures in the Sun's corona, just 3.8 million miles from the surface of our star.

- "There are many milestones to come for Parker Solar Probe and the amazing team of men and women who have worked so diligently to make this mission a reality," said Driesman. "The installation of the TPS will be our final major step before encapsulation and integration onto the launch vehicle."

- Parker Solar Probe will be launched from Launch Complex-37 at NASA's Kennedy Space Center, Florida. The two-hour launch window opens at approximately 4 a.m. EDT on July 31, 2018, and is repeated each day (at slightly earlier times) through Aug. 19.

- Throughout its seven-year mission, Parker Solar Probe will explore the Sun's outer atmosphere and make critical observations to answer decades-old questions about the physics of stars. Its data will also be useful in improving forecasts of major eruptions on the Sun and the subsequent space weather events that impact technology on Earth, as well as satellites and astronauts in space. The mission is named for University of Chicago Professor Emeritus Eugene N. Parker, whose profound insights into solar physics and processes have guided the discipline. It is the first NASA mission named for a living individual.

- Parker Solar Probe is part of NASA's Living With a Star Program to explore aspects of the connected Sun-Earth system that directly affect life and society. Living With a Star is managed by the agency's Goddard Space Flight Center in Greenbelt, Maryland, for NASA's Science Mission Directorate in Washington. Johns Hopkins APL designed, built and manages the mission for NASA. Instrument teams are led by researchers from the University of California, Berkeley; the University of Michigan in Ann Arbor; Naval Research Laboratory in Washington, D.C.; Princeton University in New Jersey; and the Smithsonian Astrophysics Observatory in Cambridge, Massachusetts.


Figure 18: NASA's Parker Solar Probe is wheeled into position in a clean room at Astrotech Space Operations (image credit: NASA/JHU-APL/Ed Whitman)

• March 6, 2018: NASA is inviting people around the world to submit their names online to be placed on a microchip aboard NASA's historic Parker Solar Probe mission launching in summer 2018. The mission will travel through the Sun's atmosphere, facing brutal heat and radiation conditions — and your name will go along for the ride. 27)


Figure 19: Eugene Parker, professor emeritus at the University of Chicago, visits the spacecraft that bears his name, NASA's Parker Solar Probe, on Oct. 3, 2017. Engineers in the clean room at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, where the probe was designed and built, point out the instruments that will collect data as the mission travels directly through the Sun's atmosphere (image credit: NASA, JHU/APL)

• On 17 January 2018, NASA's Parker Solar Probe was lowered into the 12 m tall thermal vacuum chamber at NASA/GSFC (Goddard Space Flight Center) in Greenbelt, Maryland. The spacecraft will remain in the chamber for about seven weeks, coming out in mid-March for final tests and packing before heading to Florida. Parker Solar Probe is scheduled to launch from NASA's Kennedy Space Center on July 31, 2018, on a Delta IV Heavy launch vehicle. 28)

- "This is the final major environmental test for the spacecraft, and we're looking forward to this milestone," said Annette Dolbow, Parker Solar Probe's integration and test lead from the Johns Hopkins Applied Physics Lab. "The results we'll get from subjecting the probe to the extreme temperatures and conditions in the chamber, while operating our systems, will let us know that we're ready for the next phase of our mission – and for launch."

- During thermal balance testing, the spacecraft will be cooled to -292 degrees Fahrenheit (-180ºC). Engineers will then gradually raise the spacecraft's temperature to test the thermal control of the probe at various set points and with various power configurations.


Figure 20: NASA's Parker Solar Probe descends into the thermal vacuum chamber at NASA's Goddard Space Flight Center. The spacecraft will be inside the chamber for about seven weeks (image credit: NASA, JHU/APL, Ed Whitman)

• On 6 November 2017, NASA's Parker Solar Probe spacecraft arrived at NASA/GSFC (Goddard Space Flight Center) in Greenbelt, Maryland, for environmental tests. During the spacecraft's stay at Goddard, engineers and technicians will simulate extreme temperatures and other physical stresses that the spacecraft will be subjected to during its historic mission to the Sun. 29)

- Before arriving at Goddard, Parker Solar Probe was at the JHU/APL (Johns Hopkins University /Applied Physics Laboratory) in Laurel, Maryland, where it was designed and built.


Figure 21: Parker Solar Probe arrives at the integration and testing facility at NASA/GSFC in Greenbelt, Maryland (image credit: NASA/JHU/APL, Ed Whitman)

• September 27, 2017: Now less than one year away from launch, the Parker Solar Probe began as an idea in the Outer Planet/Solar Probe program of NASA in the 1990s. 30)

- The original mission concept, the Solar Orbiter, was canceled in 2003 as part of the George W. Bush Administration's restructuring of NASA to focus more on research and development and address management shortcomings in the wake of the 1 February 2003 breakup of the Space Shuttle Columbia that claimed the lives of all seven astronauts aboard. — Six years later, the mission concept was resurrected as a "new mission start" in 2009 with an aim to launch a new solar probe in 2015.

- By 2012, as the mission moved into its design phase, the launch was pushed to 2018.

- Originally called the Solar Probe Plus (SPP), the mission was renamed earlier this year on 31 May 2017, and in so doing NASA radically departed from of its previous mission naming practices.

• On Sept. 21, 2017, the revolutionary heat shield that will protect the first spacecraft to fly directly into the Sun's atmosphere was installed for final integrated vehicle testing ahead of launch. This is the only time the spacecraft will have its thermal protection system—which will reach temperatures of 2,500 degrees F (1370ºC) while at the Sun—attached until just before launch. 31)


Figure 22: On 21 Sept. 2017, engineers at JHU/APL in Laurel, Maryland, lowered the thermal protection system – the heat shield – onto the spacecraft for a test of alignment as part of integration and testing (image credit: NASA/JHUAPL)

• July 14, 2016: Following a successful NASA management review on July 7, the Solar Probe Plus mission — which will send a spacecraft on several daring data-collecting runs through the sun's atmosphere — is moving into the system assembly, integration, test and launch stage of the project. NASA terms this period as Phase D, during which the mission team will finish building the spacecraft, install its science instruments, test it under simulated launch and space conditions, and launch it. 32)


Figure 23: Engineers at JHU/APL in Laurel, Maryland, prepare the developing Solar Probe Plus spacecraft for thermal vacuum tests that simulate conditions in space. Today, the spacecraft includes the primary structure and its propulsion system; still to be installed over the next several months are critical systems such as power, communications and thermal protection, as well as science instruments. The probe is scheduled for launch in summer 2018 (image credit: NASA, JHU/APL)

• April 8, 2015: NASA's SPP (Solar Probe Plus) mission reached a major milestone in March when it successfully completed its CDR (Critical Design Review). An independent NASA review board met at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland, from March 16 to 20 to review all aspects of the mission plan; APL has designed and will build and operate the spacecraft for NASA. The CDR certifies that the Solar Probe Plus mission design is at an advanced stage and that fabrication, assembly, integration and testing of the many elements of the mission may proceed. 33)

• In March 2014, Solar Probe Plus will begin advanced design, development and testing — a step NASA designates as Phase C — following a successful design review in which an independent assessment board deemed that the mission team, led by JHU/APL ( Johns Hopkins University/Applied Physics Laboratory) in Laurel, MD, was ready to move ahead with full-scale spacecraft fabrication, assembly, integration and testing. 34)

Launch: A launch of the Parker Solar Probe spacecraft is targeted for 31 July 2018 (at the opening of a 20-day launch period) from SLC-37B (Space Launch Complex -7 B) of the Cape Canaveral Air Force Station, Florida. The launch vehicle is a Delta-4 Heavy rocket of ULA (United Launch Alliance), augmented by Orbital ATK's Star-48 solid motor as a third stage, in order to cope with the extremely high energy required for this flagship mission. 35) 36)

Orbit: The trajectory able to send the spacecraft within 10 RS (Solar Radii) of the Sun center is a Venus-Venus-Venus-Venus-Venus-Venus-Venus-Gravity-Assist (V7GA) trajectory, a unique trajectory developed to enable the Parker Solar Probe mission without a Jupiter gravity assist. Even with the most powerful launch vehicle and upper stage, a spacecraft cannot get close to the Sun from Earth directly. Extra energy must be shed off the spacecraft's orbit to further reduce its heliocentric orbital velocity in order to encounter the Sun under 10 RS. The V7GA trajectory allows for the spacecraft to reduce the necessary orbital speed via multiple Venus gravity assists.

The amount of required orbital speed reduction required at aphelion is too large to come from one or two Venus flybys. Attaining the aphelion orbital speed reduction will require seven Venus flybys. Following each Venus flyby, the orbital speed at aphelion will decrease, resulting in a smaller orbit with a shorter perihelion distance. After seven Venus flybys, orbit perihelion distance will gradually decrease to 9.86 RS, the minimum solar distance required for the baseline mission. Throughout the mission there are no additional deep space maneuvers; all the orbit changes as well as the phasing (Venus-to-Venus transfer location and timing) between each Venus flyby are achieved through the control of the Venus flybys by appropriate selection of the Venus flyby target parameters. To minimize the mission duration, both resonant and non-resonant Venus flybys are utilized in this trajectory design (Ref. 20).


Figure 24: Overview of the V7GA mission trajectory (image credit: JHU/APL)

The SPP mission is comprised of 24 highly elliptical, heliocentric orbits with decreasing orbital periods from 168 days for orbit 1, settling into an 88 day orbit period midway through the mission. Each orbit is broken into two distinct periods, the Solar Encounter period and the Cruise/Downlink period. Figure 25 highlights the primary characteristics of each period (Ref.22) .


Figure 25: SPP Orbital Operations Concept (image credit: JHU/APL)

Note: The extreme difficulty of getting close to the Sun — as first an enormous amount of velocity must be canceled out from the Earth orbital velocity in order for a probe to leave Earth's orbit and head towards the Sun — then the tremendous gravitational acceleration towards the Sun has to be partially offset with the seven Venusian (decelerating) ‘slingshots'.