Nasa’s Parker Solar Probe: set the controls for the heart of the Sun
Image credit: Nasa/ESA
Nasa engineers are working on the first‑ever mission to make a close approach to the Sun. We look at the technologies behind the Parker Solar Probe, due to be launched this summer.
When Pink Floyd’s Roger Waters wrote ‘Set the Controls for the Heart of the Sun’ in the late 1960s, he could not have predicted Nasa’s latest attempt to push the boundaries of space exploration. The Parker Solar Probe (formerly Solar Probe Plus) will be launched to the Sun in summer 2018 and placed in orbit within 3.9 million miles (about 6.2 million km) of the surface. This is just 4 per cent of the distance between the Earth and the Sun and much closer than any previous spacecraft.
The engineering challenges of diving into the Sun’s outer corona (which, incidentally, is hotter than the surface) are immense. One only has to think how hot it can get on Earth, 150 million km from the source of the heat. The probe will experience a solar intensity more than 500 times that of a spacecraft in Earth orbit – which puts the pressure firmly on the thermal design engineers.
In common with all spacecraft, the Parker Solar Probe (PSP) contains electronics and other components designed, manufactured and tested in typical cleanroom environments. The role of the thermal engineer is to create an environment within the spacecraft such that science instruments and other hardware experience something close to the classical ‘room temperature’ of physics experiments here on Earth (about 20°C). Thus most spacecraft designs incorporate reflective surfaces to reject solar heating and radiator panels to reject excess heat generated by electronic and microwave components. On the other hand, to maintain a stable internal temperature and counteract radiation from the shadowed side of the spacecraft, the main body is heavily insulated and incorporates strategically placed heaters to avoid cold spots.
‘This is an extremely exciting, but also scary mission.’
As PSP approaches the Sun, it will begin to experience excessively high heat loads that require something extra: a specialised thermal protection system (TPS), more commonly known as a heat shield. According to Jim Kinnison, mission system engineer for PSP at the Johns Hopkins Applied Physics Laboratory, the TPS comprises “an 11cm-thick, 3m-across irregular disc made of carbon foam”. The foam is bonded at the top and bottom to thin graphite-composite sheets, he explains, and then annealed so that “the resulting article is pure carbon”.
The TPS is bolted on to a supporting structure that is, in turn, attached to the spacecraft, forming a shadow zone that protects the spacecraft platform. “The sunward face of the TPS is covered with a white ceramic coating developed for this mission to reflect as much of the solar illumination as possible back into space,” adds Kinnison, “but it will still reach a temperature of 1,377°C during closest approach to the Sun.” Thermal models predict that, despite this high external temperature, the spacecraft platform will only reach “about 30°C”.
Power for the probe – as with most other spacecraft – is derived from the Sun using solar arrays. Usually, proximity to the Sun represents an advantage for power generation efficiency, but most of PSP’s array has to withdraw behind the shield for closest approach, leaving a smaller, actively-cooled section in the sunlight.
Among the few items that peer out from behind the shield are a number of hardy science instruments (electric field antennas and a small plasma detector known as a Faraday cup). Kinnison confirms that these instruments are “designed to operate in the full solar illumination at closest approach, and have been fully tested to demonstrate this, [while] other instruments living in the shadow of the TPS do not need line-of-sight to the Sun to make measurements”.
Another challenge is communications, because the Sun is a powerful broadband-microwave noise source. Satellite Earth station operators are familiar with the ‘sun outages’ experienced when satellites pass in front of the Sun, but the issues are magnified for a probe designed to be close to the Sun for most of its mission.
Kinnison explains that the spacecraft is designed to be highly autonomous, so that it can make science measurements without ground contact. “The autonomous nature of the mission was a significant challenge, both in design and test,” he says, “but the spacecraft is also highly fault-tolerant, so if operations are interrupted it will detect the fault, recover from it and continue with science operations.”
The core of the fault management system is an avionics system built around three mutually-redundant single-board computers – one prime and two in ‘hot standby’ mode – with individual solid-state recorders. Data is stored on an SSR and downlinked when the spacecraft orbits further away from the Sun.
What about that data? What is PSP designed to do?
PSP is part of Nasa’s ‘Living with a star’ programme, designed to explore aspects of the Sun-Earth system that affect life here on Earth. The mission is managed by Nasa’s Goddard Space Flight Center (GSFC) in Greenbelt, Maryland, and implemented by Johns Hopkins APL as prime contractor. The Parker Solar Probe is the first Nasa spacecraft to be named after a living individual.
The mission is designed to improve our knowledge of how the Sun’s activity affects the Earth’s immediate space environment and so improve forecasts of ‘space weather’ (solar flares and related phenomena), which can detrimentally affect aspects of modern life from satellite transmissions to the reliability of terrestrial power grids.
Science payloads are provided by teams, based at University of California-Berkeley, University of Michigan, Princeton University, and the Naval Research Laboratory, respectively:
FIELDS – Measures the electric and magnetic fields and EM waves at the spacecraft using four electric field antennas that extend beyond the heat shield, a fifth electric sensor and three magnetometers (of two different types) that extend off the back of the spacecraft in the shadow of the heat shield.
SWEAP (Solar Wind Electrons, Alphas, and Protons) – Measures the number, velocity and temperature of the three most abundant parts (e-, He++, p) of the local plasma (solar wind) using three sensors: two behind the heat shield and one that extends beyond the shield to measure ions and electrons directly.
ISIS (Integrated Science Investigation of the Sun; pronounced ‘E-sis’.) – Measures high-energy ions and electrons (solar energetic particles) that move at 1 per cent to 10 per cent of the speed of light using two energetic particle instruments, one for low-energy and one for high-energy particles.
WISPR (Wide-field Imager for Solar Probe) – Provides images of the solar corona in visible light and of the solar wind and other structures using two telescopes that observe to the side of the spacecraft in the direction of movement. Similar to coronagraphs in that observations are limited to dimmer parts of the corona.
PSP’s launch window opens on 31 July and closes 19 August 2018. Its route to the Sun is less straightforward than one might imagine, requiring no fewer than seven Venus gravity-assist flybys over a nominal seven-year design life to gradually reduce its orbital radius and thereby distance from the Sun.
The spacecraft carries four instrument suites designed to study electric and magnetic fields, plasma, energetic particles and to image the solar wind. At closest approach, the instruments will have a good view of the Sun, whose disc will subtend an angle of about 12.5° compared with about 0.5° at Earth (a direct result of it being 25 times closer).
Eric Christian, a deputy principal investigator for one of the instrument suites based at Nasa-Goddard, is under no illusions regarding the challenge. “This is an extremely exciting, but also scary mission,” he says. “Not only do we have to protect the spacecraft from a Sun that is 600 times brighter than it is at Earth, we also have to protect ourselves from dust,” which at the high velocities involved “can do a lot of damage.” He also acknowledges the difficulty in communicating with the spacecraft when close to the Sun, frustrating because it is “the most important part of the orbit for data collection”.
The main scientific questions include why the corona is so much hotter than the photosphere, how the solar wind is accelerated and where the high-energy particles originate from. Some 60 years of space-based study has failed to answer these and other questions to the satisfaction of solar scientists, which is why they need to get as close to the Sun as possible – the raison d’etre for PSP.
Christian considers the three questions of coronal heating, solar wind acceleration and solar energetic particles to be not only “missing pieces to our basic understanding of the Sun”, but also “some of the most important questions in space science”. The need for answers dates back to the dawn of the Space Age, he says.“Only now has the technology caught up to the desire.”
The signatures of the various mechanisms are “completely smeared out by the time the plasma reaches Earth”, he explains, but PSP will be “right where the action is”, able to observe and measure events and interactions as and where they occur.
The astronomical scale is so far removed from human-related distances measured in miles and metres that most people find it difficult to understand just how far away the Sun is. NASA has attempted to explain the scale issue by producing a circular card, 22cm in diameter, printed with a photo of the solar surface on both sides. The face containing the standard ‘brochure material’ includes a tiny push-out panel containing the Earth to the same scale as the 22cm Sun (it’s about 2mm across). If this little Earth is held 23 metres from the Sun-card, it represents the scale layout of the Sun-Earth system; at this scale PSP will orbit at about 90cm from the card.
The European Space Agency’s Dr Yannis Zouganelis agrees on the importance of PSP to the scientific community. In his opinion, “PSP is an exploratory mission that will revolutionise our understanding of the Sun, the various manifestations of space plasma physics processes and their underlying mechanisms”.
Zouganelis is deputy project scientist for ESA’s own Solar Orbiter spacecraft, which is part of the agency’s Cosmic Vision 2015-25 programme and is scheduled for launch in 2020. Like PSP, it will close in on the Sun using gravity-assist manoeuvres, but will operate from a highly elliptical, high-inclination orbit and approach within about 45 million km, some seven times further away than PSP. Although Solar Orbiter is not quite the Sun-diver that PSP is, Zouganelis characterises the probes as “highly complementary”. Many members of the Solar Orbiter instrument teams are also members of the PSP team, he says, and “are currently studying the possibility of coordinated observations”.
Interestingly, Solar Orbiter is not the first spacecraft to view the Sun from this position: in 1976, the Helios 2 spacecraft approached to within 43 million km of the Sun to measure the properties of the interplanetary space plasma. However, as Zouganelis points out, “the Helios spacecraft should be compared to PSP rather than to Solar Orbiter,” because they had similar missions.
Solar Orbiter, by contrast, has “many novelties that were not on Helios 2 and will not be on PSP either”, explains Zouganelis. For a start, it carries a full set of imagers covering “all relevant wavelengths of the electromagnetic spectrum”, which, he adds, is why it can’t go much closer to the Sun. In addition, for part of its mission, Solar Orbiter will orbit above the same region of the solar atmosphere, allowing relatively long-term data gathering. “If you observe from Earth, the Sun is rotating too fast, so you cannot observe the same region for a long time,” says Zouganelis, and the opposite applies if a spacecraft goes too close because it passes over the region “too quickly”. Solar Orbiter, he argues, observes from “an optimal distance”.
Yet “probably the biggest novelty”, according to Zouganelis, is that Solar Orbiter will eventually observe from an inclination of some 32° relative to the ecliptic plane (in which the planets orbit the Sun). “This will allow the first-ever images of the Sun’s poles, which many researchers believe hold the key to understanding what drives the constant activity and eruptions on the Sun,” he says.
Ever since the Greek myth of Icarus, writers of science-fiction novels, progressive rock songs and space mission plans have been fascinated by the concept of flying close to the Sun. Now engineers have found a way to do just that.
Parker Solar Probe
ESA Solar Orbiter
Who is Parker?
The Nasa mission to the Sun has been named after Dr Eugene N Parker, a solar physicist who – in the 1950s – developed a number of theories for how stars radiate energy. In addition to naming the ‘cascade of energy’ emitted by the Sun, the solar wind, he proposed the concept that the superheated corona was due to ‘nanoflares’, which in sufficient number could cause the heating. Designers of the probe that bears his name are hoping that this foray into space may be able to confirm the theory.