Space Sunday: to touch the face of the Sun

Ignition! The three main stages of the Delta 4 Heavy fire, starting the Parker Solar Probe on its mission to examine the Sun up close and personal. Credit: NASA

On the morning of Sunday, August 12th, 2018, NASA launched the Parker Solar mission, which it describes as being “to touch the face of the Sun”. It will be the first mission to fly through the Sun’s corona – the hazardous region of intense heat and solar radiation in the Sun’s atmosphere that is visible during an eclipse, and it will gather data that could help answer questions about solar physics that have puzzled scientists for decades. Over the course of its initial 7-years the Parker Solar Probe mission will allow us to better understand the fundamental processes going on in, on, and around the Sun, improving our understanding how our solar system’s star influences, affects and changes the space environment, through which we travel as the Earth orbits the Sun.

The probe and mission are named for Dr Eugene Parker, an American solar astrophysicist, who in 1958 first posited  the theory of the supersonic solar wind, and who also predicted the Parker spiral shape of the solar magnetic field in the outer solar system. Now 91, he was present at NASA’s Kennedy Space Centre as a distinguished guest of the agency, to witness the probe’s launch, the mission (and vehicle) being the first in NASA’s history to be named after a still-living person.

The Delta 4 Heavy carrying the Parker Solar Probe sits on the pad of Space Launch Complex (SLC) 37 at Canaveral Air Force Station, Florida, following the aborted launch attempt of Saturday, August 11th, 2018. Credit: Vikash Mahadeo / SpaceFlight Insider

Lift-off came at 03:31 EDT (6:31 GMT / 7:31 BST) on Sunday, August 12th, after the initial launch attempt was scrubbed on Saturday, August 11th, when a troubled countdown was halted just one-minute, 55 seconds before the engines on the United Launch Alliance (ULA) Delta 4 Heavy rocket were to ignite. The halt was called following a gaseous helium red pressure alarm, and investigations into its cause extended beyond the 65-minute launch window, resulting in the launch scrub.

The Sunday morning launch countdown proceeded without any significant hitches, and the Delta 4 Heavy – the most powerful rocket in ULA’s fleet of launch vehicles, comprising 3 Delta 4 first stages strapped side-by-side, the outer two functioning as “strap-on boosters” – lit up the Florida coastline as it took to the early morning skies.

Although a flight to the Sun might sound an easier proposition than reaching the outer solar system, it actually isn’t; it actually requires 55 times more launch energy than a launch to Mars. Hence why the relative small and light Parker Solar Probe, weighing just 685 kg (1,510 lb) at launch, required the massive Delta 4 and a rarely-used Star 48BV variant of the Payload Assist Module (PAM).

Originally developed as the upper stage for Delta 2 launch vehicles in the 1965, the Star family of solid-fuel PAM units were commonly used with the space shuttle for satellite launches from orbit: the shuttle would carry them aloft, release the PAM / Satellite combination, then move to a safe distance before the PAM motor was ignited to push the satellite on to its require Earth orbit. For the Parker Solar Probe, the Star 48BV was used to impart as much velocity as possible into the vehicle at is starts on it journey.

Dr. Eugene Parker, now 91, watches the launch of the probe named in his honour as it lifts-off from SLC-37, Sunday, August 12th, 2018. Credit: NASA / Glenn Benson

What makes a flight to the Sun so hard is that the Earth is moving “sideways” relative to the Sun at about 107,000 km/h (67,000 mph), and the probe has to cancel out a whopping 84,800 km/h (53,000 mph) of that “sideways” motion as it makes its way to the Sun in order to achieve orbit. At the same time, the probe needs to gain velocity as it moves in towards the centre of the solar system in order for it to balance the Sun’s enormous gravitational influence and achieve the required elliptical orbit.

The use of the Delta 4 / Star 48BV combination got both of these requirements started, by pushing the probe towards Venus in an arc that will both start to shed the “sideways” velocity, whilst also accelerating the craft in towards the Sun. But it will be Venus that does the real grunt work for the mission.

On October 1st, 2018, the probe will make the first of a series of flybys of Venus, where it will use the Venusian gravity to shed still more of the angular velocity imparted by Earth’s orbit and increase its velocity towards the Sun.

In all, seven such fly-bys of Venus will occur  over the 7 year primary mission for the probe, and while only the first is required to shunt the vehicle into its core heliocentric orbit, the remaining six play an important role in both maintaining the vehicle’s average velocity across the span of the mission and in gradually shrinking its elliptical orbit around the Sun as the mission progresses.

The first pass around the Sun  – and the start of the science mission – will occur in November / December 2018. At perihelion, the vehicle will be just 6.2 million km (3.85 million mi) from the Sun’s photosphere (what we might call its “surface”). During this time, the vehicle will be well within the corona, and will also temporarily become the fastest human-made vehicle ever made, achieving a velocity of around 700,000 km/h (430,000 mph) – that’s 200 km per second (120 mi/s), or the equivalent of travelling between London and Tokyo in around 50 seconds! At aphelion – the point furthest from the Sun, and brushing Earth’s orbit, the craft will be travelling a lot slower.

The corona is a very hot place – hotter than the “surface” of the Sun, however, it is also comparatively thin as far as an “atmosphere” goes. The distance at which Parker Solar Probe will be travelling from the Sun at perihelion, combined with its speed, mean that the ambient heat of the corona isn’t a significant issue. Direct sunlight radiating out from the Sun, however, is a significant problem.

An artist’s impression of the Parker Solar Probe passing over the Sun’s photosphere. Credit: NASA / JHU/APL

To protect it against the extreme direct heating it will experience each perihelion, Parker Solar Probe will rely on a solar shadow-shield, designed to perform three tasks: absorb and reflect sunlight away from the vehicle; prevent radiation penetrating its instrument bay and burning-out its circuits and instruments (incident solar radiation at perihelion is approximately 650 kW/m2, or 475 times the intensity at low Earth orbit); and casting a long shadow behind it, in which the rest of the vehicle can remain relatively “cool”.

The Sun-facing shield is thus a hexagonal unit 2.3 m (7.5 ft) across made of  reinforced carbon–carbon composite 11.4 cm (4.5 in) thick with an outer face is covered in a white reflective alumina surface layer to minimise heat absorption. It is so effective that at perihelion the Sunward face will be heated to around 1,370oC (2,500oF), but behind it and within its shadow, the probe will remain at a relatively “balmy” 30oC (85oF). Also, the probe’s two solar panels designed to power it, are on hinged “wings”, allowing them to be folded back into the shadow cast by the shield, ensuring they are also protected during perihelion.

Conceptual art showing the solar panel configurations for Parker Solar Probe. On the left, the panels are full stored for launch. In the right they are fully deployed for power generation during the “cruise” phase of the probe’s orbit around the Sun. In the centre they are shown in the “folded” configuration used when the vehicle is passing close to the Sun, and designed to keep them within the shadow cast by the probe’s solar shadow shield. Credit: NASA / JHU/APL

The probe is also the most autonomously capable vehicle ever launched. Conditions around the Sun can change rapidly, and given the 16-minute time delay in two-way communications, the probe cannot rely on orders for Earth to protect itself. Inside, it will have to think and react for itself in response to changes within the corona that might threaten it.

It will take between 188 and 170 days for the mission to complete an orbit around the Sun, for a total of 24 such loops during the initial 7-year mission period. The majority of each orbit will be a “cruise” phase, the craft moving either away from or back towards the Sun. The core Sun science period will last, on average, around 11 days of each orbit, as the vehicle approaches, passes through and departs perihelion.

Built by the Applied Physics Laboratory at John Hopkins University, Parker Solar Probe has a long history. A mission to the Sun – then simply called the Solar Orbiter (not to be confused with the 2019 European Space Agency mission of that name) – was first proposed in the 1990s, and can be considered the direct progenitor of the Parker mission. It was one of three major missions – the others being the Pluto Kuiper Express and the Europa Orbiter – intended to by the centrepieces of NASA’s Outer Planet/Solar Probe (OPSP) programme for the early part of the 21st century.

However, OPSP was cancelled in 2003 under the Bush Administration. The Pluto Kuiper Express idea (by then superseded by the New Horizons mission), ultimately survived that cancellation, while the Solar Orbiter and Europa Orbiter missions went into a long period of gestation / reformulation. The Solar Orbiter mission eventually re-emerged as the Solar Probe Plus in 2010, having gained funding under the Obama Administration (2009), with the vehicle Christened the Parker Solar Probe in May 2017 – which saw the mission as a whole renamed to match.

Parker Solar Probe being assembled at the Applied Physics Laboratory of John Hopkins University. Cresit: JHU / APL

The primary goals of the mission are:

  • Trace the flow of energy that heats the corona and accelerates the solar wind.
  • Determine the structure and dynamics of the magnetic fields at the sources of solar wind.
  • Determine what mechanisms accelerate and transport energetic particles.

To achieve this, the vehicle carries a suite of scientific instruments and experiments:

  • Electromagnetic Fields Investigation (FIELDS) / flux-gate magnetometer: designed to make direct measurements of electric and magnetic fields, radio waves, Poynting flux, absolute plasma density, and electron temperature.
  • Integrated Science Investigation of the Sun (ISIS): two instruments, EPI-Hi and EPI-Lo, intended to measure energetic Electrons, Protons and heavy Ions emitted from the sun.
  • Wide-field Imager for Solar PRobe (WISPR): the optical element of the mission, designed to capture views of the corona and inner heliosphere.
  • Solar Wind Electrons Alphas and Protons (SWEAP): two instruments (an electrostatic analyser and a Faraday cup intended to count the electrons, protons and helium ions, and measure their properties such as velocity, density, and temperature. Its main instruments are two.
  • Heliospheric Origins with Solar Probe Plus (HeliOSPP) a special theory and modeling investigation designed to maximise the scientific return from the mission.

Perseids 2018

Every July / August, the Earth passes through a haze of stellar debris left by Comet 109P/Swift-Tuttle. The result is of this passage is the Perseid meteor shower (called this because they appear to originate from the constellation of Perseus), is one of the brightest meteor displays one can see in the northern hemisphere. The shower tends to last around a month, from July 17th (ish) through to August 24th, this year rising to a peak on the 12th-13th August, before slowly fading.

The Perseids and where to see them in the night sky

At the peak of the shower, between 60 and 100 meteors can be seen streaking across the sky. In Europe, the best time to see them is after midnight, while America gets it a little easier and earlier. To check time in your location try, which should give local observation times.

Of course, you need to be somewhere that’s dark enough to see the night sky without it being blotted too heavily by surrounding Earthly light pollution,  but the Perseids should be visible to anyone with a good, dark view of the night sky.

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