I’m not a physicist, or anything else scientific, I’m just an ageing sponge, trying to suck up knowledge and understandings in the diminishing time I have left. Physics is just one vast web of knowledge that I’ve barely stepped upon, to mix metaphors, but that won’t stop me trying to make some sense of orbital mechanics in this post, with the help of the Skeptics’ Guide to the Universe (episode 826), and other sources.
The NASA Parker Solar Probe (PSB) is the fastest human-built object, and also holds the record for closeness to the sun. It was launched in August 2018 and weighs around 73 kgs. Named for Eugene Parker – a multi-award-winning solar astrophysicist who worked out the effects of the solar wind and predicted the spiral shape of the solar magnetic field in the outer solar system – the PSB recently (only a month ago) got to within about ten million kilometres of the sun’s service. The next closest artificial object was the Helios spacecraft, in 1976, at a distance of about 43 million kms. Mercury, which has a highly elliptical orbit, only gets as close as 47 million kms at perihelion.
The PSB is part of NASA’s Living With a Star (LWS) program, which investigates the Sun-Earth system as it affects our sun-dependent and sometimes sun-threatened lives. For example, the SGU references the Carrington Event, the largest geomagnetic storm on record, caused by a solar coronal mass ejection hitting the Earth’s magnetosphere in early September 1859. If such an event occurred today, it would cause massive damage to our electrical grid systems and satellites. So the PSB is designed to study the sun’s corona and solar wind, presumably in the hope of providing an early warning of future events. For this purpose it’s loaded with various forms of detecting and measuring instruments. However, my interest here is in trying to understand how the probe gets from Earth to the Sun’s corona, how it’s expected to reach speeds of up to 690,000 km/h, and how it can withstand the temperatures in the corona.
It has apparently been calculated that it takes 55 times the energy to get to the Sun as it takes to get to Mars. This is all about orbital mechanics – the sun is spectacularly massive, making up 99.8% of the mass of the solar system. That means it also has a spectacularly massive gravitational pull on the Earth, and all other orbiting bodies. It’s the Earth’s ‘sideways’ velocity (107,208 km/h) that keeps it from falling into the sun. So the Earth’s orbital velocity needs to be taken into account – cancelled out – in planning a trip to the Sun. It turns out that it’s inordinately difficult to do so. With current technology they have only managed to cancel out about 80% of this velocity – which will bring the PSB close to the Sun but not close enough. Travelling to the outer planets is much easier. The probe would leave Earth at 40,000 km/h (escape velocity) and would require a relatively slight boost (6-7,000 km/h) to reach Mars, and further small boosts to reach the other outer planets.
The solution to this cancellation problem is to employ an orbital manipulation called Venus Earth Gravity Assist (VEGA). The PSB was sent to Venus to reduce the sideways orbital motion. Every swing around Venus further reduces this motion, and allows the PSB to decrease the orbital perihelion ultimately to about 7 million kms, at which time it will be travelling at its maximum speed. This will occur at Christmas Eve 2024 (they can be quite precise, apparently), after the last of its planned seven swings around Venus. The probe’s orbit around the Sun will be highly elliptical, with a minimum of time spent around perihelion, to prevent radiation damage to the craft and instrumentation.
Of course the PSB will also come equipped with probably the most sophisticated heat shield or thermal protection system ever built, which will protect it not only from the intense heat and radiation but from high-velocity dust particles. It measures about 2.5 metres in diameter and is made from carbon foam between layers of superheated carbon-carbon composite, aka reinforced carbon-carbon (carbon fibre in a matrix of graphite). Its outer aluminium oxide coating is, naturally, reflective white, to protect probe and equipment from a maximum temperature at perihelion of about 1370 degrees celsius. NASA expects that the inner side of the shield will be at a little under 30 degrees – so cool in fact that some instruments will be independently heated to operate at maximum efficiency. The probe has been created to be as autonomous as possible, given its distance from Earth. For example, if instrumentation somehow becomes exposed to radiation, four light sensors will ‘detect the first traces of direct sunlight coming from the shield limits and [engage] movements from reaction wheels to reposition the spacecraft within the shadow again’, to quote the Wikipedia article on the probe.
This solar probe concept was first mooted in the late 1950s but was regularly postponed due to costs. The initial idea was for a less direct route using a gravity assist from Jupiter, which would have created a longer and more expensive mission and would have required a nuclear battery called a radioisotope thermal generator. Something to research in another post maybe.
So I won’t pretend that I understand all the mathematics of this probe’s voyage, but I do know that it has been successful so far, at least in terms of its travel – the Venus assists and the solar orbits, which will all come to an end on August 29 2025. As to whether it will be successful in its research tasks, that will have to be evaluated over time. What precisely are those research tasks? There are three main ones: to trace the flow of energy that heats the corona and accelerates the solar wind, to determine the structure and dynamics of the magnetic fields that create the solar wind, and to determine what mechanisms accelerate and transport energetic particles.
Whether the knowledge gained will protect us from future solar wind and electromagnetic activity nobody knows. Predictions about the future are probably the most uncertain predictions of all.