A tenuous analogy
Keith S. Taber
The Parker Solar Probe
I recently listened to NASA's Nicky Fox being interviewed about the Parker Solar Probe which (as the name suggests) is being used to investigate the Sun.
There is a website for the project which, when I accessed it (28th December 2021), suggested the spacecraft was 109 279 068 km from the Sun's surface (which I must admit would have got a marginal comment on one of my own student's work along the lines "is the Sun's surface so distinctly positioned that this level of precision can be justified?") and travelling at 57 292 kph (kilometers per hour). This unrealistic precision derives from the details being based on "mission performance modeling [sic] and simulation and not real-time data…" Real-time data is not necessarily available to the project team itself – the kind of shielding needed to protect the spacecraft from such extreme conditions also creates a challenge in transmitting data back to earth.
But the serious point is that returning to the website at another time it is possible to see how the probe's speed and position have changed (as shown on 'the Mission' webpage – indeed by the time I took the 'screenshot' it had moved about 7000 km), as the spacecraft moves through a sequence of loops in space orbiting the Sun on a shifting elliptical path that takes it periodically very close (very close, in solar system terms, that is) to the sun. Like any orbiting body, the probe will be moving faster when closest to the sun and slowest when furthest from the sun. (The balance shifts between its kinetic and potential energy – as it works to move away against the sun's gravity when receding from it 1.)
Touching the Sun
Getting too close the Sun – with its high temperature, the 'solar wind' of charged particles emitted into space, occasional solar flares, and the high flux of radiation from across the electromagnetic spectrum – is very dangerous, making the design and engineering of any craft intended to investigate our local star up close very challenging. A key feature is a protective heat shield facing the Sun . This was the premise of the sci-fi film 'Sunshine' 2.
For the Parker probe
"the spacecraft and instruments will be protected from the Sun's heat by a …11.43 cm carbon-composite shield, which will need to withstand temperatures outside the spacecraft that reach nearly …1,377 degrees Celsius"
"At closest approach to the Sun, while the front of Parker Solar Probe' solar shield faces temperatures approaching … 1,400° Celsius, the spacecraft's payload will be near room temperature, at about [29˚C.]."
http://parkersolarprobe.jhuapl.edu
Dr Fox, who is Director of NASA's Heliophysics [physics of the Sun] Division, was being interviewed about data released from an earlier close approach on a BBC Science in Action podcast.
"The Parker Solar probe continues its mission of flying closer and closer to the sun. Results just published show what the data the probe picked up when it dipped into the surrounding plasma. NASA's Nicky Fox is our guide."
Item on BBC Science in Action
The project is framing that event as when, "For the first time in history, a spacecraft has touched the Sun". Although the visible surface of the sun has a temperature of about 6000K (incredibly hot by human standards), the temperature of the 'atmosphere' or corona around it is believed to reach several million Kelvins. On the programme, Dr Fox was asked about how the spacecraft could survive in the sun's corona, given its extremely high temperatures.
A teaching analogy?
In response she used an analogy from everyday experience:
"We talk about the plasma being at a couple of million degrees, it's like putting your hand inside an oven, and you don't touch anything. You won't burn your hand, you'll feel some heat but you won't actually burn your hand, and so the solar wind itself, or the corona, is a very tenuous plasma, there are just not that many particles there. So, even though the whole atmosphere is at about two million degrees, the number of particles that are coming into contact with the spacecraft are [sic] very small.
The temperatures that we have to deal with are about fourteen, fifteen hundred degrees Celsius, at the maximum, which is still hot, don't…let me kid you, that's still hot, but it is not two million degrees."
Dr Fox interviewed on Science in Action
Analogies are commonly used in science, science communication and science education as one means of 'making the unfamiliar familiar' by showing how something novel or surprising is actually like something the audience is already aware of and comfortable with.
Read about making the unfamiliar familiar
If the probe had been dipped in a molten vat of some hypothetical refractory liquid at two million degrees it would have quickly been destroyed. But because the Corona is not only a plasma (an 'ionised gas')3, but a very tenuous one, this does not happen. NASA sending the probe into the corona is similar to putting one's hand in the oven when cooking. If you touch the metal around the outside you will burn yourself, but you are able to reach inside without damage as long as you do not touch the sides – as although the air in the oven can get as hot as the metal structure, it has a very low particle density compared with a solid metal. So, your hand is in a hot place, but is not in contact with much of the hot material.
Do not try this at home – at least not unless you are quick
Of course, this is not the whole story. You can reach in the oven to put something in or (with suitable protection) take something out, but you cannot safely leave your hand in there for any length of time.
When two objects at different temperature are placed in contact, heating will occur with 'heat' passing from the hotter to colder object until they are in thermal equilibrium (i.e., at the same temperature). But this is not instantaneous – it takes time.4 If the Parker Solar Probe had been flown into the Sun's atmosphere and left there it would have been heated till it eventually matched the ambient temperature (not 'just' 1400˚C) regardless of how effective a heat shield it had been given. Or rather, it would have been heated till its substance reached the ambient temperature, as it would have lost structural integrity long before this point.
Of course, the probe has been designed to spend some time in the coronal atmosphere collecting data, but to only dip in for short visits, as NASA is well aware that it would not be wise to leave one's hand in the oven for too long.
Note:
1 This at least is the description based on Newtonian physics. There is an attractive, gravitational force between the Sun and the probe. As the spacecraft moves towards the sun it accelerates, and then its momentum takes it away, being decelerated by gravity.In this model gravity is a force between two bodies. (The path is actually more complex than this, as it has been designed to fly past Venus several times to adjust its trajectory round the Sun.)
In the model offered by general relativity the probe simply moves in a straight line through space which has a complex geometry due to the presence of matter/energy: a straight line which seems to us to be a shifting series of ellipses. Gravity here is best understood as a distortion from a 'flat' space. Perhaps it is clear why for most purposes scientists stick with the Newtonian description even though it is no longer the account considered to best describe nature.
2 The movie poster gives a slight clue to the hazards involved in taking a manned mission to the Sun!
3 Plasma is considered a fourth state of matter: solid, liquid, gas, plasma. The expression that 'a plasma is an ionised gas' may suggest plasma is a kind of gas, but then we might also say that a gas is a boiled liquid or that a liquid is melted solid! So, perhaps what we should say is that a plasma [gas/liquid] is what you get when you ionise [boil/melt] a gas [liquid/solid].
4 In theory, modelling of such a process suggests it takes an infinite time for this to occur. 5 In practice, the temperatures become close enough that for practical purposes we consider thermal equilibration to have occurred.
5 This is an example of a process that can be understood as having a negative feedback cycle: temperature difference drives the heat flow, which reduces temperature difference, which therefore also reduces the driver for heat flow; so the rate of heat flow is reduced, so therefore the rate of temperature change is reduced… This is a similar pattern to radioactive decay – both follow an 'exponential decay' law.