Space diary FIERY FALL by Kamal Lodaya In this Space diary, we talk about a spacecraft launched by the United States last year, called the {Parker solar probe}. The idea behind ths probe is equally simple: to get into the Sun's {corona}, the white-hot outer atmosphere that we see during a total solar eclipse. Box: Annular solar eclipse this year [Pics by Shankar of earlier ASE at Kanyakumari, Kamal's webpage] This has to be filled in. I don't have the info. Niruj? TVV? From www.timeanddate.com: Annulus means "ring". An annular solar eclipse happens when the Moon covers the Sun's center, leaving the Sun's visible outer edges to form a “ring of fire” or annulus around the Moon. Solar eclipses happen when the Moon casts a shadow on Earth. Annular solar eclipsey can only take place when it is new moon. At the same time, the Earth, the Moon, and the Sun are aligned in a straight (or nearly straight) line. This does not happen all the time since the orbits are tilted a little with respect to each other. In addition, the Moon is not in a circular orbit around Earth, but it is squashed into an elliptic orbit. So the distance to Earth varies. When the Moon is near its farthest point from Earth, called apogee, it cannot fully cover the Sun. The outer edge of the Sun then remains visible as a ring of sunlight. If the Moon were closer, it would fully cover the Sun and we would get a total solar eclipse. Annular eclipses can last over 3 hours in locations where annularity is visible. The annularity, when only a ring of fire is visible in the sky, can range from less than a second to over 12 minutes. Look out for the annular solar eclipse that will be visible from India on the morning of Dec 26, 2019. (Can you tell why the eclipse will be in the morning?) The regions where you can see at least partial annularity will be over south India. In places such as Coimbatore you will see full annularity. End Box The Sun's interior is extremely hot, millions of degrees, so hot that hydrogen atoms fuse into helium. This is the ultimate source of solar energy. The Sun's surface, which we see every day, is about 6000 degrees Celsius. It is so hot that if we look at it with some magnification for some time, we will eventually lose our eyesight. This is what happened to the 16th century Italian scientist Galileo Galilei with his primitive telescope. The corona spreads far outside from the Sun's surface. Its temperature reaches around 20 lakh degrees C. So, travelling from the centre of the Sun outwards, its temperature decreases to thousands of degrees at the surface, and then going out further, increases to lakhs of degrees. Then it decreases out into space, for example the temperature of the Moon's surface (150 million kilometres from the Sun) is only a few hundred degrees. The Earth's surface is a little cooler because its atmosphere shields the planet. [Pic: SDO picture of Sun, eg, en.wikipedia.org/wiki/Corona#/media/File:Arcing_Active_Region.jpg] Why is the corona hotter than the surface? You couldn't have asked a simpler question. We don't know. This is one of the great mysteries of astronomy. From satellites which are placed in space going around the Sun, we can see that there are very strong magnetic forces which connect the Sun to its corona. Charged particles, called plasma, are transported from the Sun to the corona in solar flares and by {coronal streamers}. From the corona they emanate outwards as the solar {wind}. This was first described by American scientist Eugene Parker. He also connected the solar wind to the ion tail formed behind a comet as it goes around the Sun. The Parker solar probe is the first spacecraft named after a living person, he is in his nineties. [Pic: Eugene Parker] Somehow these magnetic forces and travelling particles build up the temperature of the corona. How? Again, a simple question, we don't know the answer. When he was 60, in 1987, Parker suggested a mechanism of very small {nanoflares} happening on the Sun's surface, which we cannot see, which take particles into the corona. It is now theorized that these might transmit "blobs" of plasma every 90 minutes. Can we get into the corona and see? That is the job of the Parker solar probe. Engineering sungrazer comets What happens if you, or your spacecraft, gets heated to lakhs of degrees? Both will vaporize. So how do we get into the corona? We learn from comets A few years ago comet Ison went behind the Sun and disintegrated, all that came out was dust. The picture shows comet Lovejoy W3 emerging after it passed very close to the Sun in December 2011. [Pic: comet Lovejoy W3 coming out from behind the Sun, 15 Dec 2011, attached] Like most other sungrazing comets, Comet Lovejoy was not expected to survive its close encounter with the Sun. But it did. This image in the picture was taken from a coronograph on the SOHO spacecraft which keeps staring at the Sun. You can see the remnants of the tail, with the brilliant head or coma emerging from the solar glare on December 16, 2011. The Sun's position has been "blackened" out to block the glare and is indicated by the white circle. Separated from its tail, Comet Lovejoy's coma is so bright it saturates the camera's pixels creating the horizontal streaks. However, the comet disintegrated soon after. In 1965, comet Ikeya-Seki went very close to the Sun and came back out again, displaying a splendid tail of dust and ions knocked out from its nucleus. The picture shows a painting by David Nicholls from Canberra, Australia. The nucleus did not disintegrate. [Pic: Ikeya-Seki, www.home.aone.net.au/~byzantium/comet.jpg] What is the secret of their survival? Here is the sungrazer comet's trick. If you go to a place which is lakhs of degrees, and you {stay} there, you will raapidly be heated up to lakhs of degrees, of course disintegrating long before that. But if you whiz through the corona at very high speed, travelling kilometres per second, the corona is so sparse that you may not get that heated up, you may escape. The comet's elliptical orbit keeps it falling closer and closer to the Sun, and its orbital speed brings it back out. This is the trick adopted by the Parker solar probe. It first got to the planet Venus. By using its gravity, it got shot into a highly elliptical comet-like path taking it very close to the Sun. On November 6 last year, it reached within 2 crore km of the Sun, travelling at 95 kilometres per second. It came back out unhurt! Over the next few months it has sent back the data it collected. On April 4 this year it did its second graze of the Sun. The third perihelion was on September 1st. The data collected will soon be analysed and released. Over the next six years, the spacecraft will keep using Venus to make its orbit shorter and faster, and its perihelion closer and closer to the Sun. In December 2024 it will reach within 7 million kilometres of the Sun, travelling at 192 kilometres per second. For a comparison, comet Ikeya-Seki was just half a million kilometres from the Sun. [Pic: en.wikipedia.org/wiki/Parker_Solar_Probe#/media/File:Parker_Solar_Probe_coronal_stream_wispr-big_1-st_flyby.jpg] NASA/NRL/Parker Solar Probe - http://parkersolarprobe.jhuapl.edu/News-Center/Show-Article.php?articleID=115 This image from Parker Solar Probe's WISPR (Wide-field Imager for Solar Probe) instrument shows a coronal streamer, seen over the east limb of the Sun on Nov. 8, 2018, at 1:12 a.m. EST. Coronal streamers are structures of solar material within the Sun's atmosphere, the corona, that usually overlie regions of increased solar activity. The fine structure of the streamer is very clear, with at least two rays visible. Parker Solar Probe was about 27 million kilometers from the Sun's surface when this image was taken. (In contrast, our Earth is about 150 million km away from the Sun). The bright object near the center of the image is Mercury, and the dark spots are a result of background correction. The mission engineers have designed the probe to survive nearly 30 Sun encounters. The key is a reinforced carbon composite solar shield which can withstand about 1300 degrees C. It is calculated that this is the effective temperature the probe will face as it whizzes through the corona. The instruments are behind the shield facing away from the Sun (otherwise they would get destroyed). The picture shows a coronal streamer image from the first encounter. The bright dot is the planet Mercury, which never gets closer than 46 million kilometres from the Sun. The dark dots are due to the imaging process. It may not seem very exciting to you but for astronomers this is among the first ever pictures from inside the corona. The spacecraft gets measurements of the magnetic field, coronal plasma and solar wind speed. We have to wait and see whether PSP survives all its encounters. Meanwhile solar astronomers are eager to see the data from the second encounter.