Sunday, 3 May 2015

Yeah Baby, She's got it - Observing Venus

Apologies for the corny title but that was the song stuck in my head as I was writing this post.
Venus at the moment is the brightest object in the evening sky, unmistakable in its dazzling luminosity, so it seems an appropriate subject for another astronomically themed post.

Soviet Artist's impression - Venera 7 approaches Venus

As you might expect, Venus has played a hugely important role in man's understanding of the universe and our place within it. If it dazzles today, how much brighter must the planet have seemed in the inky Persian night skies over Isfahan in the 11th Century? From a convenient rooftop, the great polymath Ibn Sina, imagined below, known in the West as Avicenna and best known for his medical treatises, considered the works of Ptolemy and pondered the heavens anew, no doubt stroking his luxuriant beard as he did so. It was in the light of day however, that Avicenna made his most important discovery when in 1032 he is believed to have observed a transit of Venus across the disc of the sun. He correctly concluded that the planet must lay in between the Earth and the sun, albeit within a framework that still had the Earth at the centre of the universe.

It would take the invention of the telescope for observation of Venus to cast further light on our understanding of the universe. During his remarkable year of observations in 1610, Galileo viewed Venus and noted that at different times it exhibited different phases just like the moon, appearing sometimes full and round, sometimes gibbous and sometimes as a crescent. He concluded that for this phenomenon to be observed,Venus must circle the sun. Galileo asserted as much in a series of letters mostly concerned with sun spots written in 1613 to the Austrian banker Marc Welser; the wealthy patron of his rival Apelles, declaring that the observed phases of Venus provided proof of Copernicus' theory that the planets circle the sun.

In 1627 Galileo's great contemporary Johannes Kepler had concluded his monumental work the Rudolphine Tables; a catalogue of the stars and predicted movements of the planets, named in honour of Holy Roman Emperor Rudolph II. The tables' calculations incorporated Kepler's key discovery that the planets moved in elliptical orbits, and this ensured that they were far more accurate than anything that had gone before. The Rudolphine Tables correctly predicted a transit of Venus in 1631, though Kepler did not live to see it.

Galileo's sketch of the phases of Venus

Throughout the 1630's the Rudolphine tables were carefully studied and painstakingly improved upon by two English astronomers; William Crabtree and Jeremiah Horrocks, who corresponded regularly on their observations and findings. Theirs was a remarkable collaboration, though it is not believed that they ever met. Manchester merchant Crabtree's meticulous observations allowed watchmaker's apprentice and Cambridge drop-out Horrocks to predict that another transit of Venus would occur in 1639. It was Horrocks who established that transits of Venus occurred eight years apart in a 243 year repeating cycle. On 24th November 1639 (by the Julian calendar) Crabtree and Horrocks independently observed the transit, projecting an image of the sun through their telescopes onto paper and making sketches which allowed Horrocks to make estimates as to the size of Venus and its distance from the earth and by extrapolation the distance from the earth to the sun. Horrocks died two years later aged just 22. Many of his papers were subsequently lost, but over twenty years after his death his treatise on the transit of Venus was obtained and published by German astronomer and corresponding fellow of the Royal society Johannes Hevelius. It was a sensation.

Jeremiah Horrocks observes the 1639 transit

In 1716 Edmund Halley wrote a passionate appeal for future scientists to build upon Horrocks' work when the next transits came around in 1761 and 1769, calling for an international effort to observe the transit from various points on the globe and thereby obtain a more accurate estimate of Earth-Venus and Earth - Sun distances using parallax and trigonometry. A good explanation of how this is done is available here. So the scene was set for far-flung expeditions by European astronomers to all corners of the earth to gather as much data from the two transits as possible.

Unfortunately when the time came for the 1761 transit, war was raging across the globe between England and France, making these expeditions even more difficult and challenging. French astronomer Guillaume le Gentil was thwarted from making his observations from dry land when his intended landfall of Pondicherry fell to the British and was forced to try to observe from the pitching deck of the ship. Undeterred, he determined to wait for the next transit in eight years’ time. He spent the intervening years fruitfully, travelling the Indian Ocean on mapmaking expeditions, carrying out astronomical observations and recording details of flora and fauna. When the time came around for the next transit, Le Gentil returned to Pondicherry only to be thwarted once more by storm clouds. When at last he returned to France he found that he had been given up for dead and his relatives were squabbling over his estate. He has gone down as the unluckiest man in the history of astronomy.

Chappe d'Auteroche terrifies the locals

Another French expedition led by Jean Baptiste Chappe d'Auteroche trekked across the vastness of Russia to observe the transit from Tobolsk in Siberia, where he received a rough reception from the superstitious peasants who were terrified by his equipment and bizarre electrical experiments. He was suspected of interfering with the weather and causing flooding and had to be protected by a hired band of mercenary Cossacks. Despite the challenges he managed to obtain excellent observations. For the 1769 transit, Chappe d'Auteroche led an expedition to Baja California, where the entire expedition was struck down with fever and Chappe d'Auteroche himself perished whilst caring for the sick. Only one member of the expedition survived to bring the observations back to France.

Equally eventful were the adventures of Charles Mason and Jeremiah Dixon who are best known for establishing the boundary between the colonies of Pennsylvania and Maryland from 1765 to 1768, drawing the cultural dividing line that bears their name. In 1761 astronomer Mason and surveyor Dixon set out aboard HMS Seahorse, a frigate of 24 guns, bound for Sumatra. Their mission was to observe the transit of Venus. On the way however, Seahorse encountered French frigate Le Grand and the two ships fought a close action, leaving 11 dead and 38 wounded aboard the Seahorse including the captain. The appearance of a second frigate HMS Unicorn saw the French ship turn tail. Seahorse was too badly damaged to continue and returned to port to refit. By the time she set out again, there was insufficient time to make it to Sumatra in time to observe the transit and so Mason and Dixon were put ashore in Cape Town, from where they successfully observed the transit.
Their partnership ended after the completion of the Mason-Dixon line survey as they were sent to different destinations to observe the 1769 transit, Mason to Ireland, Dixon to Norway.

Mikail Lomonosov

Future Astronomer Royal Neville Maskelyne had a less eventful journey to St Helena to observe the 1761 transit, using the voyage to also put the latest theories on the calculation of longitude to the test. Meanwhile in St Petersburg, Mikail Lomonosov detected a ring of refracted light around the planet as it made first contact, concluding that Venus must possess an atmosphere. Despite the University of St Petersburg distributing 200 copies of his paper around Europe, it took 200 years for his discovery to be credited.

Most famously of all James Cook sailed to Tahiti aboard the Endeavour to observe the 1769 transit, accompanied by botanist Joseph Banks and astronomer Charles Green. Despite the generally friendly reception from the native population, a fortification known as Fort Venus had to be constructed in order to protect the observation equipment from looting. Nevertheless, as Cook recorded in his diary all their efforts were in vain as an enterprising thief still managed to steal their prized astronomical quadrant, which had to be tracked down. Nevertheless as Banks noted, the natives were fascinated by the proceedings and some joined the Europeans to watch the transit. Despite perfect conditions, efforts were hampered by the 'black drop effect' which made it difficult to discern the precise moments of first and second contact and the associated timings which were crucial to the whole enterprise. The gentlemen of the Royal Society were most displeased. Nevertheless when all the observations were combined the figure established for the earth-sun distance or 1 astronomical unit (AU) was within 0.8% of  the modern figure.

Joseph Banks in Tahiti

As they stared at a tiny black dot moving across the projected surface of the sun, none of these intrepid observers can surely have conceived of the possibility of a spacecraft being sent from Earth to explore Venus itself, yet within 200 years of their grand enterprise, that was precisely what occurred.

Both the United States and Soviet Union began missions to Venus in the 1960's and it would be the Soviet programme that would enjoy the greatest success. In March 1960 the US launched Pioneer 5; the first spacecraft to carry a digital telemetry system into interplanetary space. Pioneer 5 had originally been intended to reach Venus but the mission had been scaled back to an interplanetary fact finding expedition; investigating radiation and magnetic fields in deep space. The probe maintained contact until it was 36 million km from Earth before falling into a heliocentric orbit.
The following February the Soviet Union launched Venera 1. This too had been scaled down from a Venus landing to a fly by mission. In the end failure of the communication and stabilisation systems prevented this but the probe got to within 100,000 km of Venus before falling into a heliocentric orbit. It also sent back data on radiation and interplanetary magnetic fields.

Mariner 2
1962 saw a reversal of fortunes with all 3 Soviet attempts failing to escape Earth orbit whilst the US Mariner 2 made it to within 35,000 km of Venus and took measurements of the planet's magnetic field and estimated surface temperatures. Further Soviet efforts throughout 1964 met with little success although the probe known as Zond 1 came within 100,000 km of Venus before losing contact. Finally in November 1965 Venera 2 and 3 were launched. Venera 2 was a fly-by mission which passed within 24,000 km of Venus but was unable to transmit its measurements back to earth. Venera 3 successfully deployed its landing module but again communications were lost although it became the first man-made object to make it to the surface of another planet.
1967 saw both nations achieve their greatest successes thus far in the exploration of Venus. In June the Soviets launched Venera 4; a major redesign conducted by the Lavochkin aircraft company. One key improvement was in the cooling of the instrumentation, using the antennae as a thermal radiator. This allowed the craft to keep transmitting all the way to Venus. Venera 4 would release a landing module 1m across - the round section on the bottom of the probe shown below. Venera 4 successfully transmitted as it descended through the Venusian atmosphere, allowing Soviet scientists to discover that it comprised 95% carbon dioxide. The probe registered a temperature of 271 degrees centigrade and a pressure of 18 atmospheres until transmissions stopped shortly before it hit the surface. Later that same year the US Mariner 5 carried out a 4000km fly by of the planet, measuring the radio refractivity of its atmosphere.

Venera 4

The following year, despite Cold War tensions, US and Soviet Scientists held a conference to compare notes, both acknowledging that they had yet to detect conditions on the planet's surface. The Soviets kept plugging away, launching Venera 5 & 6 in 1969 with both successfully deploying their landing modules but both modules being destroyed before reaching the surface. Casings were cracked open by the extreme pressure and parachutes incinerated by the high temperatures. Nevertheless, more atmospheric data was collected before contact was lost 18km above the surface.

The Venera 7 mission of 1970 was designed to make it all the way to the surface and survive. Soviet scientists by now expected pressures of up to 100 atmospheres and temperatures in excess of 500 degrees Celsius and the titanium hulled Venera 7 descent module was built to withstand pressures and temperatures well in excess of these. The parachute was reduced in size and made from glass nitron fibre capable of withstanding the 700km/h winds in the middle atmosphere of Venus and the 500 degrees Celsius temperatures as it approached the surface. The parachute could be reefed to allow a quicker descent through the Venusian atmosphere in the initial stages before being fully deployed at a lower altitude. If the descent was too long then the 100 hour battery life could be exhausted before the lander reached the surface.

Venera 7 lander
On December 15th 1970 Venera 7 successfully landed on Venus with communications still intact.  It was the first spacecraft to send a signal back from the surface of another planet. Its parachute had been torn during the descent and it hit the ground at 38mph, coming to rest on its side with its antenna not facing towards Earth. With only 3% of its transmitting power available, the lander still managed to transmit for 23 minutes, its signal barely discernible from the background static. The probe returned temperature readings of 474 degrees Celsius and pressure readings of 90 atmospheres.

The Venera missions continued until 1984, each carrying more sophisticated equipment and returning more details of the Venusian atmosphere and surface. In 1975 Venera 9 sent back the first photograph of the surface of Venus. The US Space programme had focused more on Mars and of course the enormous effort of manned missions to the moon. Their Pioneer Venus 2 mission succeeded in landing a small probe on the surface in 1978 which survived for 50 minutes. Of course, for all the successful missions, just getting there and surviving at all was a tremendous accomplishment.

Galileo on Venus
Observing the transits in 1639, 1761, 1769
The Venera missions
More Slings and Arrows posts on astronomy and space


1 comment:

  1. The link to observing the transits in 1639, 1761 & 1769 has now moved to: - ATRamsey.