Category: Venus

Venus Express atop a Soyuz rocket. Image credit: ESA. Click to enlarge.
The European spacecraft Venus Express has been successfully placed into a trajectory that will take it on its journey from Earth towards its destination of the planet Venus, which it will reach next April.

A virtual twin sister of the Mars Express spacecraft which has been orbiting the Red Planet since December 2003, Venus Express is the second planet-bound probe to be launched by the European Space Agency.

Venus Express will eventually manoeuvre itself into orbit around Venus in order to perform a detailed study of the structure, chemistry and dynamics of the planet’s atmosphere, which is characterised by extremely high temperatures, very high atmospheric pressure, a huge ‘greenhouse effect’ and as-yet inexplicable ‘super-rotation’ which means that it speeds around the planet in just four days.

The European spacecraft will also be the first orbiter to probe the planet’s surface while exploiting the ‘visibility windows’ recently discovered in the infrared waveband.

The 1240 kg mass spacecraft was developed for ESA by a European industrial team led by EADS Astrium with 25 main contractors spread across 14 countries. It lifted off on board a Soyuz- Fregat rocket, the launch service being provided by Starsem.

The lift-off from the Baikonur Cosmodrome in Kazahkstan this morning took place at 09:33 local time (04:33 Central European Time).

Initial Fregat upper-stage ignition took place nine minutes into the flight, manoeuvring the spacecraft into a low-earth parking orbit. A second firing, one hour and 22 minutes later, boosted the spacecraft to pursue its interplanetary trajectory.

Contact with Venus Express was established by ESA’s European Space Operations Centre (ESOC) at Darmstadt, Germany approximately two hours after lift-off. The spacecraft has correctly oriented itself in relation to the sun and has deployed its solar arrays.

All on-board systems are operating perfectly and the orbiter is communicating with the Earth via its low-gain antenna. In three days’ time, it will establish communications using its high-gain antenna.

Full speed ahead for Venus
Venus Express is currently distancing itself from Earth at full speed, heading on its five-month, 350 million kilometre journey inside our Solar System. After check-outs to ensure that its on-board equipment and instrument payload are in proper working order, the spacecraft will be ‘mothballed’, with contact with Earth being reduced to once daily. If needed, trajectory correction manoeuvres can go ahead at the half-way stage in January.

When making its closest approach, Venus Express will face far tougher conditions than those encountered by Mars Express on nearing the Red Planet. For while Venus’s size is indeed similar to that of Earth, its mass is 7.6 times that of Mars, with gravitational attraction to match.

To resist this greater gravitational pull, the spacecraft will have to ignite its main engine for 53 minutes in order to achieve 1.3 km/second deceleration and place itself into a highly elliptical orbit around the planet. Most of its 570 kg of propellant will be used for this manoeuvre.

A second engine firing will be necessary in order to reach final operational orbit: a polar elliptical orbit with 12-hour crossings. This will enable the probe to make approaches to within 250 km of the planet’s surface and withdraw to distances of up to 66 000 km, so as to carry out close-up observations and also get an overall perspective.

Exploring other planets to better understand planet Earth
“The launch of Venus Express is a further illustration of Europe’s determination to study the various bodies in our solar system,” stressed Professor David Southwood, the Director of ESA’s science programmes.

“We started in 2003 with the launch of Mars Express to the Red Planet and SMART-1 to the Moon and both these missions have amply exceeded our expectations. Venus Express marks a further step forward, with a view to eventually rounding off our initial overview of our immediate planetary neighbours with the BepiColombo mission to Mercury to be launched in 2013.”

“With Venus Express, we fully intend to demonstrate yet again that studying the planets is of vital importance for life here on Earth,” said Jean Jacques Dordain, ESA Director General.

“To understand climate change on Earth and all the contributing factors, we cannot make do with solely observing our own planet. We need to decipher the mechanics of the planetary atmosphere in general terms. With Mars Express, we are studying the Martian atmosphere. With Huygens, we have explored that of Saturn’s satellite Titan.

“And now with Venus Express, we are going to add a further specimen to our collection. Originally, Venus and the Earth must have been very similar planets. So we really do need to understand why and how they eventually diverged to the point that one became a cradle for life while the other developed into a hostile environment.”

The Venus Express mission is planned to last at least two Venusian days (486 Earth days) and may be extended, depending on the spacecraft’s operational state of health.

Twin sister of Mars Express
Venus Express largely re-uses the architecture developed for Mars Express. This has reduced manufacturing cycles and halved the mission cost, while still targeting the same scientific goals. Finally approved in late 2002, Venus Express was thereby developed fast, indeed in record time, to be ready for its 2005 launch window.

However, Venusian environmental conditions are very different to those encountered around Mars. Solar flux is four times higher and it has been necessary to adapt the spacecraft design to this hotter environment, notably by entirely redesigning the thermal insulation.

Whereas Mars Express sought to retain heat to enable its electronics to function properly, Venus Express will in contrast be aiming for maximum heat dissipation in order to stay cool.

The solar arrays on Venus Express have been completely redesigned. They are shorter and are interspersed with aluminium strips to help reject some solar flux to protect the spacecraft from temperatures topping 250ºC.

It has even been necessary to protect the rear of the solar arrays – which normally remain in shadow – in order to counter heat from solar radiation reflected by the planet’s atmosphere.

An atmosphere of mystery
Following on from the twenty or so American and Soviet missions to the planet carried out since 1962, Venus Express will endeavour to answer many of the questions raised by previous missions but so far left unanswered.

It will focus on the characteristics of the atmosphere, its circulation, structure and composition in relation to altitude, and its interactions with the planet’s surface and with the solar wind at altitude.

To perform these studies, it has seven instruments on board: three are flight-spare units of instruments already flown on Mars Express, two are from comet-chaser Rosetta and two were designed specifically for this mission.

The PFS high-resolution spectrometer will measure atmospheric temperature and composition at varying altitudes. It will also measure surface temperature and search for signs of current volcanic activity.

The SPICAV/SOIR infrared and ultraviolet spectrometer and the VeRa instrument will also probe the atmosphere, observing stellar occultation and detecting radio signals; the former will in particular seek to detect molecules of water, oxygen and sulphuric compounds thought to be present in the atmosphere.

The VIRTIS spectrometer will map the various layers of the atmosphere and conduct multi-wavelength cloud observation in order to provide images of atmospheric dynamics.

Assisted by a magnetometer, the ASPERA 4 instrument will analyse interaction between the upper atmosphere and the solar wind in the absence of magnetospheric protection such as that surrounding Earth (for Venus had no magnetic field). It will analyse the plasma generated by such interaction, while the magnetometer will study the magnetic field generated by the plasma.

The VMC camera will monitor the planet in four wavelengths, notably exploiting one of the ‘infrared windows’ revealed in 1990 by the Galileo spacecraft (when flying by Venus en route for Jupiter), making it possible to penetrate cloud cover through to the surface. The camera will also be used to monitor atmospheric dynamics, notably to observe the double atmospheric vortex at the poles, the origin of which still remains a mystery.

Original Source: ESO News Release

Venus Express on top of its launcher. Image credit: ESA. Click to enlarge.
Following the announcement of the Venus Express launch delay due to particulate contamination found in the launcher fairing where the spacecraft was installed, ESA staff and industry teams have started an inspection of the spacecraft. This recovery ‘investigation procedure’ has so far revealed a spacecraft in good status.

Having been removed from the Soyuz rocket, the upper composite, consisting of the Venus Express spacecraft attached to the Fregat upper stage and all housed in the rocket fairing, was transported to the Baikonur cosmodrome’s Upper Composite Integration Facility in the early morning of Sunday 23 October. On Monday 24 October the fairing was removed and engineers started the inspection to assess the status of the spacecraft.

The scenario is so far very encouraging, as only fairly large particles, pieces of the insulating material initially covering the launcher’s Fregat upper stage, have been found on the body of the spacecraft. These have been easy to identify by naked eye or with UV lamps, and are being carefully removed with tweezers, vacuum-cleaners or nitrogen gas airbrushes, according to size.

In the next couple of days the inspections and cleaning of Venus Express will continue, focussing on the instrument optics and apertures. After this step, Venus Express will be ready for the electric tests, routine checks that precede the final cleaning done just before the encapsulation with the fairing. The upper composite will then be complete again and will be ready for re-integration with the launcher.

ESA and Starsem, the company responsible for the Soyuz-Fregat launcher, are merging the results of their parallel investigations and recovery measures to define a new launch date in the shortest time frame. The ESA Project team is confident that Venus Express will be launched well within the launch window, which closes on 24 November this year.

Original Source: ESA News Release

Scientists using measurements from NASA’s Solar Radiation and Climate Experiment (SORCE) satellite have discovered that Venus and sunspots have something in common: they both block some of the sun’s energy going to Earth.

Using data from NASA’s SORCE satellite, scientists noticed that, when Venus came between the Earth and the sun on June 8, the other planet reduced the amount of sunlight reaching Earth by 0.1 percent. This Venus transit occurs when, from an earthly perspective, Venus crosses in front of the sun. When it happens, once every 122 years, there are two transits eight years apart. The next crossing happens in 2012 and will be visible to people on the U.S. West Coast.

“Because of its distance from Earth, Venus appeared to be about the size of a sunspot,” said Gary Rottman, SORCE Principal Investigator and a scientist at the Laboratory for Atmospheric and Space Physics (LASP), at the University of Colorado at Boulder. The SORCE team had seen similar reductions in the sun’s energy coming Earthward during the October 2003 sunspot activity.

In October 2003 the Earth-bound sunlight dimmed 0.3 percent for about four days, due to three very large sunspot groups moving across the face of the sun.

“This is an unprecedented large decrease in the amount of sunlight, and it is comparable to the decrease that scientists estimate occurred in the seventeenth century,” Rottman said. That decrease lasted almost 50 years, and was likely associated with the exceptionally cold temperatures throughout Europe at that time, a period from the 1400s to the 1700s known as the “little ice age.”

Solar conditions during the little ice age were quite different, as there were essentially no sunspots. Astronomers of the time, like Galileo, kept a good record of sunspot activity before and during the period, encountering only about 50 sunspots in 30 years.

Rottman said, “Something very different was happening during the seventeenth century, and it produced a much more permanent change in the sun’s energy output at that time.” Today, the large sunspots are surrounded by bright areas called “faculae.” Faculae more than compensate for the decrease in sunlight from sunspots, and provide a net increase in sunlight when averaged over a few weeks.

The large number of sunspots occurring in October/November 2003 indicated a very active sun, and indeed many very large solar flares occurred at that time. SORCE observed the massive record-setting solar flares in x-rays. The flares were accompanied by large sunspots, which produced a 0.3 percent decrease in the sun’s energy output. SORCE simultaneously collected the energy from all wavelengths, something that had never been done before.

“The SORCE satellite instruments provide measurements of unprecedented accuracy, so the sun’s energy output is known with great precision, and precise knowledge of variations in the sun’s energy input to Earth is a necessary prerequisite to understanding Earth’s changing climate,” said Robert F. Cahalan, SORCE Project Scientist and Head of the Climate and Radiation Branch at NASA’s Goddard Space Flight Center, Greenbelt, Md.

The SORCE measurements provide today’s atmospheric and climate scientists with essential information on the sun’s energy input to the Earth. These measurements also will be valuable to future scientists, who will be relating their view of the world back to conditions existing today. Likewise Galileo’s findings about the sun almost 400 years ago have increased in value as understanding of the sun and its importance for Earth has advanced.

For more SORCE information and images on the Internet, visit:

hthttp://www.gsfc.nasa.gov/topstory/2004/0730sunblockers.html
and
http://lasp.colorado.edu/sorce/

Original Source: NASA News Release

If you’re lucky, you’ve got a front row view of Venus as it transits across the face of the Sun. That means you’re in Europe, Africa or Asia, you’ve got the proper equipment to filter the Sun, and the weather is cooperating. If you’re like me, you lack all three. Don’t worry, though, the Internet is coming to our rescue.

Astronomers and spacecraft are going to be watching the show and broadcasting what they see in real time so anyone with an Internet connection can stay tuned as the transit progresses.

For starters, educate yourself about the transit, including safety tips and the locations of groups viewing it live. There’s great information from ASTRONET, the European Southern Observatory and NASA.

Next, tune into some spacecraft. You can see the view from SOHO, TRACE, and IMAGE. Finally, settle in with a ground-based observatory. Here’s a complete list of more than 100 observatories broadcasting from Astronet.

The show begins at 0513 UTC (aka Greenwich Mean Time). That’s the same as 1:13 am EDT or 10:13 pm PDT (June 7). The whole transit will take about 6 hours to complete.

Let me know how the transit goes for you. Did you make a special trip, or just look from your backyard? Or, like me, do you have to watch it through the Internet?

And send in your pictures, either of Venus, or of you and your friends out in the sunshine, enjoying the show. 🙂

Good luck!

Fraser Cain
Publisher
Universe Today

Image credit: NASA
Want to see the 2004 Transit of Venus? Be prepared to wake up early.

On Tuesday morning, June 8th, for the first time since 1882, Venus will pass directly between Earth and the Sun. For six hours the planet’s black silhouette will crawl across the face of our star. It might be a pretty sight, or not. No one can say for sure because no one alive today has seen a transit of Venus.

If you live near the east coast of North America, you can see the crossing. The transit will be underway at dawn and visible for as much as two hours after sunrise.

Before you read the rest of this story, a reminder: Never look at the Sun without eye protection. The early morning Sun rising through the mist, reddened and dimmed by distant clouds, is so tempting; it seems safe to stare. Don’t! Even a low-hanging Sun can cause eye damage. Proper transit-watching techniques are discussed below.

The transit begins at 1:13 a.m. EDT (in the middle of the night) and ends at 7:26 a.m. EDT. If you can see the Sun before 7:26 a.m. EDT, then you can see the transit. Sky watchers east of the Mississippi River are favored. The transit will not be visible at all from Mexico, British Columbia and the following US states: Arizona, California, Colorado, Idaho, Nevada, New Mexico, Montana, Oregon, Texas, Utah, Washington, Wyoming.

A transit of Venus isn’t like a solar eclipse. The Sun won’t be blotted out or even noticeably dimmed. Venus is too small; the disk of the planet covers only 0.1% of the Sun.

Although Venus is tiny, you can probably see it without magnification. Try looking through a safe solar filter, for example, #13 or #14 welder’s glass or special “eclipse glasses” designed for solar viewing. (Do NOT use stacked sunglasses, metallized candy wrappers or compact disks; these are unsafe filters often recommended in error.) Seen through a good filter, the Sun looks like a glowing disk, about the size of the Moon, marked with a black speck–Venus.

The view is much better through a telescope. But beware: sunlight focused through a telescope can blind you instantly. There are two ways to to safely observe using a telescope:

Solar projection is one way. Align your telescope with the Sun. Do not look through the telescope or its finder scope; use shadows on the ground to effect the alignment. The shadow of a telescope looks skinniest when it is pointing directly at the Sun. Once the Sun is in the field of view, an image will shoot out of the eyepiece. Hold a white screen behind your ‘scope and, voila: a picture of the Sun. Adjust the focus of the telescope (or the distance between the eyepiece and the screen) until Venus looks crisp and round.

Solar filters are another way. Capping your telescope with a suitable sun-filter can reduce the intensity of sunlight to safe levels. Then you can look right through the eyepiece. If you’re not sure what filter is safe, contact the vendor of your telescope to ask for advice.

In addition to Venus, you’ll likely see one or two sunspots. These are planet-sized islands of magnetism floating on the Sun’s surface. Compare the two: Venus’ silhouette is dark and round like a planet. Sunspots are not so dark; and they are delightfully irregular.

A moment of special interest is “third contact” at 7:07 a.m. EDT. This is when Venus’ silhouette touches the Sun’s limb and starts its 20-minute egress from the solar disk. Third contact is the beginning of the end of the transit.

Moments before third contact, watch out for the infamous “black drop effect.” The black of space beyond the Sun’s limb will seem to reach in and touch Venus, merging with the planet to form an elongated black drop. You can recreate the black drop effect with your thumb and index finger: Hold the two in front of one eye and narrow the distance between them. Just before they touch, a shadowy bridge will spring across the gap. According to John Westfall, writing for Sky & Telescope magazine in June 2004, “this is simply the result of how two fuzzy bright-to-dark gradients add together.” The black drop effect was troublesome to 18th and 19th century astronomers who tried to measure the solar system by timing transits of Venus.

Troublesome then, fun and challenging now.

If you plan to try observing Venus on June 8th, you might want to practice on June 6th or 7th. Set up your telescope in a place where you’ll have a clear view of the Sun rising over the eastern horizon. Can you project an image of sunspots onto a wall or screen? If so, you’re ready for Venus.

And if you oversleep … mark your calendar for the next transit of Venus: June 6, 2012. Good luck!

Original Source: NASA Science Article

Image credit: NASA
NASA invites you to safely view a rare celestial event, one not seen before by any person alive. On June 8, Venus will appear to cross in front of the sun as viewed from Earth. The last “Venus transit” occurred in 1882. The next two Venus transits are on June 6, 2012, and Dec. 11, 2117.

NASA has formed partnerships with observatories, museums, and amateur astronomers to help people safely observe the event. Special precautions are necessary to safely observe the sun. NASA’s Office of Space Science is offering exciting activities and resources for classrooms and museums. Information, resources, opportunities for educational participation, local events and viewing times, are available on the Internet at:
http://sunearthday.nasa.gov

The event may be safely observed over the Internet with images from solar observatories and satellites. For Internet viewing options, including a live webcast from Athens, Greece, made in partnership with the Exploratorium in San Francisco, Calif., visit:
Centennial Challenges Workshops

The Venus transit will be visible from approximately 75 percent of the Earth. For a map of the transit visibility on the Internet, visit:
http://sunearth.gsfc.nasa.gov/eclipse/transit/TV2004/TV2004-Map1b.GIF

Transit times for cities worldwide are available on the Internet at:
http://sunearth.gsfc.nasa.gov/eclipse/transit/TV2004.html

“People using a filter approved for safe solar viewing can expect to see a small black dot, about 1/30 the size of the solar disk, very slowly moving across the sun,” said Fred Espenak, an eclipse expert at NASA’s Goddard Space Flight Center, Greenbelt, Md.

During the 19th century, Venus transits were essential for astronomers to determine the scale of the heavens. Transits were used to calculate a relatively accurate distance from the Earth to the sun. Once that distance was determined, astronomers calculated the size of our solar system. They also calculated distances to nearby stars by measuring how much they appeared to shift against remote background stars, as the Earth progressed in its orbit around the sun.

So critical was this measurement that, beginning in 1761, leading nations sent expeditions to remote corners of the globe to exactly time when Venus appeared to begin its transit of the sun. The precise timing of the transit depended on location, because different places on the Earth observed the event from different angles. The times were compared, and the distance to the sun calculated using the known distances between expedition locations on the Earth and trigonometry.

The transit phenomenon also has relevance for the future of astronomy. Scientists with NASA’s Kepler mission hope to discover Earth-like planets outside our solar system by searching for transits of other stars by planets that might be orbiting them.

NASA’s Kepler mission is scheduled for launch in October 2007. It will allow astronomers to find planets, perhaps the size of Earth, orbiting other stars by looking for tiny dips in the brightness of a star when a planet crosses in front of it. Periodic brightness dips will signal the presence of a planet in orbit around the star, even if the planet is not directly visible. For information about the Kepler mission on the Internet, visit:

NASA Kepler

Image credit: NASA/JPL
On Tuesday 8 June, observers throughout Europe, as well as most of Asia and Africa, will be able to witness a very rare astronomical phenomenon when the planet Venus lines up directly between Earth and the Sun. Seen as a small black disk against the bright Sun, Venus will take about 6 hours to complete its crossing of the Sun’s face – known as a ‘transit’. The whole event is visible from the UK, weather permitting.

The last transit of Venus took place on 6 December 1882, but the last one that could have been seen in its entirety from the UK, as on this occasion, was in 1283 (when no one knew it was happening) and the next will not be until 2247! (The transit of 6 June 2012 will not be visible from the UK). The first transit of Venus to be observed was on 24 November 1639 (Julian Calendar). Transits also occurred in 1761, 1769 and 1874.

Venus and Mercury both orbit the Sun closer than Earth. Both planets regularly line up roughly between Earth and the Sun (called ‘conjunction’) but on most occasions they pass above or below the disc of the Sun from our point of view. Since 1631, transits of Venus have been occurring at intervals of 8, 121.5, 8 then 105.5 years and this pattern will continue until the year 2984. Transits of Mercury are more common; there are 13 or 14 each century, the next being in November 2006.

WHEN AND WHERE
The Venus transit of 8 June begins shortly after sunrise at about 6.20 BST, when the Sun will be about 12 degrees above the eastern horizon. It will take about 20 minutes from ‘first contact’ until the planet is fully silhouetted against the Sun, roughly at the ‘8 o’clock’ position’. It will then cut a diagonal path across the southern part of the Sun. Mid-transit is at about 9.22 BST. Venus begins to leave the Sun near the ‘5 o’clock’ position at about 12.04 BST and the transit will be completely over around 12.24. Timings differ by a few seconds for different latitudes, but clouds permitting, the transit will be visible from any place where the Sun is up, including the whole of the UK and almost all of Europe.

For a diagram of Venus’s track across the Sun, see:

http://sunearth.gsfc.nasa.gov/eclipse/OH/tran/Transit2004-2a.GIF (hi-res)
http://sunearth.gsfc.nasa.gov/eclipse/OH/tran/Transit2004-2b.GIF (low-res)
http://www.transit-of-venus.org.uk/transit.htm

For map showing where transit is visible, see:

http://sunearth.gsfc.nasa.gov/eclipse/OH/tran/Transit2004-1b.GIF

HOW TO VIEW
Venus is large enough to be just visible to someone with normal eyesight without the help of binoculars or a telescope. Its diameter will appear about 1/32 the diameter of the Sun. However, NO ONE SHOULD EVER LOOK DIRECTLY AT THE SUN, WITH OR WITHOUT A TELESCOPE OR BINOCULARS WITHOUT USING A SAFE SOLAR FILTER. TO DO SO IS VERY DANGEROUS AND IS LIKELY TO RESULT IN PERMANENT BLINDNESS.

For safe viewing of the transit, much the same rules apply as those for observing an eclipse of the Sun. Eclipse viewers can be used (as long as they are undamaged), and observing is limited to a few minutes at a time. (Note that they must NOT be used with binoculars or a telescope.) For an enlarged view, an image of the Sun can be projected onto a screen by a small telescope. Pinhole projection, however, will not produce a sharp enough image to show Venus clearly.

More detailed information on safety from:

http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/safety2.html
http://www.transit-of-venus.org.uk/safety.htm

IMPORTANCE OF THE TRANSIT
In the 18th and 19th centuries, transits of Venus presented rare opportunities to tackle a fundamental problem – finding an accurate value for the distance between Earth and the Sun. The unit astronomers use for distance measurements in the solar system is based closely on its average value and is called the astronomical unit (AU). It is approximately 93 million miles, or 150 million km.

In the end, though observations of transits produced rough answers, they were never as accurate as originally hoped (see more on this below). But the quest was the stimulus for unprecedented international scientific cooperation and for expeditions that produced discoveries far beyond their original intended scope. Today, distances in the solar system are known with great precision through very different means.

In the 21st century, the main interest in the transits of Venus of 2004 and 2012 is their rarity as astronomical phenomena, the educational opportunities they present, and the sense of a link with important events in scientific and world history.

However, astronomers are now particularly interested in the general principle of planet transits as a way of hunting for extrasolar planetary systems. When a planet crosses in front of its parent star, there is a minute dip in the star’s apparent brightness. Identifying such dips will be a useful method of finding planets orbiting other stars. Some astronomers intend to use the transit of Venus as a test to help design searches for extrasolar planets.

The transit will be observed by two solar observatories in space: TRACE and SOHO. From where SOHO is positioned, it will not see a transit across the visible disc of the Sun, but it will observe Venus’s passage across the Sun’s corona (its outer atmosphere).

VENUS TRANSITS OF THE PAST
The first person to predict a transit of Venus was Johannes Kepler, who calculated that one would take place on 6 December 1631, just a month after a transit of Mercury on 7 November. Though the transit of Mercury was observed, the transit of Venus was not visible from Europe and there is no record of anyone seeing it. Kepler himself died in 1630.

Jeremiah Horrocks (also spelled Horrox), a young English astronomer, studied Kepler’s planetary tables and discovered with just a month to go that a transit of Venus would occur on 24 November 1639. Horrocks observed part of the transit from his home at Much Hoole, near Preston, Lancashire. His friend William Crabtree also saw it from Manchester, having been alerted by Horrocks. As far as is known, they were the only people to witness the transit. Tragically, Horrocks’s promising scientific career was cut short when he died in 1641, aged about 22.

Edmond Halley (of comet fame) realised that observations of transits of Venus could in principle be used to find how far the Sun is from Earth. This was a major problem in astronomy at the time. The method involved observing and timing a transit from widely spaced latitudes from where Venus’s track across the Sun would appear slightly different. Halley died in 1742, but the transits of 1761 and 1769 were observed from many places around the world. Captain James Cook’s expedition to Tahiti in 1769 is one of the most famous and went on to become a world voyage of discovery. However, results on the Sun-Earth distance were disappointing. The observations were plagued by many technical difficulties.

Nevertheless, 105 years later, optimistic astronomers tried again. The results were equally disappointing and people began to realise that the practical problems with Halley’s simple idea were just too great to overcome. Even so, by the 1882 tr ansit, there was enormous public interest and it was mentioned on the front page of most newspapers. Thousands of ordinary people saw it for themselves.

In his 1885 book, “The Story of Astronomy” Professor Sir Robert Stawell Ball described his own feelings on watching the transit 3 years earlier:

“… To have seen even a part of a transit of Venus is an event to remember for a lifetime, and we felt more delight than can be easily expressed… Before the phenomenon had ceased, I spared a few minutes from the somewhat mechanical work at the micrometer to take a view of the transit in the more picturesque form which the large field of the finder presents. The sun was already beginning to put on the ruddy hues of sunset, and there, far in on its face, was the sharp, round, black disk of Venus. It was then easy to sympathize with the supreme joy of Horrocks, when, in 1639, he for the first time witnessed this spectacle. The intrinsic interest of the phenomenon, its rarity, the fulfilment of the prediction, the noble problem which the transit of Venus helps us to solve, are all present to our thoughts when we look at this pleasing picture, a repetition of which will not occur again until the flowers are blooming in the June of A.D. 2004.”

For an excellent historical summary, see:

Venus Transit

THE FAMOUS ‘BLACK DROP’ PROBLEM
One of the chief problems visual observers of transits faced was pinpointing the exact time when Venus was first fully on the visible face of the Sun. Astronomers call this point ‘second contact’. In practice, as Venus crossed onto the Sun, its black disc seemed to remain linked to the edge of the Sun for a short time by a dark neck, making it appear almost pear-shaped. The same happened in reverse when Venus began to leave the Sun. This so-called ‘black drop effect’ was the main reasons why timing the transits failed to produce consistent accurate results for the Sun-Earth distance. Halley expected second contact could be timed to within about a second. The black drop reduced the accuracy of timing to more like a minute.

The black drop effect is often mistakenly attributed to Venus’s atmosphere but Glenn Schneider, Jay Pasachoff and Leon Golub showed last year that the problem is due to a combination of two effects. One is the image blurring that naturally takes place when a telescope is used (described technically as ‘the point spread function’). The other is the way the brightness of the Sun diminishes close to its visible ‘edge’ (known to astronomers as ‘limb darkening’).

More experiments will be done on this phenomenon at the 8 June transit of Venus using the TRACE solar observatory in space.

VENUS – THE PLANETARY EQUIVALENT TO HELL.
At first glance, if Earth had a twin, it would be Venus. The two planets are similar in size, mass and composition, and both reside in the inner part of the Solar System. Indeed, Venus comes closer to Earth than any of the other planets.

Before the advent of the Space Age, astronomers could only speculate over the nature of its hidden surface. Some thought that Venus might be a tropical paradise, covered in forests or oceans. Others believed that it was a totally barren, arid desert. After investigations by numerous American and Russian spacecraft, we now know that Earth’s planetary neighbour is the most hellish, hostile world imaginable. Any astronaut unlucky enough to land there would be simultaneously crushed, roasted, choked and dissolved.

Unlike Earth, Venus has no ocean, no satellites and no intrinsic magnetic field. It is covered by thick, yellowish clouds – made of sulphur and droplets of sulphuric acid – that act like a blanket to trap surface heat. The upper cloud layers move faster than hurricane-force winds on Earth, sweeping all the way around the planet in just four days. These clouds also reflect most of the incoming sunlight, helping Venus to outshine everything in the night sky (apart from the Moon). At the present time, Venus dominates the western sky after sunset.

Atmospheric pressure is 90 times that of Earth, so an astronaut standing on Venus would be crushed by pressure equivalent to that at a depth of 900 m (more than half a mile) in the Earth’s oceans. The dense atmosphere consists mainly of carbon dioxide (the greenhouse gas that we breathe out every time we exhale) and virtually no water vapour. Since the atmosphere allows the Sun’s heat in but does not allow it to escape, surface temperatures soar to more than 450 deg. C – hot enough to melt lead. Indeed, Venus is hotter than Mercury, the planet closest to the Sun.

Venus rotates sluggishly on its axis once every 243 Earth days, while it orbits the Sun every 225 days – so its day is longer than its year! Just as peculiar is its retrograde, or “backwards” rotation, which means that a Venusian would see the Sun rise in the west and set in the east.

Earth and Venus are similar in density and chemical composition, and both have relatively young surfaces, with Venus appearing to have been completely resurfaced 300 to 500 million years ago.

The surface of Venus comprises about 20 per cent lowland plains, 70 per cent rolling uplands, and 10 per cent highlands. Volcanic activity, impacts, and deformation of the crust have shaped the surface. More than 1,000 volcanoes larger than 20 km (12.5 mls) in diameter dot the surface of Venus. Although much of the surface is covered by vast lava flows, no direct evidence of active volcanoes has been found. Impact craters smaller than 2 km (1 ml) across do not exist on Venus because most meteorites burn up in the dense atmosphere before they can reach the surface.

Venus is drier than the driest desert on Earth. Despite the absence of rainfall, rivers or strong winds, some weathering and erosion does occur. The surface is brushed by gentle winds, no stronger than a few kilometres per hour, enough to move grains of sand, and radar images of the surface show wind streaks and sand dunes. In addition, the corrosive atmosphere probably chemically alters rocks.

Radar images sent back by orbiting spacecraft and ground-based telescopes have revealed several elevated “continents”. In the north is a region named Ishtar Terra, a high plateau larger than the continental United States and bounded by mountains almost twice as high as Everest. Near the equator, the Aphrodite Terra highlands, more than half the size of Africa, extend for almost 10,000 km (6,250 miles). Volcanic lava flows have also produced long, sinuous channels extending for hundreds of kilometres.

Original Source: RAS News Release