Space Sunday: Conjunctions, China & the Sun as a telescope

Jupiter and Saturn Great Conjunction” – Jupiter (the brighter object) and Saturn, imaged by astronomer Tom Wildoner on December 8th, 2020. Credit: Tom Wildoner

For those who have not already seen it, the next two weeks present an opportunity to witness a unique event – a very close conjunction between Jupiter and Saturn.

“Conjunction” is the term astronomers used to describe two astronomical objects or spacecraft having either the same right ascension or the same ecliptic longitude, and thus when seen from Earth, appear to be close together.

With the planets, such events are not especially rare – in fact as they and the Earth circle the Sun, conjunctions between Jupiter and Saturn tend to occur once every 20 years. However, most of these only see Jupiter and Saturn close to around one degree of one another, or about one-fifth the diameter of the Moon as seen from Earth. But sometimes they appear to get much closer, creating what is referred to as a “great conjunction”. This year, the two planets will appear to be just 6 arc minutes apart as seen from Earth on December 21st, 2020; so “close” (remembering that their respective orbits around the Sun will still be separated by 883 million km), they will almost, but not quite, appear as a single point of light when seen with the naked eye.

The great conjunction between Jupiter and Saturn,, tracked from October through to December 21st. Credit: Pete Lawrence

These “great conjunctions” occur, on average, once every 300-400 years, although such is the nature of orbital mechanics, they can actually occasionally occur more frequently, or have longer time gaps between them. As it is, the last time Jupiter and Saturn appeared as close as the will be between December 20th and 22nd was in 1623, not long after Galileo had observed both planets – although he was unable to witness the event, as the rising Sun would have rendered them invisible in its glare.

What is most rare is a close conjunction that occurs in our night time sky. I think it’s fair to say that such an event typically may occur just once in any one person’s lifetime, and I think ‘once in my lifetime’ is a pretty good test of whether something merits being labelled as rare or special.

Astronomer David Weintraub

However, the two planets can appear to be much closer. In 1226, and in the skies over the Mongol Empire, when the planets appear to be just 2 arc minutes apart.

Jupiter (again, the brighter object) and Saturn, seen in the sky over the Shenandoah National Park, Virginia, on December 13th, 2020. Credit: Bill Ingalls/NASA

Tracing these great conjunctions back in time reveals that Jupiter and Saturn may well have played a role in the legend of the Star of Bethlehem. In 7 B.C. not one, but three great conjunctions occurred, with the two planets again being within 2 arc minutes of one another as seen from Earth.

The first occurred in May of that year, when Jupiter and Saturn appeared as a morning star over the middle east. As  the Magi were practitioners of (among other things) astronomy and astrology – both at that time pretty much joined at the hip – such an event may well have caused them to start out on their long journey towards Judea, the second conjunction, in September of the year, encouraging them to continue. The third conjunction occurred in December, 7 B.C., the time at which they were said to have met with Herod the Great.

Using Stellarium, open-source astronomy software, it is possible to reproduce how the great conjunctions of the 6th century B.C. might have looked to the Magi, as a wondrous new star, causing them to set out for Judea. Credit: Stellarium

This year’s conjunction will be not long after sunset, with the two planets located low over the south-west horizon. With a reasonable telescope or good pair of binoculars, you’ll have an ideal opportunity to see both planets and their major moons in the same field of view.  Should you do so, you’ll be looking at over 90% of the planetary mass of the entire solar system.

Beyond the 21st, the two planets will gradually move “apart” as noted, until by the 25th December, they’ll be separated in the night sky by roughly the diameter of a full Moon, and will continue to draw apart relative to Earth as they pass below the horizon.

How to see the “great conjunction” of Jupiter an Saturn

And if you miss this close conjunction between the two, the next will be along in a relatively (and unusually) short period, occurring on March 15th 2080. The next time they’ll be as apparently close as they were in 7 B.C. will be on Christmas Day, 2874.

China News

Chang’e 5 Returns Samples

China’s Chang’e 5 lunar sample return mission has successfully delivered its cargo of lunar samples to Earth, where they have been recovered and transported for analysis.

As I’ve previously reported, Chang’e 5 was launched on November 23rd on what was to be a mission of up to 23 days in length, in which it would send two vehicles  – a lander and an ascender – to the surface of the lunar nearside at Mons Rümker in Oceanus Procellarum (Ocean of Storms), a relatively “young” part of the Moon in an a to collect samples of lunar material from up to 2 metres below the surface  for return to Earth and analysis.

Following a safe landing on Monday, November 30th, the lander / ascender successfully gathered the desired samples, and after the lander has completed a number of other tasks, the ascender used it as a launch platform to return to orbit and a rendezvous with the orbiter / sample return capsule combination. Docking with the latter, the ascender transferred the container with its precious cargo to the capsule, and was then jettisoned, later crashing back into the Moon.

Then on Saturday December 12th, and after waiting for the optimal return window to open, the two remaining elements of Chang’e 5 started back to Earth, a low-speed journey that took the vehicles 4 days to complete. On  approaching Earth on December 16th, the return capsule detached from the orbiter and performed an initial “skip” into the upper atmosphere before bouncing back out into space very briefly, like a stone skipping the water of a pond. This manoeuvre allowed the capsule to slow itself in an aerobraking manoeuvre before it re-entered the atmosphere once more, this time passing through re-entry before deploying its parachutes to land in the Siziwang district of the Inner Mongolia region.

Members of the Chinese recovery team inspect the Chang’e 5 sample return capsule shortly after it had landing at around 2:00am local time in Inner Mongolia. Credit: CCTV

Following recovery, the capsule and cargo were whisked to Beijing to begin the process of disassembly and analysis.

As our nation’s mostly complex and technically ground-breaking space mission, Chang’e 5 has achieved multiple technical breakthroughs … and represents a landmark achievement.

– China National Space Administration

The spacecraft’s return marked the first time scientists have obtained fresh samples of lunar rocks since the former Soviet Union’s Luna 24 robot probe in 1976. As with the 382 kg of lunar samples brought back by U.S. astronauts from 1969 to 1972, Chang’e 5’s payload will be analysed for age and composition and is expected to be shared with other countries. Given the samples are much “younger” than the samples returned by Apollo by around 3 billion years, it is hoped the Chang’e 5 samples will help scientists better understand the history of the Moon.

A FAST Response to the Loss of Arecibo

Following the catastrophic  loss of the Arecibo radio observatory (see Space Sunday: returns and a collapse), China is to open its own radio telescope array to international use.

FAST, the Five-hundred-metre Aperture Spherical Telescope, took the crown of the world’s largest radio telescope from the 305-metre Arecibo in 2016, when its five-year construction officially came to  a close, and the array commenced a 3-year testing an commissioning programme that was completed at the start of 2020.

China’s massive Five-hundred-metre Aperture Spherical Telescope,. Credit: CCTV

Not only is FAST bigger than Arecibo, as I reported at the time of its completion, it is far more sensitive in certain areas, and more capable in terms of its pointing capability. In fact, it is so sensitive, despite being located in a remote area of Guizhou Province in south-west China, it required the paid relocation of over 9,000 local villagers and the introduction of a 5-km wide exclusion zone around it to prevent even the output of modest microwave ovens from upsetting its instruments.

China will start accepting proposals for using the telescope from scientist and astronomers around the world starting in early 2021, with this first invitations to attend the facility being awarded later in the year.

Long March 8 Set to Fly

As I’m writing this piece, China is making final preparations to conduct a test launch of their latest launch vehicle: the Long March 8.

Developed from the Long March 7, the new vehicle is designed to lift a maximum of 5 tonnes to geosynchronous transfer orbits (GTO) or 2.8 tonnes to a Sun-synchronous orbit (SSO). Like all of China’s new Long March launch systems, it is designed to be “environment friendly”, burning a much cleaner mix of fuels – kerosene and liquid oxygen in all of its motors.

However, the most interesting aspect of the launcher is that the core stage is intended to be reusable. While this won’t be the case for the first flight, future launches will see the core stage of the rocket making a return to Earth and a landing on a floating platform a-la the SpaceX Falcon 9. However, the Long March 8 core will return with its strap-on boosters still attached, allowing them to also be reused.

For the test flight, the core stage will not be recovered, however, it will carry a classified Chinese government payload, alongside commercial satellites – Long March 8 is also designed to compete in the growing commercial launch services market.

How to See an Exoplanet

In the last several decades, we have discovered thousand exoplanets orbiting other stars, and through various means, we’ve been able to assess many of them in terms of their size, mass, potential composition (gas giant, solid rocky world), the potential for them to have an atmosphere and its possible composition, and so on.

The one thing that remains difficult is directly imaging them, particularly the more Earth-like, solid worlds, simply because they are too small to be adequately imaged in the detail we would like. In fact, to directly image any Earth-sized exoplanet up to 100 light years away at just one or two pixels resolution, we would need a telescope with a primary mirror approximately 90 km in diameter. And even then, in order to obtain a good signal-to-noise ratio and actually resolve any real surface details, it would need several thousand years of careful image processing.

However, physicists lead by Slava G. Turyshev at the Jet Proplusion laboratory, have postulated a novel approach that could potentially reveal the surfaces of exoplanets relatively close to us at resolutions where surface details could be seen – by using the Sun in what is called Solar Gravitational Lensing (SGL).

As Einstein predicted, gravity is all-pervasive, affecting everything it encounters – including light, which it can bend and focus. Astronomers have used the effect in what is called gravitational lensing to reveal information about very distant  objects – such as galaxies – by focusing and amplifying them using the gravity of a closer, massive object (such as another galaxy).

Einstein rings around galactic objects, as captured by the Hubble Space Telescope. The rings are light from most distant galaxies than has been bent by the gravitational influence of the much closer galaxy at the centre of each image. While only bands of colour, (visible light in these images), the rings can reveal much about the composition of the object they represent. Credit: NASA/ESA/A.Bolton (CfA)/SLACS Team

Up until now, gravitational lensing hasn’t required precise focusing; rather, by “bending” the light of those distant objects using the gravitational well of a closer object, astronomers have created “Einstein rings”, circles, arcs and crosses of light that appear around the “lensing object”, which can then be analysed across the full light spectrum to reveal information about the originating object.

In their paper, Turyshev and his colleagues propose taking the idea a stage further – using the Sun’s massive, and local gravity well as the primary “mirror” in a giant “telescope”, allowing it to focus the light reflected by exoplanets on a point in deep space, where it could be received by a robotic imaging system..

Doing so would allow astronomers to view a planet like Proxima b, 4.2 light years away, at a equivalent resolution 0f 1024×1024 pixels. While this may not sound particularly high, with proper post-processing, it could reveal a lot about the terrain and conditions on that world – and even, by revealing any lights on the part of the planet in shadow,tell us if some kind of civilisation lives there.

In this simulated image, (l), how a raw image of Earth might look were it as far from us (4.2 light years) as the exoplanet Proxima b, and imaged at 1024×1024 resolution using SGL. Centre: how the image might appear when post-processed, revealing cloud formations and the discernible land masses of the United States (top), Mexico & the Gulf of Mexico (middle) and the top of South America (credit: S.G. Turyshev, et al). On the right, an image of the Earth at the same resolution and in colour, demonstrating the level of detail that might be achieved using SGL on nearby exoplanets – note the town and city lights, in the upper left shadows portion of the image. Credit: NASA.

However, there is are several catches to this idea. The first is that such is the size of the Sun, the use of SGL means that the receiving imaging system would need to be an average of 550 AU (astronomical units – the average distance between the Earth and the Sun) away from the Sun. That’s about four times the distance from Earth to Voyager 1, the most distant man-made object from this planet.

The second is that at this distance, the imaging receiver would be well beyond our ability to maintain it, should it encounter operational issues. Plus, it would be facing towards the Sun, and so the light from any exoplanet would be obliterated by sunlight, and any data on a distant planet subject to possible spherical aberrations. None of these problems are insurmountable – a suitable ion drive system could help the receiver reach the desired distance from the Sun, while the sunlight issue could be resolved by using a coronagraph, and any aberrations in the image could be carefully processed out – although Turyshev’s team note that any image post-processing is liable to be on the order of a year or more to complete.

Even so, such a capability would be an invaluable tool for astronomy and astrobiology, and the study suggests that, with suitable funding over the next couple of decades, together with the time needed to get the receiver out into interstellar space, an SGL “telescope” could be operational before the end of this century.

In the next 10-15 years we will discover thousands of new exoplanets by using indirect methods (transit spectroscopy, radial velocity, astrometry, microlensing, etc.). Once we have a set of exciting targets, SGL will help us to study them. We could launch a mission towards the focal region of the SGL for a particular target and study this pre-selected target or target system.

– Slava G. Turyshev

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