Viewing orbital images of the rovers as they go about their business on the surface of Mars is pretty cool. Besides being of great interest to anyone keen on space in general, they have scientific value as well. New images from the High Resolution Imaging Science Equipment (HiRise) camera aboard the Mars Reconnaissance Orbiter (MRO) help scientists in a number of ways.
Recent images from HiRise show the Mars Science Laboratory (MSL) Curiosity on a feature called the Naukluft Plateau. The Plateau is named after a mountain range in Namibia, and is the site of Curiosity’s 10th and 11th drill targets.
Orbital imagery of the rovers is used to track the activity of sand dunes in the areas the rovers are working in. In this case, the dune field is called the Bagnold Dunes. HiRise imagery allows a detailed look at how dunes change over time, and how any tracks left by the rover are filled in with sand over time. Knowledge of this type of activity is a piece of the puzzle in understanding the Martian surface.
But the ability to take such detailed images of the Martian surface has other benefits, as well. Especially as we get nearer to a human presence on Mars.
Orbital imaging is turning exploration on its ear. Throughout human history, exploration required explorers travelling by land and sea to reconnoiter an area, and to draw maps and charts later. We literally had no idea what was around the corner, over the mountain, or across the sea until someone went there. There was no way to choose a location for a settlement until we had walked the ground.
From the serious (SpaceX, NASA) to the fanciful (MarsOne), a human mission to Mars, and an eventual established presence on Mars, is a coming fact. The how and the where are all connected in this venture, and orbital images will be a huge part of choosing where.
Tracking the changes in dunes over time will help inform the choice for human landing sites on Mars. The types and density of sand particles may be determined by monitoring rover tracks as they fill with sand. This may be invaluable information when it comes to designing the types of facilities used on Mars. Critical infrastructure in the form of greenhouses or solar arrays will need to be placed very carefully.
Sci-Fi writers have exaggerated the strength of sand storms on Mars to great effect, but they are real. We know from orbital monitoring, and from rovers, that Martian sandstorms can be very powerful phenomena. Of course, a 100 km/h wind on Earth is much more dangerous than on Mars because of the density of the atmosphere. Martian air is 1% the density of Earth’s, so on Mars the 100 km/h wind wouldn’t do much.
But it can pick up dust, and that dust can foul important equipment. With all this in mind, we can see how these orbital images give us an important understanding of how sand behaves on Mars.
There’s an unpredictability factor to all this too. We can’t always know in advance how important or valuable orbital imagery will be in the future. That’s part of doing science.
But back to the cool factor.
For the rest of us, who aren’t scientists, it’s just plain cool to be able to watch the rovers from above.
We’ve had an abundance of news stories for the past few months, and not enough time to get to them all. So we are now using a tool called Trello to submit and vote on stories we would like to see covered each week, and then Fraser will be selecting the stories from there. Here is the link to the Trello WSH page (http://bit.ly/WSHVote), which you can see without logging in. If you’d like to vote, just create a login and help us decide what to cover!
We record the Weekly Space Hangout every Friday at 12:00 pm Pacific / 3:00 pm Eastern. You can watch us live on Google+, Universe Today, or the Universe Today YouTube page.
When we think of ring systems, what naturally comes to mind are planets like Saturn. It’s beautiful rings are certainly the most well known, but they are not the only planet in our Solar System to have them. As the Voyager missions demonstrated, every planet in the outer Solar System – from Jupiter to Neptune – has its own system of rings. And in recent years, astronomers have discovered that even certain minor planets – like the Centaur asteroids 10199 Chariklo and 2006 Chiron – have them too.
This was a rather surprising find, since these objects have such chaotic orbits. Given that their paths through the Solar System are frequently altered by the powerful gravity of gas giants, astronomers have naturally wondered how a minor planet could retain a system of rings. But thanks to a team of researchers from the Sao Paulo State University in Brazil, we may be close to answering that question.
In a study titled “The Rings of Chariklo Under Close Encounters With The Giant Planets“, which appeared recently in The Astrophysical Journal, they explained how they constructed a model of the Solar System that incorporated 729 simulated objects. All of these objects were the same size as Chariklo and had their own system of rings. They then went about the process of examining how interacting with gas giant effected them.
To break it down, Centaurs are a population of objects within our Solar System that behave as both comets and asteroids (hence why they are named after the hybrid beasts of Greek mythology). 10199 Chariklo is the largest known member of the Centaur population, a possible former Trans-Neptunian Object (TNO) which currently orbitsbetween SaturnandUranus.
The rings around this asteroid were first noticed in 2013 when the asteroid underwent a stellar occultation. This revealed a system of two rings, with a radius of 391 and 405 km and widths of about 7 km 3 km, respectively. The absorption features of the rings showed that they were partially composed of water ice. In this respect, they were much like the rings of Jupiter, Saturn, Uranus and the other gas giants, which are composed largely of water ice and dust.
This was followed by findings made in 2015 that indicated that 2006 Chiron – another major Centaur – could have a ring of its own. This led to further speculation that there might be many minor planets in our Solar System that have a system of rings. Naturally, this was a bit perplexing to astronomers, since rings are fragile structures that were thought to be exclusive to the gas giants of our System.
As Professor Othon Winter, the lead researcher of the Sao Paulo team, told Universe Today via email:
“At first it was a surprise to find a Centaur with rings, since the Centaurs have chaotic orbits wandering between the giant planets and having frequent close encounters with them. However, we have shown that in most of the cases the ring system can survive all the close encounters with the giant planets. Therefore, Centaurs with rings might be much more common than we thought before.”
For the sake of their study, Winter and his colleagues considered the orbits of 729 simulated clones of Chariklo as they orbited the Sun over the course of 100 million years. From this, Winter and his colleagues found that each Centaur averaged about 150 close encounters with a gas giant, within one Hill radius of the planet in question. As Winter described it:
“The study was made in two steps. First we considered a set of more than 700 clones of Chariklo. The clones had initial trajectories that were slightly different from Chariklo for statistical purposes (since we are dealing with chaotic trajectories) and computationally simulated their orbital evolution forward in time (to see their future) and also backward in time (to see their past). During these simulations we archived the information of all the close encounters (many thousands) they had with each of the giant planets.”
“In the second step, we performed simulations of each one of the close encounters found in the first step, but now including a disk of particles around Chariklo (representing the ring particles). Then, at the end of each simulation we analyzed what happened to the particles. Which ones were removed from Chariklo (escaping its gravitational field)? Which ones were strongly disturbed (still orbiting around Chariklo)? Which ones did not suffer any significant effect?”
In the end, the simulations showed that in 90 percent of the cases, the rings of the Centaurs survived their close encounters with gas giants, whereas they were disturbed in 4 percent of cases, and were stripped away only 3 percent of the time. Thus, they concluded that if there is an efficient mechanism that creates the rings, then it is strong enough to let Centaurs keep them.
More than that, their research would seem to indicate that what was considered unique to certain planetary bodies may actually be more commonplace. “It reveals that our Solar System is complex not just as whole or for large bodies,” said Winter, “but even small bodies may show complex structures and even more complex temporal evolution.”
The next step for the research team is to study ring formation, which could show that they in fact picking them up from the gas giants themselves. But regardless of where they come from, its becoming increasingly clear that Centaurs like 10199 Chariklo are not alone. What’s more, they aren’t giving up their rings anytime soon!
The target date for the ‘Return to Flight’ launch of Antares on a cargo resupply mission for NASA to the International Space Station (ISS) is “likely to result in an updated launch schedule in the August timeframe,” Orbital ATK spokeswoman Sean Wilson told Universe Today.
The company had most recently been aiming towards an Antares launch date around July 6 from NASA’s Wallops Flight Facility – for its next NASA contracted mission to stock the ISS via the Orbital ATK Cygnus cargo freighter on a flight known as OA-5.
Meanwhile the firms most recently launched Cygnus OA-6 cargo ship departed the space station and completed its planned destructive reentry into the Earth’s atmosphere on Wednesday, June 22.
A thorough analysis of the hot fire test results and its implications is underway.
“Our Antares team recently completed a successful stage test and is wrapping up the test data analysis,” Wilson said.
“Final trajectory shaping work is also currently underway, which is likely to result in an updated launch schedule in the August timeframe.”
In the meantime, company engineers continue to ready the rocket and payload.
“We are continuing to prepare for the upcoming launch of the Antares rocket and Cygnus spacecraft for the OA-5 cargo logistics mission to the International Space Station from NASA’s Wallops Flight Facility,” Wilson noted.
It’s also clear that a decision on a launch date target is some weeks away and depends on the busy upcoming manifest of other ISS missions coming and going.
“A final decision on the mission schedule, which takes into account the space station traffic schedule and cargo requirements, will be made in conjunction with NASA in the next several weeks.”
And it also must take into account the launch of the intervening SpaceX ISS cargo flight that was just postponed two days to no earlier than July 18.
Another factor is the delayed launch of the next manned crew on a Russian Soyuz capsule from late June into July. Blastoff of the three person crew from Russia, the US and Japan is set for July 7. OA-5 will deliver some 3 tons of science experiments and crew supplies.
So the primary goal of the stage test was to confirm the effectiveness of the new engines and all the changes in the integrated rocket stage.
It’s not entirely clear at this time whether the Antares launch delay to August is due to changes in the ISS manifest scheduling or any lingering questions from the hot fire test or both.
“A final decision on the mission schedule definitely takes into account the completion of data analysis combined with the busy space station traffic schedule and NASA’s cargo requirements,” Wilson told me in a response requesting clarification.
Following a quick look immediately following the May 31 test, Orbital ATK officials initially reported that all seemed well, with the caveat that further data review is needed.
“Early indications show the upgraded propulsion system, core stage and launch complex all worked together as planned,” said Mike Pinkston, Orbital ATK General Manager and Vice President, Antares Program.
“Congratulations to the combined NASA, Orbital ATK and Virginia Space team on a successful test.”
Orbital ATK engineers will now “review test data over the next several days to confirm that all test parameters were met. ”
The test used the first stage core planned to launch the OA-7 mission from Wallops late this year.
With the engine test completed, the OA-7 stage will be rolled back to the HIF processing hanger at Wallops and a new stage fully integrated with the Cygnus cargo freighter will be rolled out to the pad for the OA-5 ‘Return to Flight’ mission in August.
The mission of the OA-6 Cygnus ended on Wednesday, with a planned destructive reentry into the Earth’s atmosphere at 9:29 a.m. EDT.
Also known as the SS Rick Husband, it had spent 3 months in orbit since launching in March on a ULA Atlas V.
It departed the ISS on June 14 and continued several science experiments. Most notable was to successfully create the largest fire in space via the Spacecraft Fire Experiment-I (Saffire-I).
Stay tuned here for Ken’s continuing Earth and Planetary science and human spaceflight news.
Boeing thinks it can have its Starliner spacecraft ready to fly crewed missions by February, 2018. This is 4 months later than the previous date of October 2017. It isn’t yet clear what this will mean in Boeing’s race against SpaceX to relieve NASA’s dependence on Russian transportation to the ISS.
Currently, astronauts travel to the ISS aboard the Russian workhorse Soyuz capsule. Ever since the end of the Space Shuttle program, NASA has relied on Russia to transport astronauts to the station. Both Boeing and SpaceX have received funds to develop a crewed capsule, and both companies are working at a feverish pace to be the first to do so.
Boeing has a long history of involvement with NASA. It’s the prime contractor for ISS operations, and is also the prime contractor for NASA’s Space Launch System (SLS), which will be the most powerful rocket ever built and will power NASA’s exploration of deep space. So Boeing is no stranger to complex development cycles and the types of delays that can crop up.
In a recent interview, Boeing’s Chris Ferguson acknowledged that everything has to go well for the Starliner to meet its schedule. But things don’t always go well in such a complex engineering program, and that’s just the way things are.
The Starliner, and every other spacecraft, has to undergo extensive testing of each component before any flight can be attempted. Various suppliers are responsible for over 200 pieces of equipment, just in avionics alone, and each one of those pieces has to assembled, integrated, and tested. Not just by Boeing, but by NASA as well. This takes an enormous amount of time, and requires great rigor to carry out. In some cases, a problem with one piece of equipment can delay testing of other pieces. It’s the nature of complex systems.
Another challenge that Boeing engineers face is limiting the mass of the spacecraft. Recent wind-tunnel testing of a Starliner model produced aero-acoustic issues when mated to a model of the Atlas 5, the rocket built by United Launch Alliance (ULA) which will carry the Starliner into space. Now Boeing is modifying the exterior lines of the vehicle to get the airflow just right.
The spacecraft also has to be tested for emergencies. Though the Starliner is designed to land on solid ground, it’s also being tested for emergency landings on water.
NASA blames the delays in the development of the Starliner, and the SpaceX Dragon, on funding cuts from Congress. Administrator Charles Bolden has criticized Congress for consistent under-funding since the retirement of the Space Shuttle fleet in 2011. According to NASA, this has caused a 2 year delay in development of the Dragon and the Starliner. This delay, in turn, has meant that NASA has had to keep paying Russia for trips to the ISS. And like everything else, that cost keeps rising.
But it looks like the end, or maybe the beginning, is in sight for the Starliner. Boeing has paid deposits to ULA for four flights with the Atlas 5. A 2017 un-crewed test flight, a 2018 crewed test flight, and two crewed flights to the ISS.
Beyond that, the future looks a little hard to predict for Boeing and the Starliner. With both SpaceX and Blue Origin developing re-usable rockets, the future viability of the Atlas 5 might be in jeopardy. Compounding the uncertainty is NASA’s stated plan to stop funding the ISS by 2024 or 2028.
By that time, NASA should be focused on establishing a presence in cislunar space, which would require different spacecraft.
But you can’t wait forever to develop spacecraft. The only way to stay in the game is for Boeing to develop the spacecraft that are required right now, and let the knowledge and experience from that feed the development of the next spacecraft, whether for cislunar space or beyond.
In the big scheme of things, a four month delay for the first flight of the Starliner is not that big of a deal. If the Starliner is successful, and there’s no reason to think it won’t be, considering Boeing’s track record, the four month delay in the initial flight won’t even be remembered.
Whether its SpaceX or Boeing who get America back into space first, that moment will be celebrated, and all the delays and funding cuts will be left in the dust-bin of history.
A dispute may be brewing between SpaceX and the Canaveral Port Authority, where the private space company brings its recovered boosters back to land. Citing concerns over wear and tear on the port’s facilities, the Authority is considering raising SpaceX’s fees by 14 times, to a total of $15,000 for each booster passing through.
Port Canaveral is the facility that SpaceX relies on in its operations. Spent boosters are recovered aboard their drone ship, which docks at the Port. They are then offloaded from the drone ship with SpaceX’s special crane, loaded onto a truck and delivered to Kennedy Space Center.
All of this activity puts a special strain on the Port’s facilities, according to Rodger Rees, the port’s deputy executive director and chief financial officer. In a memo to port commissioners, he said “Due to the heavy weight and the effect of this weight on the port’s berths, staff is recommending that the tariff be expanded to include a wharfage charges category for aerospace/aircraft items.”
So far, SpaceX has transported 3 recovered boosters through Port Canaveral. The rationalization for the fee increase is based on some minor damage caused to the Port, and on the increased wear and tear that 30 ton boosters will have on the Port and its structures. SpaceX’s special crane also takes up space at the Port.
But SpaceX isn’t being singled out. The Port is trying to develop a fee structure for private space companies, who are expected to proliferate in the future and require port facilities the same way SpaceX does.
“As new aerospace companies relocate to the Space Coast, it is anticipated that the port will need to accommodate items of a similar nature in the future, and will retain the right to negotiate these future charges, if needed,” said Rees in the same memo.
The fees themselves are a result of research into what other ports charge for oversized items. Staff at Port Canaveral have recommended charging $500 a ton or $15,000 per item, whichever amount is greater. In his memo to the port’s commissioners, Rees also said “Staff understands that the current Falcon first stage weighs approximately 30 tons when it arrives in the port on the drone ship. Under this weight, it is anticipated that each time the rocket stage is transported over the berth, a charge of $15,000 will be assessed and collected from the owner of the item.”
Rees made note of the cool factor that having SpaceX recover boosters at their facility gives the Port. SpaceX’s use of the Port attracts a lot of public interest, which also creates additional security and logistical considerations for the Port.
SpaceX has indicated that it is concerned with the raise in fees. Representatives from Port Canaveral and SpaceX are due to discuss the issue at a meeting on Wednesday, June 22nd.
The residue of the Falcon sailed into home port at Port Canaveral, Fl under cover of darkness and covered by a big blue tarp late Saturday night, June 18, at around 9 p.m. EDT.
It arrived atop SpaceX’s ASDS drone ship landing platform known as “Of Course I Still Love You” or “OCISLY” – that had already been dispatched several days prior to the June 15 morning launch from the Florida space coast.
And check out this exquisite hi res aerial video of the tarp ‘Blowing in the Wind’ – showing an even more revealing view of the remains of the Falcon 9 after much of the tarp was blown away by whipping sunshine state winds.
Video Caption: SpaceX booster remains from Eutelsat-ABS launch seen in Port Canaveral on 06-19-2016 the day after arrival. The wind blew off part of the tarps covering what is left of Eutelsat-ABS booster. Credit: USLaunchReport
Recovering and eventually reusing the 156 foot tall Falcon 9 first stage to loft new payloads for new paying customers lies at the heart of the visionary SpaceX CEO Elon Musk’s strategy of radically slashing future launch costs and enabling a space faring civilization.
The latest attempt to launch and propulsively land the Falcon booster on a platform a sea took place on Wednesday, June 15 after the on time liftoff at 10:29 a.m. EDT (2:29 UTC) from Space Launch Complex 40 on Cape Canaveral Air Force Station in Florida.
The 229 foot-tall (70 meter) Falcon 9 successfully accomplished its primary goal of delivering a pair of roughly 5000 pound commercial telecommunications satellites to a Geostationary Transfer Orbit (GTO) for Eutelsat based in Paris and Asia Broadcast Satellite of Bermuda and Hong Kong.
The Falcon 9 delivered the Boeing-built EUTELSAT 117 West B and ABS-2A telecommunications satellites to orbits for Latin American and Asian customers.
“Ascent phase & satellites look good,” SpaceX CEO and founder Elon Musk tweeted.
After first stage separation, SpaceX engineers attempted the secondary and experimental goal of soft landing the 15 story tall first stage booster nine minutes after liftoff, on an ocean going ‘droneship’ platform for later reuse.
OCISLY was stationed approximately 420 miles (680 kilometers) off shore and east of Cape Canaveral, Florida in the Atlantic Ocean.
However, for the first time in four tries SpaceX was not successful in safely landing and recovering the booster intact and upright.
The booster basically crashed on the drone ship because it descended too quickly due to insufficient thrust from the descent engines.
The rocket apparently ran out of fuel in the final moments before droneship touchdown.
“Looks like early liquid oxygen depletion caused engine shutdown just above the deck,” Musk explained via a twitter post.
The first stage is fueled by liquid oxygen and RP-1 propellant.
A SpaceX video shows a huge cloud of black smoke enveloping the booster in the final moments before the planned touchdown – perhaps soot from the burning RP-1 propellant.
In the final moments the booster is seen tipping over and crashing with unrestrained force onto the droneship deck – crushing and flattening the boosters long round core and probably the nine Merlin 1D first stage engines as well.
“But booster rocket had a RUD on droneship,” Musk noted. RUD stands for rapid unscheduled disassembly which usually means it was destroyed on impact. Although in this case it may be more a case of being crushed by the fall instead of a fuel related explosion.
“Looks like thrust was low on 1 of 3 landing engines. High g landings v sensitive to all engines operating at max,” Musk elaborated.
Watch these incredible launch videos showing many different vantage points:
Video caption: SpaceX Falcon 9 launch video compilation – Eutelsat and ABS satellites launched on 06/15/2016 from Pad 40 CCAFS. Credit: Jeff Seibert
Video caption: SpaceX Falcon 9 lifts off with Eutelsat 117W/ABS-2A electric propulsion comsats on June 15, 2016 at 10:29 p.m. EDT from Space Launch Complex 40 at Cape Canaveral Air Force Station, Fl, as seen in this up close video from Mobius remote camera positioned at pad. Credit: Ken Kremer/kenkremer.com
The dynamic relationship between Earth and the Sun two sides. The warmth from the Sun makes life on Earth possible, but the rest of the Sun’s intense energy pummels the Earth, and could destroy all life, given the chance. But thanks to our magnetosphere, we are safe.
The magnetosphere is our protective shield. It’s created by the rotation of the molten outer core of the Earth, composed largely of iron and nickel. It absorbs and deflects plasma from the solar wind. The interactions between the magnetosphere and the solar wind are what create the beautiful auroras at Earth’s poles.
In the inner regions of Earth’s magnetosphere are the Van Allen belts, named after their discoverer James Van Allen. They consist of charged particles, mostly from the Sun, and are held in place by the magnetosphere. Usually, there are two such belts.
But the output from the Sun is not stable. There are periods of intense energy output from the Sun, and when that happens, a third, transient belt can be created. Up until now, the nature of this third belt has been a puzzle. New research from the University of Alberta has shown how this phenomena can happen.
Researchers have shown how a so-called “space tsunami” can create this third belt. Intense ultra-low frequency plasma waves can transport the outer part of the radiation belt into interplanetary space, and create the third, transient belt.
The lead author for this study is physics professor Ian Mann from the University of Alberta, and former Canada Research Chair in Space Physics. “Remarkably, we observed huge plasma waves,” said Mann. “Rather like a space tsunami, they slosh the radiation belts around and very rapidly wash away the outer part of the belt, explaining the structure of the enigmatic third radiation belt.”
This new research also sheds light on how these “tsunamis” help reduce the threat of radiation to satellites during other space storms. “Space radiation poses a threat to the operation of the satellite infrastructure upon which our twenty-first century technological society relies,” adds Mann. “Understanding how such radiation is energized and lost is one of the biggest challenges for space research.”
It’s not just satellites that are at risk of radiation though. When solar wind is most active, it can create extremely energetic space storms. They in turn create intense radiation in the Van Allen belts, which drive electrical currents that could damage our power grids here on Earth. These types of storms have the potential to cause trillions of dollars worth of damage.
A better understanding of this space radiation, and an ability to forecast it, are turning out to be very important to our satellite operations, and to our exploration of space.
The Van Allen belts were discovered in 1958, and classified into an inner and an outer belt.
In 2013, probes reported a third belt which had never before been seen. It lasted a few weeks, then vanished, and its cause was not known. Thanks to Mann and his team, we now know what was behind that third belt.
“We have discovered a very elegant explanation for the dynamics of the third belt,” says Mann. “Our results show a remarkable simplicity in belt response once the dominant processes are accurately specified.”
An understanding of the radiation in and around Earth and the Van Allen belts is of growing importance to us, as we expand our presence in space. Our technological society relies increasingly on satellite communications, and on GPS satellites. Radiation in the form of high-energy electrons can wreak havoc on satellites. In fact, this type of radiation is sometimes referred to as a satellite killer. Satellites require robust design to be protected from them.
Organizations like the United Nations Committee on the Peaceful Uses of Outer Space (COPUOS) and the International Living with a Star (ILWS) Program are attempts to address the threat that radiation poses to our system of satellites.
The planets of our Solar System vary considerably in size and shape. Some planets are small enough that they are comparable in diameter to some of our larger moons – i.e. Mercury is smaller than Jupiter’s moon Ganymede and Saturn’s moon Titan. Meanwhile, others like Jupiter are so big that they are larger in diameter than most of the others combined.
In addition, some planets are wider at the equator than they are at the poles. This is due to a combination of the planets composition and their rotational speed. As a result, some planets are almost perfectly spherical while others are oblate spheroids (i.e. experience some flattening at the poles). Let us examine them one by one, shall we?
With a diameter of 4,879 km (3031.67 mi), Mercury is the smallest planet in our Solar System. In fact, Mercury is not much larger than Earth’s own Moon – which has a diameter of 3,474 km (2158.64 mi). At 5,268 km (3,273 mi) in diameter, Jupiter’s moon of Ganymede is also larger, as is Saturn’s moon Titan – which is 5,152 km (3201.34 mi) in diameter.
As with the other planets in the inner Solar System (Venus, Earth, and Mars), Mercury is a terrestrial planet, which means it is composed primarily of metals and silicate rocks that are differentiated into an iron-rich core and a silicate mantle and crust.
Also, due to the fact that Mercury has a very slow sidereal rotational period, taking 58.646 days to complete a single rotation on its axis, Mercury experiences no flattening at the poles. This means that the planet is almost a perfect sphere and has the same diameter whether it is measured from pole to pole or around its equator.
Venus is often referred to as Earth’s “sister planet“, and not without good reason. At 12,104 km (7521 mi) in diameter, it is almost the same size as Earth. But unlike Earth, Venus experiences no flattening at the poles, which means that it almost perfectly circular. As with Mercury, this is due to Venus’ slow sidereal rotation period, taking 243.025 days to rotate once on its axis.
With a mean diameter of 12,756 km (7926 mi), Earth is the largest terrestrial planet in the Solar System and the fifth largest planet overall. However, due to flattening at its poles (0.00335), Earth is not a perfect sphere, but an oblate spheroid. As a result, its polar diameter differs from its equatorial diameter, but only by about 41 km (25.5 mi)
In short, Earth measures 12713.6 km (7900 mi) in diameter from pole to pole, and 12756.2 km (7926.3 mi) around its equator. Once again, this is due to Earth’s sidereal rotational period, which takes a relatively short 23 hours, 58 minutes and 4.1 seconds to complete a single rotation on its axis.
Mars is often referred to as “Earth’s twin”; and again, for good reason. Like Earth, Mars experiences flattening at its poles (0.00589), which is due to its relatively rapid sidereal rotational period (24 hours, 37 minutes and 22 seconds, or 1.025957 Earth days).
As a result, it experiences a bulge at its equator which leads to a variation of 40 km (25 mi) between its polar radius and equatorial radius. This works out to Mars having a mean diameter of 6779 km (4212.275 mi), varying between 6752.4 km (4195.75 mi) between its poles and 6792.4 km (4220.6 mi) at its equator.
Jupiter is the largest planet in the Solar System, measuring some 142,984 km (88,846 mi) in diameter. Again, this its mean diameter, since Jupiter experiences some rather significant flattening at the poles (0.06487). This is due to its rapid rotational period, with Jupiter taking just 9 hours 55 minutes and 30 seconds to complete a single rotation on its axis.
Combined with the fact that Jupiter is a gas giant, this means the planet experiences significant bulging at its equator. Basically, it varies in diameter from 133,708 km (83,082.3 mi) when measured from pole to pole, and 142,984 km (88,846 mi) when measured around the equator. This is a difference of 9276 km (5763.8 mi), one of the most pronounced in the Solar System.
With a mean diameter of 120,536 km (74897.6 mi), Saturn is the second largest planet in the Solar System. Like Jupiter, it experiences significant flattening at its poles (0.09796) due to its high rotational velocity (10 hours and 33 minutes) and the fact that it is a gas giant. This means that it varies in diameter from 108,728 km (67560.447 mi) when measured at the poles and 120,536 km (74,897.6 mi) when measured at the equator. This is a difference of almost 12,000 km, the greatest of all planets.
Uranus has a mean diameter of 50,724 km (31,518.43 mi), making it the third largest planet in the Solar System. But due to its rapid rotational velocity – the planet takes 17 hours 14 minutes and 24 seconds to complete a single rotation – and its composition, the planet experiences a significant polar flattening (0.0229). This leads to a variation in diameter of 49,946 km (31,035 mi) at the poles and 51,118 km (31763.25 mi) at the equator – a difference of 1172 km (728.25 mi).
Lastly, there is Neptune, which has a mean diameter of 49,244 km (30598.8 mi). But like all the other gas giants, this varies due to its rapid rotational period (16 hours, 6 minutes and 36 seconds) and composition, and subsequent flattening at the poles (0.0171). As a result, the planet experiences a variation of 846 km (525.68 mi), measuring 48,682 km (30249.59 mi) at the poles and 49,528 km (30775.27 mi) at the equator.
In summary, the planets of our Solar System vary in diameter due to differences in their composition and the speed of their rotation. In short, terrestrial planets tend to be smaller than gas giants, and gas giants tend to spin faster than terrestrial worlds. Between these two factors, the worlds we know range between near-perfect spheres and flattened spheres.
Welcome back to Messier Monday! Today, in our ongoing tribute to Tammy Plotner, we take a look at the M15 globular cluster, one of the oldest and best known star clusters in the night sky. Enjoy!
In the 18th century, French astronomer Charles Messier began noticing a series of “nebulous objects” in the night sky while looking for comets. Not wanting other astronomers to make the same mistake, he began compiling a list of these objects into a catalog. In time, this list would include 100 objects, and came to be known by future astronomers as the Messier Catalog.
One of these objects is the globular cluster known as M15. Located in the northern constellation Pegasus, it is one of the brightest clusters in the night sky (with a visual brightness that is roughly 360,000 times that of our Sun). It is also one of the finest globular clusters in the northern section of the sky, the best deep-sky object in the constellation of Pegasus, and one of the oldest and best known globular clusters.
Messier 15 is probably the most dense globular cluster in our entire Milky Way galaxy – having already undergone a process of contraction. What does that mean to what you’re seeing? This ball of stars measures about 210 light years across, yet more than half of the stars you see are packed into the central area in a space just slightly more than ten light years in size.
By looking for single stars within globular clusters, the Hubble Space Telescope was either looking for a massive black hole or evidence of a “core collapse” – the intense gravity of so many stars so close together. Although it was peeking nearly 37,000 light-years away, the Hubble was able to resolve hundreds of stars converging on M15’s core. Like magnetism, their gravity would either cause them to attract or repel one another – and a black hole may have formed at some point in the cluster’s 12-billion-year life.
The study which addressed this data – which appeared in the January 1996 issue of the Astronomical Journal, was led by Puragra Guhathakurta of UCO/Lick Observatory, UC Santa Cruz – asked the question of whether or not the speed of the cluster’s stars could tell us if M15’s dense core was caused by a single huge object, or just mutual attraction. As Guhathakurta stated in the study:
“It is very likely that M15’s stars have concentrated because of their mutual gravity. The stars could be under the influence of one giant central object, although a black hole is not necessarily the best explanation for what we see. But if any globular cluster has a black hole at its center, M15 is the most likely candidate.”
John Bahcall and astrophysicist Jeremiah Ostriker of Princeton University were the first to forward the idea that Messier 15 might be hiding a black hole. While it is distinct from many other globular clusters by having such a dense core, it really isn’t that much different than all the rest of the globular clusters we see. Yet, no where else in our galaxy, except at its core, are the stars that dense!
It is estimated that 30,000 distinct stars exist in the inner 22 light-years of the cluster alone. The closer the Hubble telescope looked, the more stars it found. This increase in stellar density continued all the way to within 0.06 light-years of the center – about 100 times the distance between our Sun and Pluto. “Detecting separate stars that close to the core was at the limit of Hubble’s powers,” says Brian Yanny of the Fermi National Accelerator Laboratory.
At this point, even the great Hubble could not distinguish individual stars, or locate the exact position of the core. Guhathakurta and is colleagues theorized that the stars crowd even closer inside the radius, so they plotted the distribution of the stars as a function of distance from the core. When the results came back, they had two answers – either a black hole was responsible, or a gravothermal catastrophe called core collapse was the culprit.
“It’s a catastrophe in the sense that once it starts, this process can run away very quickly,” said Guhathakurta. “But other processes could cause the core to bounce back before it collapses all the way.”
At an estimated 13.2 billion years old, it is one of the oldest known globular clusters, but it isn’t done throwing some surprises at us. M15 was the first globular cluster in which a planetary nebula, Pease 1 or K 648 (“K” for “Kuster”), could be identified – and can be seen with larger aperture amateur telescopes. Even stranger is the fact that Messier 15 contains 112 variable stars, and 9 known pulsars – neutron stars which are the leftovers of ancient supernovae. And one of these is a double neutron star system – M15 C.
History of Observation:
M15 was discovered by Jean-Dominique Maraldi on September 7, 1746 while he was looking for a comet. Says he:
“On September 7 I noticed between the stars Epsilon Pegasi and Beta Equulei, a fairly bright nebulous star, which is composed of many stars, of which I have determined the right ascension of 319d 27′ 6″, and its northern declination of 11d 2′ 22”. About 25 years later, Charles Messier would independently rediscover it to add to his own catalog, describing it as: “In the night of June 3 to 4, 1764, I have discovered a nebula between the head of Pegasus and that of Equuleus it is round, its diameter is about 3 minutes of arc, the center is brilliant, I have not distinguished any star; having examined it with a Gregorian telescope which magnifies 104 times, it had little elevated over the horizon, and maybe that observed at a greater elevation one can perceive stars.”
Sir William Herschel would be the first to resolve some of its stars, but not the core. It would be his son John who would later pick up structure. However, like the dutiful and colorful observer that he was, Admiral Smyth will leave us with this lasting impression:
“Although this noble cluster is rated as globular, it is not exactly round, and under the best circumstances is seen as in the diagram, with stragglers branching from a central blaze. Under a moderate magnifying power, there are many telescopic and several brightish stars in the field; but the accumulated mass is completely insulated, and forcibly strikes the senses as being almost infinitely beyond those apparent comets. Indeed, it may be said to appear evidently aggregated by mutual laws, and part of some stupendous and inscrutable scheme of involution; for there is nothing quiescent throughout the immensity of the vast creation.”
Considering Smyth’s observations were made nearly two centuries before we really began to understand what was going on inside Messier 15, you’ll have to admit he was a very good observer!
Locating Messier 15:
Surprisingly enough, globular cluster M15 is easy to find. Once you’ve located the “Great Square” of Pegasus, simply choose its brightest and southwesternmost star – Alpha. Now identify the small, kite shape of the constellation of Delphinus. Roughly halfway between these two (and slightly south), you’ll spy a slightly reddish star – Epsilon Peg (Enif).
By placing Enif in your binoculars or image correct finderscope at the 7:00 position, you can’t miss this bright, compact beauty. Even the smallest of optics will reveal the round glow and telescopes starting at 4″ will begin resolution – while large telescopes will simply amaze you. However, don’t expect to open this globular up to the core region. As already noted, its pretty dense in there!
And here are the quick facts for Messier 15, for your convenience:
Object Name: Messier 15 Alternative Designations: M15, NGC 7078 Object Type: Class IV Globular Cluster Constellation: Pegasus Right Ascension: 21 : 30.0 (h:m) Declination: +12 : 10 (deg:m) Distance: 33.6 (kly) Visual Brightness: 6.2 (mag) Apparent Dimension: 18.0 (arc min)