We tend to think of the Earth orbiting around the Sun along a path largely free from debris. However, this is not strictly true. Twenty-five years ago, scientists discovered that Earth orbits the Sun along with a giant ring of dust which appears to have originated within the asteroid belt that lies between Mars and Jupiter. This belt is made up of millions of rocks of all sizes, many of which over the millennia crash into one another and grind together, producing a lot of dust. This gradually falls towards the Sun as a result of gravity – but along the way, some of it is influenced by the Earth’s gravity, becoming trapped along and either side of the Earth’s orbit, forming a ring.
Observations of Mars by NASA’s Maven orbiter have also given indications that the Red Planet could have a ring – or at least, a proto-ring – occupying its orbit, while 10 years ago, astronomers discovered a ring straddling the orbit of Venus. Now a new study reveals little Mercury has a ring of dust lying along its orbit – although by rights, it shouldn’t.
Mercury’s ring was discovered entirely by accident – ironically, those responsible for its discovery, Guillermo Stenberg and Russell Howard of Naval Research Centre in Washington, DC, were attempting to find a dust-free region that is thought to surround the Sun, created by solar energy radiating outwards from our star. The idea being that determining the size of this dust-free region would both reveal more about the nature of the Sun and the evolution of the solar system. But instead of locating this area of “empty” space, the astronomers discovered the ring sharing Mercury’s orbit.
People thought that Mercury, unlike Earth or Venus, is too small and too close to the Sun to capture a dust ring. They expected that the solar wind and magnetic forces from the Sun would blow any excess dust at Mercury’s orbit away.
– Astronomer Guillermo Stenberg
The two scientists worked with images from NASA’s STEREO solar observatory. This pair of satellites follow highly elliptical geocentric orbits. Over time, one of them pulls farther ahead of Earth while the other falls further behind. This means that together they provide stereo images of the Sun. In studying the images from the satellites, Stenberg and Howard noticed an area of enhanced brightness along Mercury’s orbit, indicative of a dust ring being present.
The question is – how did it form? There’s no answer to this yet; as Stenberg notes, the ring shouldn’t be there, and the lesson of Venus has revealed that it’s better not to assume common factors in the formation of these rings.
This is because initially, it was assumed the ring around Venus was the result of the same gravitational forces that have created the dust ring along Earth’s orbit. However, when astrophysicists Petr Pokorny and Mark Kuchner from NASA’s Goddard Space Flight Centre attempted to use extensive computer modelling to try to reproduce a dust ring matching the one in Venus’ orbit, they were unable to do so.
As a result, the two started researching and modelling possible explanations, and in a paper published on March 12th, 2019, the two suggest that the Venusian ring is the result of a previously undiscovered group of asteroids occupying the same orbit as Venus with a 1:1 resonance (that is, they complete one orbit of the Sun for every orbit Venus makes). Further, their research suggests that the group of asteroids are the remnants of a much larger asteroid ring that existed when the solar system was born.
The asteroid themselves have yet to be located – no easy task, assuming they do exist, as the Venusian dust ring is 25.5 million km (16 million mi) deep, and 9.6 million km (6 million mi) across, and bright enough to hide larger objects within it. However, if the asteroid are discovered, they would not only confirm the theory about how the dust ring around Venus’ orbit formed, but also hold clues to how the solar system formed.
Further SLS Changes
In my previous Space Sunday report, I covered the announcement by NASA that suggested the Space Launch System rocket might have its initial launch delayed. Now it seems the system is to undergo further changes to both its initial flights and its future development.
As it was originally planned, the SLS was to have been initially launched in its Block 1 configuration. This would see the vehicle use what is called the Interim Cryogenic Propulsion Stage (ICPS) as its upper stage. After that, launches would switch over to using the Block 1B version, intended to use a more powerful upper stage called the Exploration Upper Stage (EUS), being built by Boeing Aerospace.
Given issues with the development of the EUS, in late 2018 NASA announced the first two SLS launches, referred to as EM-1 and EM-2, and designed to send a Orion vehicle on a month-long trip around the Moon, the first uncrewed, the second crewed, will utilise the Block 1 version of the rocket, with flights thereafter shifting to the Block 1B rocket to undertake tasks such as launching elements of the Lunar Gateway. Now, under the Trump Administration’s 2020 budget request, it appears the introduction of the EUS is to be deferred – possibly indefinitely, with NASA ordered to carry out all initial flights using the Block 1 variant of the rocket.
While the ICPS stage is more than sufficient to achieve the objectives established for EM-1 and EM-2, it is not powerful enough to meet all of the demandd of the proposed Lunar Gateway development. Instead, NASA is expected to supplement SLS flights to build the Gateway with the use of commercial launch vehicles, such as the United Launch Alliance Delta V, the SpaceX Falcon Heavy and – potentially – Blue Origin’s New Glenn.
These changes also mean that SLS will be unable to launch NASA’s Europa Clipper, the mission to study Jupiter’s icy moon Europa. While that mission will still go ahead, NASA is to turn to using a commercial vehicle; most likely a Falcon Heavy. The White House indicate that switching to a commercial launcher would shave around $600 million from the mission’s cost; however, use of the Falcon Heavy will mean Europa Clipper will have to complete a longer journey to Jupiter than would be the case when using the SLS as the launch vehicle.
Nor is that all; such are the concerns around the status of SLS development, that in order to achieve the planned June 2020 launch window for the EM-1 mission, NASA Administrator Jim Bridenstine has indicated he is considering switching to commercial rockets. Were this to happen, it would be something of a body blow for the SLS programme; however, it is also an option perhaps easier said than done.
For one thing, there is no launch vehicle currently available capable of carrying the Orion capsule, its service module and the upper stage needed to push it to the Moon in a single go. Thus, a switch to using any commercial launch system would require two launches: one to place the Orion and its service module into orbit around Earth, and the other to do the same with the upper stage engine and fuel needed to push Orion to the Moon. This raises the spectre of an on-orbit rendezvous and docking between the Orion and the upper stage – something Orion is not outfitted to complete. Thus, any switch to using commercial vehicles for the EM-1 flight will incur both additional launch costs for NASA and require the mission be delayed while the required rendezvous and docking capability is developed for Orion – something that will likely delay the launch of EM-1 – possibly beyond the date by which SLS will be cleared to fly if it cannot be ready by June 2020.
China’s Chang’e and Yutu 2 Face Third Lunar Night
China’s lander and rover mission to the southern region of the Moon’s far side have been powered down ready to face their third lunar night period.
The lander, carrying the 140 kg rover, arrived on the surface of the Moon in the 186 km (115 mi) diameter Von Kármán crater within the South Pole-Aitken Basin, on January 2nd, 2019. Since that time, the rover has covered a total drive distance of 163 metres, carrying out a range of science studies – including manoeuvring up to a 20 cm rock to analyse its composition. The lander has also carried out a range of scientific studies of the local environment.
Both vehicles have functioned flawlessly thus far, surviving both the cold extremes of the 2-week lunar night periods, and the searing heat of the “midday” Sun during the 2-week “day” periods, when both vehicles again have to be shut down to prevent sensitive electronics being impacted by solar radiation.
Designed to last 3 months, Yutu-2 continues to function well, and providing it comes through the latest lunar night period unscathed, the operations team plan to resume driving operations once the sun has risen late on March 28th, 2019. At that time, the rover will continue travelling in a north-westerly direction away from the landing site, which has been officially named Statio Tianhe (“Location of Harmony of the Heavens”) by the International Astronomical Union.
Both the rover and the lander entered their dormant states within minutes of one another, as confirmed by the communications relay satellite Queqiao (Magpie Bridge), occupying a halo orbit in the L2 position beyond the Moon, where it can “see” both the landing site and Earth. As well as being a communications relay, Queqiao carries its own science packages, including the Netherlands-China Low-Frequency Explorer (NCE), which deployed its three 5-metre (15 ft) antennae ready to make observations of the solar system and, potentially, record signals from the cosmic “dark ages”.
Opportunity’s Last Panorama
This is the final image returned to Earth by NASA’s Mars Exploration Rover Opportunity. Captured over 29 days between May 13th and June 10th, 2018, it offers 360-degree panorama of Oppy’s resting place in Perseverance Valley, Endeavour Crater, an impact crater at Meridiani Planum.
The images were captured in sets of three, each one with a different filter. They have all been combined to create one coloured panorama, although the lower left elements are black-and-white as a result of the global dust storm that ended Opportunity’s life blocking communications before all the captured images could be transmitted.
This final panorama embodies what made our Opportunity rover such a remarkable mission of exploration and discovery. To the right of centre you can see the rim of Endeavour Crater rising in the distance. Just to the left of that, rover tracks begin their descent from over the horizon and weave their way down to geologic features that our scientists wanted to examine up close. And to the far right and left are the bottom of Perseverance Valley and the floor of Endeavour crater, pristine and unexplored, waiting for visits from future explorers.
– Opportunity project manager John Callas