Space Sunday: farewell and welcome back

One of the last images of Ceres returned by the Dawn mission which was officially declared ended on November 1st, 2018. Note the bright carbonate mineral deposits in Occator Crater to the right of the image. Credit: NASA/JPL

Two important space missions came to an end at the end of October 2018. The Kepler observatory, which spent nine years in deep space collecting data that detected thousands of planets orbiting stars outside our solar system; and the Dawn spacecraft, which spent 11 years orbiting and studying the main asteroid belt’s two largest objects, Vesta and Ceres.

Concerns had been growing for months over Kepler’s ability to continue working as a result of dwindling on-board propellant supplies, as the space observatory has had to use it thrusters a lot more than originally planned, following the failure of some of its pointing gyroscopes several years ago. Similarly, the end of the Dawn mission had been signed as a result of that vehicle also running low on orientation propellants.

Launched in 2007, Dawn was the first spacecraft to orbit a body between Mars and Jupiter, and the first to orbit more than one deep-space destination. From 2011 to 2012, the spacecraft studied the asteroid Vesta before pulling off an unprecedented manoeuvre by leaving orbit and travelling to the dwarf planet Ceres, which it observed for over 3.5 years. Even with the mission now officially over, Dawn will remain in a stable orbit around Ceres for decades, while among its many findings, Dawn helped scientists discover organics on Ceres and evidence that dwarf planets could have hosted oceans over a significant part of their history—and possibly still do.

Both missions were extended past their originally anticipated lifetime because of the innovative work of their engineers and scientists. In 2016, Dawn’s mission at Ceres was extended. In 2017, its mission at Ceres was extended again to study the dwarf planet from altitudes as low as 35 km (22 mi) above the surface, with the main goal of understanding the evolution of this dwarf planet.

Dawn depleted its hydrazine propellant on October 31st, 2018 while still actively engaged in studying Ceres. Without it, the vehicle could not keep its solar panels oriented towards the Sun in order to provide energy to its battery systems, resulting in a complete loss of contact with Earth. Attempts were made to re-establish communications through NASA’s Deep Space Network, but the loss of propellants had been expected, and the US space agency officially announced the mission as concluded on November 1st, 2018.

Ceres’ lonely mountain, Ahuna Mons, seen in a simulated perspective view with the elevation has been exaggerated by a factor of two. The view was made using enhanced-colour images from NASA’s Dawn mission. Credit: NASA/JPL

Among the more surprising discoveries Dawn made was the fact that small bodies in the solar system like Vesta and Ceres are more diverse in nature that had even been thought. Dawn also revealed that geological activity on Ceres had once been sufficient to raise a massive 5 km (3 mi) high cryovolcano, Ahuna Mons (or informally, The Lonely Mountain), and to create more than 300 bright features, called faculae. On Earth, these bright deposits of carbonate minerals are associated with water, suggesting Ceres may have, or had, a liquid water interior. The brightest of these deposits, in Occator Crater is also the largest deposit of carbonate minerals found beyond Earth.

Such is the amount of data returned by Dawn, analysing it all will still take several more years, as noted by the mission’s Principal Investigator, Carol Raymond:

In many ways, Dawn’s legacy is just beginning. Dawn’s data sets will be deeply mined by scientists working on how planets grow and differentiate, and when and where life could have formed in our solar system. Ceres and Vesta are important to the study of distant planetary systems, too, as they provide a glimpse of the conditions that may exist around young stars.

Kepler, meanwhile, was launched in 2009 and completed its primary mission in 2012, leading to the first mission extension. Then, in 2013, a second gyroscope failure left the observatory unable to continue in its primary operating mode. Instead, engineers found a way to use both solar pressure and the observatory’s manoeuvring jets to keep it pointing in a desired direction. This allowed a new mission, dubbed K2, to commence in 2014. It has been running ever since, gathering science from 19 different patches of sky with populations of stars, galaxies and solar system objects.

Kepler was officially retired on October 30th, 2018. For most of the year it had been showing signs of running out of propellants, and without them, it would be unable to maintain the correct orientation to either continue observations or turn itself to communicate with Earth.

As NASA’s first planet-hunting mission, Kepler has wildly exceeded all our expectations and paved the way for our exploration and search for life in the solar system and beyond. Not only did it show us how many planets could be out there, it sparked an entirely new and robust field of research that has taken the science community by storm. Its discoveries have shed a new light on our place in the universe, and illuminated the tantalizing mysteries and possibilities among the stars.

– NASA’s official announcement on Kepler’s retirement

Kepler by the numbers. Credit: NASA

Overall, Kepler revealed that there is statistically at least one planet around every star in our galaxy, and opened our eyes to the variety of worlds beyond our solar system, discovering more than 2,600 exoplanets. Among these worlds are rocky, Earth-sized planets, some of which orbit within their stars’ habitable zones, where liquid water could pool on the surface. Kepler also characterised a class of planets that don’t exist in our solar system: worlds between the sizes of Earth and Neptune, or “super-Earths.”

Kepler also revealed just how different planetary systems can be from our own. For instance, the most common type of planet it observed (those that are between the size of Earth and Neptune) doesn’t even exist in our Solar System. In addition, Kepler found planetary systems that were so packed with planets orbiting close to their stars that it made the Solar System look sparsely populated by comparison.

Kepler’s passing does not mark the end of space-based planet hunting, however. In April 2018, NASA launched TESS, the Transiting Exoplanet Survey Satellite, which is just getting started on its mission. Intended to last 2 years, it is anticipated TESS will examine the 200,000 brightest stars around us during its primary mission phase in the hope of detecting planetary bodies in orbiting them. It will do this using the transit method of observation used by Kepler – looking for dips in the brightness of stars which might indicate the passage of an orbiting planet between the star and the telescope.

TESS will also attempt to measure the masses of at least 50 small planets whose radii are less than four times that of Earth, offering the opportunity to characterise their likely structure and composition. Many of TESS’s planets should be close enough to our own that, once they are identified, scientists will be able to study them using telescopes such as Hubble and the forthcoming James Webb Space Telescope, to detect atmospheres, characterise atmospheric conditions, and even look for signs of habitability.

Russia Pins Soyuz Lunch Failure on Assembly Error

On Thursday, November 1st, Russia issued the findings of its investigation into the failure of a Soyuz launch vehicle in October 2018. En-route to the ISS, the launch vehicle was completely destroyed, but the two crew, US astronaut Nick Hague and Russian cosmonaut Aleksey Ovchinin, were unharmed, the launch abort system hauling their Soyuz orbital vehicle clear of the rocket prior to the latter exploding, allowing them to make a safe return to Earth.

Oleg Skorobogatov, the head of the commission that probed the accident, said the flight was aborted because part of a sensor that indicates the separation of the stages of the rocket was damaged during the rocket’s assembly at the Baikonur cosmodrome in Kazakhstan. As a result, one of the four strap-on boosters that form the vehicle’s first stage failed to detach at its forward end when the jettison was triggered, causing it to pivot into the core stage, precipitating the loss of the vehicle. Space expert Scott Manley explains more in the video below.

As no actual vehicle component has been found to be defective, Roscosmos plan to resume Soyuz crewed launches on December 3rd, 2018, when Russian cosmonaut Oleg Kononenko, Canadian astronaut David Saint-Jacques, and NASA’s Anne McClain, are due to depart for the ISS. However, to prevent further errors in future vehicle assembly, Roscosmos will be retesting assembly personnel at Baikonur, and implementing additional training and safety checks.

Hubble Back In Action, Chandra Almost So

In my October 15th, 2018 Space Sunday update, I wrote about the Hubble Space Telescope and Chandra X-Ray Observatory both entering “safe” modes. Both situations occurred within 48 hours of one another and both – as it turned out – were related to gyroscope issues.

As with Kepler, both Hubble and Chandra use gyroscopes to maintain a stable orientation when performing their science operations. For Chandra, the safe mode was triggered when a glitch in one of its gyroscopes resulted in a 3-second period of bad data being sent to the main computer, causing it to incorrectly calculate the spacecraft’s momentum.

An artist’s impression of the Chandra X-Ray telescope above the Earth. Credit: NASA

Identifying this root cause for the issue on October 15th, 2018, the mission team were able to get Chandra to swap the problematic gyroscope with one of its reserve systems. This took several days to complete, but allowed the observatory to exit its safe mode. It was then instructed to perform a set of manoeuvres to change its pointing and orientation in order to confirm the gyroscopes were behaving as expected. As a result of these tests, carried out over a number of days, on October 24th, NASA confirmed Chandra would require the upload of a small software patch to completely eliminate the remaining issues. This is expected to happen soon, after which the observatory should be able to resume science operations.

Meanwhile, Hubble’s issues were a little more troublesome. As a result of one of the platform’s three remaining operational gyroscopes, the mission team were forced to take it off-line and try to replace it with the last remaining reserve gyro – which itself had previously suffered issues. The first attempt to do this failed, triggering Hubble putting itself into a safe mode.

Normally, Hubble requires three gyros to operate – but can manage on two. However, as this could shorten the telescope’s remaining operational life, mission engineers ordered Hubble to performs multiple manoeuvres, switching the gyro between different operational modes, eventually persuading the unit to start behaving itself, and Hubble returned to normal science operations on October 27th, 2018.

Park Solar Probe Sets Records

In August 2018, I covered the launch of the Parker Solar Probe, a mission to uncover the secrets of the Sun. Due to the complexes in getting into a suitable heliocentric orbit – which is actually harder than getting to the outer planets of the solar system – Parker will actually spend seven years effectively making loops around the Sun, making fast-fly-bys, whilst also using similar fly-bys of Venus along the way to keep it on track.

The first of the probe’s solar fly-bys is due to take place on 6th November 2018, when it will pass just 24 million km (15 million mi) from the Sun’s photosphere (what we might call its “surface”). Eventually, as these loops continue it will come to within just 6.9 million km (4.3 million mi) of the photosphere.

An artist’s impression of the Parker Solar Probe swinging around the Sun at a distance of 6.2 million km (3.85 million mi) . Credit: NASA

Ahead of this, on October 29th, 2018, Parker set a new record as the closest human-made object to the Sun when in passed inside  the orbit of the German-American Helios mission, which circles the Sun at a distance of 42.73 million km (26.55 million mi).

And if it sounds like the probe has a very long way to go in a very short time to make that first perihelion of just 6.2 million km, it’s also worth pointing out that at the end of October Parker became the fastest human-made object moving relative to the Sun, travelling at 246,961 km/h (153,454 mph). That’s 68.6 kilometres a second, or London to New York in less than 90 seconds.

As the mission continues, Parker will briefly touch a velocity of 700,000 km/h (430,000 mph) – or London to Tokyo in 50 seconds. The vehicle won’t maintain this level of velocity, however, as it uses Venus to achieve its loops around the Sun, it will also use the planet to gradually decrease its speed, allowing it to study the Sun in greater detail as it loops around it.