Tag: ESA

Space Sunday: BepiColombo and Mercury + a RST Update

Posted on by Inara Pey
An infographic on the ESA / JAXA BepiColombo mission, which is now approaching it’s destination: Mercury. Credit: Airbus – click for full size

Mercury, the smallest planet in our solar system is also one of the closest to Earth – the average separation between the two being around 77 million kilometres (although obviously, this varies widely depending on which side of the Sun they are on relative to one another). When you compare that to say, the 714 million km average distance between Earth and Jupiter (with the same caveat of position relative to one another and the Sun), you’d think Mercury would be an easy place to get to.

Not so. It actually takes more energy for a spacecraft launched from Earth to reach “nearby” Mercury than it does to get to the outer solar system. There are two reasons for this. The first is that – as we all know – Mercury is racing around the Sun much faster than Earth, with a sidereal period of just under 88 terrestrial days. This means it is moving more than half as fast again as Earth: 47.6 kilometres per second compared to our 30 km/s. So to catch it, anything launched from Earth has to be going very fast- and it’s going to be accelerating all the way as it will effectively be falling “down” the Sun’s gravity well.

And therein lies the second problem: the speed required to reach Mercury + the acceleration gained under the Sun’s influence means that the spacecraft is going to be zipping along way too fast to actually slow itself in order to achieve orbit – not unless it is carrying a huge amount of propellants – so much in fact, that the mass of those propellants couple with the size of their storage requirements make such a mission a non-starter; not without a means to get around this propellant problem.

Mercury infographic. Credit: sciencenotes.org

And there just so happens that there is a means to do so – and it was actually proposed specifically to enable the very first mission ever launched to Mercury. It is called the gravity assist; using the gravity of one or more planets to either accelerate or slow a spacecraft, whilst also alter its course sufficiently enough so that it can intercept the orbit of its intended destination.

Gravity assists are commonplace today, used in many missions – both of the current mission to Jupiter – NASA’s Europa Clipper and the European Space Agency’s (ESA) Juice mission – are using multiple gravity assists to boost them to Jupiter and reduce the propellant load they would otherwise need. But it was with NASA’s Mariner 10 mission of the 1970s where the idea was first used. The mission used the gravity of Venus to both accelerate the spacecraft and swing it on to an orbit around the Sun such that it would be able to make 3 fly-bys of Mercury, the first two of which (1974 and 1975) would additionally adjust its orbit to enable the next fly-by (the last also occurring in 1975). Even so, the spacecraft was travelling far to fast to ever enter orbit around Mercury.

Launched in 2004, NASA’s MESSENGER mission utilised a fly-by of Earth (2005), two of Venus (2006/07) and three with Mercury itself (2×2008, 1×2009)in order to initially accelerate it and adjust its orbit so it could reach Mercury, with the 3 fly-bys of the planet then being used to slow the spacecraft so that in 2011, it could use its thrusters to slow itself sufficiently to enter orbit, becoming the first probe from Earth to orbit Mercury at the start of a 4-year mission there.

Now a joint mission by the European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA) is about to become only the second mission to achieve orbit around Mercury and commence a joint period of study. It is, without hyperbole, one of the most complex missions ever undertaken, involving a multi-part space vehicle which includes two orbiters, and eight-year flight to Mercury, and no fewer than nine gravity assist fly-bys. The mission is called BepiColombo – named for the man who developed the concept of the gravity assist fly-by as noted above: mathematician, engineer and scientist, Giuseppe “Bepi” Colombo.

Giuseppe “Bepi” Colombo working on his idea of the spacecraft gravity-assist. Credit: ESA

Four in One, and One for All!

The mission comprises four main elements: three spacecraft stacked together for the voyage to Mercury, together with a special thermal shade and power relay unit. These are:

  • The Mercury Transfer Module (MTM): a vehicle built by ESA and combining chemical and solar electric propulsion as well as serving as the initial power bus and navigation system for the rest of the stack.
  • The Mercury Planetary Orbiter (MPO): a 1.15 tonne science satellite, again built by ESA and carrying a suite of 11 science instruments comprising cameras, spectrometers (IR, UV, X-ray, γ-ray, neutron), a radiometer, a laser altimeter, a magnetometer, particle analysers, a Ka-band transponder, and an accelerometer. The majority of these are located on the nadir side of the vehicle (the side constantly facing away from the Sun) for optimal performance.
  • The Mercury Magnetospheric Orbiter (MMO, also called Mio – meaning “water channel”, Mercury being known as the “water star” in Japan): a small, 285 kg satellite built by JAXA carrying a further 10 instruments, 6 of which are packaged into the combined Mercury Plasma Particle Experiment (MPPE), with the remaining four operating as independent instruments.
  • The Magnetospheric Orbiter Sunshield and Interface (MOSIF): a protective shroud for MMO to help with its thermal regulation during the mission’s time orbiting the Sun in order to reach Mercury, and to provide a power interface for MMO during this time.
2018: The MTM, with solar electric thruster ring fitted and red-back solar arrays in their stowed position, is prepared for lowering onto its mobility platform (left). To the right, the MPO sheathed in its white thermal blanketing and with its high gain antenna and solar array in their stowed positions, sits with the black cylinder of the MMO on top of it, the pair waiting to be mated to the top of the MTM. Credit: Astrium

First selected for development by ESA in 2000 as the last of the agency’s Horizon 2000+ programme, the mission was to have included a 44 kg lander. Had this been flown, it could have carried a dual imaging system (descent camera and surface camera), an x-ray spectrometer, magnetometer, a seismometer, a drilling system for obtaining sub-surface samples and a micro-rover! Unfortunately, budgetary constrains meant it was cancelled well before contracts for its design and construction were arranged.

The science packages carried by the mission are multi-national, with instruments supplied by ESA member states (Austria, Finland, France, Germany, Italy, Spain, Sweden, Switzerland and the UK) together with Japan, Russia and the United States.

The Long Slog

Issues with developing the flight hardware – particularly the MTM’s solar electric propulsion system – led to a series of delays such that the mission did not launch until October 2018, just over 4 years after its target launch date of July 2014. Launch was aboard an Arianespace Ariane 5 booster – marking the 101st launch of that vehicle – out of Europe’s Spaceport in Kourou, French Guiana.

It was the easiest part of the mission, putting the combined vehicle into a heliocentric orbit close to that of Earth but with a hyperbolic excess velocity of 3.475 km/s. This meant that 18 months after launch, the mission swung by Earth in April 10, 2020, performing a fly-by / gravity assist at an altitude at closest approach of 12,700 km. A combination of the time of the fly-by (04:25 UTC), the size of the spacecraft and the degree to which it reflected sunlight meant that in some quarters, it was mistaken for a previously unknown near-Earth asteroid and gained a temporary designation as such – 2020 GL2.

Passing Earth successfully, the vehicle moved onto a trajectory heading in towards the orbit of Venus, where it would eventually complete two further gravity assist in 2020 and 2021, accelerating it on towards Mercury. The first of the fly-bys of Venus came just after reports had been made that the biomarker phosphine (PH3) had been detected in Venus’s upper atmosphere, suggesting it might contain microbial life. Requests were made for the BepiColombo to use the instruments on MPO and MMO to try to detect PH3 as well, although doubt was expressed as to whether they would be able to, as they were not calibrated for phosphine detection. No report on any attempts to do so has ever been released by ESA or JAXA, suggesting either the attempts were not made, or they yielded negative results. The original claim was also cast into doubt after a series of Earth-based follow-up studies failed to locate any phosphine in Venus’ atmosphere.

The August 2021 fly-by of Venus was significant as it put BepiColombo on a direct intercept with Mercury, the fly-by taking place in October 2021. This allowed instruments aboard both MPO and MMO to be directed at their intended target, together with camera mounted on the MTM.

September 5, 2024, BepiColombo moves away from Mercury after its 4th fly-by of the planet, and the first one in which it passed over the planet’s south pole. This image was captured by the dual imaging system on the MTM module. Credit: ESA

Between October 2021 and January 2025, the mission completed six fly-bys of Mercury, allowing it to carry out a number of tasks. These included direct observations of the planet and measurements of its immediate environment, as well as helping to reduce the vehicle’s velocity so that it would eventually be able to enter orbit around the planet.

Originally, orbital insertion had been planned for December 2025, with the MTM to be jettisoned shortly ahead of that time, its job done. However, April 2024 saw the MTM’s solar electric thrusters develop a fault which prevented them for operating at full power, and would thus be unable to generate the thrust required to brake the spacecraft into orbit in December 2025. This resulted in the cruise phase of the mission being extended through November 2026, thus allowing the thrusters to be used in a series of lower-power firings over the extended cruise phase to help reduce its velocity such that it would be able to achieve orbital insertion.

On January 8, 2025, the sixth and final final fly-by of Mercury produced the shot taken along the planet’s terminator by the MTM imaging system. The tall impact peaks at the centre of craters Prokofiev and Tolkien can be seen reflecting sunlight. Credit: ESA

The last of these braking manoeuvres was completed in June 2026, successfully reducing the spacecraft’s velocity such that both the MPO and MMO would be able to enter orbit around Mercury under their own power. The solar electric thrusters on the MTM were therefore shut down for the last time, and the vehicle started its coast towards the mission’s Mercury Arrival Phase (MAP).

On September 3, 2026, this MAP phase of the mission commenced with the successful separation of the MTM, its work done. This means that the MPO is now the provider of power, navigation and propulsion for the mission. The next major milestone will come on November 21, 2026 as the spacecraft enter an initial 60-hour polar orbit around Mercury. Then, for a period of roughly 20 days, the MPO’s thrusters will be periodically used to drop that orbit to 9.3 hours and occupying with a perihermion of 590 km extending out to an apohermion of 11,640 km. At this point, MMO Mio will be spun-up and released into this orbit on December 10, 2026, allowing it to commence its primary mission, after which the MOSIF protective shield for MMO will be jettisoned, its job also done.

MPO will then continue to refine its orbit to 480 km x 1,500 km and a period of 2.36 hours through early 2027, prior to commencing its primary science mission around the start of April 2027.

And the Objective Is?

The primary objectives of the BepiColoumbo mission are to study the likely origin and evolution of a planet so close to its parent star; gain insight into Mercury’s actual form, interior, composition and internal structure; study the planet’s tiny and unstable exosphere (comprising, in decreasing order, hydrogen, helium, oxygen, sodium, calcium, potassium and other trace elements) and its magnetosphere, gain a greater understanding of its interactions with the solar atmosphere and – entirely separate to the planetary studies – test Einstein’s general theory of relativity.

The primary mission is expected to last around a year, with the potential for an extended mission (subject to funding and propellants) through until the end of December 2029. The overall cost of the mission from development through to launch and the end of the primary mission of €1.65 billion (US $1.86 billion).

Nancy Grace Roman Space Telescope – Update

In my previous Space Sunday piece, and at the time of its launch, I covered some of the history of the Nancy Grace Roman Space Telescope (NGRST, or more simply, RST) together with some biographical notes about the woman after whom it was named. Since then, three further milestones have been passed:

  • On August 31, 2026, the critical mid course, 3-minute burn of the observatory’s thrusters system was successfully performed. At the time of writing, NASA were still evaluating whether a second mid-course correction manoeuvre would be required, the first was so accurate.
  • On September 1, 2026, the high-gain communications antenna was unfolded from the rear of the observatory and commenced testing.
  • Also on the same day, the deployable aperture cover, designed to protect the telescope’s optics from the light of the Sun, successfully unfurled and one of the two main science systems, the Coronagraph Instrument (CGI), was powered-up for initial testing.
An animation showing the opening of the deployable aperture coveron RST, September 1, 2026. Credit: NASA

If the second mid-course correction burn is not required at it heads for its halo orbit around the Sun-Earth Lagrange L2 position, it will mean the observatory will have a small additional reserve of propellants to help with it regime of on-orbit station-keeping manoeuvres, one of which will be required roughly every 28 days once it enters its planned operational orbit around the L2 position.

Still to come is the powering-up of the Wide Field Instrument (WFI) suite and its initial check-out, after which the telescope will enter fully into a pre-commissioning phase of operations.

Space Sunday: ESA’s Hera and catching a rocket in mid-air

Posted on by Inara Pey
Seconds from capture: Super Heavy Booster 12 descends between the “chopsticks” of the Mechazilla lifting system of the tower from which it and Ship 30 launched less than 8 minutes previously, as the arms close around it in readiness for a safe capture during the fifth integrated flight test of SpaceX’s starship / super heavy launch system. Story below. Credit: SpaceX via the NSF.com livestream.

Hera: Return to Didymos

On November 24th, 2021, NASA launched the Double Asteroid Redirection Test (DART) mission, a vehicle aimed at testing a method of planetary defence against near-Earth objects (NEOs) that pose a real risk of impact, by smashing an object into them and using kinetic energy  to deflect them from their existing trajectory.

To achieve this, the spacecraft was both a science probe and impact device, and it was launched to rendezvous with the binary asteroid 65803 Didymos (Greek for “twin”), comprising a primary asteroid approximately 780 metres across, and a smaller companion called Dimorphos (Greek: “two forms”). These sit within a heliocentric orbit which periodically cross that of Earth whilst also reaching out beyond Mars , which occupy a heliocentric orbit that periodically crosses that of Earth. On reaching the pair, DART smashed into Dimorphos, successfully altering its orbit around Didymos.

A SpaceX Falcon 9 lifts-off from Cape Canaveral Space force station’s SLC-41 on Monday, October 7th, 2024, carrying the European Space Agency’s Hera asteroid mission to the binary asteroids Didymos and Dimorphos. Credit: ESA/SpaceX

I covered the launch of the mission in Space Sunday: a DART plus JWST and TRAPPIST-1 updates, and the aftermath of the impact two years ago in Space Sunday: collisions, gamma bursts and rockets. Since then there has been much reported on what has happened to Dimorphos in the wake of the impact, but scientists have been awaiting a planned follow-up mission to the Didymos pairing which could survey the outcome up close. And that mission is now underway, courtesy of the European Space Agency (ESA).

Launched at 14:52:11 UTC on Monday, October 7th from Canaveral Space Force Station, Florida atop a SpaceX Falcon 9 rocket, ESA’s Hera mission made it away from Earth just ahead of the arrival of Hurricane Milton. Lift-off marked the start of a two-year journey for the 1.1 tonne solar-powered spacecraft – also called Hera, after the mythological Greek goddess, rather than the name being an acronym –, as it heads first for Mars, which it will pass in March 2025 at a distance of between 5,000 and 8,000km. Taking the opportunity to test its science instruments in studying the tiny outermost Martian moon, Deimos as it does so, Hera will use the Martian gravity well to swing itself onto a trajectory so it can rendezvous with Didymos in December 2026.

Hera spacecraft design. The locations of the different payload elements are indicated (AFC = Asteroid Framing Cameras; TIRI = Thermal InfraRed Imager; PALT = Planetary ALTimeter; SMC = Small Monitoring Cameras. Credit: Michael, Kuppers, et al, ESA

The cube-shaped vehicle will have a primary mission of six months orbiting the Didymos pair, split into 5 phases:

  • Initial characterisation (6 weeks): determine the global shape and mass/gravity together with the thermal and dynamical properties of both asteroids.
  • Payload deployment (4 weeks): release two small cubesats, Juventas and Milani. The former will attempt to land on Didymos to conduct direct surface and sub-surface science, the latter will gather spectral data on the two asteroids and the surrounding dust cloud resulting from the DART impact.
  • Detailed characterisation (6 weeks): metre-scale mapping of the asteroids and determination of thermal, spectral, and interior properties.
  • Dimorphos observations (6 weeks): High-resolution investigations of a large fraction of the surface area of Dimorphos, including the DART impact crater.
  • Experimental (6 weeks): study the morphological, spectral, and thermal properties of Dimorphos.

Overall, the mission is designed to accuracy access the overall success of the DART mission if deflecting Dimorphos in its orbit around Didymos (and thus the effectiveness of using kinetic impact to deflect NEOs threatening Earth with an impact) and to characterise both asteroids to help us better understand the composition, etc., of typical NEOs, so that the data obtained might help further refine plans for potential future asteroid redirect missions.

Hera, with the crescent Earth to one side, seen from the SpaceX Falcon 9 upper stage following vehicle separation and prior to solar array deployment. October 7th, 2024. Credit: SpaceX/ESA

One of the major elements of the mission has been the development of sophisticated guidance and mapping software which will allow Hera, using a series of compact sensor systems, to autonomously construct a map of the Didymos system and the space around it. It will then use this map to determine for itself the safest orbital trajectories around the asteroids to avoid impacts with any remaining rock and dust debris remaining in orbit around both bodies from the DART impact, and of a sufficient size to damage it in a collision.

Following launch, Hera successfully separated from the upper stage of the Falcon 9 launch vehicle and called ESA’s mission control to confirm it was operating correctly and ready to start crucial operations such as deploying its solar panels. In November 2024, the vehicle will perform a “mid-flight” adjustment to better align its trajectory to Mars.

Starship Flight 5

October 13th saw the launch of the fifth Starship / Super Heavy combination from the SpaceX facilities at Boca Chica – and the first attempt to bring a booster back to the launch pad and catch it using the “chopsticks” of the Mechazilla mechanism on the launch tower.

A lot of people – myself included – severely doubt(ed) the ability of both the long-term viability of the idea of catching boosters and launch vehicles out of the air, or whether this flight could prove the concept. Credit falls where due, and for this flight we were proven wrong.

A drones-eye via of the starship / super heavy launch facility, Boca Chica, Texas as IFT-5 propellant loading is underway. Note the clouds of liquid oxygen forming as a result of venting from the propellant feeds and vehicle tank vents. Credit: SpaceX livestream

The launch came at 13:25 UTC, with the ignition of the 33 Raptor 2 motors lifting the roughly 5,000 tonne mass of the combined Ship 30 and Booster 12 into the morning skies above south Texas. All 33 motors had a good clean burn, and the stack quickly gained altitude. At 2m 40s after launch, and approximately 50km altitude, the majority of the engines on the booster shut down and the six motors on Ship 30 ignited in the “hot staging” burn ahead of separation. Following separation, the booster immediately commenced a manoeuvre to steer away from the starship, in readiness to commence a flight back towards the launch pad.

This started the critical phase of Booster 12’s flight. Initially it continued to gain height ballistically, reaching an altitude of approximately 100 km whilst performing a “boost back” engine burn to slow its ascent and then start a fall back towards the launch site. The manoeuvre was completed with a level of accuracy such that SpaceX confirmed they would proceed with the “return to base” and attempted booster capture. Had the boost-back been off, the capture phase would have been abandoned and the booster allow to make a controlled splashdown in the Gulf of Mexico.

Boost back: with the hot staging ring a bright dot at the bottom of the image, Booster 12 fall back towards Earth heading towards the launch site. Credit: SpaceX livestream

There followed a series of visible pulses from the booster as it purged excess vapour from this primary propellant tanks while the three central motors gimballed to direct their thrust and steer it away from the jettisoned hot staging ring falling below it. Canting over to being close to horizontal, the booster descended to some 10 km altitude, racing back towards the launch facilities with a speed of 2,860 km/h, before the inner ring of 10 motors fired committing it to an initial braking manoeuvre.

At this point, and abort and splashdown was still possible, but the guidance system on the launch tower was working perfectly, allowing the booster to home into it. At 5km and still travelling at over 1750 km/h the 13 motors that have been firing all shut down, the booster gradually righting itself and decelerating through 1200 km/h before all thirteen re-fired in a final deceleration move before the inner ring of ten engines shut down and the three centre engines took over at at 1 km altitude to steer the booster in for capture.

With propellant vapours also burning form the mid-point vents, Booster 12 approaches the launch tower in readiness for capture. Credit: SpaceX livestream

The final part of the descent witnessed flames rising along two sides of the booster. The first, and larger of the two appeared to originate at the Quick Disconnect ports at the bottom of the booster (the connectors for loading propellants into the booster). The second appears part-way up the booster, possibly at vent ports for the main propellant tanks. This may have been ignited by flames from the lower fire reaching around the vehicle and setting vapours from the vents alight. Neither fire affected the vehicle’s performance as it slowed rapidly and descend precisely between the Mechazilla “chopsticks”, although it did actually come quite close to striking the tower in the process.

At precisely the same time, the “chopsticks” started to close on either side of the booster such that once it was vertical, the arms were close enough for it to gently lower itself onto them using four hard points around its hull (called “pins”, and specifically designed to allow the “chopsticks” take the booster’s unladen weight when raising / lowering it), which came to rest precisely on “shock absorbers” running along the length of the arms, designed to dissipate the weight of the booster as it dropped onto them. At this point, the Raptor engines shut down, and because of the fire, the onboard fire suppression system appeared to activate.

Even so, the fire rocket continued for several minutes, giving rise to fears of a possible post-capture explosion, but vent valves at the top of the booster were opened, allowing any remaining propellant vapours in the header tanks (smaller propellant tanks used for the final decent and capture) to be released away from the vehicle, greatly reducing the rick of explosion, and the vehicle remained intact on the launch tower.

In all, a remarkable achievement for a first attempt. Kudos to SpaceX.

However, the booster’s successful capture just under 8 minutes after launch wasn’t the end of the flight. As Booster was making its return, Ship 30 continued on its way to orbit, reaching a peak altitude of some 211 km as it cruised half-way around the world.

As it passed across Africa, the vehicle started a slow decent back into the atmosphere, passing over the tip of southern Madagascar as it gently dropped from 119 km to 115km. At around 100m altitude, it started to show the first indications of plasma built-up due the frictions created as it pushed the air molecules around it against their neighbours in the increasing atmospheric density, signs which quickly grew in intensity.

Plasma flow around the side of the starship as it passes through the re-entry interface and enters into the period of maximum dynamic stress during descent. Thanks to Starlink, transmissions from the vehicle were largely uninterrupted during the re-entry phase. Credit: SpaceX livestream

At around 75 km altitude, the vehicle entered the period of peak heating – the roughly 10 minute period when the plasma generated around the vehicle reaches its highest temperatures. It was during the period during IFT-4 in June 24, that the starship started to suffer significant burn-through issues and structure loss with one it its aft aerodynamic flaps, and which continued through its decent, destroying pretty much all of the flap in the process. Not of this was evident at this point with Ship 30.

As re-entry progressed, propellant from the header tanks in the vehicle started to be pumped through the three motors that would be used during the final phase of the flight in a “chill down” process to get them down to the desired temperature for full ignition.

At 47 km altitude, and slightly lower than the previous flight, one of the aft flaps on Ship 30 (top left) shows evidence of burn-through along the hinge mechanism. Whilst showing there is is still an issue with the hinges, this time the burn-through did not result in the partial loss of the entire flap. Credit: SpaceX livestream

It was after the period of peak re-entry heating, as the vehicle entered the period of maximum  dynamic stress on its structure that the first hints of plasma burn-through began to make their presence visible on one of the two aft flaps (at roughly 48 km altitude), although there was no visible sign of large pieces of the flaps disintegrating, as had been the case in June.  Transmissions did break up at this point, resuming as the vehicle entered aerodynamic fee-fall (the “bellyflop”), which showed all four flaps functioning despite the burn-through damage to one.

With less than a kilometre to fall, the three Raptors ignited, and the vehicle tipped upright, and 1 hour 5 minutes after launch, it splashed-down at night, precisely on target in the Indian Ocean. There was around a 20-second period where the vehicle appeared to settle in the water prior to it exploding, the event caught via a remote camera on a buoy positioned a short distance from the target splashdown zone.

20 seconds after splashing down in the Indian Ocean and precisely on target, Ship 30 exploded, the moment caught by a remote camera mounted on a buoy anchored close the the landing zone. Even so, IFT-5 can be counted as nothing short of a successful flight. Credit: SpaceX livestream

The cause of the explosion has yet to be determined – but given that Starship isn’t actually designed to land on water, and the mix of super-heated engine elements and cold sea water isn’t a particularly good one, the explosion shouldn’t be surprising, and doesn’t negate the overall success of the flight.

There is still much more to do in testing this system – such as demonstrating these kinds of “return to base” flights and captures can be achieved consistently. There is also much that is questionable about the starship  / super heavy launch system as a whole, particularly in terms of crewed missions to Mars and even in supporting NASA’s Project Artemis lunar aspirations. However, none of this negates what is a remarkable first time achievement for SpaceX with IFT-5.

And here’s another view of the Booster 12 capture – from a camera mounted on the launch tower:

 Europa Clipper  Update

Previewed in my previous Space Sunday update (see: Space Sunday: Europa Clipper, Vulcan Centaur and Voyager 2), Europa Clipper, NASA’s mission to study the Jovian moon Europa, which had been due to lift-off on Thursday, October 10th, suffered a launch postponement courtesy of Hurricane Milton. The launch is now targeted for 16:06 UTC on Monday, October 14th for launch from Launch Complex 39A at Kennedy Space Centre, Florida.