Space Sunday: collisons, rockets, and telescopes

Official poster for the DART mission, a joint NASA-John Hopkins University Applied Physics Laboraroty (JHUAPL) mission. Credit: NASA

Monday, September 26th 2022 will see NASA’s Double Asteroid Redirection Test (DART) reach its primary goal when a small space probe will collide with an asteroid called Dimorphos in an attempt to test a method of planetary defence against near-Earth objects (NEOs) by deflecting their path around the Sun via a kinetic impact.

The risk we face from Earth-crossing NEOs – asteroids and cometary’s fragments that routinely zoom across or graze the Earth’s orbit as they follow their own paths around the Sun – is not insignificant. More that 8,000 of such objects are currently being tracked, and that number is still rising. Such objects range in size from the relatively small to objects like the infamous 99942 Apophis (370m along one axis). which were it to strike Earth, would result in an estimated explosive force equivalent to 1,000 megatons, through to objects large enough to result in possible extinction events.

In 2013, a cometary fragment roughly 20m across entered Earth’s atmpsohere to explode 26km above the the Russian oblast of Chelyabinsk with a force of 400–500 kilotons of TNT. The resulting shockwave damaged some 7,200 buildings and injured over 1,500 people in 6 cities. This image captures the fragment’s path as it burnt up through the denser atmosphere. with the poiint of its explosive destruction marked by a distinctive “mushroom cloud” towards the right-hand end of the trail. Credit: Alex Alishevskikh

Over the years, various means of prevent such an impact have been suggested, with one of the most popular being the use of the kinetic energy from one or more impacts against the threat to alter its orbital track around the Sun so it would miss Earth. It is a popular option because if we get sufficient warning about a threatening object, it should be possible to plan an intercept mission to strike it at a point in its orbit where only a very small deflection in its track would be enough to ensure it misses Earth, allowing smaller, more manageable payloads to be used.

DART is the final incarnation of what started as two independent missions by NASA and the European Space Agency (ESA) to achieve the same goal. These were then combined into a single mission –  AIDA (for Asteroid Impact & Deflection Assessment(, which would have seen ESA launch a observation platform intended to fly to the designated target asteroid and carry out observations and analysis prior to NASA’s DART impactor arriving, and then observing the impact on the latter and the effect it had on the target’s orbit.

However, the ESA element of the mission was cancelled, leaving NASA to push ahead with DART, with the role of observing the impact taken over by Earth-based based observatories and a small payload carried by DART. To compensate, ESA now plans to launch Hera in October 2024, a mission and vehicle that will rendezvous with the target asteroid in 2027 to observe the overall results of the DART mission.

Dimorphos, the target for DART, is actually a relatively small asteroid, some 170m across (but still large enough to result in considerable destruction and loss of life were it to enter Earth’s atmosphere and explode). It has been selected for a combination of reasons, the most pertinent being it is actually the moon of a much larger asteroid, 65803 Didymos (Greek for “twin”), itself a NEO forming part of the Apollo group, and noted as being potentially hazardous to Earth. It is around 780m across, and it orbits the Sun every 770 days, its orbit eccentric enough  for it to cross both the orbits of Earth and Mars, and thus present a potential impact hazard to both.

Dimorphos (Greek: “having two forms” and discovered in 2003, seven years after Didymos was first located) occupies an equatorial and near-circular orbit around Didymos with a period of 11.9 hours. This makes it an attractive target because its position is easy to calculate / track, and the fact that it is orbiting a large object means that the angle of deflection as a result of DART’s impact can be directly measured against its motion around Didymos, and from this it will be possibly to extrapolate the amount of deflection achieved had Dimorphos been a solo asteroid en route to a collision with Earth.

DART launched on November 24th, 2021 atop a Falcon 9 rocket. In order to impact the asteroid at a speed sufficient to affect its velocity, the vehicle has been propelled towards its target by a solar-powered NEXT ion thruster, and will strike Dimorphos head-on at a speed of 6.6 kilometres per second. This should be sufficient to effectively slow it in its orbit around Didymos and result in a charge to the orbital period and shape. Given Dimorphos is large enough to exert some gravitational influence over its parent, it is expected that Didymos’ velocity and orbit will also be affected to a small degree.

An artist’s impression of how the LICIACube cubesat might witness the outflow of ejecta from DART’s impact into Dimorphos. Credit: ESA / Italian Space Agency

Exactly how small or obvious all these changes will be is unknown – we simply do not know the topography of Dimorphos to know where and how DART will strike it. However, to assist with Earth-based observations of the impact, earlier this month DART released the Light Italian CubeSat for Imaging of Asteroids (LICIACube).

Built by the Italian Space Agency, this cubesat is now on a trajectory that will carry it through the Didymos / Dimorphos pairing, allowing it to observe and hopefully record DART’s impact and also gather initial data on the immediate results of the impact – although it is estimated that it will be a week or so before the overall effects of the impact can be properly interpreted. Similar cubesats, originally dubbed “Luke” and “Leia” but now officially called Milani and Juventas (a case of football winning out over Star Wars in the Italian science team?) will accompany the Hera mission in 2024.

DART itself carries little in the way of science instruments related to the mission, other than a 20 cm aperture camera called Didymos Reconnaissance and Asteroid Camera for Optical navigation (DRACO, which should record and return images of both Didymos and Dimorphos right up to the actual impact). However, it is in many respects also a technology demonstrator, making use of the  Roll Out Solar Array (ROSA) system recently deployed to the International Space Station, and which allows for more efficient harvesting of sunlight over a smaller area of solar array surfaces to generate power, and also RLSA, the spiral Radial Line Slot Array, a new type of compact and lightweight high gain communication antenna.

An artist’s impression of the NASA DART vehicle under propulsive thrust from its ion engine, moments before impacting with the asteroid Dimorphos. Credit: NASA

Currently, DART remains on course for an impact with Dimorphos at 23:14 UTC on Monday, September 26th, 2022. The images returned by the DRACO camera ahead of the impact will mark only the 6th time we have received close-up images of the surface of an asteroid.

Big Boosters: SpaceX Booster 7’s Seven and Artemis 1’s Weather Delay

It’s been a week of ups and downs for the two big boosters which are most prominently on spaceflight enthusiasts’ minds.

At the SpaceX Starbase Facility in Boca Chica, Texas, Booster 7, the vehicle seen as the favourite to lift the company’s massive Starship into the sky on the system’s first orbital attempt, completed a second spin-start test of seven of its 33 Raptor 2 engines  on September 19th. This looked to be a different selection of motors to those tested the previous week, meaning that between 14 and 17 of the booster’s motors have now completed spin-starts. Nor was this the end of things: just a few hours after the spin-start – which lasted around 13 seconds – the booster was re-pressurised with fuel and warning given of a further engine test.

This was a full static fire of seven of the engines, marking the largest number of Raptor 2 motors to go through such a test thus far. Slow-motion payback of high-speed film shot of the event reveals that – as with the spin-start tests – rather than igniting all seven engines simultaneously, engine ignition was staggered, which might be indicative of how actual orbital launches will be managed; staggering engine starts by just a few milliseconds could help with reducing noise vibration resulting from all 33 engines coughing into life at the same time, and may even help reduce the amount of sound being deflected back up against the vehicle and the launch stand.

Following this test, SpaceX announced that, rather than remaining at the orbital launch facility for further engine tests, Booster 7 would be returned to the production centre at Starbase for “robustness upgrades”, and Booster 8 would replace it on the orbital launch mount to undergo its own testing. Whilst not entirely clear from the tweets given, it appears these tests will include a full wet dress rehearsal (WDR), which could involve stacking the booster with Starship 24, then fully tanking them and proceeding through a launch countdown that stops short of engine ignition. Then, after this, there will be a full 33-motor static fire test for a booster.

Whether this means Booster 8 will overtake Booster 7 to become the vehicle to make the first orbital launch attempt with a Starship on top, or whether the two boosters will again be swapped to allow Booster 7 make the attempt – which SpaceX appear to be hoping to make in November (still subject to the granting of an FAA license) – is unclear.

Either way, Booster 7 was removed from the launch mount mid-week, and the launch mount itself then went through a series of tests of its upgraded sound suppression system, which appears to deliver both water and nitrogen as to the flame pit of the launch table to both absorb sound (and reduce the potential for it causing damage to the vehicle or launch facilities) and reduce the risk of unexpected fire.

Booster 8 (centre left) imaged on Highway 4, Boca Chica, on its way to the Starbase test and launch facilities. Just to the right of the booster stands Starship 24, located on a sub-orbital test stand. Centred in the photo is the orbtial launch tower, with the mechazilla lifting arms lowered and rotated away from the launch table and Booster 7 (hidden by the bulk of the launch tower). Credit: NASASpaceflight.com (not a NASA afilliate)
Meanwhile, on September 21st, NASA held a further fuelling test of the massive Space launch System rocket that will launch the uncrewed Artemis 1 mission to cislunar space. Earlier attempts to complete this test – a critical final step in readying the massive launcher for its maiden flight – had to be curtailed due to leaks in the liquid hydrogen fuel feed system at the base of the rocket, leading to padside repairs, as I noted in my previous Space Sunday update.

While the September 21st test also encountered leaks with the liquid hydrogen propellant flow, they were now sufficient to curtail operations, and the tst was successfully completed with both the core and upper stage liquid oxygen and liquid hydrogen tanks being fully fuelled roughly 6 hours after operations commenced.

One September 23rd, and after post-test checks on the vehicle, NASA held a press conference to confirm they would be making a launch attempt on Tuesday, September 27th, 2022 – only to have to call off the attempt on the 24th September due to tropical storm Ian threatening to roll across Florida and over the Space Coast, potentially requiring the vehicle to be rolled back to the safety of the Vehicle Assembly Building (VAB).

The Artemis 1 SLS booster on launch pad 39-B at Kennedy Space Centre. Credit: NASA

At the time of writing, no final decision had been announced regarding the roll-back proceeding. Should it occur, it is likely to occur overnight (local time) on Sunday 25th / Monday 26th September). This roll-back would mean the earliest launch opportunity would be October 2nd; however, this is a date in doubt due to the planned October 3rd launch for the NASA / SpaceX Crew 5 mission to the ISS from neighbouring Pad 39A. As both pads within launch Complex 39 at Kennedy Space Centre use the same infrastructure, back-to-back launches from the two pads are logistically difficult, and was there are further windows for the Artemis 1 launch, letting this slip is seen as preferrable to disrupting ISS operations.

The one good piece of news for Artemis 1, is that the flight termination system (FTS) has received a recertification waiver from the US Space Command at Cape Canaveral Space Centre. The FTS is used to destroy a rocket should it veer off-course post-launch. However, its batteries have a limited service life, and so packages need routine re-certification to state their batteries are suitable for use – or the batteries require replcing. Re-certification  / replacement means returning the vehicle to at VAB, further delaying any launch. However, the USSC has agreed that the package on the SLS could have the recertification delayed until mid-October, allowing the vehicle o be available for the late September / early October launch windows.

JWST Update: Images and Issues

On September 24th, NASA released images of the solar system’s outermost planet, as captured by the James Web Space Telescope. The pictures, taken in July 2022, show not only Neptune’s thin rings, but its faint dust bands, never before observed in the infrared, as well as seven of its 14 known moons.

Neptune, its rings and some of its moons as seen by JWST in July 2022. Credit: NASA

Neptune has fascinated researchers since its discovery in 1846. Located 30 times farther from the Sun than Earth, it is characterised as an ice giant due to the chemical make-up of its interior, whilse because of the great amounts of methane and heavier elements within its atmosphere, it has a disntinctive ocean blue colouring when seen in visible light.

The JWST images capture Neptune in the near-infra-red wavelengths which are readily absorbed by the planet’s atmosphere. This results in it appearing very differently to how it appears in visible light, looking light a misty, crystal marble lit from within by bright streaks – actually the atmospheric interactions only previously hited at b the passge of high-althitude cloud zipping around the planet. Beyond it, and more particularly, the planet’s ring and dust system is revealed in the clearest detail seen in more than 30 years.

Three views of Neptune over the decades, each revealing different information about the planet and its rings. Credit: NASA

Among the seven moons also captured in the JWST images is massive Triton, which appears to float over Neptune like a giant star – the result of the moon reflecting around 70% of the sunlight striking it, thanks to the frozen sheen of condensed nitrogen covering it.

The images of Neptune came at a time when it was confirmed the observatory has developed a minor issue. This lays with a grating wheel mechanism within the Mid-Infrared Instrument (MIRI), resulting in suspension of one of the instrument’s four operating modes (medium-resolution spectroscopy observations).  The other three observing modes — imaging, low-resolution spectroscopy and coronagraphy — are not affected, and observations using those modes of MIRI are continuing.

Naptune and its rings and moons, as omaged by JWST in July 2022. Credit: NASA

The cause of the friction within the mechanism is not clear. Hoever, NASA made it clear the decision to suspend the affected operations with MIRI was not as a result of failure, but rather “an abundance of caution” so that engineers could review telemetry data from the instrument and the mechanism in order to understand the extent of the issue, what might be done to correct it and the potential for impact on mid-range spectroscopy data already gathered by the instrument. In the meantime, mission managers remain confident MIRI will return to full operations in the near future.

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