At 12:55 UT (13:55 BST, 08:55 EST, 05:55 PDT) the very last signal was received from the NASA / ESA Cassini spacecraft as it entered the upper reaches of Saturn’s atmosphere before disintegrating and burning-up. It was received 83 by NASA’s ground station near Canberra, Australia, 83 minutes after being transmitted – by which time the probe had already been destroyed.
At mission control, at the Jet Propulsion Laboratory, operated jointly by NASA and Caltech in Pasadena, California, it was an emotional moment. For many, the mission had been a part of their daily lives for nigh-on 20 years.
“The signal from the spacecraft is gone and, within the next 45 seconds, so will be the spacecraft,” Cassini programme manager Earl Maize announced, his voice catching, to the team gathered in mission control. “I’m going to call this the end of mission.” He then turned to Spacecraft Operations Team manager Julie Webster and hugged her, before giving Linda Spilker, the Cassini Project Scientist a hug as well. That loss of signal came within 30 seconds of the time predicted ahead of Cassini’s final dive.
As I reported last week, The Cassini-Huygens mission has been an incredible voyage of discovery, revealing so much about Saturn, its rings and retinue of moons, including hints on the evolution of life itself and revealing how moons Titan and Eceladus may have all the right conditions to support basic life while Tethys could – like Enceladus – have a liquid water ocean under its ice.
Cassini’s final approach commenced on September 11th, as it started back towards Saturn having made a final pass between the planet and its rings and looping away from both the week before. Passing by Titan, and once more using the moon’s gravity to push it into the correct trajectory, the probe headed back for its final encounter with Saturn. The Titan fly-by presented a last opportunity to image and study the moon before Cassini’s imaging system was focused on Saturn for the first part of the final approach. Imaging Saturn ended on Thursday, September 14th as the vehicle re-oriented itself to gather as much data on its brief passage into the upper reaches of Saturn’s atmosphere.
As I’ve previously noted in my Cassini mission updates, the primary reason for sending the probe into Saturn’s atmosphere was because it had exhausted almost all of its on-board fuel supplies used to orient itself and to adjust its flight through the Saturnian system, and the mission team didn’t want to leave the probe tumbling around Saturn’s moons where it might one day impact one of them and contaminate it with both Earthly microbes which may be dormant inside the vehicle, and which radioactive debris from its electrical power generators.
However, an alternative would have been to use the last of the vehicle’s fuel to boost it away from Saturn and out into space, but the scientific return promised by a final plunge into the planet was too good to refuse. “Saturn was so compelling, so exciting, and the mission we finally came up with was so rich scientifically that we just couldn’t — we had to finish up at Saturn, not some place else.” Earl Maize stated during a press conference after the probe’s fiery end.
There are currently no planned missions that will follow Cassini-Huygens to Saturn, although there are proposals to send missions to Titan. However, while the active part of the mission has come to an end, it’s not an end of the mission’s science.
“We have collected this treasure trove of data, so we have decades of additional work ahead of us,” Linda Spilker, the Cassini Mission Scientist said. “With this fire hose of data coming back basically every day, we have only been able to skim the cream off the top of the best images and data. But imagine how many new discoveries we haven’t made yet! The search for a more complete understanding of the Saturn system continues, and we leave that legacy to those who come after, as we dream of future missions to continue the exploration we began.”
As a closing note – for now – it’s not often that a space mission gains an official music video; but Cassini-Huygen has been a major inspiration for many over the past two decades, it has earned not one, but three official music videos which form a suite of music by three composes: Iniziare (Italian: “to start” by Sleeping At Last, aka Ryan O’Neal), Kanna (Icelandic: “Explore” by Sarah Schachner) and Amaiera (“end” or “stop” by Joseph Trapanese). I’ve embedded the first part below.
SpaceX Launch X-37B
On Thursday, September 7th, a SpaceX Falcon 9 booster launched the US Air Force X-37B secret mini-shuttle into orbit ahead of the Florida coast being hit by hurricane Irma. It marked the 13th Falcon 9 launch of 2017, and the fifth flight overall for the X-37B.
OTv-5 (Orbital Test Vehicle flight 5) saw the automated spaceplane placed into a higher inclination orbit than previous missions – thus expanding the vehicle’s flight envelope. However, in keeping with previous missions, the USAF has remained mostly silent on the mission’s objectives or its intended duration, revealing only that one experiment flying is the Advanced Structurally Embedded Thermal Spreader II (ASETS-II), which will measure the performance of an oscillating heat pipe.
Previous OTV missions have been long-duration flights, with the maiden flight in 2010 lasting 224 days and 9 hours, which each mission lasting longer than the last, with the last mission completed, OTV-4, totalling 717 days and 20 hours in orbit. The flights have, up until now, alternated between the two known X-37B vehicles, so although it has not been confirmed, it is believed this mission is being carried out by the first X-37B to fly in space.
The launch took place from Kennedy Space Centre’s Launch pad 39A, which SpaceX has leased from the US space agency and refurbished to handle Falcon 9 and Falcon Heavy launches – and which is now liable to be the pad from which the company’s massive ITS super-heavy rocket will depart when it enters operations in the 2020s. After separating from the upper stage and its cargo, the Falcon 9 first stage performed a “burn-back” manoeuvre and flew back to SpaceX’s dedicated Landing Zone-1 (LZ-1) at Cape Canaveral Air Force Station alongside Kennedy Space Centre, offering spectators a superb view of the landing.
More Exoplanet News
News on exoplanets keeps pouring out on an almost weekly basis. Here’s an update on a couple of the more recent announcements.
Gas Giants Orbiting TRAPPIST-1?
TRAPPIST-1, with its seven Earth-sized planets (three of them orbiting in the star’s habitable zone), is fast becoming the most famous exoplanet system in the public consciousness. Studies have bounced back and forth as to the habitability of the planets in the system – with the most recent swinging the pendulum back towards the potential for liquid water to exist on the surface of some (see my September 7th, 2017 Space Sunday report). Now comes a study suggesting the system might have yet-to-be discovered gas giants as well.
A team of scientists, led by researchers from the Carnegie Institute of Science used observations made of TRAPPIST-1 over a period of five years (from 2011 to 2016) using the Carnegie Astrometric Planet Search Camera (CAPSCam), an instrument on the du Pont telescope at the Las Campanas Observatory in Chile.
This instrument uses the astrometric method to locate possible exoplanets by measuring the wobble of this host star around the system’s centre of mass (aka. its barycenter). By using several years of observations of TRAPPIST-1 gathered by CAPSCam, the researchers have been able to determine the upper mass limits for any potential gas giants orbiting the star, concluding that planets that were up to 4.6 Jupiter masses could orbiting with a period of about a terrestrial year, while planets up to 1.6 Jupiter Masses could be orbiting the star with 5-year periods.
As well as opening the debate on how many planets might yet be discovered around TRAPPIST-1, the study also may help resolve an ongoing debate about the formation of the Solar System.
Currently, there are two competing hypotheses on how our gas giants were formed, both of which are founded on the Nebular Hypothesis which states that the solar system planets were formed out of the disk of dust and gas surrounding the young Sun. In the first, the Core Accretion theory, the gas giants were formed much like the inner planets: dust and material in this cloud accreted into solid cores which then attracted the gasses around them. In the Disk Instability theory, clumps of dust and gas rapidly coalesced to form baby gas giants, attracting more dust and gas and growing in size, the solid matter in them sinking under gravity to form their solid cores.
The Carnegie study suggest that for low-mass star like TRAPPIST-1, the Core Accretion theory would not account for the formation of any gas giants, whilst the Disk Instability theory suggested gas giants could – and that they might therefore exist around other red dwarf stars (exoplanets found around most M-class stars thus far show a bias towards being sold bodies rather than gas giants). Morre directly, scaling the models used by the team to account for a star the mass of the Sun and a system as big as the solar system, potentially suggests the Disk Instability would be more likely to account for our own gas giants.
Three More Super-Earths?
Three more “super-Earths” may have been discovered orbiting a star 100 light years away. Gliese 9827 (GJ 9827), is a late K-type dwarf star, also known as an orange dwarf, around 0% the size of our own Sun and between 15 to 30 billion years old. Indications of the three planets came from data gathered during the Kepler Observatory‘s K2 mission, making this the closest star to Earth thought to have a planetary system so far discovered by this phase of the Kepler mission.
The three planets have been designated GJ 9827 b, c, and d, and have the following characteristics: GJ 9827 b is 1.6 times the radius of Earth and approximately 4.5 times its mass. It orbits its parent once every 1.2 terrestrial days at a distance of 0.02AU. GJ 9827 c is 1.2 Earth radii and a mass 2.62 times its mass. It orbits its parent star every 3.6 terrestrial days at a distance of 0.04 AU. GJ 9827 d orbits 0.06 AU from its parent every 6.2 terrestrial days and is 2.1 times the radius of Earth and is 5.3 Earth masses. All three have extremely high temperatures – rivalling those at the surface of Venus – so even if solid, they are unlikely to offer environments favourable to life gaining a foothold.
Given their estimated masses, all three planets are on the transitional boundary between solid and gas giants, so there is still doubt about which they may actually be. However, this is itself interesting, as thus far, “super-Earth” type planets have fallen broadly into two categories: those with radii of 1.6 that of Earth or larger, tend to be less dense, suggesting they have a hydrogen / helium atmosphere leaning towards gas giant status, while those will radii below 1.6 that of Earth lean more towards being “Earth-like” in that they are solid, rocky bodies, potentially with an atmospheric envelope.
With GJ 9827, the three planets potentially offer one of each kind of “super-earth” – rocky in the case of GJ 9827 c, and having a puffy hydrogen / helium atmosphere (GJ9827 d), while GJ 9827 b sits right on the boundary between the two. This makes the system a unique opportunity to study the evolution of “super-Earth” worlds within a single system, allowing for direct comparison and contrast investigation. As such, the system will not only continue to be studied from Earth, but it has been added to the list of candidate systems for study by the James Webb Space Telescope, due to be launched in 2018.
And Now For Something Completely Different…
Launching rockets is a risky business. Launching and trying to land rockets is not so much nail-biting as finger chewing. Things can go wrong at both ends of the spectrum; so you need something of a stoic disposition. When your entire company and its future depends on you being able to do both successfully and repeatedly, it also helps if you have a sense of humour to deal with things.
Elon Musk, founder, CEO and lead engineer for SpaceX is certainly stoic and has often demonstrated a sense of humour. After an at-sea landing attempt with the first stage of a Falcon 9 booster resulted in the lost of the vehicle as it ran out of hydraulic fluid just is it was about to touch down, he commented, “Well, technically it did land – just not in one piece!” In April the same year, he wryly referred to the explosion of another vehicle which smashed into the deck of its landing drone as “a rapid and unscheduled disassembly”.
On September 14th, 2017, SpaceX issued a video entitled How Not To Land An Orbiter Rocket Booster. It’s a humorous charting of the company’s problems in trying to land a rocket in the run-out to their more recent successes. It’s set to the music of Liberty Bell, the march composed by John Philip Sousa – which many might be more familiar with as “the Monty Python Flying Circus theme”. The captions are worth a read, as they reference some of Musk’s own comments and can include plays on the names of the company’s automated landing platforms Just Read the Instructions and Of Course I Still Love You.