Space Sunday: BepiColombo and giant planets

An Ariane 5 rocket carrying the European-Japanese BepiColombo mission to Mercury rises from the pad at the Guiana Space Centre in Kourou, French Guiana on the 19th October, 2018 (local / 20th October, 2018 GMT). Credit: ESA-CNES-Arianespace

At 01:45 GMT on Saturday, October 19th, 2018, the European / Japanese BepiColumbo mission lifted-off from the European Spaceport in Kourou, French Guiana at the start of a 7-year voyage to Mercury, the innermost planet of the solar system.

Named after Giuseppe “Bepi” Colombo, an Italian scientist, mathematician and engineer, who took a particular interest in Mercury, and first formulated the use of the gravity-assist as a part of an interplanetary mission (Mariner 10, 1973/75).

The mission actually comprises four elements. There are two individual satellites, the Mercury Planetary Orbiter (MPO) and Mio (Mercury Magnetospheric Orbiter, MMO), an propulsion / power unit called the  Mercury Transfer Module (MTM) and a Sun shield designed to protect the more sensitive instruments on Mio.

BepiColombo elements (l to r) Mercury Transfer Module (MTM) with solar panels folded; Mercury Planetary Orbiter (MPO) also with solar panel stowed;, sun shield and vehicle interface; Mercury Magnetospheric Orbiter (MMO). Credit; ESA

Built by the European Space Agency, MPO weighs 1,150 kg (2,540 lb), and carries a payload of 11 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. It also carries the smaller Mio, and will supply it with power until such time as the two separate once in orbit around Mercury.

Mio, built primarily in Japan, masses of 285 kg (628 lb) and carries five groups of science instruments with a total mass of 45 kg (99 lb). The is a spin-stabilised platform, meaning that prior to detaching from MPO, it will be set spinning at 15 rpm so it can remain stable as it operates in a polar orbit around Mercury.

The overall goal of the mission is to carry out the most comprehensive study of Mercury to date, examining its magnetic field, magnetosphere, interior structure and surface, with a primary mission period of one year. In addition, during the flight, BepiColombo will make the most precise measurements of the orbits of the Earth and Mercury around the Sun made to date as a part of further investigations of Einstein’s theory of general relativity.

As noted above, it will take BepiColombo seven years to reach Mercury. This is because of a couple of reasons. The first is, contrary to what logic might suggest, getting closer to the Sun is actually harder than moving away from it when starting from Earth. The is because a vehicle departing Earth does so with a “sideways” motion relative to the Sun of around 67,000 mph (107,000 km/h), the speed the Earth is orbit the Sun, and this has to be overcome. At the same time, speed has to be managed so that the vehicle can also approach Mercury at a slow enough velocity to allow it to brake its way into orbit.

To achieve both of these goals, the MTM on BepiColombo is equipped with the most powerful ion propulsion system yet flown in space. This is capable for maintaining a low rate of thrust over exceptionally long periods – much long that could be achieved by rocket motors and for far less fuel, given the ion system is electrically powered, using two 14 metre (46 ft) long solar panels to generate the power. The motor will be used to help slow BepiColombo in its flight, acting as a long-slow-burning brake. However, the ion motors aren’t sufficient to get the mission to Mercury; more is required.

Computer composite rendering of the stacked BepiColombo spacecraft making a flyby of Mercury with the ion propulsion system of the MTM firing. Credits: Spacecraft: ESA/ATG medialab; Mercury: NASA/JPL

This “more” take the form of using no fewer than nine planetary fly-bys. The first of these will happen in April 2020, when BepiColumbo, now in an extended orbit around the Sun, will encounter Earth once more. This will bend the vehicle’s flight path inwards towards the Sun which will swing it past Venus in October of that year, the first of two Venus fly-bys. The second of these will occur in August 2021, and will bend BepiColombo’s orbit further in towards Mercury, which it will reach at the start of October 2021.

But things don’t end there. While planetary fly-bys serve to bend a space vehicle’s trajectory, allowing it to “hop” from planet to planet, it also increases the vehicle’s velocity. Even with the long periods of braking possible using the ion motors, BepiColombo will be travelling too fast to achieve orbit around Mercury at that first encounter. Instead, the spacecraft will be placed in a solar orbit that periodically intercepts Mercury in is orbit, and over a series of five such encounters between June 2022 and January 2025, BepiColombo will use Mercury’s gravity in conjunction with its ion engines to slow itself down to around the threshold at which it can make orbit.

BepiColombo’s flight to Mercury, via Phoenix7777

This will occur in December 2025, as the vehicle makes its seventh approach to Mercury. However, with a mass of around 4 tonnes combined, the vehicle will still have too much inertia for the ion motors to bring it into orbit. Instead, the MTM will be jettisoned, and the smaller, lighter MMO will use its own high-thrust conventional motor systems to brake itself into an initial orbit around Mercury. At the same time, Mio will be separated, so it can enter a more distant orbit around the planet.

Such are the complexes of the mission that, following the successful launch on October 20th, ESA Director General Jan Wörner acknowledged the challenges still to be faced – but they come with considerable science rewards to be gained:

Launching BepiColombo is a huge milestone for ESA and JAXA, and there will be many great successes to come. Beyond completing the challenging journey, this mission will return a huge bounty of science. It is thanks to the international collaboration and the decades of efforts and expertise of everyone involved in the design and building of this incredible machine, that we are now on our way to investigating planet Mercury’s mysteries.

– ESA Director General Jan Wörner

An artist’s impression of JAXA’s Mio separating from the Mercury Planet Orbiter (MTO) after BepiColombo enters orbit around Mercury. Credit: Astrium

BepiColombo will operate for at least a terrestrial year following orbital insertion. This is the funded period for the primary mission, but both craft could operate for around two years following orbital insertion, if ESA / JAXA decide to fund the mission through a second year of science gathering.

This is first joint interplanetary mission between ESA and the Japan Aerospace Exploration Agency (JAXA). It is also only the third mission to Mercury and the second to make orbit around the planet. The first was the aforementioned Mariner 10, in 1973/5. Unable to carry sufficient fuel to achieve planetary orbit, Mariner 10 remained in a solar orbit from which it made three fly-bys of Mercury, in March 1974 (at a distance of 703 km / 437 mi), September 1974 (at a distance of 48,069 km / 29,869 mi) and March 1975 (at a distance of 327 km / 203 mi). Due to Mercury’s tidally locked orbit around the Sun, Mariner 10 only saw the same side of the planet with each pass.

The second mission to Mercury was NASA’s MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission launched in 2004. This performed three fly-bys of Mercury in January 2008, October 2008 and September 2009, before going into orbit around the planet in March 2011. Some of MESSENGER’s major discoveries were made once in orbit, including evidence of water ice in permanently shadowed craters at the planet’s poles, depressions called “hollows” that are only known to exist on Mercury, and extensive volcanic activity across the planet. It is hoped the BepiColombo’s finding will at least match MESSENGER’s.

The Young Star with Four Giant Planets

As stars go, CI Tauri is a baby. Estimated to be only around 2 million years old, it lies some 500 light years from Earth in the constellation Taurus. Given its relative immaturity, the star still sits within a gaseous disk, the remnants of the dust cloud that led to its creation.

According to accepted thinking, planets coalesce over millions of years out of the stellar nebula disk surrounding a new-born star like CI Tauri – but the process is thought to be slow. In the case of our own solar system, for example, the Sun formed about 5 billion years ago, but it took almost half a billion years for the outer planets to form, and half a billion for the inner planets. These time frames are the planets all came into existence around 4.5 or 4.6 billion years ago.

Given this kind of time frame, CI Tauri shouldn’t, in theory, have any planets orbiting it; only four now appear to have been found in orbit around it, and they’re not exactly small.

An artist’s impression of a protoplanetary disk around C1 Tauri. Gaps in the disk indicate the presence of three previously undetected gas giant worlds joining a “hot Jupiter” already known to orbit the star at extremely close range. The research raises questions about how solar systems form. Image: Amanda Smith, Institute of Astronomy

The first planet, CI Tauri b, was discovered in 2016. Estimated to be around 12 times the mass of Jupiter, it lies very close to its parent, orbiting it every nine terrestrial days, marking the planet as a “hot Jupiter” – as the gases comprising it will be super-heated by the nearby star. The remaining three have apparently been found as a result of studies carried out by a team of astronomers using the Atacama Large Millimetre/sub-millimetre Array (ALMA) in Chile have discovered. These three sit further out from the star than CI Tauri b, although the first of them is thought to be about the same overall size. The remaining two are thought to be about the size of Saturn, with the outermost lying about a thousand times further from its parent than CI Tauri b.

This again All of this turns many of the theories around planetary formation upside down. The theory behind “hot Jupiters” is that they form much further out from their star, then slowly migrate inwards for reasons yet to be determined. But again, the process takes millions of years, and CI Tauri hasn’t been in existence for this to have happened with CI Tauri b.

How the three outer planets were located is also different from the normal means of planetary discovery. This, as I’ve frequently noted in these pages, is generally via the transit method: as a planet passes between its star and Earth, it causes a regular dipping in the star’s brightness as see from Earth. With CI Tauri, however, the disc of matter surrounding the star makes identification of such dimming difficult, if not impossible. Instead, the astronomers used ALMA to study the disk itself in a range of wavelengths, looking for gaps in it that are likely to have been created by planet-sized bodies effectively “sweeping up” the dust and matter lying along their orbital paths.

Further observations are required to confirm the presence of the three new planets, but all of this leaves a long list of questions for astronomers to answer: how could planets form around so young a star? Could whatever mechanism that created CI Tauri b also be responsible for the formation of other hot Jupiters, and the initial theory incorrect? Could the outer planets actually be responsible for influencing the formation of CI Tauri b? And could other young stars in the galaxy also have their own unusual planetary systems? It could be some time before these questions are all answered.

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