Space Sunday: return to the extra-solar visitor

An artist’s impression of 1I/2017 U1 (or `Oumuamua), which was first seen by the Pan-STARRS 1 telescope in Hawaii on October 19th, 2017, and subsequently studied by a number of telescopes around the world, including the VLT of the European Southern Observatory (ESO) Credit: ESO / M. Kornmesser

On October 30th, 2017 I wrote about the extra-solar body which had crossed the orbit of Earth after swinging around the Sun during a rapid flight into and back out of the solar system. The object, originally designated A/2017 U1 and then as 1I/2017 U1 (the “1I” indicating it is the first positively identified interstellar object we’ve observed in 2017), was initially spotted on October 18th in Hawaii by the Pan-STARRS 1 telescope. Since then it has been closely tracked by astronomer around the world. What is particularly interesting about it is that Sun-orbiting eccentricity of between 0 (a circular orbit), and 1 (a parabolic orbit). Anything above 1 would tend to point to an object being entirely extra-solar in origin. A/2017 U1 has an orbital eccentricity of 1.2.

Since that time, the object has been under intense study, as has been reported in the media, and is proving to be most unusual. Now dubbed `Oumuamua, roughly translated as “scout” (ou being Hawaiian for “reach out for” and mua meaning “first, in advance of” – which is repeated for emphasis). At first thought to be a comet on account of initial observations, it was reclassified as an asteroid following more details observations.

In particular, observations made using the Very Large Telescope (VLT), operated by the European Southern Observatory (ESO) at the Paranal Observatory in Chile revealed the object to be cigar-shaped, rather than being a more rounded shape, as had been expected. Overall, it is estimated to be around 400 metres (1312 ft) in length, and approximately 40-50 metres (130-162.5 ft) in height and width. It is tumbling .

Using the VLT, ESO were able to accurately measure the brightness, colour and orbit of the asteroid and refine measurements of its trajectory as it leaves the solar system at a stunning 95,000 km/h (59,000 mph). These have revealed that `Oumuamua varies dramatically in terms of brightness (by a factor of ten) as it spins on its axis every 7.3 hours. As Karen Meech of the Institute for Astronomy in Hawaii explained in an ESO press release, this was both surprising and highly significant:

This unusually large variation in brightness means that the object is highly elongated: about ten times as long as it is wide, with a complex, convoluted shape. We also found that it has a dark red colour, similar to objects in the outer Solar System, and confirmed that it is completely inert, without the faintest hint of dust around it.

These observations also allowed Dr. Meech and her team to constrain `Oumuamua’s composition and basic properties. Essentially, the asteroid is now believed to be a dense and rocky asteroid with a high metal content and little in the way of water ice. It’s dark and reddened surface is also an indication of tholins, which are the result of organic molecules (like methane) being irradiated by cosmic rays for millions of years.

The measurements confirmed that the asteroid came to us from the general vicinity of Vega  in the Constellation of Lyra, and has taken around 300,000 years to reach the solar system, which it has been passing through for the last 20,000. However, whether it originated around Vega is still being debated. Some of those observing the object believe it could have been wandering the interstellar void for 45 million years, having originally been ejected from a stellar system in the Carina–Columba association, which had once been far more aligned with the constellation of Lyra, relative to the solar system.

Passing through most of the solar system at a speed of around 80.0oo km/h (58,000 mph), the asteroid gradually accelerated under the Sun’s gravity so that it reached a velocity of 315,700 km/h (196,000 mph) at perihelion – the point closest to the Sun, which it reached on September 17th, 2017. Since then, the object has been heading away from the Sun and decelerating, again under the influence of gravity, passing the orbit of Earth in October. It will pass Jupiter’s orbit in May 2018, Saturn’s orbit in January 2019, and Neptune’s orbit in 2022, passing onwards through the solar system. It will be another 20,000 years before the object re-enters the interstellar medium.

Even it is of extra-solar origin, `Oumuamua is seen as being of significant import for our understanding of the formation of other solar systems. If nothing else, a study of the asteroid as it continues onward and outward from the Sun could potentially teach us a lot about its origins and the likely conditions within the system where it was born.

To this end, there have been numerous calls for the development of one or more missions to investigate the asteroid, some of which, such as Project Lyra, are already being mapped out.  However, planning such a mission is one thing – actually pulling it off is quite another. `Oumuamua is currently travelling at 95,000 km/h (59,375 mph) – a velocity it will now more-or-less maintain.That is equivalent to 5.5 AU (Astronomical Units – the average distance from Earth to the Sun) per year, or 26 metres (84.5 ft) per second – what is technically referred to as its hyperbolic excess velocity.

Project Lyra points to NASA’s Space Launch System rocket (left and centre) and the SpaceX Interplanetary System launcher (aka the BFR, right), as possible launch vehicle for a mission to intercept an extra-solar body. Credit: SpaceX

No space vehicle launched from Earth has been able to attain that kind of velocity – even the fastest human-made objects in space, Voyager 1, and the fastest space probe at launch, New Horizons, are both only managing around two-thirds of that velocity. So just getting to a point where we can launch a vehicle capable on eventually matching the speed of the asteroid is a major challenge  – without the worry of getting it to a speed where it might eventually catch with `Oumuamua at a speed which would allow it sufficient time to gather data on the rock as it flies by, rather than shooting right on past it at such a speed, it has next to no time to gather data of significant value. Nevertheless, the proponents of Project Lyra are going so far as to suggest a mission might rendezvous with  `Oumuamua and gather samples for on-board analysis.

Of course, the asteroid will be travelling through the outer solar system – and by that I mean the Kuiper Belt outwards to, and through, the Oort cloud – for thousands of years; it’s not just going to vanish in a decade or so. So this does give some leeway. An encounter with  `Oumuamua within the Kuiper Belt for example (say, 50-200 AU from Earth) wouldn’t need to be launched for another 5-10 years. This could potentially allow for the use of an upcoming launch vehicle, such as NASA’s Space Launch System rocket or even SpaceX’s gigantic Interplanetary Transport System launcher, the BFR.

However, looking towards an encounter that far from earth still means that the probe would have to achieve a hyperbolic excess velocity of up to 76 metres (247 ft) per second – or half as much again as the asteroid’s velocity – again calling into question the effectiveness of a mission in gathering and returning data. Certainly, at those kinds of speeds, an actual rendezvous with `Oumuamua to gather a sample would be out of the question.

An alternative approach might be more “slow and steady” approach using solar sail technology – such as that being developed with projects such as the Breakthrough Initiatives’ Starshot. This might allow a vehicle propelled by an earth-based array of lasers to eventually catch the asteroid, and with a rate of steady acceleration, overhaul it at a rate at which data can be gathered in earnest. However, such technology is in its infancy; thus the chances of such a mission being used for catching `Oumuamua are perhaps slim. However, development of the technology and a mission for intercepting an extra-solar object in the future a distinct possibility – particularly as it is now estimated at least one extra-solar object passes through the solar system a year.

Whether intended to study `Oumuamua or one of these other interstellar wanderers, any such mission – using rockets, ion drive propulsion, solar sail technologies -, if pursued, could led to technological breakthroughs as well as scientific rewards. As the project authors note:

As 1I/‘Oumuamua is the nearest macroscopic sample of interstellar material, likely with an isotopic signature distinct from any other object in our solar system, the scientific returns from sampling the object are hard to understate. Detailed study of interstellar materials at interstellar distances are likely decades away, even if Breakthrough Initiatives’ Project Starshot, for example, is vigorously pursued. Hence, an interesting question is if there is a way to exploit this unique opportunity by sending a spacecraft to 1I/‘Oumuamua to make observations at close range.

[A] mission to the object will stretch the boundary of what is technologically possible today. A mission using conventional chemical propulsion system would be feasible using a Jupiter flyby to gravity-assist into a close encounter with the Sun. Given the right materials, solar sail technology or laser sails could be used… Future work within Project Lyra will focus on analysing the different mission concepts and technology options in more detail and to down select 2 – 3 promising concepts for further development.

 

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