Did we have visitors?

Above: Proxima Centauri, the nearest star to us.. for the moment. Courtesy of hubblesite.org

Our solar system has a very, very long history, and it's not even the oldest solar system like ours around - that honour goes to Kepler 444, a Sun-like-star system that is a staggering 11 billion years young. We've never found convincing evidence of any kind of life beyond Earth (that we didn't put there). But space teems with the kind of molecules and chemistry that go into making life, and the jury is very much out on whether or our kind of chemistry is the only kind that could support life -  just look at the chemical imbalances discovered on Titan or Venus, imbalances that might be the result of very alien kinds life. On top of this a recent result from SETI listed 1072 signals from deep space that were 'statistically significant' - or, put another way, unusual enough to warrant following up with more investigation.

 

Of course following those signals might have a down side...

It's a very long way to the nearest star system, Proxima centauri, though - and that solar system isn't even looing very promising for habitable worlds.

Interstellar on the cheap(ish):
There is a recent find that might change the odds of our solar system getting an alien visitor. This week a paper was published describing how, 70,000, years ago another star passed within less that a light year of Earth. We know it as Scholz's star, and it came within the outer boundaries of the Oort cloud, our solar system's cloud of comets.  Encounters like this might happen on a fairly regular basis according to Dr Mamajek,  who helped discover the star's flyby. Although previou simulations suggested these close encounters happen only once in 9,000,000 years, he believes it's likely a small dim star like this probably passes close every hundred thpusand years or so:

...It is a bit of a strange coincidence that we happen to have caught one that passed so close within the past 100,000 years or so.

- Dr Mamajek, speaking to the BBC

Could a similar encounter in the deep past, a flyby by a star with an inhabited world,  have left the relic of someone else's exploration effort hanging above our skies?



No, we'd notice one of those. I'm almost sure 

If a star with a habitable world passed so close by today then our fastest spacecraft, New Horizons,  would take 16,000 years to make the trip. The fastest proposed space craft that'd use current tech, the Innovative Interstellar Explorer, would need 6000 years. The fastest proposed spacecraft that'd use technology we could realistically build today, the TAU (Thousand Astronomical Units) space probe, would need 2500 years. We would need a spacecraft 25 times faster than TAU to make the trip in under a century.
That sounds un-promising, but the gap between humans reaching New Horizons top speed and the Wright brothers top speed was only a century of technological development. People have undertaken projects over many lifetimes often enough in the past, like York Minster Cathedral, and space agencies are developing new engines all the time - take the NEXT or VASIMIR space drives , which are currently undergoing testing.
We don't know how an alien race's technological history might play out - they could develop fast and not need a close stellar encounter to make the trip - but they'd still find it easier. Or they might hit a technological plateau at some point. In either case, a star that was very near to their own at some point in their history would surely get more attention; even for a very advanced civilization interstellar travel (using physics we understand today) would still be a huge undertaking, and consume massive amounts of energy.

Looked at like that, the idea of a civilisation just a little more advanced than us, making the trip in the distant past when our solar system happened to be exceptionally close, seems worth considering. But would we be able to tell?

A probe that simply did a fast flyby without stopping wouldn't have left any trace. But a probe that went into high orbit about one of the solar system's worlds, or the Sun, could remain here for millions of years. Our solar system is 4,500,000,000 years old - so if Dr Mamajek is right there have been 45,000 close encounters where a probe could have made an easier crossing. What's more, some researchers believe that 'mail'  might be the best way for an alien probe to communicate, which entails a physical messenger we could observe moving back and forth.

Yoda lived to 900, patient little alien is he.

Alien space probe?
Have we ever seen something that could fit the M.O. of an alien probe? Probably not.. um.. here's the abstract for a paper by Duncan Steele, describing a strange little object that seems to have been following Earth (here's a link to the paper) 

Abstract: A ~ 10-metre object on a heliocentric orbit, now catalogued as 1991 VG, made a close approach to the Earth in 1991 December, and was discovered a month before perigee with the Spacewatch telescope at Kitt Peak. Its very Earth-like orbit and observations of rapid brightness fluctuations argue for it being an artificial body rather than an asteroid. None of the handful of man-made rocket bodies left in heliocentric orbits during the space age have purely gravitational orbits returning to the Earth at that time, and in any case the a priori probability of discovery for 1991 VG was very small, of order one in 100,000 per anmun. In addition, the small perigee distance observed might be interpreted as an indicator of a controlled rather than a random encounter with the Earth, and thus it might be argued that 1991 VG is a candidate as an alien probe observed in the vicinity of our planet.

Since then some of the unusual aspects of 1991VG have been explained, but not all. And, although an extra-terrestrial probe is not likely, it does opened a path of thought on the subject: Look at our own currently advancing trends of miniaturisation and we can wonder: How small, and how hard to find , might an alien probe loitering in our solar system be?

The answer seems to be 'very' - we ourselves have fist sized space craft called cubesats. Fingernail sized chipsats, which are touted as a technology that could get to interstellar speeds, are being developed. If anything 1991 VG is too big, and too obvious to fit the bill.

Above: A Cubesat spacecraft being built. Courtesy of M.I.T.

It may be that we will never know, but if one day we stumble across something.... well, given the 4.5 billion years our solar system has had to be visited in, perhaps we shouldn't be surprised...

Elsewhere in the Universe:

The New Horizons probe has captured its first views of Pluto, and it's absurdly big moon Charon. You can see here that Charon is so big it and Pluto orbit about a point in the space between them (called a barycentre)

The DAWN mission has captured its best view yet of the dwarf planet Ceres - and the clever folk over at unmannedspaceflight.com have managed to match dawns black and white images with lower resolution colour shots taken by Hubble, to produce coloured versions.

Above: Ceres in (exaggerated) colour. Courtesy of Phil Stooke and Ian R.

What's amazing is how closely the images match. We're already seeing structure and shape to the white spots littering one hemisphere, and we're starting to see things like pancake shaped plateaus, long chains of mountains and gigantic slopes stretching over hundreds of kilometres. Phil nStooke has also created this animation, based on the best images to date:

News from the Moon: As if in response to the last few weeks' moon theme, the latest data from the Chang'e lunar mission has arrived! Here and here.

Elsewhere on the internet:

Cubesats come of age?

Micro spacecraft to explore Jupiter?

Strange clouds on Mars

Why comets are like fried ice cream with chocolate?