Interstellar Intruder ‘Oumuamua
– Harvard Scientists Now Speculate about a Light Sail
What We Know About ‘Oumuamua
In a paper published last year (Astrophysical Journal Letters:
Interstellar Interloper 1I/2017 U1: Observations from the NOT and WIYN
Telescopes, David Jewitt et al), researchers showed that a newly discovered
object, since named ‘Oumuamua, must be of
interstellar origin, due to its velocity (26 km per second) exceeding the escape
velocity from the solar system.
Furthermore, it was shown that
its shape must be unusually elongated.
It must have at least a 5:1 ratio of length to width, possibly even much
more. That’s based on its light curve,
or how the brightness changes over a short interval (about 7 hours). The best explanation for that is a long, thin
object that is tumbling with respect to the Earth, sometimes showing a
shortened profile (and thus being less bright) and sometimes showing a longer
profile (and thus being more bright). If
you spin a pencil around its long axis and observe how it apparent size changes,
you can get the idea.
A following paper in Nature (Non-gravitational acceleration in the
trajectory of 1I/2017 U1 (‘Oumuamua)) demonstrated that ‘Oumuamua has a
non-gravitational component to its motion.
In other words, its trajectory has been influenced by some other push
than the gravity of the major bodies of the solar system. The effect is very small, so it took a lot of
observational data to detect the anomalous motion. This was based on a comprehensive
analysis of multiple observations, so the evidence is quite persuasive. Plus, this paper was in Nature, so it no
doubt had fairly intensive peer review.
That paper speculated that ‘Oumuamua
is probably a comet that had been kicked out of another solar system long ago,
and is slightly off-gassing as it goes through our solar system. That produces a slight rocket-like effect, as
the off-gassing is essentially reaction mass.
Its best fit velocity profile indicates that the acceleration is of a (1/R**2)
form, as would be expected if the “driving force” was related to distance from
the sun.
What the Latest Harvard Paper Says About ‘Oumuamua
The latest pre-print paper (COULD
SOLAR RADIATION PRESSURE EXPLAIN ‘OUMUAMUA’S PECULIAR ACCELERATION? SHMUEL BIALY
AND ABRAHAM LOEB, Harvard Smithsonian Center for Astrophysics), disagrees with
the cometary explanation (it should be noted that this paper is by researchers
from Harvard’s Smithsonian Astrophysics Center, so it has a credible pedigree). Rather, it proposes that the
non-gravitational acceleration that has been seen is more likely due to solar
radiation pressure than off-gassing. In
other words, the light from the sun is doing the pushing, rather than it
heating up material that off-gasses, thus producing a rocket-like effect.
This paper takes issue with the
off-gassing comet theory for the simple reason that none of the usual features
associated with a comet were observed in the ‘Oumuamua case. There was no cometary trail, nor were gas
emission/absorption lines seen in the observations.
In addition, a different paper that they cite (SPIN
EVOLUTION AND COMETARY INTERPRETATION OF THE INTERSTELLAR MINOR OBJECT 1I/2017
’OUMUAMUA Roman R. Rafikov) has argued from theoretical grounds that an
outgassing comet would have evolved a much different spin than that actually
observed. These arguments are based on
conservation of angular momentum considerations. So, that is claimed to be supporting evidence
that ‘Oumuamua is not a comet.
The Harvard paper goes through a
chain of calculations to show that, if the object is not a comet, then a
reasonable alternative explanation is a very thin object with a very small mass
to area ratio (about 0.1 to 0.3 grams per square centimeter). In other words, the object must be extremely
thin, if radiation pressure is the driving force behind its non-gravitational
acceleration, rather than off-gassing.
It also examines the
probability that a very thin object could actually survive a long trip through
the galaxy, rather than being destroyed (ablated or torn apart) or dramatically
slowed down by collisions with matter in the interstellar medium such as gas or
dust. Again, a chain of theoretical
reasoning is given to show that such an object could survive a long trip, as
these collisions would not absorb much of the objects momentum, nor ablate much
of its mass. The interstellar medium is
simply too sparse, and ‘Oumuamua’s velocity is too low to compromise its
survival to a significant degree.
As indicated in the attached
graph, an object of this type could travel many kiloparsecs before being
appreciably slowed down or vaporized (for context, the distance from Earth to
the center of the galaxy is on the order of a kiloparsec).
The possibility that a very thin
object might also be torn apart by rotational torques or tidal forces is also
examined. Their calculations indicate
that those would only be a problem if such an object passed very close to a
star, much closer than is likely.
Presumably if it passed that close, it would be captured by the star,
anyway.
Given the unusual physical
constraints that these calculations give for such an object (very thin and
elongated), what could it be? There are
no known solar system objects like this, but that doesn’t mean some natural astrophysical
process might not produce such thin objects.
We just don’t know of any such process, and can’t think of one at the
moment, but we also know that the universe has produced many unexpected
surprises for us, as revealed by recent observations (e.g. hot Jupiters,
planets around neutron stars).
Then, of course, there is always
the possibility of the object having a non-natural genesis. As the paper notes, a solar sail is an
obvious possibility. It would fit the
possible geometry, and we know they exist, because we have built them ourselves. Debris from some other sort of craft is also
possible – perhaps it’s a quarter panel from a broken-up spaceship?
The paper also notes that an
artificial source would have a bit of Occam’s razor on its side (my wording,
not theirs). Some earlier calculations
about the probability of us observing a natural interstellar object that had
been ejected randomly from another solar system, resulted in the conclusion
that there must be a surprisingly large amount of these things produced during
stellar evolution, just floating around in interstellar space (maybe a thousand
trillion per cubic parsec). If the
object was artificial, and directed to our solar system on purpose, that surprising
conclusion wouldn’t be necessary, though it would be replaced by the even more
surprising conclusion that aliens sent a probe to our solar system.
Oh, and for what its worth, I actually included the idea of
a solar sail in my last blog on the subject of ‘Oumuamua, as reproduced below.
Other Possible Explanations
Some other physical models that
might produce a non-gravitational acceleration were discussed. For fun, I have included a Science Fiction
possibility, which I have called SF response, where possible:
·
1. Solar Radiation Pressure, such as that
which drives a light-sail spacecraft.
This was rejected on the basis that the object would have to have an
unrealistically low density, three or four orders of magnitude lower than
natural solar system objects.
·
SF response – what if actually was a solar
sail?
You can check it out for yourselves: http://dodecahedronbooks.blogspot.com/2018/07/oumuamua-non-gravitational-acceleration.html
It’s nice to see that these Harvard researchers went to the
trouble of looking into that possibility :).
Sources:
1.
(Astrophysical Journal Letters: Interstellar
Interloper 1I/2017 U1: Observations from the NOT and WIYN Telescopes, David Jewitt
et al)
2.
(Non-gravitational acceleration in the
trajectory of 1I/2017 U1 (‘Oumuamua))
3.
(COULD SOLAR RADIATION PRESSURE EXPLAIN
‘OUMUAMUA’S PECULIAR ACCELERATION? SHMUEL BIALY? AND ABRAHAM LOEB, Harvard
Smithsonian Center for Astrophysics
4.
(SPIN EVOLUTION AND COMETARY INTERPRETATION OF
THE INTERSTELLAR MINOR OBJECT 1I/2017 ’OUMUAMUA Roman R. Rafikov)
Now that you
have read about a real interstellar interloper (natural or not), you should
consider reading some Science Fiction.
How about a short story, also about interstellar interlopers. It also features one possible scenario to
explain why we haven’t met ET yet (as far as we know, anyway). Only 99 cents on Amazon.
The Zoo Hypothesis or The News of the World: A Science Fiction Story
Summary
In the field known as Astrobiology, there is a research
program called SETI, The Search for Extraterrestrial Intelligence. At the heart of SETI, there is a mystery
known as The Great Silence, or The Fermi Paradox, named after the famous
physicist Enrico Fermi. Essentially, he
asked “If they exist, where are they?”.
Some quite cogent arguments maintain that if there was
extraterrestrial intelligence, they should have visited the Earth by now. This
story, a bit tongue in cheek, gives a fictional account of one explanation for
The Great Silence, known as The Zoo Hypothesis.
Are we a protected species, in a Cosmic Zoo? If so, how did this come about? Read on, for one possible solution to The
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The short story is about 6300 words, or about half an hour
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The Magnetic Anomaly
Summary
An attractive woman in a blue suit handed a dossier to an
older man in a blue uniform. “Give me a quick recap”, he said.
“A geophysical crew went into the Canadian north. There were some regrettable accidents among a few ex-military who had become geophysical contractors after their service in the forces. A young man and young woman went temporarily mad from the stress of seeing that. They imagined things, terrible things. But both are known to have vivid imaginations; we have childhood records to verify that. It was all very sad. That’s the official story.”
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