Comets are regular visitors to the inner solar system. Some are spectacular, known as Great
Comets, which are visible to the naked eye, sometimes stretching across a huge portion
of our sky. However, most comets come and go without much of a fuss, not visible to
the naked eye, and we wouldn’t even know about them were it not for telescopes constantly
monitoring sections of our sky. It is thought that there are billions of comets in our solar
system. Comets come in three categories, short, intermediate and long periods. Short period
comets orbit as far out as Jupiter. Intermediate comets can be found around the orbit of Neptune
and into the Kuiper belt. And many more beyond that are found in the Oort cloud, which are
known as long period comets. Comets tend to have very elliptical orbits, with most taking
tens of thousands of years to make one orbit around the Sun. Due to their tiny size and
huge distance from us, most only get discovered when they approach the Sun in their orbits.
Volatile material on their surface, like water and carbon dioxide ices, heat up upon approach,
subliming into space with some force, producing the bright coma and tails comets are most
famous for. This makes their detection a lot easier for astronomers, but once they whip
around the Sun and head back out towards the outer solar system, we often never see them
again, not in our lifetimes anyway. Most of the Great Comets we see are long period comets,
meaning we’ll only see them once in their millennia long orbits. All, except for one:
Halley’s Comet. While Halley’s Comet is an intermediate period comet, every orbit
has always produced a fantastic spectacle to those observing it, making it the only
intermediate period Great Comet. Its orbit takes roughly 75 years, meaning someone could
see it twice in their lifetime. And incredibly, there have been records of Halley’s comet
dating all the way back to 467 BC. With the regularity and predictability of it lighting
up our night skies, it was no surprise that as it was approaching in the 1980s, many space
agencies scrambled at the chance to get a close-up look of a Great Comet’s nucleus
for the very first time. And the result? Something known as Halley’s Armada. I’m Alex
McColgan, and you’re watching Astrum, and together we will investigate the story of
the Halley Armada, plus what it did and what it discovered around the comet Halley. The
Halley Armada is the unofficial name of five separate missions from three different space
agencies that all approached Halley at roughly the same time. One probe, called Giotto was
an ESA mission, Vega 1 and 2 came from a joint venture between the Soviet Union and France,
and the final two, Suisei and Sakigake came from ISAS, later to be known as JAXA, or the
Japanese space agency. Each of them has a pretty interesting story.
We’ll start with Vega 1 and 2. These Vega missions primarily were designed to drop
probes off at Venus, but it was determined that the mothership portion of the mission
could be redirected using Venus’ gravity to do a flyby of Halley if it was timed just
right. This gravity assist would prove useful not just because of the speed boost the probes
gains from the Venus flyby, but the redirection was particularly important as Halley has a
rather unconventional orbit which makes a rendezvous very tricky. Its orbit is highly
elliptical, is retrograde compared to the planets in our solar system, and is inclined
by 18° to the ecliptic, meaning most of the time it is under, or south of the solar system’s
plane. These two probes are twins, identical spacecraft that launched within days of each
other from Kazakhstan. They arrived on the 6th March and the 9th March 1986 respectively.
These two probes, while not perhaps the highlight of the armada, collected a host of data, including
photos of the nucleus, observations of its temperature, its surface properties and composition
of the coma. They both flew by Halley at a distance of only 8000km. But interestingly,
their positioning data was then passed on to the Giotto team, due to be arriving a few
days later, which allowed them to increase the accuracy of their flyby, meaning Giotto
could fly past Halley at an altitude of only 600km! But before Giotto arrived, the Japanese
space agency’s probes were approaching. This was Japan’s first deep space mission,
and as such, the Suisei and Sakigake probes were quite basic, and the missions were more
about a demonstration of the launch vehicle rather than the probes themselves. Sakigake
was launched first, and didn’t even have a camera onboard, but instead had instruments
to measure magnetic fields and plasma in interplanetary space. Upon the success of the Sakigake probe,
Suisei was launched a few months later, but this time with an imaging system and a solar
wind detecting instrument. Japanese scientists were interested in the comet’s huge coma,
which was imaged continuously as Suisei passed through it on the 8th March, at a distance
of 150,000km from the comet nucleus. Even at this far distance, Suisei hit two dust
particles during its flyby. Sakigake observed from a much greater distance of 7 million
km at its closest approach on the 11th March. Then, it was ESA’s turn. On the 13th March,
Giotto made the most ambitious flyby yet. Equipped with a colour camera, scientists
were keen to get their first close up view of a comet’s nucleus. Flying only 600km
over a very active comet, scientists were worried that the spacecraft wouldn’t survive.
As such, all the most important scientific instruments were tucked into the body of the
probe as much as possible, with a shield at the front to protect it from collisions. As
Giotto approached, it took many pictures, and in this motion interpolated video of those
images, this is what is saw. For the 1980s, this view is pretty incredible. Just after
this approach however, and just as scientists feared, Giotto was hit by some particles which
sent it spinning out of control, meaning its antenna was no longer pointed at Earth. For
an agonising 35 minutes, the mission controllers’ screens showed no contact with the probe.
But then, as perhaps hope seemed lost, contact. Giotto had stabilised itself again and re-established
contact with Earth, sending back the valuable data it had collected. Giotto largely survived
the encounter, although a particle had struck and disabled the camera onboard, but thankfully
not before it had taken images of its closest encounter. So what did all these missions
discover about Halley? Well, the main thing that surprised scientists greatly was that
Halley was a lot darker than they anticipated, as dark as coal. It was so dark in fact, that
it was beyond the limitations of the camera to distinguish detail. The image you are seeing
here is the result of serious image processing that bring out details from the original images.
Also, only a small portion of the comet seemed to be outgassing, seen mainly around these
beams here. This contradicted the ‘dirty snowball’ theory regarding comets, instead
this data indicated comets were more like ‘snowy dirtballs’. The armada also discovered
that the comet’s coma and tail included mainly water ice, but also carbon dioxide,
methane, ammonia, plus iron, hydrocarbons and sodium. The dusty particles are likely
caught up in the jets as the comet outgasses, blasting them away from the comet and into
the tail. It’s the dust particles ejected from comets that we see as shooting stars.
That is how scientists are able to predict when meteor showers are likely to happen,
as we can predict when Earth will pass through the remnants of a comet’s tail. The brightness
of the coma is due to sunlight being absorbed and re-radiated by the ejected ices, and to
a lesser extent due to the reflection of the sunlight off the dust particles. So the brightness
of a comet depends on how much it outgasses during its approach with the Sun. Scientists
also found Halley to be very ‘fluffy’ with a density of only roughly 0.6g/cm³,
so it is likely to be porous, or like a rubble pile. Measurements showed that Halley’s
surface temperature was 27c-127c, meaning only 10% of the surface was active as these
missions passed by. But perhaps the most astounding discovery of all was the deuterium ratio found
in its water ices. Scientists up until recently believed that comets are the source of water
found on Earth, but this comet - and again with Churyumov-Gerasimenko - have shown that
this likely isn’t the case. The deuterium ratio, or the key indicator to show whether
water shares the same origin or not, is different between water found on Earth and water found
on comets. This likely means we need a new theory regarding where Earth’s water came
from, perhaps asteroids could be the next option? So, there we have it. The Halley Armada,
an ambitious multi-national project to examine a frequent visitor to the inner solar system,
which uncovered a wealth of information about the outer solar system and the role of comets
in the Earth’s formation. And remember, make sure you have 2061 in your calendars
for when Halley comes by again, it promises to be a fantastic spectacle and you wouldn’t
want to miss it! Find it incredible how scientists can calculate the trajectory of probes accurately
enough to come within a few hundred km of a comet hurtling through space? It’s all
to do with classical mechanics and gravitational physics, subjects which don’t have to be
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for watching! I’ve been looking forward to making this video since I first learned
about the Armada. Want more old missions? Let me know in the comments. Thanks to everyone
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