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23Comets
SCI-TECH & ENGINEERING
FIRST COMET VISITED
BY A SPACECRAFT
NASA’s International Cometary Explorer passed
through the plasma tail of comet Giacobini-Zinner,
around 7,800 km (4,846 mi) from its nucleus on
11 Sep 1985. The first comet encountered at close
range was 1P/Halley, which was visited by an
armada of five probes in 1986. ESA’s Giotto was
the only craft to pass the comet at close range –
596 km (370 mi) from the nucleus, on 14 Mar 1986.
SMALLEST COMET VISITED BY
A SPACECRAFT
Launched on 12 Jan 2005, NASA’s Deep Impact
spacecraft was re-tasked as the EPOXI mission
on 3 Jul 2007 with the goal of studying extrasolar
planets and performing a flyby of comet 103P/
Hartley. The flyby occurred on 4 Nov 2010.
“Small” in cosmic terms, the comet is around
2.25 km (1.39 mi) long with a mass of some
300 million tonnes (330.6 million tons).A comet consists primarily of ice, gas, dust and rock. This
composition has earned it the nickname “dirty snowball”.When it passed through the Solar System in 1997,
Hale-Bopp expelled 250 tons (226.7 tonnes) of
gas and dust – almost one-and-a-half times the
weight of a blue whale – every secondLargest comet
Discovered in Nov 1977, the comet known as
95P/Chiron has a diameter of 182 km (113 mi).Largest observed coma
The Great Comet of 1811 – discovered on 25 Mar
that year by Honoré Flaugergues (FRA) – had
a coma with an estimated diameter of around
2 million km (1.2 million mi).Longest measured comet tail
The tail of Comet Hyakutake measured
570 million km (350 million mi) long - more
than three times the distance from Earth
to the Sun. A team lead by Geraint
Jones of Imperial College London
discovered the prodigiously
long tail on 13 Sep 1999. To do
so, the scientists used data
gathered by the ESA/NASA
spacecraft Ulysses after its
chance encounter with the
comet on 1 May 1996.Closest approach to Earth by a comet
On 1 Jul 1770, Lexell’s Comet, travelling at
138,600 km/h (86,100 mph) relative to the Sun,
was within 2,200,000 km (1,360,000 mi) of Earth.Largest recorded impact in the Solar System
From 16 to 22 Jul 1994, more than 20 fragments
of Comet Shoemaker–Levy 9 struck the planet
Jupiter. The “G” fragment exploded with the
energy of around 600 times the world’s nuclear
arsenal, equivalent to six million megatons of TNT.Largest source of comets
Beyond the orbit of Neptune lie the Kuiper Belt,
the Scattered Disc and the Oort Cloud, which are
collectively known as Trans-Neptunian Objects.
The Oort Cloud is a spherical cloud of thousands
of billions of cometary nuclei. It surrounds the Sun
at a distance of around 50,000 AU (Astronomical
Units: 1 AU = the distance from Earth to the Sun),
or around 1,000 times the distance from the Sun
to Pluto. It is believed to be the source of most of
the comets that visit the inner Solar System.LEAST DENSE SOLID
As reported in Nature on 27 Feb 2013, a team from
Zhejiang University in China, led by Professor Gao
Chao (CHN), has made a graphene aerogel with
a density of just 0.16 mg/cm^3 (0.04 grains/cu in).
More than seven times lighter than air (which
weighs 1.2 mg/cm^3 , or 0.3 grains/cu in), it can
balance on a blade of grass (see inset). Below is a
block of aerogel without graphene. Among many
applications, aerogel is
used to gather
dust from
comets’
tails.DARKEST OBJECT IN THE SOLAR SYSTEM
The least reflective body discovered in the Solar
System to date is Comet Borrelly. This 8-km-long
(5-mi) comet nucleus was imaged by the Deep
Space 1 unmanned spacecraft on 22 Sep 2001.
Dust coating the surface of Borrelly makes it so
dark that it reflects less than 3% of the sunlight
it receives. By way of comparison, Earth reflects
around 30% of the sunlight it receives.MOST COMETS VISITED BY A SPACECRAFT
As of Oct 2016, three spacecraft have each visited two comets. The European Space Agency’s (ESA)
Giotto ( 1 ) encountered 1P/Halley in 1986 and then 26P/Grigg–Skjellerup in 1992. NASA’s Deep Impact ( 2 )
visited 9P/Tempel in 2005 and then (under the name EPOXI) encountered 103P/Hartley in 2010. NASA’s
Stardust spacecraft ( 3 ) visited 81P/Wild in 2004 and 9P/Tempel in 2011.The average comet is
the size of a small townAs it nears the Sun, a comet’s
nucleus heats up and its ice
begins to sublime, turning
into a huge cloud of gas (or
“coma”) larger than many
planets. The solar wind and
radiation act on the coma,
creating a tail that points
away from the Sun and may
be millions of kilometres
long (see below).FIRST SAMPLE RETURNED FROM A COMET
The earliest sample returned from a comet came
from Comet Wild 2. The Stardust spacecraft,
which launched on 7 Feb 1999, encountered the
comet on 2 Jan 2004. Flying through its coma (a
cloud of dust and gas around the comet’s centre),
Stardust swept up precious tiny samples of
cometary dust in an aerogel collector. The valuable
material was returned to Earth on 15 Jan 2006.
Its ongoing analysis is providing insights into the
chemical make-up of this icy primordial body.