[meteorite-list] Herschel Intercepts Asteroid Apophis

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 9 Jan 2013 10:14:56 -0800 (PST)
Message-ID: <201301091814.r09IEuaN026662_at_zagami.jpl.nasa.gov>

http://www.esa.int/Our_Activities/Space_Science/Herschel_intercepts_asteroid_Apophis

Herschel intercepts asteroid Apophis
European Space Agency
9 January 2013

ESA's Herschel space observatory made new observations of asteroid
Apophis as it approached Earth this weekend. The data shows the
asteroid to be bigger than first estimated, and less reflective.

Catalogued as asteroid (99942) Apophis (previously 2004 MN4), it
is often nicknamed "the doomsday asteroid" in popular media, after
initial observations made after its discovery in 2004 gave it a
2.7% chance of striking Earth in April 2029.

With additional data, however, an impact in 2029 was soon ruled
out, although the asteroid will pass within 36 000 km of Earth's
surface, closer even than the orbits of geostationary satellites.

The asteroid will return to Earth's neighbourhood again in 2036,
but quite how close it will come then is uncertain, as the 2029
approach is predicted to alter its orbit substantially. Obtaining
improved physical parameters for Apophis and its orbit is thus of
great importance in being able to make better predictions of its
future trajectory.

Herschel had a good opportunity at the weekend, observing the
asteroid for about two hours on its approach to Earth, ahead of
today's closest encounter at a little less than one tenth of the
distance from Earth to the Sun: about 14.5 million km. The
observations were made as part of Herschel's Guaranteed Time
Programme MACH-11.

"As well as the data being scientifically important in their own
right, understanding key properties of asteroids will provide
vital details for missions that might eventually visit potentially
hazardous objects," says Laurence O'Rourke, Principal Investigator
of the MACH-11 observing programme, from the European Space
Astronomy Centre (ESAC), near Madrid, Spain.

"Apophis is only the second near-Earth asteroid observed by
Herschel, and these were the fastest tracked observations by the
space telescope - the asteroid moved at a rate of 205 arcseconds
per hour as seen from Herschel's viewpoint."

Herschel provided the first thermal infrared observations of
Apophis at different wavelengths, which together with optical
measurements helped refine estimates of the asteroid's properties.
Previous estimates bracketed the asteroid's average diameter at
270 +/- 60 m; the new Herschel observations returned a more precise
diameter of 325 +/- 15 m.

"The 20% increase in diameter, from 270 to 325 m, translates into
a 75% increase in our estimates of the asteroid's volume or mass,"
says Thomas Muller of the Max Planck Institute for
Extraterrestrial Physics in Garching, Germany, who is leading the
analysis of the new data.

By analysing the heat emitted by Apophis, Herschel also provided a
new estimate of the asteroidl's albedo - a measure of its
reflectivity - of 0.23. This value means that 23% of the sunlight
falling onto the asteroid is reflected; the rest is absorbed and
heats up the asteroid. The previous albedo estimate for Apophis
was 0.33.

Knowing the thermal properties of an asteroid indicates how its
orbit might be altered due to subtle heating by the Sun. Known as
the Yarkovsky effect, the heating and cooling cycle of a small
body as it rotates and as its distance from the Sun changes can
instigate long-term changes to the asteroid's orbit.

"These numbers are first estimates based on the Herschel
measurements alone, and other ongoing ground-based campaigns might
produce additional pieces of information which will allow us to
improve our results," adds Muller.

"Although Apophis initially caught public interest as a possible
Earth impactor, which is now considered highly improbable for the
foreseeable future, it is of considerable interest in its own
right, and as an example of the class of Near Earth Objects," says
Goran Pilbratt, ESA's Herschel Project Scientist.

"Our unique Herschel measurements play a key role for the
physical characterisation of Apophis, and will improve the
long-term prediction of its orbit."


Notes for editors

The observations of asteroid Apophis were made as part of a
Herschel Science Centre Guaranteed Time Programme called MACH-11.
The 11 asteroids and comets observed in this programme have been
or will be visited by spacecraft or are undergoing studies to that
end, as is the case for Apophis.

Four observations were made with PACS, the Photodetector Array
Camera & Spectrometer instrument, which contains an imaging
photometer (camera) and an imaging spectrometer. The camera
operates in three bands centred on 70 ??m, 100 ??m, and 160 ??m,
respectively.

The observations of Apophis began at 23:54:35 on 5 January 2013,
and ended at 02:04:12 on 6 January 2013, a total of 2 hours and 10
minutes.

Apophis is only the second near-Earth asteroid observed with
Herschel, after asteroid 2005 YU55 , which was observed during its
close approach to Earth in November 2011.

Herschel is an ESA space observatory with science instruments
provided by European-led Principal Investigator consortia and with
important participation from NASA. PACS has been developed by a
consortium of institutes led by MPE (Germany) and including: UVIE
(Austria); KU Leuven, CSL, IMEC (Belgium); CEA, LAM (France); MPIA
(Germany); INAF-IFSI/OAA/OAP/OAT, LENS, SISSA (Italy); IAC
(Spain). This development has been supported by the funding
agencies BMVIT (Austria), ESA-PRODEX (Belgium), CEA/CNES (France),
DLR (Germany), ASI/INAF (Italy), and CICYT/MCYT (Spain).

For further information, please contact:

Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Tel: +31 71 565 6799
Mob: +31 61 594 3 954
Email: Markus.Bauer at esa.int

Laurence O'Rourke
MACH-11 Principal Investigator
Email: Laurence.O'Rourke at esa.int

Thomas Muller
Max Planck Institute for Extraterrestrial Physics
Email: tmueller at mpe.mpg.de

Goran Pilbratt
ESA Herschel Project Scientist
Tel: +31 71 565 3621
Email: gpilbratt at rssd.esa.int
Received on Wed 09 Jan 2013 01:14:56 PM PST