Observing Asteroid 99942 (2004 MN4) Apophis with the Herschel Space Observatory.

99942 (2004 MN4) Apophis was discovered on 19 June 2004 by the Spacewatch Project at  University of Arizona's Lunar and Planetary Laboratory operating out of the Kitt Peak Observatory. The designation 2004 MN4 indicates that it was the 113th asteroid (asteroid N13) discovered in the second half of June 2004 (period 2004 M) while the number 99942 indicates that it was the 99 942nd asteroid discovered (asteroids are not given such a name immediately, to avoid numbering false or duplicate observations) and named Apophis refers to an Egyptian god of darkness and destruction. 

It has an eccentric orbit which takes it from 0.746 AU from the Sun (74.6% of the average distance between the Earth and the Sun, ad slightly outside the orbit of the planet Venus) to 1.0985 AU from the Sun (109.85% of the average distance between the Earth and the Sun), making it an Aten type Near Earth Asteroid (an asteroid that crosses the Earth’s orbit but is closer to the Sun than we are most of the time); although it is predicted that a close encounter with the Earth in April 2029 will result in the asteroids orbit being altered in such a way that it becomes an Apollo type asteroid (one that crosses the Earth's orbit but is on average further away from the Sun than the Earth). 

It was initially estimated to have an equivalent diameter of between 320 m and 970 m (i.e. a spherical object with the same volume would have a diameter of between 320 m and 970 m), and was calculated to have a 2.7% chance of impacting the Earth in April 2029. An object of this size would be capable of passing straight through the Earth’s atmosphere resulting in an explosion up to 250 000 times as large as the one caused by the Hiroshima bomb, the formation of a crater between 5 and 12 km in diameter, and climatic effects that would last decades if not centuries. As such 99942 (2004 MN4) Apophis was also classified as a Potentially Hazardous Asteroid.

The current orbit of (99942) 2004 MN4 Apophis. JPL Small Body Database Browser.

Subsequent observations have ruled out the possibility of 99942 (2004 MN4) Apophis impacting the Earth in 2029, though it is still thought that it will pass extremely close, reaching about 38 310 km from the Earth at its closest, roughly 10% of the average distance between the Earth and the Moon, and within orbits of some satellites, however the possibility of such a collision occurring at some point in the future cannot be ruled out.

The size of the asteroid has also been recalculated, and reduced to about 270 m, still very bad news should it hit us, but a considerable improvement on the original size estimation. Observations made in the near infrared part of the spectrum suggested that 99942 (2004 MN4) Apophis is a Sq-class asteroid (a stoney asteroid with a high olivine and pyroxine content) with a composition similar to an LL ordinary chondrite type meteorite (Low iron, Low metal ordinary chondrite). This is similar to the asteroid 25143 (1998 SF36) Itokawa, which was visited by the Japan Aerospace Exploration Agency's Hayabusa Probe in 2005. 25143 (1998 SF36) Itokawa has an estimated porosity of 40%, and if the same is true for 99942 (2004 MN4) Apophis then it is likely to have a mass of about 20 000 000 tons. Such an object impacting the Earth would be predicted to result in a 375 Megatonne explosion, over 22 000 times as large as that caused by the Hiroshima bomb, resulting in the formation of a crater 3 km in diameter.

In a paper published in the journal Astronomy & Astrophysics on 24 April 2014, and on the arXiv database at Cornell University Library on 23 April 2014, a team of scientists led by Thomas Müller of the Max-Planck-Institut für extraterrestrische Physik, describe the results of a series of observations of 99942 (2004 MN4) Apophis in the far infrared part of the spectrum using the Herschel Space Observatory’s Photodetector Array Camera and Spectrometer instrument on 6 January and 14 March 2013. 

Each observation consisted of a series of scans at different wavelengths, used to create a mini scan-map of the Asteroid. These scan-maps were then compared to a physical shape model of 99942 (2004 MN4) Apophis created by a team of scientists led by Petr Pravec of the Astronomical Institute of the Academy of Sciences of the Czech Republic to create a three dimensional thermal map of the asteroid. 

Viewing geometry during the two Herschel observing epochs at phase angles of roughly 60˚ angle before (left) and after opposition (right). Top: calculated observing geometry on basis of the nominal solution in Pravec et al. (2014). L is fixed vector of angular momentum, the Aries sign is the X axis of the ecliptical frame, S is a direction to the Sun, and x, y, z are the axes of the asteroid co-rotating coordinate frame (corresponding to the smallest, intermediate and the largest moment of inertia of the body, respectively). Middle: The solar insolation in [W/m²]. Bottom: TPM temperature calculations assuming a Itokawa-like thermal inertia of 600 Jm¯²s¯⁰˙⁵K¯¹. Müller et al (2014).

Using this thermal profile it was possible to form a revised model of the size of 99942 (2004 MN4) Apophis, using the apparent distances between the two ends of the asteroid on the two observation dates. This suggests an asteroid with an equivalent diameter of 371-385 m, considerably larger than the previous model. 

The temperature differentiation between the two ends of the object suggest that it has a thermal inertia of between 250 and 800 Jm¯²s¯⁰˙⁵K¯¹ (this is the capacity of the object to redistribute its heat internally by conduction as one side is heated by the Sun and the other is in darkness), probably about 600 Jm¯²s¯⁰˙⁵K¯¹. This suggests that 99942 (2004 MN4) Apophis is probably comprised of a small amount of fine regolith (which has a low conductivity) and a larger volume of denser rocks and boulders (with higher conductivities). This suggests that Apophis is slightly larger and more dense than 25143 (1998 SF36) Itokawa, and with a mass of about 53 000 000 tonnes.

Such an object would clearly be more dangerous than previously thought, and if it were to collide with the Earth it would result in an explosion around 16 000-16 500 times as large as the Hiroshima bomb, and the formation of a crater up to 6 km across. However in addition to raising the danger level associated with 99942 (2004 MN4) Apophis, the new more refined model of the asteroids mass and density should allow a more detailed modelling of its orbital properties, enabling us to make better predictions of the threat it presents.

See also…

 Asteroid 388838 (2008 EZ5) passes the Earth.




 Asteroid 2014 HS4 passes the Earth.






 Asteroid 2003 HM passes the Earth.

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What Hayabusa brought back from 25143 (Itokawa).

In November 2005, the Japan Space Agency's probe Hayabusa touched down on the near Earth asteroid 25143 (Itokawa) in order to collect samples. The probe landed in the Muses Sea are of the asteroid, and was due to fire a projectile into the surface in order to dislodge material from the surface for collection. In the event this projectile did not fire, but the probe was able to collect a small number of mineral grains floating above the surface, of the asteroid, which has negligible gravity. This material was returned to Earth for analysis in June 2010.

Close up image of taken from Hayabusa. A & B indicate possible recent impact sites. Circles represent possible hydrological sinks. Arrows point to areas of talus (rubble). Curved lines indicate possible debris flow. Japan Space Agency.

25143 (Itokawa) is a 558 m long, 288 m diameter asteroid with a 556 day period (year) on an orbit that crosses that of the Earth. It is roughly bean-shaped and has a surface covered in rubble; in fact it may be rubble all the way through. The asteroid rotates on its axis every 12 hours, and has a gravity of about a millionth of the Earth's (though this varies from place to place, dependent on the local density of the asteroid). 25143 (Itokawa) has a number of areas on the surface that appear to be hydrological sinks.

These hydrological sinks are surprising in on an asteroid, which is not somewhere we would expect to find water, but are attributed to the former presence of ice. It is thought that the asteroid may have formed further out in the solar system, where chunks of ice (not necessarily water ice) were incorporated into its makeup. At some point it was shifted onto its current orbit, where it passes closer to the sun. This caused the asteroid to heat up, and the ice to sublimate (turn directly from a solid to a gas) in a similar way to material evaporating from the surface of a comet. After this happened the loose rocky material covering the new void subsided forming a sinkhole.

The orbit of 25143 (Itokawa). Bellatrix Astronomical Observatory.

On 27 February 2012 a paper was published in the Proceedings of the National Academy of Sciences, by a group of scientists lead by Eizo Nakamura of the The Pheasant Memorial Laboratory for Geochemistry and Cosmochemistry at the Institute for Study of the Earth’s Interior at Okayama University, detailing the results of a study of five mineral grains brought back from 25143 (Itokawa) by Hayabusa, and the deductions made from these studies.

The grains were made of the minerals olivine, pyroxene, diopside and plagioclase, all common in igneous rocks from the Earth and the Moon, with small inclusions of other common minerals. They ranged in size from 30 × 40 μm to 90× 110 μm, and all were covered in tiny pits, apparently impact craters caused by the action of tiny grains 10-20 nm across. This is interesting as these may not have originated from 25143 (Itokawa), or a similar asteroid. In the Solar System objects larger than 5 μm tend to fall towards the sun, whereas those smaller tend to be carried outwards by the solar winds.

Scanning Electron Microscope images of an olivine grain from 25143 (Itokawa). (A) Detail of part of the grain as shown in inset. F1, F2 & F3 represent fracture plains (the plains along which a mineral will split). C, D & E are areas of magnification. (B) Back Scattered Electron Microscope image of (A) showing mineral textures; Ol is olivine, Pl is plagioclase. (C, D & E) Detail of (A) showing craters made by tiny impacts. These are 100-200 nm across, implying impactors 10-20 nm across. From Nakamura et al. (2012).

While the minerals from which 25143 (Itokawa) are made are not unusual they do tell us something about the history of the asteroid. The minerals present would generally form at a temperature of about 900°C, far hotter than the temperature likely to be reached during the formation of a 300 m radius asteroid. From this Nakamura et al. conclude that 25143 (Itokawa) was formed as part of a larger body, from which it has become separated at some point.

See also The composition of Comet C/2009 P1, The composition of Comet 10P/Tempel 2, Asteroid 2012 BX34 to pass within 59 000 km of Earth, The dust-tail of Asteroid P/2010 A2 and Fragments of Mars found in Morocco.