One plan becomes two plans

Editor’s note: Those who have been following our blog will know that the MEX Flight Control Team at ESOC have been actively preparing for the flyby of comet C/2013 A1/Siding Spring on 19 October. Initial estimates gave the possibility that Mars Express might have to contend with a large particle flux – and that several (2? 3?) very high-speed (~56 km/sec!) particles might bash into the spacecraft. Happily, additional observations by ground and space telescopes (including the ESA/NASA Hubble Space Telescope) have allowed initial estimates to be refined and the risk is now understood to be much lower – and perhaps even as low as zero. In today’s blog post, the team explain how this (happy!) real-life, real-time development is affecting their preparations for fly-by.

Comet C/2013 A1 Siding Spring seen on 6 September 2014 from Argentina. Image credit: César Nicolás Fornari https://www.facebook.com/cesar.fornari

Comet C/2013 A1 Siding Spring seen on 6 September 2014 from Argentina. Image credit: César Nicolás Fornari https://www.facebook.com/cesar.fornari

Late last year, estimates given in scientific papers estimated that over the duration of the encounter, the number of large cometary particles per square metre would be around 1. As MEX’s area in the most protected attitude is about 3m2, we could then expect about 3 potentially significant impacts. Not good!

By the middle of this summer, published estimates (based on new images and additional modelling) were indicating a flux of around 10-6 particles per m2, which, for Mars Express, very roughly equates to a 1-in-300,000 chance of being hit. It’s starting to look like our comet C/2013 A1/Siding Spring will manifest itself as a more friendly passer-by than initially thought and that it won’t be hurling clouds of large particles at unthinkable speeds towards Mars and its man-made satellites.

Closest approach: If Mars were Earth. Credit: NASA/JPL

Closest approach: If Mars were Earth. Credit: NASA/JPL

So why the big change?

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Cosmic bully spotted by ESA and NASA

There are some great new Hubble images of our ‘friend’, Comet Siding Spring, due to pass by Mars at less than 136 000 km on 19 October – less than half the distance between Earth and our moon.

Comet Siding Spring imaged by ESA/NASA Hubble. Credit: NASA, ESA, and J.-Y. Li (Planetary Science Institute)

Comet Siding Spring imaged by ESA/NASA Hubble. Credit: NASA, ESA, and J.-Y. Li (Planetary Science Institute)

The passage of the comet may affect spacecraft in orbit around Mars, including ESA’s Mars Express.

The image on the left above, captured 11 March by the NASA/ESA Hubble Space Telescope, shows comet C/2013 A1, also called Siding Spring, at a distance of 568 million km from Earth. Hubble can’t see Siding Spring’s icy nucleus because of its diminutive size. The nucleus is surrounded by a glowing dust cloud, or COMA, that measures roughly 19 000 km across.

The right image shows the comet after image processing techniques were applied to remove the hazy glow of the coma revealing what appear to be two jets of dust coming off the location of the nucleus in opposite directions. This observation should allow astronomers to measure the direction of the nucleus’s pole, and axis of rotation.

Hubble also observed Siding Spring on 21 January as Earth was crossing its orbital plane, which is the path the comet takes as it orbits the Sun. This positioning of the two bodies allowed astronomers to determine the speed of the dust coming off the nucleus.

“This is critical information that we need to determine whether, and to what degree, dust grains in the coma of the comet will impact Mars and spacecraft in the vicinity of Mars,” said Jian-Yang Li of the Planetary Science Institute in Tucson, Arizona.

Compass and Scale Image for Comet C/2013 A1 Siding Spring (3 Epochs)

Compass and Scale Image for Comet C/2013 A1 Siding Spring (3 Epochs)
Source: Hubblesite.org

The image above shows a series of HST pictures of comet C/2013 A1 Siding Spring as observed on 29 October 2013, 21 January 2014 and 11 March 2014. The distances from Earth were, respectively, 605 million km, 552 million km, and 568 million km. The solid icy nucleus is too small to be resolved by Hubble, but it lies at the center of a dusty coma that is roughly 19 000 km across in these images.

When the glow of the coma is subtracted through image processing, which incorporates a smooth model of the coma’s light distribution, Hubble resolves what appear to be two jets of dust coming off the nucleus in opposite directions. The jets have persisted through the three Hubble visits, with their directions in the sky nearly unchanged. These visible-light images were taken with Hubble’s Wide Field Camera 3.

Discovered in January 2013 by Robert H. McNaught at Siding Spring Observatory, Australia, the comet is falling toward the Sun along a roughly 1-million-year orbit and is now within the radius of Jupiter’s orbit. The comet will make its closest approach to our Sun on 25 October at a distance of 209 million km – well outside of Earth’s orbit. The comet is not expected to become bright enough to be seen by the naked eye.

The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA’s Goddard Space Flight Center in Greenbelt, Md., manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington.