Help Shape the Future of Space Exploration

Join The Planetary Society Now  arrow.png

Join our eNewsletter for updates & action alerts

    Please leave this field empty

Mars Express artist's concept at Mars

Mars Express team readies for Siding Spring

Posted by ESA Mars Express Team

26-02-2014 13:31 CST

Topics: comets, Mars Express, comet Siding Spring, Mars, optical telescopes

This article was originally posted on the Mars Express blog and is reposted here with permission.

One of the most interesting events in planetary exploration in 2014 is potentially also one of the most threatening for spacecraft orbiting Mars. This post was contributed by the MEX operations team here at ESOC and marks the start of our coverage of their efforts to safeguard the mission during the close flyby of Comet Siding Spring in October – while doing some unique science.

On Sunday, 19 October 2014, at around 18:30 UTC (20:30 CET), comet C/2013 A1 – known widely as 'Siding Spring' after the Australian observatory where it was discovered in January 2013 – will make a close fly-by of Mars.

Orbit of comet C/2013 A1 Siding Spring as it swings around the sun in 2014


Orbit of comet C/2013 A1 Siding Spring as it swings around the sun in 2014

It will be the second comet to visit the Red Planet in 12 months, following Comet ISON in October 2013. However, where ISON passed some 10,000,000 km from the planet, current estimates put Siding Spring's miss distance at just 136,000* km from the surface.

To give some perspective, Siding Spring will approach Mars by about 1/3 the average distance from Earth to the Moon (about 385,000 km).

At the scale of our Solar System, this is a very, very close shave...

Siding Spring


Siding Spring
Comet Siding Spring as seen from ESA's Optical Ground Station, Tenerife, Spain, 31 January 2014, 19:50 UTC.

While we know the comet will not hit Mars, nor our spacecraft, Mars Express, initial observation data lead us to expect that the coma (the cloud of dust particles surrounding the comet's nucleus) will be big enough to envelop Mars and therefore the spacecraft orbiting it.

Three orbiters are currently active at Mars: NASA's Mars Reconnaissance Orbiter (MRO) and Mars Odyssey, and our Mars Express. Two more that departed Earth in late 2013 are due to enter orbit around Mars about three weeks before the comet Siding Spring flyby: NASA's Mars Atmosphere and Volatile Evolution (MAVEN) and India's Mars Orbiter Mission.

Further observation of the comet will allow better predictions of the actual size of the coma, and the resulting level of risk to Mars-orbiting spacecraft, but this may not come for several months. Nonetheless, the mission operations team at ESOC have already begun considering ways to best protect Mars Express from the cloud of cometary dust.

The particles in the coma – ranging from 1 to 1/10,000th of a cm in diameter – are not expected to be large. However, they will be travelling toward Mars Express at a staggering 56 km/second (200,000 kph!).

At these speeds, even dust can be dangerous.

Consider that man-made space debris in orbit around Earth, where the relative velocities are 'merely' 7 km/second, can seriously harm satellites. The relative velocity for the Siding Spring dust particles will be about eight times faster – but the energy of an impact goes up with the square of the speed, meaning that the energy levels are 64 times higher!

Plus, it is not only the risk of physical damage from an impact the must be considered. Hyper-velocity impacts such as these can generate plasma clouds and electromagnetic pulses, which can cause disruptions with the many electronic systems onboard Mars Express.

The team have been doing a great deal of brainstorming to 'work the issue', and one of the obvious solutions lies in how we could adjust our orbit to shield the spacecraft behind the bulk of Mars, for at least part of the encounter if not all.

We also need to determine how can we best orient the spacecraft to reduce the exposure of instruments and critical systems to the coma and comet debris.

The team are also looking at how the many subsystems on board the spacecraft can be configured to ensure the highest possible resilience to the potential risks. Finally, given this opportunity to observe a comet as it passes so close to a rocky planet, we must co-ordinate spacecraft operations with the ESA science teams to accommodate as much science observations of this unique event as possible, consistent with safety.

Our close encounter with Siding Spring is still over nine months away, but the Mars Express team have already begun preparing for it, consulting with experts, industry and scientists and researching a complex set of details, possibilities and what-ifs.

It's a major challenge, and even if we design and implement the best possible way to deal with the close approach, there's no guarantee that Mars Express remains unaffected.

We'll keep you updated here in the blog (and in the main ESA website) to share how we're tackling these issues, ensuring that both the team and the spacecraft are ready for this incredibly challenging, once-in-a-lifetime encounter.

Siding Spring is currently around 670 million km from Mars, a distance it will cover in just nine months.

The countdown has begun!

Ed: Thanks to Andy, Kees, Simon, Luke and Michel for this great report.

Siding Spring by the numbers

Source: ESA and NASA

  • Date of comet closest approach (CA): 19 October 2014
  • Time of CA: ~18:28 UT
  • *Estimated distance of comet from Mars at CA: 136 000 km from centre || 132 000 km from surface
  • Comet nucleus diameter: Unknown
  • Coma radius: Likely to engulf Mars
  • Time for Mars to pass through coma: Several hours (MEX now orbits Mars every 7 hours)
  • Velocity of cometary dust particles: 56 km/second
  • Dust particles produced by comet (as of 28/1/14): 100 kg/second
See other posts from February 2014


Or read more blog entries about: comets, Mars Express, comet Siding Spring, Mars, optical telescopes


Telluric: 02/26/2014 08:08 CST

Yes. Safety first; no ifs or buts. These spacecraft cost $100s of millions, if not $Billions, usually 4 or 5 years to develop. Siding Spring is an amazing event but even still it is not worth losing one's mission, time and funds. I watched the JPL Small-Body Database over the last year. Early on, the minimum and maximum closest approach distances were large. It included Mars at closest approach, i.e. Mars was in the bulls-eye. More observational data was needed. The observations accumulated, now up to 625 and the accuracy improved to the present. As ESA scientists state, it won't collide with Mars. Precision is now at +/- 3000 miles - nominal distance at 83,500 miles. The uncertainty of arrival time, i.e. the precision, was 1 hour. Now it is 1 minute. My gut feeling is that out-gassing of the comet cannot cause a significant departure from the present course. Someone more knowledgeable could respond to this. Inaccuracy of all measurements due to systematic error is unlikely.

Telluric: 02/26/2014 08:25 CST

Pardon my wordiness but it is an incredible event. Siding Spring is relevant to the present discussion of asteroid impacts with Earth. While cometary impacts are much more rare, Earth just like Mars must receive close brushes such as Siding Spring and quite frequently, probably on nearly a human lifetime scale. Not until the last 100 years have we had such frequent observational coverage that we would see every comet pass by the Earth. Siding Spring will pass Mars very quickly, 2 or 3 hours , minutes at closest approach. Humanity could have easily missed viewing similar celestial apparitions involving Earth; day time event, odd approach angles or opposing side from where our population lies. If we are confronted with our own Siding Spring, there would be global shock. With early uncertainty in the path such as with Siding Spring, it would lead to a brief window of opporutunity during which we could launch a vehicle to push it aside e.g. with a nuclear explosion. Siding Spring at Earth would have caused World panic. We should have a plan on paper, e.g. UN agreement for such an event. The consequences of such an event are severe - destruction of civilization and all complex life forms. The contingencies would have to include being helpless, without sufficient time to respond and also working very fast with perhaps just weeks to launch something. To imagine that we would do nothing and sit and wait for precise measurements to be made to allay our fears of impact is pretty unimaginable.

Jonathan Ursin: 02/26/2014 11:11 CST

Is it known what the likelihood is of spacecraft survival? Does each orbiter have the same chances or are some better off than others? Last year Congress let NASA work through the government shutdown to launch MAVEN, it would be an unfortunate irony if the orbiter gets destroyed when the government shutdown would have saved it. :(

Leave a Comment:

You must be logged in to submit a comment. Log in now.
Facebook Twitter Email RSS AddThis

Blog Search

Planetary Defense

An asteroid or comet headed for Earth is the only large-scale natural disaster we can prevent. Working together to fund our Shoemaker NEO Grants for astronomers, we can help save the world.


Featured Images

Jupiter from Juno at Perijove #4
Jupiter in approximate true color during Juno perijove 4
More Images

Featured Video

Class 9: Saturn, Uranus, and Neptune

Watch Now

Space in Images

Pretty pictures and
awe-inspiring science.

See More

Join The Planetary Society

Let’s explore the cosmos together!

Become a Member

Connect With Us

Facebook, Twitter, YouTube and more…
Continue the conversation with our online community!