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Cosmic Ray Delays Spacecraft Headed for Dwarf Planet

Late last week, an errant, high-energy particle struck part of the Dawn spacecraft‘s electrical system, disabling the NASA craft’s ion thrusters and sending it into safe mode on Sept. 11. The Dawn mission team spent the weekend investigating what had happened, and after four days were able to restart Dawn’s journey to dwarf planet Ceres, the largest body in the asteroid belt.

As a result of that cosmic strike, Dawn’s science orbits around Ceres will begin in April 2015, about a month later than expected*. But not to worry.

“The spacecraft is healthy,” says Marc Rayman, Dawn Mission Director and chief engineer. “We are still on course for Ceres, and we have every expectation of conducting the full exploration of Ceres, which is the first dwarf planet ever discovered.”

Soon, the team will repair the disabled ion engine and get it back online. It’s not the first time mission engineers have done this; in fact, a similar radiation strike disabled the ion propulsion system three years ago, while Dawn was approaching its first target, the dry and dusty asteroid Vesta. “We restored the same part simply by turning it off and turning it back on,” Rayman says. Really. It’s that simple. He notes that there is some actual physics explaining why this solution works for this particular component — it’s not being done out of an absence of better ideas.

High-energy particles, commonly called cosmic rays, are one of the hazards of space travel. In space, there’s no protective magnetic field or atmosphere to deflect them, as there are on Earth. So, even though interplanetary spacecraft wear radiation shields, some of the highest-energy particles still manage to slip through occasionally (sometimes, one of these particles will even strike a camera and produce a weird image, causing all kinds of wild speculation on Earth).

If you’re like me, any mention of a problem on Dawn sends you into a slight panic. The spacecraft has been assigned an exciting and unprecedented task: Orbit and study Vesta and Ceres, the two largest bodies in the asteroid belt. Never mind that these worlds are nowhere near one other.

Should Dawn succeed, it will be the first spacecraft to slip into orbit around two distinct extraterrestrial worlds — and it would be a sad day indeed if that doesn’t happen.

Vesta and Ceres, shown in relation to other similar solar system worlds. (NASA/JPL-Caltech/UCLA)
Vesta and Ceres, shown in relation to other similar solar system worlds. (NASA/JPL-Caltech/UCLA)

Launched in 2007, Dawn took less than four years to reach its first target, the giant, 525-kilometer wide asteroid Vesta. It then spent 14 months circling the pockmarked space rock, peering closely at its unusual shape. Vesta proved to be a more exotic object than scientists had anticipated. The source of many meteorites on Earth, Vesta is more a failed planetary seed than just an overgrown, lumpy asteroid. Its south pole had been mostly obliterated by two giant impacts and now hosts one of the tallest mountains in the solar system. Grooves and fractures deform its equator. There’s evidence for past volcanic activity, and Vesta’s interior is divided into a core, mantle, and crust — just like a planet’s.

At this point, Dawn’s mission is already halfway complete — but it’s possible the best part of the journey lies ahead, at Ceres. Hurtling through space, Dawn is well on its way to the watery dwarf planet, propelled by blasts from its next-gen ion thrusters. Now about 4.8 million kilometers away from Ceres, Dawn is so far from Earth that radio signals traveling at the speed of light take a bit more than 25 minutes to reach the spacecraft.

Dawn's location on Sept. 16, 2014. (Gregory L. Whiffen, JPL)
Dawn’s location on Sept. 16, 2014. (Gregory L. Whiffen, JPL)

Some scientists have suggested that 950-kilometer-wide Ceres could be a life-friendly world. What little we know about the faraway icy sphere suggests that it could have a super thin atmosphere, and there’s quite a bit of water there. Its interior could be differentiated, like Vesta’s, with a mantle containing more water than all of Earth’s oceans. In January, scientists reported twin tufts of water vapor wafting from the dwarf planet; whether those tufts are produced by volcanic activity or the sun’s warmth is a mystery that Dawn should be able to solve.

The year Dawn is supposed to spend studying Ceres from orbit will be crucial for unlocking its secrets, and the secrets of the dwarf planets in general. These runty worlds are the most common type of planet in the solar system — and we know next to nothing about them.

Of course, a few months after Dawn arrives at Ceres, a second dwarf planet will emerge from the darkness when the New Horizons spacecraft arrives at Pluto. By the end of next summer, if all goes well, we will know more about dwarf planets than ever before. So let’s all hope that Dawn and New Horizons stay healthy on their journeys into the unknown.

By early next year, the images we have of Ceres should be more detailed than this Hubble Space Telescope image. (Image Source)
By early next year, the images we have of Ceres should be more detailed than this Hubble Space Telescope image. (Image Source)

This post has been updated with comments from Marc Rayman, Dawn’s Mission Director and chief engineer.

*Because of the way Dawn’s ion thrusters work, it slips into orbit a bit differently than other spacecraft. Rather than zooming over and slamming on the brakes, Dawn constantly shapes its orbit around the sun to match that of its target. So the situation is actually a bit more complicated. Those four days without propulsion — during which Dawn’s trajectory wasn’t being massaged to match Ceres’ — mean that Dawn might well arrive at Ceres a bit earlier than expected. But it will be going faster and will take longer to spiral down into data-collecting orbit, which is why the science portion of the mission will be a bit delayed.

9 thoughts on “Cosmic Ray Delays Spacecraft Headed for Dwarf Planet

  1. I glad that we are reach further the before. I wonder whether what was next on the agenda after Pluto? What is the next desalination? When exploring Ceres, what will be the test taking.

  2. Not doubting, but trying to understand. The earth is usually referenced as being 70 percent water. Ceres is shown by the graphic as being less than half the size of the moon, let alone earth. How could it possibly have more water than all the oceans? (the pdf was over my head as well). Is there an easy way to explain? Just cant see how it could be possible.

  3. Darrel Armstrong: Ceres was discovered in 1801, Pluto in 1930.

    John B: Be aware that Earth is not 70% water, but about 70% of its surface area is covered in water. For example, if you covered 70% of your skin with peanut butter you would not BE 70% peanut butter, you would just need a shower. Ceres may actually BE composed of a large % of peanut b- er water, and so may have much more water buried in it than is puddled on Earth’s rocky skin. Hope that helps!

  4. John B: over 70% of the Earth’s surface is ocean, but even the deepest oceans are shallow compared to the enormous size of the Earth. In fact only about 0.02% of the whole Earth is water, so it’s perfectly possible that Ceres’ mantle could contain more water than Earth’s oceans.

    And darrell: Ceres was classified as a dwarf planet at the same time Pluto was. Since it was discovered in 1801, long before Pluto, it is the first dwarf planet to have been discovered.

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