Capturing the Solar Wind
Interview with Don Burnett
|Hexagonal array to capture Genesis’ solar wind particles Credit: NASA|
NASA’s Genesis mission is the first attempt to collect samples of the solar wind and return them to Earth. Genesis launched in August 2001, and three months and about one million miles later, hexagonal wafers on the spacecraft began to collect solar wind particles. Even though the sun is mostly hydrogen and helium, scientists believe it also contains over 99 percent of all matter that makes up the solar system.
The Genesis spacecraft is currently on route back to Earth. On September 8, 2004, the sample-return capsule will leave the spacecraft and re-enter Earth’s atmosphere, parachuting down towards the U.S. Air Force Test and Training Range in Utah.
Don Burnett, a professor of geochemistry at the California Institute of Technology, is Principal Investigator for the Genesis mission. In this interview with Astrobiology Magazine, he explains what a sample smaller than a few grains of salt can tell us about the birth of the solar system.
Astrobiology Magazine (AM): Why did we need to launch a collector into space to capture solar particles? Isn’t the Earth bombarded with the solar wind and solar particles every day?
Don Burnett (DB): No, the Earth’s magnetic deflects the solar wind, and you have to go far away from Earth to get out of reach of the Earth’s magnetic field.
|Don Burnett, Principal Investigator, Genesis.
There are no new kinds of atoms in the solar wind that are not already on Earth. The Earth has nitrogen and silicon; the sun has nitrogen and silicon, and so on. The difference in the solar particles is one of quantity: the relative amounts of nitrogen and silicon are different on the Earth and in the sun. Genesis will measure the relative amount of elements in solar matter.
AM: What do you expect to learn from the sample return?
DB: Planetary scientists are interested in matter from the solar surface because it has essentially preserved the chemical and isotopic composition of the solar nebula from which all planetary materials formed. The difference in composition between solar nebula material that we capture with Genesis and any other material – for example, cometary dust grains – is a major constraint on theories of how the material formed.
AM: So what will knowing the composition of the solar nebula tell us that we don’t yet know?
DB: Again, it’s an issue of quantity. For example, instruments on spacecraft have measured that the ratio of oxygen-18 and oxygen-16 on the sun is within 10 to 20 percent of that on the Earth. But, we also know that different planetary materials differ in this ratio by up to 5 percent.
There are a variety of theories on why the planets differ by 5 percent, and these theories predict what the solar composition should be. So our sample should allow us to decide which of these theories is correct – although it’s possible none of them will be correct. Our view of how we went from dust grains to planets will be correspondingly affected.
AM: Speaking of "quantity," why is the sample size so tiny – only 10 to 20 micrograms? Wouldn’t a larger sample give more information?
DB: We would love to have a kilogram of solar matter, but this just isn’t feasible. We believe that we can meet our science objectives with the amount of material that we have collected.
AM: Why isn’t it feasible – is it due to the size or composition of the collectors, the time, or the rate of solar wind?
DB: The area and exposure time are the practical issues. We made the largest collectors we could and still get them in the launch vehicle. The exposure time was a compromise between what we needed to meet our goals and how much the mission would cost.
|Nebula NGC-6751, Hubble Space Telescope.
AM: Your mention of the mission costs reminds me that the collectors are made of silicon, gold, sapphire and diamond. Why were these materials needed?
DB: Those are only 4 of 13 different collector materials we are using. The primary criterion was that the material be pure because the amounts of solar wind collected are small. The materials were also selected based on our plans to analyze specific elements. The simplest example is that we can’t use a silicon collector to measure silicon in the solar wind, but there is more to it than this.
AM: When the sample return capsule parachutes towards the ground in September, a helicopter will snatch it in mid-air. Why is this dramatic mid-air rescue necessary to retrieve the capsule?
DB: Our materials were fairly strong when launched. But they now have some holes and cracks from micrometeorite impacts and are much weaker. We also have an electrostatic mirror – a concentrator – which it is very important to recalibrate after recovery. The midair recovery eliminates these concerns.
AM: What will happen if the helicopter misses?
DB: The helicopter won’t miss if it’s there. Our main concern is really bad luck with the weather that grounds the helicopters. If the helicopters are grounded, then the capsule will parachute to the ground. Its horizontal motion and any winds will make it tumble when it hits. We will potentially than have losses of material due to breakage and scratching of the collector materials. We would then have to spend a lot of time sorting through broken pieces rather than doing science. Most importantly. if we lose the ability to recalibrate the concentrator, we degrade the quality of the data for our most important science objectives. Parachuting to the ground would not be mission failure, but we would take a significant hit.
AM: The phrase "sample return" can raise the hackles of people worried about contaminating the Earth. How do you answer the concerns raised by scientists or the general public?
DB: I can’t think of any sample return that should raise hackles. In our case, the solar wind represents material from a plasma with a temperature of a million degrees. There are no molecules, let alone organisms in our sample. We are collecting no element that is not already on Earth in billions of billions times larger amounts.
AM: How long after the retrieval of the samples can we expect to hear about the results?
DB: The one downside of sample return missions is that the laboratory analyses can take time. We have set up an "Early Science Return," which will be the initial publication of Genesis science results. In the worst case scenario, this issue could take as long as 18 months to come out. We will get it out as soon as possible, short of compromising science quality.