Interestingly, this is something the aptly named Stardust Spacecraft did not originally set out to discover, but it’s a question that its findings have provoked. And it’s all thanks to including the lightest substance in the world onboard.
Stardust is known for its involvement with the Deep Impact NASA mission to the comet Tempel-1. However, this was not Stardust’s first mission, nor its primary one. Before Tempel 1, Stardust was out collecting data on another comet by doing something that no other probe before it had done.
Its mission was to travel to the Comet Wild 2, collect some of the material from its Coma, and then deliver the sample intact back to Earth. And given that this would involve catching particles that were moving at over 23,000 kilometers per hour, all without damaging them, this was no easy task.
We explore how Stardust managed to accomplish this incredible feat and uncover what Stardust’s findings taught us about cometary origins, as well as our own. In the late 1990s, Cometary Science was still in the early stages.
Although we had sent 6 probes up to visit these enigmatic celestial bodies, not very much was known about their origins. It was believed at the time that comets were foreign visitors to our solar system, older than the sun, having been formed from the loose pre-solar grains of dust that orbit other stars before drifting through space towards us, only to be caught up in the sun’s gravitational pull.
It was believed that this theory could be confirmed by traveling to one of these comets and picking up some of this loose dust or “Stardust” that surrounds them in space.
By examining the isotopic composition, scientists would be able to tell if it was unusual when compared to the dust given off by our star. However, this was a challenging mission. As is often the case, it came down to a question of speed and energy. Comets travel through the inner solar system at speeds reaching 160,000 km.
While a probe could try to match that speed and come up alongside it, this had to be done without needing too much fuel, or the weight of the craft would be too heavy and thus too expensive to get into space in the first place. For this mission, scientists selected a comet known as Wild 2.
They believed that they would be able to get Stardust alongside Wild 2 at a relatively low velocity. However, this velocity would still be around 6.5km per second or 23,400kmph. As you can imagine, catching even particles at that speed would be extremely challenging.
Although particles would likely not do too much damage to Stardust, being too small to impact it, it would do irreparable harm to the particles themselves. When an object crashes at 23,400 mph into a surface, the odds of it keeping its original shape and structure are incredibly small.
Scientists would not learn much about the structure of these particles if they smashed those particles into pieces, not to mention the warping effect all that kinetic energy is suddenly converted into thermal would have on the molecular bonds involved.
So, what was their solution? What was their mechanism for catching objects traveling at those speeds?
Well, much like how an airbag softens the blow for you if you are involved in a car crash, scientists realized that they would need an airbag of their own.
Something that would not halt the particle all at once, but would reduce its speed over a longer distance, thus reducing the amount of crushing deceleration involved. For this, they found an incredible material that was air. Solid air. They decided to use Aerogel.
Aerogel is a fascinating substance that was discovered in 1931 by Samuel Stephens Kistler when he made a bet with fellow scientist Charles Learned about jelly. As you have probably seen if you have ever made it yourself, jelly is formed of two parts. Firstly, a relatively solid structure that acts kind of like a sponge, and secondly, water.
When you add water to solid cubes of dense jelly, it absorbs the water and expands into the wobbly substance we are all familiar with. If you were to extract the water, the solid part of the jelly would normally contract again.
Kistler’s bet with Learned was to be the first one to remove all of the liquid from the jelly without making it shrink. In short, to make a jelly that was filled with air.
An air jelly Without going into all the details, Kistler won his bet, and at the same time invented the first aerogel. Aerogel is a fascinating substance, as it is usually over 99% air, and yet has the structural strength to support bricks.
Nowadays it tends to be made from silica composites, rather than jelly, but can be made from a wide range of materials. It is incredibly light and is strangely enough an even better insulator than regular air.
And importantly for Stardust, when particles hit it, it would offer just the right amount of resistance to slow down the particle without denaturing or destroying it. The trails left behind in the aerogel would also be useful for scientists to spot where a particle had been captured.
Stardust was fitted with tennis-racket-size centimeters an aerogel collector tray made up of 90 blocks of aerogel 3cm thick, with over 1000 square centimeters of surface area, which would be deployed from inside the main body whenever sampling was to take place.
Stardust would also capture dust from the interstellar medium, to allow comparisons and to learn more about the dust in our solar system. Once it had collected these samples, it would store them on a Sample Return Capsule, which would be fired back towards the Earth for reentry and collection.
This SRC was 0.8x0.5m, weighed 45kg, and came fitted with an aero shield, navigation recovery aids, and a parachute. Also, onboard Stardust was a navigation camera, a cometary and interstellar dust analyzer, and a dust flux monitoring system, among other scientific devices.
The probe launched on 7th February 1999 and spent the next 5 years travelingmeters through space, passing the Asteroid 5535 AnneFrank along the way, which it took some photos of. But on 2nd January 2004, it finally arrived at its target Comet Wild 2. And what it found was immediately extraordinary.
Scientists had not expected much from Wild 2. Some NASA scientists described their expectation of it to be “a rather bland object looking somewhat like a black potato”. However, this is not what they found.
Instead, the surface of Wild 2 was covered with spiky pinnacles hundreds of meters tall, cliffs, massive holes jetting dust and gas out into space, even on parts of the comet that were pointing away from the Sun and thus were expected to be less reactive.
In short, the surface of the comet was unexpectedly alive and self-renewing. Something else was just as notable for its absence, Craters.
Unlike almost every other body in our solar system with surfaces exposed to space, there were no craters on the surface of Wild 2. This puts it in stark contrast to places like Mars, or our moon. Given the period Wild 2 is thought to have existed, it surely must have encountered other objects which impacted it.
So where had these craters gone?
It shows that a comet’s surface can be either self-renewing or active, reducing signs of visible craters over short time frames, astronomically speaking. And of course, during this flyby, Stardust had its aerogel collector exposed, and it was rapidly collecting dust samples.
The samples were carefully stowed away, and upon reaching the vicinity of Earth, Stardust ejected the SRC. The angle of approach had to be just right as it was travellingtravelinghas at tremendous speed. If the approach angle was too low, it would just skim off the atmosphere and fly back into space. If the angle was too high, the heat would disintegrate the capsule.
So it was with great relief that the DC-8 NASA Airplane monitoring the sky saw it approaching at just the right second at just the right angle. The SRC landed in the Utah desert, where it was recovered, everything has worked and deployed just as it was designed to.
When taking the samples back to the lab, scientists learned another completely unexpected fact about Comet Wild 2. It was not a visitor to our solar system at all. Unlike what had previously been believed, Comet Wild 2 had not originated from another star. It had been born from our own.
By comparing the isotopic composition of the particles Stardust collected with samples from our solar system, it was proven that Comet Wild 2 originated from the solar system. And contrary to what all the ice on its surface might lead you to believe, the rock at its center was formed under white-hot conditions. Chondrules and Calcium Aluminium Inclusions were both found among the samples Stardust collected.
These are structures that only form under incredibly hot conditions and can be found in other asteroids between Mars and Jupiter. So, scientists had to rethink their theory that comets formed in cold conditions at the edge of solar systems, even if they do spend some time there.
Both fire and ice go into making comets. And thanks to the careful, delicate way that the particles had been collected, scientists were able to find one last, surprising thing – the Amino acid Glycine. Amino acids are the building blocks that make up proteins, which are vital for all living things.
Although this does not mean that there was anything alive on Comet Wild 2, this does lend weight to the idea that it was from comets such as this, crashing into our Earth millions of years ago, that life’s first building blocks found their way to our planet. Which, I’m sure you will agree, offers a tantalizing make up a glimpse into our origins.
What happened next for Stardust?
As only the SRC was sent back to Earth, Stardust remained in space and had enough fuel to visit another object, Comet Tempel-1, which is just as well as the Deep Impact mission there didn’t go as planned, and you can find out more about Stardust’s involvement with that mission here.
After this extended mission, with all its fuel used up, it sent one last transmission to Earth to acknowledge that it was being turned off for good. Comets are truly fascinating things, and it was thanks to the incredible work of the Stardust probe, and all those who worked on it, that we could make these discoveries.
Who would have expected that as we looked out across the wide universe, we would discover things that would help us to understand ourselves better? But thanks to them, we now know: that life’s origins might just lie in stardust.