Frequently Asked Questions

    What is VeSpR?

    VeSpR stands for the Venus Spectral Rocket. Simply put, it's a rocket telescope.

    To be more precise, it is a telescope that will be mounted inside a sounding rocket. This is a suborbital rocket that will take the telescope to a height of 300 km in less than 5 minutes—well into outer space (defined as 100 km altitude), and above most of the earth's atmosphere so we can observe ultraviolet (UV) light from Venus that would otherwise be absorbed by the Earth's atmosphere. In fact, this is over half as high as the Hubble Space Telescope, but our rocket won't be carrying enough speed to go into orbit around the earth. Instead, it will fall back to earth on a ballistic trajectory less than 10 minutes after launch, where we will recover the payload (the telescope) to use again.

    In between, we should have almost seven minutes of flight time above our mission goal of 110 km with which to collect data—three and a half on the way up, and three and a half on the way back down. Our goal is to collect at least 250 seconds (4 minutes, 10 seconds) of observations. That leaves the telescope only about two and a half minutes to find and lock onto Venus.

    Four minutes and ten seconds of data may not sound like a lot, but our telescope's optics are specially designed for our spectroscopic observations. In fact, because our telescope is so efficient for the UV light that we will be observing, it can observe in 5 minutes what would take Hubble four hours to observe.

    Why not use Hubble anyway? At least you wouldn't have to send up another rocket!

    They won't let us—believe us, we've asked! It's ok, though, we're not offended. They have a good reason: Hubble isn't allowed to point too close to the sun to avoid damaging its instruments. Since Venus orbits closer to the sun than earth, it always appears close to the sun in the sky. This is why we can only see it in the morning or evening—just as the sun is rising or setting—and it makes it too dangerous for Hubble to look at.

    To consider the extreme case, if Hubble were to be aimed directly at the sun for observations, the result would be disastrous. We all know you can start fires by focusing sunlight with a magnifying glass just a few centimeters across—imagine what would happen to the innards of Hubble from being blasted by the focused beam of light from its 2.4 meter (7.8 foot) wide mirror!

    But even if Hubble's mirror isn't pointed directly at the sun (as would be the case with Venus), sunlight falling on the inside of the telescope tube can be dangerous to Hubble's sensitive equipment by heating its surfaces too quickly and releasing paint or gas. And when it comes to sunlight, space is a harsh environment by virtue of its sheer emptiness—without the atmosphere in the way, the sun feels roughly twice as strong in earth orbit as it does on Earth's surface.

    So what about the rocket that it will be on?

    The rocket is a two-stage Terrier-Black Brant Mk 1 system. The first stage is a Terrier missile, originally built as a surface-to-air missile (SAM) and used by the Navy in the 1960s. Since being decommissioned, they have been used regularly as first stages for research missions such as ours. In our case, it's been modified to mount a Black Brant Mk 1 sounding rocket on top of it as its second stage. The Mk 1 model is a late-90s update of the Black Brant sounding rocket, which has been in production by its Canadian manufacturer, Bristol Aerospace, since the 1960s and is renowned for its reliability.

    The Terrier stage fires for only 6 seconds after launch before it burns out and separates from the Mk 1, having accelerating from zero to 2100 km/hr (1300 mph). The Black Brant coasts upward for 6 seconds, before firing for about 30 seconds. That is all that's needed to take the payload to a speed of over 7800 km/hr (4800 mph).

    At that point the rocket is 46 km high, but it already has enough speed to coast upward for almost four more minutes, reaching its peak of 300 km (186 miles) before starting its descent back to earth. The payload will make its final descent with a parachute and touch down about 80 km (50 miles) downrange of the launch site, where we will recover it to reuse it.

    This is a video of the launch of a Terrier-Black Brant rocket similar to ours (go to 2:00 to skip past the countdown).

    The rocket in the video launched from Wallops Flight Facility in Virginia. We will be performing integration testing at Wallops. Integration testing is when we hook up the payload to the rest of the rocket and ensure the electronics are communicating with each other properly in the different sections. We also test the structural stability of the entire system by shaking and bending it to see if it will break under the stresses and strains of launch. However, all sounding rockets launched from Wallops land in the ocean and aren't recovered after launch. Since VeSpR can be reused, we will be launching from White Sands Missile Range in New Mexico, where the payload will safely land in the desert.

    What is deuterium?

    We all know the chemical makeup of water—H20. That means that every water molecule has two hydrogen atoms and one oxygen atom. Usually when we think of hydrogen, we think of its simplest form, in which its nucleus contains just one proton. But hydrogen can come in other forms (called "isotopes"), as well. One of them is deuterium, which has a proton and a neutron. This makes it twice as heavy as plain old hydrogen. Water in which one or both of the hydrogen atoms are deuterium (HDO or D20) is called heavy water.

    But while deuterium may sound strange or dangerous, in fact it's perfectly natural. To be sure, it is rare, but it is not undetectable—out of about every 6,400 hydrogen atoms in the earth's oceans, one of them will be a deuterium atom. Our models of how the solar system formed tell us that all of the inner planets—Mercury, Venus, Earth, and Mars—were formed out of the same cloud of material. Thus, we are almost certain that Venus also had water, and a similar concentration of deuterium. The questions are:
    • In what forms—ice, liquid, or water vapor?
    • Was it present on the surface? It could have been that the temperature there was so hot that it could only exist high up in the atmosphere where it was cooler.
    • How much was there?

    Today, there is no water on Venus' surface in any form, and the concentration of water vapor in the atmosphere is only 0.002 percent, compared to Earth's 0.40 percent. If all the water on Venus were condensed into a liquid and spread evenly across the entire surface, it would be a 3 centimeter (~1.2 inch) deep Global Equivalent Layer (GEL). In comparison, Earth's GEL of water is over 3 kilometers (nearly 2 miles) thick! So where did Venus' oceans go?

    The short answer is they evaporated out into space. The longer answer is that it involves the greenhouse effect and photodissociation.

    What is the greenhouse effect?

    There are two scenarios for how Venus came to be the hellish place it is today—one in which water once flowed on the surface, and one in which conditions were too hot for that to ever happen. Space scientists call the first, wet scenario a moist greenhouse—where Venus' atmosphere was warm and humid, but with a surface temperature below the boiling point of water (100° C or 212° F). The second, dry scenario is called a runaway greenhouse—where the surface temperature was hotter than the boiling point, so that any water on it would have turned instantly to steam.

    Both scenarios rely on the greenhouse effect to warm the planet.

    The greenhouse effect has become a familiar term—it's what allows atmospheres to retain heat, and makes life possible here on earth. But its mechanism isn't always understood. In short, the reason for the effect is that certain gases, known as greenhouse gases, let in heat from sunlight, but block that heat from escaping when it leaves the earth as infrared waves.

    To be precise, the gases in the atmosphere (mostly nitrogen and oxygen) have a molecular structure and shape that lets sunlight through to the surface. This visible light is the energy that warms the earth and drives almost all forms of life on its surface. Indeed, the reason why we are able to see light from the sun is because the vision systems of life forms on earth evolved to be sensitive to the available light.

    But the earth doesn't just sit there growing infinitely hotter. It also re-radiates some of this energy back into space in the form of infrared light.

    However, greenhouse gases such as water vapor (H20), carbon monoxide (CO), and carbon dioxide (CO2) are shaped in such a way that they let in sunlight, but trap infrared light. This means that the infrared waves carrying heat away from the earth can't escape the atmosphere as easily, and bounce back down around the lower atmosphere. The greenhouse gases act as an insulating blanket, trapping heat.

    Now imagine early Venus as an Earth-like planet—covered in water, but cursed to inhabit an orbit closer to the sun than Earth's. The sun would feel stronger, and the oceans would evaporate more quickly, releasing water vapor—one of the strongest greenhouse gases—into the atmosphere. This water vapor works to trap even more heat, which causes the oceans to evaporate even faster, which intensifies the greenhouse effect even more, creating a feedback loop like the screeeEEEECH of feedback through a microphone. This is what climate scientists call a runaway greenhouse effect. (You may have noticed that, as stated above, we space scientists use the term runaway greenhouse in a slightly more specific context—to denote a Venus too hot for surface water.)

    But what causes all that water vapor to actually disappear out into space?

    Just as liquid evaporates from the surface and escapes the ocean for the atmosphere, gases can also "evaporate" from the atmosphere and escape away into space. But water vapor is too heavy for it to have escaped so quickly. First, it had to be broken down into its smaller, individual parts.

    But what could be so powerful as to break the molecular bonds of matter?

    Actually, it's something we're all familiar with—the sun's harmful UV rays. When they strike matter, they can tear molecules apart in a process called photodissociation. This is what gives you sunburn (and eventually, skin cancer)—photodissociation literally destroys the DNA molecules in your skin cells.

    So why hasn't water on earth been destroyed and lost to space due to photodissociation?

    Water vapor on earth has an advantage—it's shielded by the atmosphere itself. On earth, water vapor is concentrated in the troposphere, the lowest layer of the atmosphere, where all of earth's weather systems circulate. The upper edge of the troposphere is referred to as a cold trap because water vapor cannot exist above it—it's the height where the temperature drops below the dew point and water vapor condenses to form clouds. On earth, our cold trap is at an altitude of only 12 km. This keeps the water vapor beneath most of the atmosphere, which blocks out UV rays and protects the water vapor from photodissociation.

    But when the concentration of water vapor in an atmosphere reaches 20 percent, as it did on Venus, we have reached a "moist greenhouse" (and it could have quickly become a runaway greenhouse). At this point, the sheer amount of water vapor in the atmosphere means it permeates higher up into the atmosphere. The cold trap is not reached until as high as 100 km, well above most of the atmosphere's protective shield, leaving water vapor high and exposed to the sun's UV rays. Through photodissociation, it is ripped apart into its separate atoms of oxygen, hydrogen—and occasionally, deuterium.

    Even as an individual element, oxygen is so heavy that it will leak out into space rather slowly. However, deuterium, at 1/8th the mass of oxygen, will go much faster. And hydrogen, the lightest of all elements at 1/16th the mass of oxygen, will escape the fastest—expelled from the water cycle forever, lost to space. And because hydrogen evaporates the fastest, the all-important value of D/H will start to increase.

    So how do we get from a measurement of D/H to determining how much water was on Venus?

    We can compare it to our measurements here on earth, because it appears that over geological time, we've hardly lost any water. Although some hydrogen does get past the cold trap, it's much less than on Venus, and it gets replenished from within—erupting volcanoes and other geologic activity releases hydrogen from the earth's interior. Sea levels may rise and fall due to some water being locked up in the polar ice caps during ice ages, but the overall amount of water has stayed pretty much the same. Thus, we can conclude the overall amount of hydrogen has stayed the same—as has the ratio D/H. Furthermore, since all of the inner planets were formed from the same cloud of gas, dust, and rock, we can assume they all started out with the same ratio D/H.

    Today, we've measured D/H in Earth's atmosphere and oceans and found it to be approximately 0.02%. That is, of all the hydrogen and deuterium atoms, deuterium makes up about 0.02 percent of them—and presumably it was never much lower. On Mars, it's not much different—about 0.09 percent. (Its surface water was lost not due to photodissociation, but simply because its weak gravity couldn't keep water vapor bound to it—and it likely still has oceans of water locked up in ice at the polar caps and beneath its surface.) But on Venus, according to the best measurements so far, D/H is a whopping 5 percent. That makes deuterium 250 times more abundant (relative to hydrogen) than on Earth. If our assumptions about the common origins of the inner planets are correct, that means that Venus once had 250 times more hydrogen than it currently does—and therefore, 250 times more water, at least. That would be, at minimum, enough water to blanket the entire surface of Venus to a depth of over seven meters.

    Wait... so we DO know the D/H ratio of Venus? Then why do we need VeSpR at all?!

    The problem is that our measurements of D/H on Venus are not as accurate as we would like, and they seem to vary widely depending on how high in the atmosphere we measure—in particular, D/H appears to be larger in the upper atmosphere. This leaves us with some nagging uncertainty.

    This is where VeSpR comes in. VeSpR will measure the ratio of D/H, specifically in Venus' upper atmosphere, by measuring the strength of the signature light being emitted by both hydrogen and deuterium (a process called spectroscopy), and then comparing them. With this data, we can better understand the current composition and structure of the deuterium and hydrogen in Venus' atmosphere, and refine our estimate of how much water existed on Venus, and if life ever could have.

    It will be up to the theoreticians to take that data and generate computer models of Venus' evolution to determine exactly how much water there was, and if it existed on the surface in a moist greenhouse—and for how long—or if it was locked in the atmosphere in a runaway greenhouse.

    More Resources

    Venus Express: The European Space Agency's web site for the V-enus Express spacecraft, currently orbiting Venus on a science mission.

    Akatsuki: The Japanese Aerospace Exploration Agency's web site for the Akatsuki spacecraft, also known as the Venus Climate Orbiter. Currently en route to Venus, it will arrive in December 2010.

    Taylor, Fred W. "Venus: Not evil, just a bit unfortunate." Astronomy & Geophysics Volume 51, Issue 1, Feb. 2010, p.1.26-1.31. (doi:10.1111/j.1468-4004.2010.51126.x) (PDF file)

    Kasting, James F. "Runaway and moist greenhouse atmospheres and the evolution of Earth and Venus." Icarus, Volume 74, Issue 3, June 1988, p. 472-494. (doi:10.1016/0019-1035(88)90116-9) (PDF file)

    Prinn, Ronald G. and Bruce Fegley, Jr. "The Atmospheres of Venus, Earth, and Mars: A Critical Comparison." Annual Review of Earth and Planetary Sciences. Volume 15, 1987, p. 171-212. (doi:10.1146/annurev.ea.15.050187.001131) (PDF file)