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From the beginnings of our solar system four and a half

billion years ago, there remained tantalizing clues to its

evolution. Remnant debris, asteroids and comets. They vary

in size from grains of dust to mountainsides, from footballs

to planetoids. They were the building blocks of the planets

and perhaps carry the origins of life itself. Now within our

grasp, these rocks of ice and dust are ready to give up their secrets.

Asteroids are believed to be made of chondrilles, flash

-heated grains of rock within the stellar disk of our

forming solar system. These chondrilles cluster together,

forming the first asteroids and the building blocks of the

planets. Once the solar system had evolved, there was a lot

of asteroid material left over. They cover a broad spectrum

of types. The largest of these are minor planets or

planetoids, large enough to have an ovoid shape. This

category took the previous planet Pluto off the major list

and onto the minor. The smallest remnants of debris are

often called meteoroids. There are in fact several minor

planets. Some have been seconded into planetary orbit and

become moons. The traditional asteroid belt between Mars and

Jupiter has one called Ceres, the largest and the first to

be detected. Our first close-up of an asteroid was courtesy

of Galileo on its flight through the main asteroid belt

towards Jupiter. It photographed 951 Gaspera, an S-type

asteroid with an average diameter of just over 6 kilometers.

The S stands for stony composition. Galileo then

photographed the larger 243 Ida at 15.5 kilometers wide,

revealing that it has its own moon named Dactyl. Asteroids

are not limited to the asteroid belt between the orbits of

Mars and Jupiter. Many orbit much closer to Earth and

are known as near-Earth objects or NEOs. Raider is a very

powerful instrument that we use to study near-Earth

asteroids. Asteroid Tutates was millions of kilometers away

and we were able to resolve surface rocks. We could see

boulders. There are currently only two radar facilities in

the world that have sufficient sensitivity for doing regular

observations of near-Earth objects, Arecibo and Goldstone.

Even the most powerful optical telescopes, and I'm talking

even Hubble Telescope, they can only see this asteroid as a

point of light. It is just too far and too small. It

provides an extraordinary opportunity to get very detailed

radar images. You are transmitting microwaves. It's

propagating at the speed of light toward the asteroid. It is

bouncing back. And this radar echo is containing surface

features of the asteroid. It's telling us about its

rotation. And it's very precisely pinpointing its distance

from the radar. These asteroids were imaged with ground

-based radar. BL86 revealed it has its own moon. And

asteroid HQ124 passed very close to Earth, some three and a

quarter times the distance to the moon. It is due to return

sometime in the 24th century. Scientists are looking much

more closely at these objects for their potential to pass

through Earth's orbital plane and perhaps pose a threat.

The most common type of asteroid is the C-type,

Carbonaceous, accounting for about 75% of known asteroids.

The probe near Shoemaker was the first dedicated asteroid probe launched by NASA.

It photographed 253 Matilda, C-type, then moved onto 433

Eros, the largest visited at the time, where it orbited,

took extensive measurements and, more by accident than good

planning, landed on the asteroid, the first probe to do so.

Deep Space One, an experimental NASA probe, was sent to

investigate an asteroid, 9969 Braille. Technical errors

returned poor imagery. However, the probe continued onto

its second rendezvous for the first time with a comet, 19P

Borrelli. Comets are closely related to asteroids, but

originate from the cold, dark outer boundaries of our solar

system. Comets are bodies in our solar system that have

been left over ever since the solar system formed, some 4.5

billion years ago. And therefore, when we look into comets,

we look into the past of our solar system. And so, by

investigating the details of comets, how they formed, how

they evolved, we can actually have a glimpse into how our

solar system formed and, in the end, how the Earth formed and why we are here.

Comets have been recorded throughout history, as they are

easily observed when close to the sun, often considered an

omen. One comet of note was Halley's.

In 1986, Halley's comet returned once again, and this

time it was met with a veritable armada of space probes.

The first attempt at a space rendezvous was with the

International Comet Explorer, or ICE. It passed through the

tail of Comet 21P Jacobini-Zinner on its way to meet Halley.

The European Space Agency sent Giotto. The Russian and

French sent two probes via Venus, Vega 1 and Vega 2. Japan

sent Suisei and Sakikage, that country's first deep space

probes. Their measurements went on to refine the targeting

for Giotto to make a much closer pass of the comet's nucleus

than first planned. In 1994, astronomers and scientists were

given an unexpected treat. Comet Shoemaker-Levy broke apart

and struck Jupiter in a spectacularly violent fashion.

Comets required more study. The Stardust probe was

dispatched to investigate 5535 Anne Frank, Wild 2, and then

the Temple 1 comet. It returned a sample of cometary tail to

Earth. Our biggest discovery that we did was looking at this

cometary material that was returned from NASA's Stardust

mission. And the Stardust mission went up, rendezvoused

with the comet, brought back very small amounts of material,

comet material and comet-exposed material. We had basically

one shot at looking at this and it was really pushing our

limits of detection. So I spent about two years optimizing

our technique, really rehearsing, practicing, getting

everything as perfect as possible before the one day of

doing measurements. It's sort of all leading up to one big

game, one big day. And also just working with meteorites

and working with the cometary material, I'm working with

something that's four and a half billion years old that very

few people ever get to play with. And the few days of being

able to do the actual measurements make up for all of the rehearsals that it takes.

JAXA launched Hayabusa to study asteroid 25143 Itokawa and

to retrieve a sample from the surface in a touch-and-go

maneuver. The mission took a total of seven years to

accomplish, with the sample return pod retrieved from the

Australian outback in 2010.

Launched a year earlier by the European Space Agency was a

very ambitious spacecraft called Rosetta. Its goal? To land

a probe on a comet, 67P Churyumov-Gerasimenko. Just getting

there was to prove a challenge in astro-navigation. But when

you want to rendezvous with a comet, you have to accelerate

the spacecraft and match the same velocity that the comet

has around the sun. So this is the problem, not only the

distance but also the velocity. There is no rocket that can

give us the velocity needed to be as fast as the comet. I

close to a planet and you use the gravitational attraction

of the planet to actually accelerate your spacecraft.

It passed by asteroids 2867 Steins and 21 Lutetia.

Lutetia is a very strange target, a very strange asteroid.

We believe that it may be a C-class asteroid, which means

that it is very primitive. However, it shows from ground

-based and also space-borne observations that Lutetia does

not look completely like a C-type asteroid and we are really

puzzled about what it really may be. The spacecraft then

moved on to its primary target, Comet 67P. The nucleus is

pulling the spacecraft out of its planned orbit and that can

be seen as a shift in frequency of the transmitted radio

signal from the spacecraft. And the extent of this frequency

shift is a measure of the mass of the comet nucleus, so we

are able to weigh the nucleus here. There is no ice at the

top, so it's covered by a mantle that we consider is

essentially made of organic material, that's why it's very

dark. And this material is one of the key things we would

like to explore and analyze. These organics may hold the

secret to life on Earth. What it's all about is the carbon

chemistry. How much did the comets bring to Earth? So was it

just the right elements, the right building blocks, or

was there more information in it when these comets already

arrived? To try and answer these questions, ESA attempted

one of the most daring missions mankind has ever undertaken,

to land a probe on the surface of the comet.

Landing on a comet is one of the hardest things that has

ever been done by the human species. This is the comet. It's

roughly a one in thousand model, so the real thing is a

thousand times bigger. The landing site is roughly here,

which we are aiming for to deliver the lander. It's the

flattest part we could find. What we are studying at the

comet with the instruments are basically what are the

ingredients, which materials are present, and coming back to

do one of the objectives of the mission, how complex are the

materials present at the comet. Landing means flying very,

very slowly over the comet and then gently pushing away the

lander. It's not a landing like you can imagine on the moon,

where you come with rockets and you have to break. Here, the

problem is the opposite. You have to really touch gently the

comet. The forces in board are very small. If I get

meaningful data, that would be just marvelous. If the

descent works, the landing is okay, we receive a sample, and

the whole thing runs smoothly. That would be just great. But

we need a lot of luck, really. We had a lot of luck already.

So we are sitting on the surface. Filet is talking to us.

More data to come and to be honest right now. Going down,

which it should do, of course, we are there. It's done its

job. We are on the comet. The science has started now. We

have the first results that give us the first comprehension

of what we think the comet is, where it started from. Now,

for the rest of the year, we will watch how the comet

evolves. We will unlock how the comet works. We are looking

at where the gas and the dust start to accelerate from the

surface and how that beginning of the coma, that birth of

the coma works. So how the coma develops as it does

to higher altitudes. This region has only ever been

theoretically constrained or modeled. These will be the

first measurements we make in this area or this region, and

that's a really big, important target for us. Eventually,

the tiny probe shut down. Having Filet reactivated is not so

likely, but it's not impossible. Filet was designed to

hibernate, was designed to switch off and be able to

reactivate itself. Of course, we expected this to be a

duration of a few days or a few weeks, not a few months, but

we will see. Maybe we are lucky and the units have survived

these months and will reactivate in June, July. While

observing the asteroids, scientists were surprised to find

one with what looked like a cometary tail. After careful

study, scientists realized they were observing the results

of the impact of two asteroids. 596 Schela had been struck

at high speed by a small asteroid, the impact

hit with the force of a 100 kiloton nuclear bomb.

NASA had done something similar with Deep Impact, a probe

sent to Comet Tempel 1, where it dispatched a kinetic

impactor, which struck the comet to study the

impact and the debris thrown up as a consequence.

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Soon after, NASA launched another small ion-powered probe,

Dawn, which also had an extraordinary mission, to travel

deep into the asteroid belt between Mars and Jupiter.

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Its targets? Two of the largest asteroids in the solar system.

Dawn rendezvoused with Four Vesta and orbited

it for over a year, returning a wealth of data.

Dawn then departed and cruised towards Ceres, the largest of

the asteroids, a planetoid, where it has obtained orbit and

begun its study.

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JAXA, the Japanese space agency, has recently launched a

second Hayabusa probe, this one with many improvements over

the first. Its target is the C-type asteroid 1999 JU3.

It's expected to reach its destination in three

years, collect samples and return to Earth by 2020.

Music NASA has announced the OSIRIS-REx sample return

mission to asteroid 1999 RQ-36, better known as Bennu.

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It's expected to launch sometime in the near future and,

after a two-year journey, orbit and map the surface before

touching Dawn to retrieve two kilograms of material.

The probe sample's return is expected in 2023.

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Music There is also a practical reason to study asteroids.

In 2013, an asteroid with a mass of about 9,100 tons

exploded over Chelyabinsk, Russia, with the force of 20

Hiroshima bombs, causing 1,500 injuries and damaging 7,000 buildings.

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It isn't the first asteroid strike on Earth, as the

dinosaurs can attest to, and probably not the last.

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Through the United Nations, ESA and other major

space agencies have established a safeguard program.

The NEOWISE data have returned two very important findings.

First, we've been able to determine that we've found 93

percent of all the near-Earth asteroids that are out there

that are larger than one kilometer. We've also been able to

tell that there are somewhat fewer near-Earth asteroids that

are larger than 100 meters than were previously thought.

However, fewer does not mean none. That leaves about 15,000

asteroids larger than 100 meters that remain to be found.

This advisory group is also planning intervention missions

if needed. We think that we can cope with deflecting an

asteroid with two different technologies mainly. One is what

we call kinetic impactor, hitting the asteroid and pushing

it out of the way. The second one is take a heavy spacecraft

and use it as, say, a gravity tractor. So by the mass of the

spacecraft, you pull the asteroid away.

There is one project in the planning stage to snag a small

asteroid in the near-Earth region and drag it into a lunar

orbit.

There it can be met by astronauts aboard an Orion capsule,

who will study the asteroid firsthand, take extensive

samples, and return to Earth.

For the more we know, the better prepared we

are to protect our place in the solar system.

NASA Jet Propulsion Laboratory, California Institute of Technology